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	<TITLE>XOTcl - Tutorial</TITLE>
	<META NAME="AUTHOR" CONTENT="Gustaf Neumann and Uwe Zdun">
	<META NAME="DOCNUMBER" CONTENT="2.0.0">
	<META NAME="CHANGEDBY" CONTENT="Gustaf Neumann">
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<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055" STYLE="page-break-before: always">
	<TR>
		<TD WIDTH=75%>
			<P><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>XOTcl
			- Tutorial - Index </FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic1" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>
<p align=right>Version: 2.0.0</p>

<UL>
	<LI>
	<P STYLE="margin-bottom: 0in">
	  <A HREF="#introduction">Introduction</A>
	
	<UL>
		<LI><A HREF="#langOverview">Language Overview</A>
                <LI><A HREF="#stack"> Introductory Overview Example: Stack</A>
<ul>
  <LI><A href="#object-methods">Object specific methods</<>
  <li><A href="#refining-methods">Refining the behavior of objects and classes</<>
  <li><A href="#integer-stack">Stack of integers</a>
  <li><A href="#class-specific-method">Class specific methods</a>
</ul>
                <LI><A HREF="#soccerClub"> Introductory Overview Example: Soccer Club</A>
        </UL>
	</P>
	<P STYLE="margin-bottom: 0in">
	<LI><A HREF="#object_class_system">Object and Class System </A>
	</P>
	<LI><A HREF="#basic">Basic Functionalities</A> 
	<P STYLE="margin-bottom: 0in">
	<UL>
		<LI><A HREF="#object">Objects </A>
		<UL>
			<LI><A HREF="#data_on_obj">Data on Objects </A>
			<LI><A HREF="#obj_methods">Methods for Objects</A> 
			<LI><A HREF="#obj_info">Information about Objects</A> 
		</UL>
		<LI><P STYLE="margin-bottom: 0in"><A HREF="#classes">Classes </A>
		<UL>
			<LI><A HREF="#class_instance">Creating Classes and Deriving
		  Instances</A> 
			<LI><A HREF="#class_methods">Methods Defined in Classes</A> 
			<LI><A HREF="#class_info">Information about Classes</A> 
			<LI><A HREF="#class_inheritance">Inheritance</A>
			<LI><A HREF="#class_destroy">Destruction of Classes</A> 
			<LI><A HREF="#class_method_chaining">Method Chaining</A> 
		</UL>
		</P>
		<LI><A HREF="#class_dynamics">Dynamic Class and Superclass 
	      Relationships</A> 
		<LI><A HREF="#meta-classes">Meta-Classes</A>
                <LI><A HREF="#destroy-logic">Create, Destroy, and Recreate Methods</A>
	        <LI><A HREF="#non-pos-args">Methods with Non-Positional Arguments</A>
	</UL>
        </P>
	<LI><P STYLE="margin-bottom: 0in"><A HREF="#interceptors">Message
Interception Techniques</A>

        <UL>
                <LI><A HREF="#filter">Filter</A> 
                <LI><A HREF="#mixins">Mixin Classes</A> 
                <LI><A HREF="#precedence order">Precedence Order</A>
	        <LI><A HREF="#guards">Guards for Filters and Mixins</A>
                <LI><A HREF="#updateinterceptors">Querying, Setting, Altering Filter and Mixin Lists</A>
	        <LI><A HREF="#callstack_info">Querying Call-stack Information</A> 
        </UL>
      </P>
      <LI><A HREF="#slots">Slots</A>
	<UL>
	  <LI><A HREF="#system-slots">System Slots</A>
	  <LI><A HREF="#attribute-slots">Attribute Slots</A>
          <LI><A HREF="#setter">Setter and Getter Methods for Slots</A>
          <LI><A HREF="#parameter">Backward-compatible Short-Hand Notation
          for Attribute Slots</A>
          <LI><A HREF="#slot-experimental">Experimental Slot Features</A>
          <UL>
            <LI><A HREF="#value-checking">Value Checking</A>
            <LI><A HREF="#trace-commands">Init Commands and Value Commands for Slot Values</A>
          </UL>
	</UL>
	<P>
    
        <LI><A HREF="#nesting">Nested Classes and Dynamic Object Aggregations</A>
        <UL>
          <LI><A href="#nested-classes">Nested Classes</A>
          <LI><A href="#obj-agg">Dynamic Object Aggregations</A>
          <LI><A href="#nest-agg">Relationship between Class Nesting
          and Object Aggregation </A>
          <LI><A href="#contains">Simplified Syntax for Creating
          Nested Object Structures</A>
          <LI><A href="#copy-move">Copy/Move</A>
        </UL>
        <p>
        
        <LI><A HREF="#forwarding">Method Forwarding</A>
        
        <LI><A HREF="#assertions">Assertions</A>
	<LI><P STYLE="margin-bottom: 0in">
	  <A HREF="#additional-functionalities">Additional Functionalities</A> 
	<UL>
		<LI><A HREF="#abstract-classes">Abstract Classes</A> 
                <LI><A HREF="#autonames">Automatic Name Creation</A> 
                <LI><A HREF="#meta-data">Meta-Data</A>
	</UL>
	</P>
        <LI><A HREF="#cext">Integrating XOTcl 
	    Programs with C Extensions (such as Tk)</A> 
        <LI><A HREF="#references">References</A> 
</UL>

<!-- PAGE BREAK -->

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="introduction"></A><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Introduction
			</FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic2" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>


<H2><A NAME="langOverview"></A>&#160;<BR>Language Overview
</H2>

<P>XOTcl <a href="#xotcl">[Neumann and Zdun 2000a]</a> is a successor
of the object-oriented scripting language OTcl <a
href="#otcl">[Wetherall and Lindblad 1995]</a> which itself was an
early highly flexible object oriented exitension of 
Tcl <a href="#tcl">[Ousterhout 1990]</a> (Tool Command Language).
XOTcl was so far released in more than 30 versions, is described in
its detail in more than 20 papers and serves as a basis for TclOO <a
href="#tcloo">[Donal ???]</a>. XOTcl 2.0 <a
href="#xotcl2">[Neumann and Sobernig 2009]</a>
extends the basic ideas of XOTcl 1.0 by providing support for
language-oriented programming and makes it easy to host several
object oriented languages by a common environment...


<P>XOTcl runs in the <tt>tclsh</tt> and provides a few extension
commands. These are offered via the Tcl namespaces <tt>::xotcl</tt>
and <tt>::xotcl2</tt>, and can be imported into the current namespace
to reduce typing and improve readability. All Tcl commands remain
available (and are also applicable on the extension constructs).  </P>

<P>A central property of Tcl is, that it uses strings solely for the
representation of data. Internally it uses an dynamic type system with
automatic conversion (which enables efficient type handling).  For
that reason all components (e.g. written in C) once integrated in Tcl
automatically fit together and the components can be reused in
unpredicted situations without change. The evolving <EM>component
frameworks</EM> provide a high degree of code reuse, rapid
application development, and ease of use. The application developer
may concentrate on the application task solely, rather than investing
efforts in fitting components together.  Therefore, in certain
applications scripting languages like Tcl are very useful for a fast
and high-quality development of software (see <a
href="#ousterhout">[Ousterhout 1998]</a> for more details).
</P>
<P>Tcl is equipped with appropriate functionalities for the easy
gluing of components, like dynamic typing, dynamic extensibility, and
read/write introspection. OTcl is an object-oriented extension to Tcl,
which encourages a Tcl-like programming style and is composed of
language constructs with properties similar to Tcl. It offers an
object-orientation with encapsulation of data and operation without
protection mechanisms and single and multiple inheritance.
Furthermore it enables to change the relationships dynamically, offers
read/write introspection, has a three level class system based on
meta-classes and offers method chaining. These abilities are
integrated in XOTcl with only slight changes to OTcl visible to the
programmer.
</P>
<P>Extended Object Tcl aims at complexity and adaptability issues that
may occur in context of large (object-oriented) software structures
and in the context of component glueing. In particular we added the
following support:
</P>
<UL>
	<LI><P STYLE="margin-bottom: 0in"><I>Filters</I> as a means of
	abstractions over method invocations to implement large program
	structures, like design patterns. 
	</P>
	<LI><P STYLE="margin-bottom: 0in"><I>Mixin Classes</I>, as a
	means to give an object or a classes' instances access to several different supplemental
	classes, which may be changed dynamically. 
	</P>
	<LI><P STYLE="margin-bottom: 0in"><I>Dynamic Object Aggregations</I>,
	to provide dynamic aggregations through nested namespaces. 
	</P>
	<LI><P STYLE="margin-bottom: 0in"><I>Nested Classes</I>, to reduce
	the interference of independently developed program structures. 
	</P>
	<LI><P STYLE="margin-bottom: 0in"><I>Assertions</I>, to reduce the
	interface and the reliability problems caused by dynamic typing and,
	therefore, to ease the combination of components. 
	</P>
	<LI><P STYLE="margin-bottom: 0in"><I>Forwarders</I>, to delegate
          calls efficiently to other objects or classes.
	</P>
  	<LI><P STYLE="margin-bottom: 0in"><I>Slots</I>, to manage values
          of instance variables with a common interface.
	</P> 
	<LI><P><I>Meta-data and Automatic Documentation</I>, to enhance self-documentation of objects
	and classes. 
	</P>
</UL>

<!-- PAGE BREAK -->

<P STYLE="margin-bottom: 0in"><BR>
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><A NAME="features"></A><A NAME="1176"></A>
&#160;&#160; 
</P>

<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 1:</STRONG>
Language Extensions of XOTcl <br><span class="fixme">new graphic,
extension of the features above, history with OTcl, XOTcl1, tcloo and
XOTcl2</span> </P>

<CENTER>
	<TABLE WIDTH=455 BORDER=0 CELLPADDING=2 CELLSPACING=0>
		<COL WIDTH=451>
		<TR>
			<TD WIDTH=451>
				<P><IMG SRC="features.gif" NAME="Graphic3" ALIGN=BOTTOM WIDTH=451 HEIGHT=378 BORDER=0></P>
			</TD>
		</TR>
	</TABLE>
</CENTER>

<H2><A NAME="stack"></A>&#160;<BR>Introductory Overview Example: Stack</H2>
        
<p>
To give you an impression of the language before we go into the details of the extended language constructs, we present in this section a simple, classical example, familiar to many from introductory programming courses: the <em>Stack</em> example. In the later section, we will present the <em>soccer club</em> example, which focuses more on the dynamic features of the Extended Object Tcl.
</p>
        
<p>In a first step, we define a class <tt>Stack</tt>. A new class is defined in XOTcl via the command <tt>Class create yourclass</tt>. The stack will have a constructor (in XOTcl, the method <tt>init</tt>) and the methods <tt>push</tt> and <tt>pop</tt>. In the following example, all predefined commands (some from Tcl, some from XOTcl) are emphasized.
</p>
        
<pre CLASS="code">
<it>#</it>
<it># Create a stack class </it>
<it>#</it>
<tt>Class create</tt> Stack {
        
  <tt>:method</tt> init {} { <it># Constructor</it>
    <tt>set</tt> :things ""
  } 

  <tt>:method</tt> push {thing} {
    <tt>set</tt> :things [<tt>linsert</tt> ${:things} 0 $thing] 
    <tt>return</tt> $thing
  }
  
  <tt>:method</tt> pop {} {
    <tt>set</tt> top [<tt>lindex</tt> ${:things} 0]
    <tt>set</tt> :things [<tt>lrange</tt> ${:things} 1 end]
    <tt>return</tt> $top
  }
}
</pre>
<p>
The three methods are defined via <tt>:method</tt> (which means:
define a method for the current class). Variables are set with the
Tcl command <tt>set</tt>. Variable names starting with a dot "<tt>.</tt>"
are treated as instance variables (variables of an instance of the
Stack, i.e. an Stack object). The other variables are scoped to
the methods.
</p>
<p>
The definition of the class <tt>Stack</tt> is typically saved in a file (say <tt>stack.xotcl</tt>)
and can be used e.g. in an interactive Tcl shell (<tt>tclsh</tt>) as follows. The percent sign
indicates the prompt of the Tcl shell, the reminder of the line is typed in, the result
of the command is shown in the line below. Comments are lines starting with a hash symbol <tt>#</tt>.

<pre CLASS="code">
% <tt>package require</tt> XOTcl
% <tt>::xotcl::use</tt> xotcl2
% <tt>source</tt> stack.xotcl
  
<it># Create Object s1 of class Stack</it>
% Stack <tt>create</tt> s1
::s1
% s1 push a
% s1 push b
b
% s1 push c
c
% s1 pop
c
% s1 pop
b
<it># Delete object s1</it>
s1 <tt>destroy</tt>
</pre>        

<p>In the session above, we load XOTcl into the current shell, import the names from the xotcl namespace and we load the file <tt>stack.xotcl</tt>. At this time, the class <tt>Stack</tt> is available in the scripting session. In the next step, we create an stack object named <tt>s1</tt> and push into the stack the values <tt>a</tt>, <tt>b</tt> and <tt>c</tt> via separate push calls. Then we pop values from the stack and we destroy finally the stack <tt>s1</tt>.

<H3><A NAME="object-methods"></A>&#160;<BR>Object specific methods</h3>

<p>The definition of <tt>Stack</tt> provided above is pretty similar
to stack definitions in many other object oriented languages. The next
example shows how to define purely object specific behavior. We can
define an object <tt>stack</tt> without the need of a class
<tt>Stack</tt>. Notice that the methods of the object <tt>stack</tt>
are defined exactly the same way as in the previous example with the
class <tt>Stack</tt>. Instead of defining a constructor, we can
set the instance variable <tt>things</tt> directly in the
definition block of the object.

<pre CLASS="code">
<it>#</it>
<it># Create an object named stack</it>
<it>#</it>
<tt>Object create</tt> stack {

  <tt>set</tt> :things ""

  <tt>:method</tt> push {thing} {
    <tt>set</tt> :things [<tt>linsert</tt> ${:things} 0 $thing] 
    <tt>return</tt> $thing
  }
  
  <tt>:method</tt> pop {} {
    <tt>set</tt> top [<tt>lindex</tt> ${:things} 0]
    <tt>set</tt> :things [<tt>lrange</tt> ${:things} 1 end]
    <tt>return</tt> $top
  }
}
</pre>

<p>The object <tt>stack</tt> can be used in exactly the same way as <tt>s1</tt> (the instance of class <tt>Stack</tt>) before.

<H3><A NAME="refining-methods"></A>&#160;<BR>Refining the behavior of objects and classes</h3>

<p>So far, the definition of stacks were pretty minimal. Suppose, we want to define "safe stacks", that check e.g. for stack underruns (more pop than push operations are issued).  Checking safety can be done mostly independent from the implementation details of the stack (usage of internal data structures).
With XOTcl, one can define stack-safety as a separate class using methods with the same names as the implementations before, and "mix" this behavior later into classes or objects. The implementation of <tt>Safety</tt> uses a counter to check for stack underruns.</p>

<pre CLASS="code">
<it>#</it>
<it># Create a safety class </it>
<it>#</it>
<tt>Class create</tt> Safety {
        
  <tt>:method</tt> init {} { <it># Constructor</it>
    <tt>set</tt> :count 0
    <tt>next</tt>
  } 

  <tt>:method</tt> push {thing} {
    <tt>incr</tt> :count
    <tt>next</tt>
  }
  
  <tt>:method</tt> pop {} {
    <tt>if</tt> {${:count} == 0} <tt>then</tt> { <tt>error</tt> "Stack empty!" }
    <tt>incr</tt> :count -1
    <tt>next</tt>
  }
}
</pre>

<p>When we load the classes <tt>Stack</tt> and <tt>Safety</tt> into the same script,
we can define e.g. a certain stack <tt>s2</tt> as a safe stack, while all other stacks
might be still "unsafe". This can be achieved via the option <tt>-mixin</tt> during
object creation.
</p>
<pre CLASS="code">
% Stack <tt>create</tt> s2 <tt>-mixin</tt> Safety
::s2
% s2 push a
% s2 pop
a
% s2 pop
Stack empty!
</pre>

Note that the definition of Safety can be used not only for instances
of the class <tt>Stack</tt>, but for arbitrary objects supporting the
same interface.  We can as well use <tt>Safety</tt> to create a new
class <tt>SafeStack</tt>. In this case, all instances of
<tt>SafeStack</tt> have the safety property defined above.

<pre CLASS="code">
<it>#</it>
<it># Create a safe stack class by using Stack and mixin </it>
<it># Safety </it>
<it>#</it>
<tt>Class create</tt> SafeStack <tt>-superclass</tt> Stack <tt>-mixin</tt> Safety

SafeStack <tt>create</tt> s3
</pre>

<H3><A NAME="integer-stack"></A>&#160;<BR>Stack of integers</h3>

<p>The definition of <tt>Stack</tt> is generic and allows all kind of
elements to be stacked. Suppose, we want to use the generic stack
definition, but a certain stack (say, <tt>s4</tt>) should allow only
stacking of integers. This behavior can be achieved by defining an
object specific method for the stack <tt>s4</tt> that checks the
values to be pushed. In case the pushed value is ok, the push
definition of <tt>Stack</tt> is called via <tt>next</tt>.

<pre CLASS="code">
<it># </it>
<it># Create a stack with a object-specific method </it>
<it># to check the type of entries </it>
<it>#</it>
<it># s4 is a stack of integer </it>
 
Stack <tt>create</tt> s4 {

  <tt>:method </tt> push {value} {
    <tt>if </tt> {![<tt>string is</tt> integer $value]} {
      <tt>error</tt> "value $value is not an integer"
    }
    <tt>next</tt>
  }

}
</pre>

<H3><A NAME="class-specific-method"></A>&#160;<BR>Class specific methods</h3>

<p>In extended object Tcl, classes are objects as well (objects with
certain properties; we will come to this later in more
detail). However, we can define as well methods of classes, which are
not inherited to the instances, but which are to be applied on the
class object itself. This can be achieved by the modifier
<tt>object</tt> which is placed in front of <tt>method</tt>. Such
methods defined on the class object are actually exactly same as the
object specific methods in the example with the object named
<tt>stack</tt> above.</p>

<p>In the following example, we will define the method
<tt>available_stacks</tt> on the class object, that returns the number
of the currently existing stack instances.  </p>

<pre CLASS="code">
Class <tt>create</tt> Stack {

   <it># ...</it>
    <tt>:class-object method</tt> available_stacks {} {
      <tt>return</tt> [<tt>llength</tt> [<tt>:info</tt> instances]]
   }
}

Stack <tt>create</tt> s1
Stack <tt>create</tt> s2

<tt>puts</tt> [Stack available_stacks]
</pre>
<p>The final command <tt>puts</tt> prints 2 to the console.</p>

<H2><A NAME="soccerClub"></A>&#160;<BR>Introductory Overview Example: Soccer Club
</H2>
<p>
In our second example, we will focus on an application example where one can benefit substantially from the dynamic language constructs of XOTcl, the soccer club example (the full code can
be found in the <tt>xotcl/src/scripts/soccerClub.xotcl</tt> file.  All
the persons and characters in this example are fictitious, and any resemblance to
actual persons, living or deceased, is coincidental.
</p>

<p> Before we start, we introduce an instrument for making the documentation of programs more easy. In order to document source code files, we can use the <tt>@</tt> object, which is used generally to provide any kind of information, meta-data, and documentation on a running program. Here, we just give a file description.  Then the <tt> makeDoc.xotcl</tt> tool can automatically document the program file later for us.  </p>

<pre CLASS="code">
  @ @File {
    description {
      This is a simple introductory example for the language XOTcl. 
      It demonstrates the basic language constructs on the example of
      a soccer club.
    }
  }
</pre>
<p>

All things and entities in XOTcl are objects. A special kind of objects 
are classes. Classes define common properties for other objects. For a
soccer club, we firstly require a common class for all kinds of members.
</p>
<p>
Common to all members is that they have a name. Common properties
defined across all instances of a class are called 'parameter' in
XOTcl. In this example the instance variable <tt>name</tt> will be
initialized by default with an empty string.
</p>
<pre CLASS="code">
  <tt>Class create</tt> ClubMember <tt>-parameter</tt> {{name ""}}
</pre>
<p>
A special club member is a <tt>Player</tt>. Derived classes can be
build with inheritance (specified through
<tt>superclass</tt>). Players may have a <tt>playerRole</tt> (defaults
to NONE).
</p>
<pre CLASS="code">
  <tt>Class create</tt> Player <tt>-superclass</tt> ClubMember <tt>-parameter</tt> {{playerRole NONE}}
</pre>
<p>
Other club member types are trainers, player-trainers, and presidents:
</p>
<pre CLASS="code">
  <tt>Class create</tt> Trainer <tt>-superclass</tt> ClubMember
  <tt>Class create</tt> President <tt>-superclass</tt> ClubMember
</pre>
<p>
The PlayerTrainer uses multiple inheritances by being both a player
and a trainer:
</p>
<pre CLASS="code">
  <tt>Class create</tt> PlayerTrainer <tt>-superclass</tt> {Player Trainer}
</pre>
<p>
Now we define the SoccerTeam class:
</p>
<pre CLASS="code">
  <tt>Class create</tt> SoccerTeam <tt>-parameter</tt> {name location type}
</pre>
<p>
We may add a player by using method. Methods can be defined
in XOTcl2 either by <tt>:method</tt> in the class creation block, or
via "<tt><em>ClassName</em> method ...</tt>". The added players (as
well as other club members) are aggregated in
the object of the soccer team (denoted by :: namespace syntax).
</p>
<pre CLASS="code">
  SoccerTeam <tt>method</tt> newPlayer args {
    <it># we create a new player who is part of the soccer team</it>
    <it># "eval" is needed to pass the provided arguments separately to the call of new</it>
    <tt>eval</tt> Player <tt>new</tt> -childof [<tt>self</tt>] $args
  }
</pre>
<p>
A player can be transferred to another team. The player object does
not change internally (e.g. the playerRole stays the same). Therefore we
<tt>move</tt> it to the destination team.
</p>
<pre CLASS="code">
  SoccerTeam <tt>method</tt> transferPlayer {playername destinationTeam} {
    <it># We use the aggregation introspection option <tt>children</tt> in order</it>
    <it># to get all club members</it>
    <tt>foreach</tt> player [<tt>:info</tt> children] {
      <it># But we only remove matching playernames of type "Player". We do</it>
      <it># not want to remove another club member type who has the same</it>
      <it># name.</it>
      <tt>if</tt>{[$player info has type Player] && [$player name] eq $playername} {
        <it># We simply 'move' the player object to the destination team.</it>
        <it># Again we use a unique autoname in the new scope</it>
        $player <tt>move</tt> ${destinationTeam}::[$destinationTeam <tt>autoname</tt> player%02d]
      }
    }
  }
</pre>
<p>
Finally we define two convenience to print the members/players to
the console with <tt>puts</tt>.
</p>
<pre CLASS="code">
  SoccerTeam <tt>method</tt> printMembers {} {
    <tt>puts</tt> "Members of ${:name}:"
    <tt>foreach</tt> m [<tt>:info</tt> children] {<tt>puts</tt> "  [$m name]"}
  }
  SoccerTeam <tt>method</tt> printPlayers {} {
    <tt>puts</tt> "Players of ${:name}:"
    <tt>foreach</tt> m [<tt>:info</tt> children] {
      <tt>if</tt> {[$m info has type Player]} {<tt>puts</tt> "  [$m name]"}
    }
  }
</pre>
<p>      
Now let us build to example soccer team objects.
</p>
<pre CLASS="code">
  SoccerTeam <tt>create</tt> chelsea -name "Chelsea FC" -location "Chelsea"
  SoccerTeam <tt>create</tt> bayernMunich -name "F.C. Bayern M&uuml;nchen" -location "Munich"
</pre>
<p>

With <tt>addPlayer</tt> we can create new aggregated player objects
<p></p>
Let us start some years in the past, when "Franz Beckenbauer" was
still a player.
</p>
<pre CLASS="code">
  <tt>set</tt> fb [bayernMunich newPlayer -name "Franz Beckenbauer" -playerRole PLAYER]
</pre>
<p>
<tt>playerRole</tt> may not take any value. It may either be NONE, PLAYER, 
or GOALY ... such rules may be given as assertions (here: an instinvar 
gives an invariant covering all instances of a class). In XOTcl 
the rules are syntactically identical to <tt>if</tt> statements:
</p>
<pre CLASS="code">
  Player instinvar {
    {${:playerRole} <tt>in</tt> [<tt>list</tt> "NONE" "PLAYER" "GOALY"]}
  }
</pre>
<p>
If we break the invariant and turn assertions checking on, we should
get an error message:
</p>
<pre CLASS="code">
  $fb check all
  <tt>if</tt> {[<tt>catch</tt> {$fb playerRole SINGER} errMsg]} {
    <tt>puts</tt> "CAUGHT EXCEPTION: playerRole has either to be NONE, PLAYER, or TRAINER"
    <it># turn assertion checking off again and reset to PLAYER</it>
    $fb check {}
    $fb playerRole PLAYER
  }
</pre>
<p>
But soccer players may play quite different, orthogonal
roles. E.g. Franz Beckenbauer was also a singer (a remarkably bad
one). However, we can not simply add such orthogonal, extrinsic
extensions with multiple inheritance or delegation. Otherwise we
would have either to build a lot of unnecessary helper classes, like
PlayerSinger, PlayerTrainerSinger, etc., or we would have to build
such helper objects. This either leads to an unwanted combinatorial
explosion of class or object number 
</p><p>
Here we can use a per-object mixin, which is a language construct
that expresses that a class is used as a role or as an extrinsic
extension to an object.
</p><p>
First we just define the Singer class.
</p>
<pre CLASS="code">
  <tt>Class create</tt> Singer {

    <tt>:method</tt> sing text {
      <tt>puts</tt> "${:name} sings: $text, lala."
    }
  }
</pre>
<p>
Now we register this class as a per-object mixin on the player object:
</p>
<pre CLASS="code">
  $fb <tt>mixin</tt> Singer
</pre>
<p>
And now Franz Beckenbauer is able to sing:
</p>
<pre CLASS="code">
  $fb sing "lali"
</pre>
<p>
But Franz Beckenbauer has already retired. When a player retires, we
have an intrinsic change of the classification. He *is* not a player
anymore. But still he has the same name, is club member, and
is a singer (brrrrrr).
</p><p>
Before we perform the class change, we extend the Player class to
support it. I.e. the playerRole is not valid after class change
anymore (we unset the instance variable).
</p>
<pre CLASS="code">
  Player <tt>method</tt> class args {
    <tt>unset</tt> :playerRole
    <tt>next</tt>
  }
</pre>
<p>
Now we can re-class the player object to its new class (now Franz
Beckenbauer is President of Bayern Munich.
</p>
<pre CLASS="code">
  $fb <tt>class</tt> President
  <it># Check that the playerRole isn't there anymore.</it>
  <tt>if</tt> {[<tt>catch</tt> {$fb playerRole} errMsg]} {
    <tt>puts</tt> "CAUGHT EXCEPTION: The player role doesn't exist anymore \
         (as it should be after the class change)"
  }
</pre>
<p>
But still Franz Beckenbauer can entertain us with what he believes
is singing:
</p>
<pre CLASS="code">
  $fb sing "lali"
</pre>
<p>
Now we define some new players for Bayern Munich:
</p>
<pre CLASS="code">
  bayernMunich newPlayer -name "Oliver Kahn" -playerRole GOALY
  bayernMunich newPlayer -name "Giovanne Elber" -playerRole PLAYER
</pre>
<p>
If we enlist the players of Munich Franz Beckenbauer is not enlisted
anymore:
</p>
<pre CLASS="code">
  bayernMunich printPlayers
</pre>
<p>
But as a president he still appears in the list of members:
</p>
<pre CLASS="code">
  bayernMunich printMembers
</pre>
<p>
Now consider an orthonogal extension of a transfer list. Every
transfer in the system should be notified. But since the transfer
list is orthogonal to SoccerTeams we do not want to interfere with
the existing implementation at all. Moreover, the targeted kind of
extension has also to work on all subclasses of SoccerTeam. Firstly, we
just create the extension as an ordinary class:
</p>
<pre CLASS="code">
  <tt>Class</tt> TransferObserver {
    <tt>:method</tt> transferPlayer {pname destinationTeam} {
      <tt>puts</tt> "Player '$pname' is transferred to Team '[$destinationTeam name]'"
      <tt>next</tt>
    }
  }
</pre>
<p>
Now we can apply the class as a per-class mixin, which functions
exactly like a per-object mixin, but on all instances of a class and
its subclasses.  The <tt>next</tt> primitive ensures that the original
method on <tt>SoccerTeam</tt> is called after notifying the transfer (with
puts to stdout):
</p>
<pre CLASS="code">
  SoccerTeam <tt>mixin</tt> TransferObserver
</pre>
<p>
If we perform a transfer of one of the players, he is moved to the new 
club and the transfer is reported to the stdout:
</p>
<pre CLASS="code">
  bayernMunich transferPlayer "Giovanne Elber" chelsea
</pre>
<p>
Finally we verify the transfer by printing the players:
</p>
<pre CLASS="code">
  chelsea printPlayers
  bayernMunich printPlayers
</pre>
<p>

<P><BR><BR>
</P>
<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="object_class_system"></A><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Object
			and Class System </FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic4" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>

<P>In XOTcl every object is associated with its managing class by a relationship
called <tt>class</tt>. Classes are special objects with the purpose of
managing other objects. "Managing" means that a class controls the
creation and destruction of its instances and that it contains a
repository of methods accessible for the instances.
</P>

<p>Since a class is a
special (managing) kind of object it is managed itself by a special
class called a "meta-class" (which manages itself). Meta-Classes are
used to define classes and to provides methods for these.  Most
classes are defined by the predefined meta-class <tt>Class</tt>. One
interesting aspect of meta-classes is that by providing a constructor
pre-configured classes can be derived.  Meta-classes can be used to
instantiate large program structures, like some design patterns (see
<a href="#xotcl-filter">[Neumann and Zdun 1999a]</a> for more
details), where the meta-class may holds the generic parts of the
structures. Since a meta-class is an entity of the program, it is
possible to collect these entities in pattern libraries for later
reuse easily (more details about meta-classes are given in a later <A
HREF="meta-classes">section</A>).
</P>

<P>The methods common to all objects in the XOTcl 2 object system are
defined in the root class <tt>Object</tt> (fully qualified name
<tt>::xotcl2::Object</tt>). All methods can be predefined (defined by
XOTcl) or user-defined. All objects of XOTcl 2 are either direct
instances of <tt>Object</tt> or instances of subclasses of
<tt>Object</tt>.</p>

<p>The most basic meta-class is <tt>Class</tt> (fully qualified name
<tt>::xotcl2::Class</tt>). All classes of XOTcl 2 are either direct
instances of <tt>Class</tt> or instances of subclasses of
<tt>Class</tt>. Since - as noted before - a class is a special kind of
object, <tt>Class</tt> is a subclass of <tt>Object</tt>. Therefore,
all methods available in all classes are the union of the methods of
<tt>Object</tt> and <tt>Class</tt> (see Figure 2a).</p>

<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 2a:</STRONG>
Basic Classes of the XOTcl2 object system
</P>
<CENTER>
<IMG SRC="object-class.png" NAME="Graphic5" ALIGN=BOTTOM WIDTH=400 BORDER=0>
</center>

<p>
When we create an application class such as the class <tt>Stack</tt>
in the examples above, we create it as instance of the basic
meta-class <tt>::xotcl2::Class</tt>. The application class will have
<tt>::xotcl2::Object</tt> as it superclass, unless we spefify this
differently. When we create an instance of the class <tt>Stack</tt>
(such as e.g. the stack <tt>s1</tt>) we create it by using the method
<tt>create</tt> provided by <tt>::xotcl2::Class</tt> (an instance can
use the methods provided by its class).
</p>

<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 2b:</STRONG>
Application class Stack and instance of Stack together with the Basic Classes of the XOTcl2 object system
</P>
<CENTER>
<IMG SRC="object-class-appclass.png" NAME="Graphic5" ALIGN=BOTTOM WIDTH=400 BORDER=0>
</center>

<P>XOTcl supports single and multiple inheritance. Classes are ordered
by the relationship <tt>superclass</tt> in a directed acyclic
graph. The root of the class hierarchy is the class <tt>Object</tt>.
Note that it is possible to create as well objects from this most
general class; we have done this already above by creating an object
named <tt>stack</tt>.</p>

<p>A classical problem of multiple inheritance is the problem of name
resolution, when for example two super-classes contain an instance
method with the same name. XOTcl provides an intuitive and unambiguous
approach for name resolution by defining the precedence order along a
linear "<EM>next-path</EM>" incorporating the class and mixin
hierarchies. A method can invoke explicitly the shadowed methods by
the predefined command <tt>next</tt>.  When <tt>next</tt> is executed
a shadowed method is invoked. The execution of the shadowed methods is
called "method chaining". Method chaining without explicit naming of
the targeted method is very important for languages supporting a
dynamic class system, because one cannot always predict which classes
are currently participating in the inheritance hierarchy at design
time (often necessary in inheritance models, like C++).  </P>


<P
 STYLE="margin-bottom: 0in">An important feature of all XOTcl objects
is the read/write introspection. The reading introspection abilities
of XOTcl are packed compactly into the <tt>info</tt> instance method
which is available for objects and classes. All obtained information
can be changed at run-time dynamically with immediate effect. Unlike
languages with a static class concept, XOTcl supports dynamic
class/superclass relationships. At any time the class graph may be
changed entirely using the <tt>superclass</tt> method, or
an object may change its class through the <tt>class</tt> method. This
feature can be used for an implementation of a life-cycle or other
intrinsic changes of object properties (in contrast to extrinsic
properties e.g. modeled through roles and implemented through
per-object and per-class mixins <a href="#xotcl-mixin">[Neumann and
Zdun 1999c]</a> ) . These changes can be achieved without losing the
object's identity, its inner state, and its per-object behavior (methods
and mixins).
</P>

<span class="fixme">xotcl2 changes until here, reminder is
missing;</span><br>

<P ALIGN=CENTER STYLE="margin-bottom: 0in"><A NAME="features1"></A><A NAME="11761"></A>
&#160;&#160; 
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 2b:</STRONG>
Object and Class System 
</P>
<CENTER>
	<TABLE WIDTH=469 BORDER=0 CELLPADDING=2 CELLSPACING=0>
		<COL WIDTH=465>
		<TR>
			<TD WIDTH=465>
				<P ALIGN=CENTER><IMG SRC="obj_class_system.gif" NAME="Graphic5" ALIGN=BOTTOM WIDTH=467 HEIGHT=144 BORDER=0></P>
			</TD>
		</TR>
	</TABLE>
</CENTER>
<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="basic"></A><FONT
                  COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT
                  SIZE=6>Basic Functionalities </FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
	<IMG SRC="logo-100.jpg" NAME="Graphic6" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>
<H2><A NAME="object"></A>&#160;<BR>Objects </H2>
<span class="fixme">at least the first paragraph has to be rewritten;
"2 commands" don't really hold</span><br>
<P>Initially XOTcl offers two new commands: <tt>Object</tt> and
<tt>Class</tt>. They represent hooks to the features of the language.
This section discusses both of them in detail and shows how they
function in the context of XOTcl. Note that even if most of this is
compatible to OTcl, a few changes occur. For this reason, this
section is no introduction to plain OTcl. The <tt>Object</tt> command
provides access to the <tt>Object</tt> class, which holds the common
features of all objects, and allows us to define new objects. Objects
are always instances of classes, therefore, objects defined with the
<tt>Object</tt> command are (initially) instances of the <tt>Object</tt>
class. But since they have no user-defined type, they may be referred
to as <EM>singular objects</EM>. As all other objects they may be
specialized by object-operations and -data. 
</P>
<P>The object command has the following syntax: 
</P>
<pre CLASS="code">
  <tt>Object</tt> <em>objName ?args?</em>
</pre><P>
A command of this form is a short-cut for a message to the <tt>create</tt>
instance method (forwarded automatically by the <tt>unknown</tt>
mechanism, which is invoked every time the message dispatch system
discovers an unknown message): 
</P>
<pre CLASS="code">
  <tt>Object</tt> <tt>create</tt> <em>objName ?args?</em>
</pre><P>
It creates a new object of type <tt>Object</tt> with the name <tt>objName</tt>
(in fact it invokes a <tt>create</tt> call on the <tt>Object</tt> class).
<tt>objName</tt> becomes a new command, which allows us to access the
created object. Similar to the <tt>Object</tt> command it may be
used like a normal Tcl-command (using sub-commands to access the
object's methods). Therefore, this form of access is called
<EM>object-command</EM> approach. A simple example is an object which
holds the information of a kitchen. It is created by: 
</P>
<pre CLASS="code">
  <tt>Object</tt> kitchen
</pre>
<P>
An object creation calls the constructor <tt>init</tt> of the
object's class. The destruction of an object is handled by the
<tt>destroy</tt> instance method. The general syntax of <tt>destroy
</tt>is: 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>destroy</tt>
</pre><P>
E.g. the kitchen object is destroyed by: 
</P>
<pre CLASS="code">
  kitchen <tt>destroy</tt>
</pre><P>
To invoke a user-defined destruction process, it is possible to
overload this instance method in every class derived from object.
</P>
<P>
Note that the destruction of an object is performed by the
method <tt>destroy</tt> of Object (since every object is an instance
of <tt>Object</tt>, every object can call <tt>destroy</tt>). When an application class
overloads <tt>destroy</tt>, this method should contain a <tt>next</tt> in order
to reach the base class and to actually destroy the object. 
</P>
<H3><A NAME="data_on_obj"></A>Data on Objects 
</H3>
<P>The <tt>Object</tt> class provides a range of operations to manage
objects, including those to manipulate data-structures on the
objects. They are similar to the same-named Tcl-commands: 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>set</tt> <em>varname ?value?</em>
  <em>objName</em> <tt>unset</tt> <em>v1 ?v2 ... vn?</em>
</pre>
<P>
The <tt>set</tt> instance method with given <tt>value</tt> option
allows us to manipulate an object-variable's value or to create a new
one, if the variable <tt>varname</tt> does not exist on the object so
far. Without <tt>value</tt> option the <tt>set</tt> operation queries
the variable and returns it's value, if the variable exists,
otherwise it produces an error message. The <tt>unset</tt> operation
deletes one or optionally a set of variables from an object. For
example the <tt>kitchen</tt> object can store information on the
color of the wall-paper by: 
</P>
<pre CLASS="code">
  kitchen <tt>set</tt> wallPaperColor white
</pre><P>
Similar to Tcl-variables the object variables are dynamical; they
may be set at run-time when they are needed and unset when they
become obsolete. E.g. the persons in the kitchen may be stored in an
array. If there are no persons in the kitchen the array is deleted: 
</P>
<pre CLASS="code">
  <it># Peter enters the kitchen to cook</it>
  kitchen <tt>set</tt> persons(cook) Peter
  ...
  <it># Marion enters the kitchen to take one of the seats</it>
  kitchen <tt>set</tt> persons(seat1) Marion 
  ...
  <it># Both Peter and Marion leave the kitchen</it>
  <it># the array is deleted by unset</it>
  kitchen <tt>unset</tt> persons
</pre><P>
Since XOTcl variables are internally realized through Tcl-variables
they may be treated like all Tcl-variables. For that reason they have
all Tcl-variable abilities, including the possibility to handle them
as lists or arrays (as seen in the last example). The <tt>array</tt>
command of Tcl is mapped to an XOTcl-command directly. An
object-oriented call to an object of the form 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>array</tt> <em>option arrayName args</em>
</pre><P>
forwards its arguments to an <tt>array</tt> Tcl-command for the
object&acute;s instance variable <tt>arrayName</tt>. It could be used like
the same-named Tcl-command, e.g. the command 
</P>
<pre CLASS="code">
  kitchen <tt>array</tt> names persons
</pre><P>
returns all indexes currently stored in the <tt>persons</tt> array. 
</P>
<P>Similarly Tcl&acute;s <tt>incr</tt> command is mapped to the
object system. A call with the syntax: 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>incr</tt> <em>varName ?value?</em>
</pre><P>
increments <tt>varName</tt> with the given value (or without given
value with 1). 
</P>
<H3><A NAME="obj_methods"></A>Methods for Objects 
</H3>
<P>Methods in XOTcl resemble Tcl-procedures. On objects one can define
object-specific methods, called procs. Instance methods which are
defined on classes are called instprocs. A new proc is defined using
the <tt>proc</tt> instance method of the class <tt>Object</tt>:
</P>
<pre CLASS="code">
  <em>objName</em> <tt>proc</tt> <em>name args body</em>
</pre><P>
The arguments of the <tt>proc</tt> instance method specify the name,
the arguments as a Tcl-list, and the body of the new proc. All of them
must be given, only one of <tt>args</tt> and <tt>body</tt> may be
empty. An example proc would be a method to let persons enter the
kitchen: 
</P>
<pre CLASS="code">
  kitchen <tt>proc</tt> enter {name} {
    [<tt>self</tt>] <tt>set</tt> persons($name) [<tt>clock</tt> seconds]
  }
</pre><P>
Here the predefined <tt>self</tt> command is used in one of three
possible ways, which allow us to access useful information when working
with XOTcl-methods, these are in particular: 
</P>
<UL>
	<LI><P STYLE="margin-bottom: 0in"><tt>self</tt>: returns the
	  name of the object, which is currently in execution. This
	  command is similar to <tt>this</tt> in C++. It is
	  automatically generated on each object. If it is called from
	  outside of an XOTcl method, it produces the error message
	  "<tt>Can't find self</tt>".
	</P>
	<LI><P STYLE="margin-bottom: 0in"><tt>self class</tt>: the self
	command with the  argument <tt>class</tt> returns the name of the
	class, which holds the currently executing instproc. Note that this
	may be different to the class of the current object. If it is called
	from a proc it returns an empty string. 
	</P>
	<LI><P><tt>self proc</tt>: the self command with the argument
	<tt>proc</tt> returns the name of the currently executing
	method (proc or instproc).
	</P>
</UL>
<p>The method <tt>enter</tt> can be written in XOTcl as well with
less syntactic overhead by using the predefined primitive <tt>my</tt>
instead of <tt>[<tt>self</tt>]</tt>:</p>
<pre CLASS="code">
  kitchen <tt>proc</tt> enter {name} {
    <tt>my</tt> <tt>set</tt> persons($name) [<tt>clock</tt> seconds]
  }
</pre><P>
<P>Note that there is a difference to the realization of these
object information to OTcl. XOTcl uses commands in order to make
XOTcl-methods compatible to Tcl-procedures and accessible via
namespace-paths. OTcl uses the three variables <tt>self</tt>, <tt>class</tt>
and <tt>proc</tt>, which are filled automatically with proper values
by the interpreter each time a method is called. To gain backwards
compatibility XOTcl can be compiled with <tt>-DAUTOVARS</tt> to
provide these variables additionally. By default this option is turned off.
</P>
<P>Each XOTcl-method has its own scope for definition of local
variables for the executing method. In most cases when a method uses
object-variables, it is likely that the programmer wants to make one
or more of these variables part of the method's scope. Then the
Tcl-command for variable handling, like <tt>set</tt>, <tt>lindex</tt>,
<tt>array</tt>, ... work also on these variables.  The
<tt>instvar</tt> instance method links a variable to the scope of
an executing method. It has the syntax:
</P>
<pre CLASS="code">
  <em>objName</em> <tt>instvar</tt> <em>v1 ?v2 ... vn?</em>
</pre>
<P> 
It makes the variables <tt>v1 ... vn</tt>, which must
be variables of the object, part of the current method's scope. A
special syntax is:
</P>
<pre CLASS="code">
  <em>objName</em> <tt>instvar</tt> <em>{varName aliasName} ...</em>
</pre>
<P> 
for one of the variables. This gives the variable with the name
<tt>varName</tt> the alias <tt>aliasName</tt>. This way the variables
can be linked to the methods scope, even if a variable with that name
already exists in the scope. Now the <tt>enter</tt> method can be
adapted slightly and a <tt>leave</tt> method can be added, which uses
Tcl's <tt>info</tt> command to check
whether the named person is in the object's <tt>persons</tt> array. To
demonstrate the alias-syntax this is done with the <tt>persons</tt>
array and the alias <tt>p</tt>.
</P>
<pre CLASS="code">
  kitchen <tt>proc</tt> enter {name} {
    <tt>my</tt> <tt>instvar</tt> persons
    <tt>set</tt> persons($name) [<tt>clock</tt> seconds]
  }

  kitchen <tt>proc</tt> leave {name} {
    <tt>my</tt> <tt>instvar</tt> {persons p}
    <tt>if</tt> {[<tt>info</tt> exists p($name)]} {
      <tt>puts</tt> &quot;$name leaves after [<tt>expr</tt> {[<tt>clock</tt> seconds]-$p($name)}] seconds&quot; 
      <tt>unset</tt> p($name) 
    } <tt>else</tt> {
      <tt>puts</tt> &quot;$name is not in the room&quot;
    }
  }
</pre>
A method defined via <tt>proc</tt> can be deleted by <tt>proc</tt> using 
an empty argument list and an empty body. The following example deletes the method
<tt>enter</tt>:
<pre CLASS="code">
  Room <tt>proc</tt> enter {} {}
</pre>

<H3>
<A NAME="obj_info"></A>Information about Objects 
</H3>
<P STYLE="margin-bottom: 0in">XOTcl offers reading and writing
introspection. The reading introspection abilities are packed
compactly into the <tt>info</tt> instance method which is available
for objects and classes (there are special info options for object
aggregations, nested classes, mixins, filters, meta-data and
assertions, which are explained separately in the following
sections). 
</P>
<CENTER>
      <P><A NAME="table_oinfo"></A>
	<STRONG>Options for the <tt>info</tt> method on objects</STRONG></P>

      <TABLE BORDER=1>
	<TR>
	  <TD width=40%>
	    <tt><em>objName</em> <tt>info</tt> args <em>methodName </em></tt>
	  </TD>
	  <TD>
	    <P ALIGN=LEFT>Returns the arguments of the specified proc (object specific method).</P>
	  </TD>
	</TR>
	<TR>
	  <TD>
	    <tt><em>objName</em> <tt>info</tt> body <em>methodName</tt>
	  </TD>
	  <TD>
	    <P ALIGN=LEFT>Returns the body of the specified proc.</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>objName</em> <tt>info</tt> class <em>?className?</em></tt>
	  </TD>
	  <TD>
	    <P ALIGN=LEFT>Returns the name of the class of the current
	    object, if <em>className</em> was not specified.  Otherwise it
	    returns 1 if <em>className</em> matches the object's class and 0 if
	    not.
	    </P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>objName</em> <tt>info</tt> commands <em>?pattern?</em></tt>
	  </TD>
	  <TD>
	    <P ALIGN=LEFT>Returns all commands defined on the object
	    if <em>pattern</em> was not specified. Otherwise it
	    returns all commands that match the pattern.</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>objName</em> <tt>info</tt> default <em>methodName arg var</em></tt></P>
	  </TD>
	  <TD>
	    <P ALIGN=LEFT>Returns 1 if the argument <em>arg</em> of
	    the specified proc has a default value, otherwise 0. If
	    the default value exists it is stored in <em>var</em>.</P>
	  </TD>
	</TR>

	<TR>
	  <TD><tt><em>objName</em> <tt>info</tt> precedence <em>?pattern?</em></tt>
	  </TD>

	  <TD>
	    <P ALIGN=LEFT>Returns all classes in the precedence order
	    from which the specified object inherits methods. The
	    returned list of classes contains the mixin and instmixin
	    classes as well as the classes of the superclass chain in
	    linearized order (i.e., duplicate classes are removed). If
	    the <em>pattern</em> is specified, only matching classes are
	    returned.</P>
	  </TD>
	</TR>

	<TR>
	  <TD><tt><em>objName</em> <tt>info</tt> vars <em>?pattern?</em></tt>
	  </TD>
	  <TD>
	    <P ALIGN=LEFT>Returns all variables defined on the object
	    if <em>pattern</em> was not specified, otherwise it returns
	    all variables that match the pattern.</P>
	  </TD>
	</TR>
      </TABLE>
    </FONT>
    </CENTER>
<p><br></p>
<P>For example on the <tt>kitchen</tt> object 
</P>
<pre CLASS="code">
  kitchen <tt>info</tt> procs
</pre><P>
returns <tt>enter</tt> and <tt>leave</tt> as a Tcl-list since these
are the procs defined on the object. 
</P>
<H2><A NAME="classes"></A>Classes 
</H2>
<H3><A NAME="class_instance"></A>Creating Classes and deriving
Instances 
</H3>
<P>There are different ways to create a class in XOTcl. They have in
common that they derive the new class from a meta-class. Initially the
<tt>Class</tt> command provides access to the meta-class
<tt>Class</tt>, which holds the features common to all classes. It
also allows one to derive new meta-classes. The common way to create a
new class is:
</P>
<pre CLASS="code">
  <tt>Class</tt> <em>className ?args?</em>
</pre>

<P>
Similar to the object short form, this is a short form of a call to
the <tt>create</tt> instance method of the meta-class <tt>Class</tt>,
which is also executed by the standard <tt>unknown</tt> mechanism.
This mechanism is always triggered when XOTcl does not know a method
called on an object. Supposed that there is no method with the name
<tt>className</tt>, defined on the class-object of <tt>Class</tt>,
XOTcl looks up the method <tt>unknown</tt> (which is found on the
Class <tt>Object</tt>) and executes it. The standard unknown-mechanism
of XOTcl calls <tt>create</tt> with all arguments stepping one step
to the right; in the general case: 
</P>
<pre CLASS="code">
  <tt>Class</tt> <tt>create</tt> <em>className ?args?</em>
</pre><P>
This may also be called directly. Besides the indirection when using
<tt>unknown</tt>, in most cases there is no difference in the action
performed: Firstly the memory is allocated, using the <tt>alloc</tt>
instance method; as the next step the constructor <tt>init</tt> is called
on the creating object, which is in this case the class-object of the
meta-class <tt>Class</tt>. In seldom cases the programmer may want to
suppress the <tt>init</tt> call. To do so the <tt>alloc</tt> instance
method may also be called directly: 
</P>
<pre CLASS="code">
  <tt>Class</tt> <tt>alloc</tt> <em>className ?args?</em>
</pre>
<P>
As seen in the preceding section objects are created in the same way.
The difference was, that the command <tt>Object</tt>, which accesses
a class, instead of the command <tt>Class</tt>, which accesses a
meta-class, was used. The user-defined classes may also be used in
the same way to create new objects: 
</P>
<pre CLASS="code">
  <em>className objName ?args?</em>
</pre>
<P>
Resembling the creation of classes this creates an object <tt>objName</tt>
of type <tt>className</tt> using the <tt>unknown</tt> mechanism. That
means the <tt>create</tt> instance method of the class is called. If
there is no other instance method defined on the class-path so far
(which would mean, an user defined creation process is invoked), the
<tt>create</tt> instance method of the class <tt>Object</tt> is
invoked. This method is similar to the <tt>create</tt> method of the
meta-class <tt>Class</tt>. It firstly calls the <tt>alloc</tt>
instance method on its (of the <tt>Class</tt> class) which allocates
memory for the object, and makes it an instance of it's class.
Afterwards a call to the constructor <tt>init</tt> is invoked. 
</P>
<P>Now we can specify the object for the kitchen by the class to
which it belongs. In this case a kitchen is an instance of a room. 
</P>
<pre CLASS="code">
  <tt>Class</tt> Room
  Room kitchen
</pre><P>
A <tt>set</tt> call on a class creates an instance variable on the
class-object. This variable is unique for all instances, therefore,
it may be referred to as a class variable. 
</P>
<H3><A NAME="class_methods"></A>Methods Defined in Classes 
</H3>
<P>Methods which are defined in classes and which are provided to the 
instances of these classes are called "instprocs". 
The syntax for defining an instproc is: 
</P>
<pre CLASS="code">
  <em>className</em> <tt>instproc</tt> <em>procname args body</em>
</pre>
<P>
It is similar to the definition of procs on objects, but uses the
keyword <tt>instproc</tt> to distinguish between the methods defined
on the class-object and those defined on the class. Since all rooms
(in the modeled world) have ceilings, we may want to define a simple
convenience instproc, which is able to set the color: 
</P>
<pre CLASS="code">
  Room <tt>instproc</tt> setCeilingColor color {
    <tt>my</tt> <tt>set</tt> ceilingColor $color
  }
</pre><P>
A special instproc, the constructor <tt>init</tt>, was mentioned
already. Now we are able to define such an instproc. Defined on a
class it is responsible for all initialization tasks, which needed to
be performed, when constructing a new instance object of the class.
The constructor of the <tt>Room</tt> can initialize a variable for
the color, in which the ceiling is painted, to white as default,
since this is the color of ceilings without painting. 
</P>
<pre CLASS="code">
  Room <tt>instproc</tt> <tt>init</tt> args {
    <tt>my</tt> setCeilingColor white
    <tt>next</tt>
  }
</pre>
<P>
After this definition, all instances derived from the <tt>Room</tt>
class have an instance variable <tt>ceilingColor</tt> with the value
<tt>white</tt>. The <tt>args</tt> argument used here is a special
argument in Tcl which allows us to use a list of arguments which may
change its length from call to call.
<p>
An instproc can be deleted by the method <tt>instproc</tt> as well.
If <tt>instproc</tt> is called with an empty argument list and an
empty body, the specified method is deleted, as the following example shows:
<pre CLASS="code">
  Room <tt>instproc</tt> setCeilingColor {} {}
</pre>

</P>
<H3><A NAME="class_info"></A>Information about Classes 
</H3>
<P STYLE="margin-bottom: 0in">Resembling to objects, information on
classes may be gained through the <tt>info</tt> instance method of the
meta-class <tt>Class</tt>. Note that this instance method does not
only support the class info options, but also the class-object info options,
since the accessing command refers to the class-object, which itself
is an object and, therefore, offers its information.  The following
table summarizes the additional info options available on classes.
</P>
<CENTER>
<P><STRONG>Options for the <tt>info</tt> method on classes</STRONG></P>
      <TABLE BORDER=1>
	<TR>
	  <TD width=35%><tt><em>className</em> <tt>info</tt> heritage <em>?pattern?</em></tt>
	  </TD>
	  <TD><P ALIGN=LEFT>Returns a list of all classes in the
	  precedence order of the class hierarchy matching
	  <em>pattern</em> or a list of all classes, if
	  <em>pattern</em> was not specified.</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>className</em> <tt>info</tt> instances <em>?pattern?</em></tt>
	  </TD>
	  <TD><P ALIGN=LEFT>Returns a list of the instances of the
	      class matching <em>pattern</em> or of all instances, if
	      <em>pattern</em> was not specified.
	    </P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>className</em> <tt>info</tt> instargs <em>methodName</em></tt>
	  </TD>
	  <TD><P ALIGN=LEFT>Returns the arguments of the specified instproc 
	      (method provided to objects).</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>className</em> <tt>info</tt> instbody <em>methodName</em></tt>
	  </TD>
	  <TD><P ALIGN=LEFT>Returns the body of the specified instproc.</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>className</em> <tt>info</tt> instcommands <em>?pattern?</em></tt>
	  </TD>
	  <TD><P ALIGN=LEFT>Returns all commands defined on the class, if
	      <em>pattern</em> was not specified, otherwise it returns all
	      commands provided to objects that match the pattern.</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>className</em> <tt>info</tt> instdefault <em>methodName arg var</em></tt></P>
	  </TD>
	  <TD>
	    <P ALIGN=LEFT>Returns 1 if the argument <em>arg</em> of
	    the specified instproc has a default value, otherwise 0. If
	    the default value exists it is stored in <em>var</em>.</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>className</em> <tt>info</tt> subclass <em>?className2?<em></tt>
	  </TD>
	  <TD><P ALIGN=LEFT>Returns a list of all subclasses of the class, if
	      <em>className2</em> was not specified, otherwise it returns 1 if
	      <em>className2</em> is a subclass and 0 if not.</P>
	  </TD>
	</TR>
	<TR>
	  <TD><tt><em>className</em> <tt>info</tt> superclass <em>?className2?</em></tt>
	  </TD>
	  <TD><P ALIGN=LEFT>Returns a list of all super-classes of the class,
	      if <em>className2</em> was not specified, otherwise it returns 1
	      if <em>className2</em> is a superclass and 0 if not.</P>
	  </TD>
	</TR>
      </TABLE>
</CENTER>  
<P>The full list of info options is provided in the language reference.</P>
<H3><A NAME="class_inheritance"></A>Inheritance 
</H3>
<P>Besides encapsulation of operations and state in objects, a second
central ability of object-orientation is inheritance. XOTcl supports
single and multiple inheritance with a directed acyclic class
graph. Automatically each new class created by the instance methods
<tt>create</tt> and <tt>alloc</tt> of <tt>Class</tt> inherits from
<tt>Object</tt>. Therefore, it is ensured that all instances of the
new class have access to the common features of objects stored in the
class <tt>Object</tt>.
</P>
<P>To specify further inheritance relationships the instance methods
<tt>superclass</tt> of <tt>Class</tt> is used: 
</P>
<pre CLASS="code">
  <em>className</em> <tt>-superclass</tt> <em>classList</em>
</pre><P>
E.g. in the example a kitchen may be seen as a
special room: 
</P>
<pre CLASS="code">
  <tt>Class</tt> Room
  <tt>Class</tt> Kitchen <tt>-superclass</tt> Room
</pre><P>
Now all instances of <tt>Kitchen</tt> are able to access the
methods provided by the <tt>Room</tt> and the <tt>Kitchen</tt> classes.
Note the transition the kitchen was going through: firstly it was a
singular object, then it was an object with a user-defined class,
and now it is a class. This is possible because we can provide 
a per-object behavior, and because classes are a special kind of objects.
Both properties of XOTcl's object system lead to a
seamless connection of the run-time behavior of objects
and the descriptive properties of the classes. It is possible
to avoid the strict distinction between objects and classes, known from static typed
languages, like C++, Java, etc. 
</p>
<p>Moreover, since the syntaxes of constructs expressing the same
concern are nearly identical, we can re-factor a solution with very few 
changes to the alternative. We will see similar "ease of refactoring"
throughout the XOTcl language. E.g., we can also easily re-factor the
class hierarchies or exchange class hierarchies against mixin
solutions with only slight changes in the code.
</P>
<P>Besides single inheritance, as seen, XOTcl provides also multiple
inheritance. This is syntactically solved by giving the <tt>superclass</tt>
instance method a list of classes instead of a single class as
argument. 
</P>
<pre CLASS="code">
  <tt>Class</tt> Room
  <tt>Class</tt> 4WallsRoom <tt>-superclass</tt> Room
  <tt>Class</tt> CookingPlace
  <tt>Class</tt> Kitchen <tt>-superclass</tt> {4WallsRoom CookingPlace}
</pre><P>
Now the kitchen class is specialized a bit more. It is a special room
which has four walls <EM>and</EM> it is a cooking place. Multiple
inheritance, as seen here, is as simple to apply as single
inheritance. 
</P><P>

Most often when the disadvantages of multiple inheritance are
discussed, the name resolution along the class graph is considered as
the biggest problem. The question is, which method is to be chosen and
which path through class graph is to be taken, if more then one method
of the specified name exist on the class graph.

</P>
<P ALIGN=LEFT STYLE="margin-bottom: 0in">In the example such questions
would arise for an object of the <tt>Kitchen</tt> class, if two
same-named methods are defined on <tt>CookingPlace</tt> and
<tt>4WallsRoom</tt> or if a method of the class <tt>Object</tt> is
called, which is reachable through two paths (along
<tt>CookingPlace</tt> or <tt>Room</tt>). 
</P>

<P ALIGN=LEFT STYLE="margin-bottom: 0in">Often - e.g. in the
inheritance model of C++ - the path through the graph is not clearly
determined and/or the rules are too complicated to be understood on
the first glance. The programmer often can only determine by trial
which method is found firstly. Than an explicit naming of the class is
necessary, which means storage of non-local information in
subclasses. Often different compilers of one language behave
differently. All these issues make code reuse difficult. Moreover
understandability and portability are reduced.
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><BR>
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 3:</STRONG>
The example classes and the following next-path</P>
<CENTER>
	<TABLE BORDER=0 CELLPADDING=2 CELLSPACING=0>
		<COL>
		<TR>
			<TD>
				<IMG SRC="next-path.gif" NAME="Graphic13" ALIGN=LEFT BORDER=0><BR CLEAR=LEFT></TD>
		</TR>
	</TABLE>
</CENTER>
<P>XOTcl goes an intuitive and unambiguous way to solve this problem.
It resolutes the precedence order along a ``<EM>next-path</EM>''.
Firstly the class of the object is searched, which is <tt>Kitchen</tt>
in example. Then the super-classes are searched in definition order,
which means at first <tt>4WallsRoom</tt>, then <tt>CookingPlace</tt>.
Each branch is searched completely, before changing to the next
branch. That means, <tt>Room</tt> is searched, before the
<tt>CookingPlace</tt> branch is visited. At last the top of the
hierarchy, the class <tt>Object</tt>, is searched. 
</P>

<P>The usage of <tt>next</tt> in XOTcl is different to OTcl: In OTcl,
	 <tt>next</tt> is defined as a method, in XOTcl it is a
	primitive command. Furthermore, in OTcl, 
it is always necessary to provide the full argument list for every
invocation explicitly. In XOTcl, a call of <tt>next</tt> without
arguments can be used to call the shadowed methods with the same
arguments (which is the most common case). When arguments should be
changed for the shadowed methods, they must be provided explicitly in
XOTcl as well. In the rare case that the shadowed method should
receive no argument, the flag <tt>--noArgs</tt> must be used. 
</P>

<H3><A NAME="class_destroy"></A>Destruction of Classes 
</H3>
<P>Classes are destroyed by the destruction of the class-object using
the <tt>destroy</tt> method of the <tt>Object</tt> class. The
destruction of super-classes does not destroy the subclasses. The
super-class is simply removed from the subclasses' super-class
lists. All classes have the super-class <tt>Object</tt>, if no
super-class is specified. Therefore, if all super-classes are
destroyed or removed, the new super-class is <tt>Object</tt>, not: no
super-class. The destruction of the class of an object does neither
delete the object nor leave it without class. In XOTcl a deleted class
leaves it's instances with the class <tt>Object</tt>.
</P>
<P>So all empty class- and superclass-relationships are automatically
reset to <tt>Object</tt>. Note that this are differences to OTcl,
where the destruction of a class destroys all instances and an empty
super-class list remains empty. 
</P>
<H3><A NAME="class_method_chaining"></A>Method Chaining 
</H3>
<P>A special feature of XOTcl is the method chaining without explicit
naming of the ``mix-in''-method. It allows one to mix the same-named
superclass methods into the current method (modeled after CLOS). The
previously described next-path is the basis for this functionality.
At the point marked by a call to the <tt>next</tt> primitive of XOTcl
the next shadowed method on the next path is searched and, when it is
found, it is mixed into the execution of the current method. When no
method is found, the call of <tt>next</tt> returns an empty string,
otherwise it returns the result of the called method. 
The syntax is: 
</P>
<pre CLASS="code">
  <tt>next</tt> <em>?arguments|--noArgs?</em>
</pre><P>
As stated earlier the usage of <tt>next</tt> in XOTcl differs from
OTcl, since the <tt>next</tt> call without arguments in OTcl means
per default that no arguments are passed. But most often all
arguments are passed through to the shadowed methods (since these will
most likely have the same signatures). When all
variables should be passed through, in OTcl it is necessary for
correct variable substitution to use: 
</P>
<pre CLASS="code">
  <tt>eval</tt> $self <tt>next</tt> $args
</pre><P>
To avoid such difficulties, we made the passing of all arguments the
default case; a simple 
</P>
<pre CLASS="code">
  <tt>next</tt>
</pre><P>
performs the task of passing all arguments to the shadowed methods.
These arguments are called the <EM>standard arguments</EM>. If the
standard argument feature should not be used, optionally arguments
can be given or the flag <tt>--noArgs</tt> could be set as sole
argument, which means that the shadowed method is called with no
arguments. 
</P>
<P>
E.g. the following <tt> next </tt> call ignores the standard arguments 
and sends the arguments 1 and 2 instead:
</P>
<pre CLASS="code">
  <tt>next</tt> 1 2
</pre><P>

<P>As an example all classes involved in the previous example should
get a constructor instance method, which simply sets an instance
variable on the object:
</P>
<pre CLASS="code">
  Room <tt>instproc</tt> <tt>init</tt> args {
    <tt>my</tt> <tt>set</tt> roomNumber 0
    <tt>next</tt>
  }    
  4WallsRoom <tt>instproc</tt> <tt>init</tt> args {
    <tt>my</tt> <tt>set</tt> doorPosition 0
    <tt>next</tt>
  }
  CookingPlace <tt>instproc</tt> <tt>init</tt> args {
    <tt>my</tt> <tt>set</tt> stoveType electric
    <tt>next</tt>
  }
  Kitchen <tt>instproc</tt> <tt>init</tt> args {
    <tt>my</tt> <tt>set</tt> cookName -
    <tt>next</tt>
  }
</pre><P>
After creation an object of class <tt>Kitchen</tt> gets automatically
four instance variables <tt>cookName</tt>, <tt>roomNumber</tt>,
<tt>doorPosition</tt> and <tt>stoveType</tt> set up with default
values in this order (since this is the order of the classes in the
next-path). Note that the order is important, because one missing
next call, in one of the <tt>init</tt> methods, means that succeeding
<tt>init</tt> methods will not be executed. This mechanism functions
equally on all kinds of instprocs, not only on constructors. 
</P>
<P>The constructors use the <tt>args</tt> argument, which allows us to
give a list of variable length as arguments. To ensure reusability of
our classes the constructors should use <tt>args</tt> in most cases,
since they may pass through arguments for constructors further up the
class hierarchy.
</P>
<P>If a <tt>proc</tt> with the searched name exists on the object it
shadows all instprocs. A <tt>next</tt> call in a proc leads to
the normal next-paths search, starting with the object's class. 
</P>
<p>By the way, an observant reader might notice that the example
above can be rewritten without explicit constructors, just by
using parameters with default values.
</p>
<pre CLASS="code">
  <tt>Class</tt> Room <tt>-parameter</tt> {{roomNumber 0}}
  <tt>Class</tt> 4WallsRoom <tt>-superclass</tt> Room <tt>-parameter</tt> {{doorPosition 0}}
  <tt>Class</tt> CookingPlace <tt>-parameter</tt> {{stoveType electric}}
  <tt>Class</tt> Kitchen <tt>-superclass</tt> {4WallsRoom CookingPlace} <tt>-parameter</tt> {{cookName -}}
</pre><P>
If an instance of a Kitchen is created it will contain instance
variables for <tt>doorPosition</tt>, <tt>cookName</tt>,
<tt>roomNumber</tt>, and <tt>stoveType</tt>, as the following
statements will show.</p>
<pre CLASS="code">
  Kitchen k
  <tt>puts</tt> [k <tt>info</tt> vars]
</pre>


<H2><A NAME="class_dynamics"></A>Dynamic Class and Superclass
Relationships 
</H2>
<P>Another property of XOTcl that distinguishes it from statically typed
languages are dynamics of class relationships. The realization of the
definition of super-classes as seen above with the <tt>superclass</tt>
method suggests already, that it is not only available at the class
definition time. In the above example its appended to the class
definition with &quot;<tt>-superclass</tt>&quot; as a short syntax
for method invocation at definition time (all other available methods
can also be called with a preceding dash (&quot;-&quot;) appended
to definitions). 
</P>
<P>At any time the class graph may be changed entirely using the
<tt>superclass</tt> method. Suppose the rooms and kitchens created in
modeling of a house should be displayed to a screen, but it is not
determined, whether the user of the system has the possibilities for
graphical outputs. Two classes <tt>TextOutput</tt> and
<tt>GraphicalOutput</tt> may be defined, which handle the output. Both
have an instproc <tt>paint</tt> which does the painting of the virtual
world on the chosen display type. The common output requirements are
handled by a derived class <tt>VirtualWorldOutput</tt> which calls the
<tt>paint</tt> method of the superclass using <tt>next</tt>. In
statically typed languages it would need more sophisticated constructs
to change the output class at run-time. E.g. a delegation to another
object handling the intrinsic task of the output object would be
introduced solely for the purpose of configuring the output
form. With a dynamic class system we can use the <tt>superclass</tt>
method to do so easily:
</P>
<pre CLASS="code">
  <tt>Class</tt> TextOutput
  TextOutput <tt>instproc</tt> paint args {
    <it># do the painting ...</it>
  }
  <tt>Class</tt> GraphicalOutput
  GraphicalOutput <tt>instproc</tt> paint args {
    <it># do the painting ...</it>
  }

  <it># initially we use textual output</it>
  <tt>Class</tt> VirtualWorldOutput <tt>-superclass</tt> TextOutput
  VirtualWorldOutput <tt>instproc</tt> paint args {
    <it># do the common computations for painting ...</it>
    <tt>next</tt>; <it># and call the actual output</it>
  }

  <it># user decides to change to graphical output</it>
  VirtualWorldOutput superclass GraphicalOutput
</pre>
<P>
Sometimes, such a change to new intrinsic properties should not happen
for all instances of a class (or the class hierarchy), but only for
one specific object. Then the usage of a dynamic super-class
relationship is a too coarse-grained means. A second form of such
dynamics is the changing of the relationship between object and
class. This means, objects can also change their class dynamically at
run-time. This feature may be used to model a life-cycle of an object,
without losing the object's identity, inner state or
per-object-specializations through procs. The <tt>class</tt> instance
method enables this functionality.
</P>
<P>An example would be an agent for the virtual world. Agents may be
placeholders for persons, who interactively travel the world, or
programs, which act automatically. When a person decides at run-time
to give a task it has performed formerly by hand to an automatic
agent, the agents nature changes from interactive agent to automatic
agent, but the identity and the local state (that means the parts of
the task, that are already fulfilled by the person) stay the same.
This is a scenario for changing class relationships, e.g.: 
</P>
<pre CLASS="code">
  <tt>Class</tt> Agent
  <tt>Class</tt> AutomaticAgent <tt>-superclass</tt> Agent
  <tt>Class</tt> InteractiveAgent <tt>-superclass</tt> Agent

  <it># create a new agent for a person</it>
  InteractiveAgent agent1

  <it># the person does something ...</it>
  <it># and decides the change to an automatic agent</it>
  agent1 <tt>class</tt> AutomaticAgent
</pre>

<H2>
<A NAME="meta-classes"></A>Meta-Classes 
</H2>
<P>Meta-classes are a special kind of classes. Similar as classes are
managing objects (where managing means: control the creation and
destruction of instances, know what instances exist, provide methods),
meta-classes are managing classes. So, meta-classes are used to define
classes. In other words, every Class in XOTcl is created by a
meta-class, in most cases by the meta-class named <tt>Class</tt>.  New
user-defined meta-classes can be defined as subclasses of the
predefined meta-class <tt>Class</tt>, or by adding an instmixin class
(see <A HREF="per-class-mixins">below</A>) containing <tt>Class</tt>
to the precedence chain of the class.  By defining <tt>Object
instmixin Class</tt> one can even change the object system of XOTclin a way such that every created Object is a meta-class.
</P>
<P>Since the concept of a meta-class are sometimes
confusing to people of a background of some other programming
languages, we explain meta-classes slowly with the analogy of classes
and objects.
</p>
<p>
When a class <tt>Foo</tt> is created via the command
<pre CLASS="code">
   <tt>Class</tt> Foo
</pre>
 it has no private variables and no special methods. This is 
 somewhat similar as creating an object via <tt>Object</tt>:
<pre CLASS="code">
   <tt>Object</tt> foo
</pre>
 This plain object <tt>foo</tt> can be configured directly, or
 one can create a class that configures the object. 
 Instead of writing 
<pre CLASS="code">
   <tt>Object</tt> foo 
   foo <tt>set</tt> x 1
   foo <tt>proc</tt> hi {} {<tt>puts</tt> "hello"}
</pre>
 one can use
<pre CLASS="code">
   <tt>Class</tt> C <tt>-superclass</tt> Object
   C <tt>instproc</tt> <tt>init</tt> {} {<tt>my</tt> <tt>set</tt> x 1}
   C <tt>instproc</tt> hi {} {<tt>puts</tt> "hello"}
</pre>
 and create an instance and call the method.
<pre CLASS="code">
   C c1
   c1 hi
</pre>
 The same holds for meta-classes and classes as well: Instead of writing 
<pre CLASS="code">
   <tt>Class</tt> Foo
   Foo <tt>set</tt> x 1
   Foo <tt>proc</tt> hi {} {<tt>puts</tt> "hello"}
</pre>
 the following can be used:
<pre CLASS="code">
   <tt>Class</tt> MC <tt>-superclass</tt> <tt>Class</tt>
   MC <tt>instproc</tt> <tt>init</tt> {} {<tt>my</tt> <tt>set</tt> x 1}
   MC <tt>instproc</tt> hi {} {<tt>puts</tt> "hello"}
</pre>
 The instances of meta-classes are classes which can be 
 defined the usual way:
<pre CLASS="code">
   MC Bar
   Bar hi
   Bar b1
</pre>
 Now we have a class names <tt>Bar</tt> which has a class-scoped
 variable named <tt>x</tt> with the value of 1 (set via the
 constructor); the class <tt>Bar</tt> has as well a class-method named
 <tt>hi</tt> which prints, when called, the string "hello". The class
 <tt>Bar</tt> can be used to create instances of the class like
 <tt>b1</tt>, <tt>b2</tt> and so on.
</p>
<p>Note that the command <tt>Class</tt> is a predefined definition
of the most general meta-class in XOTcl. Each time we are creating
a class, we use this meta-class.
In order to define a specialized meta-class, we can do this the
traditional object-oriented way: we subclass. Therefore, in to define
a specialized meta-class, we can use:
</P>
<pre CLASS="code">
  <tt>Class</tt> myMetaClass <tt>-superclass</tt> <tt>Class</tt>
</pre>
<P>This defines a new meta-class <tt>myMetaClass</tt>, which has all the
abilities of meta-classes. That means that the programmer is able to
specify new class features or override old ones. Later she/he may
instantiate these into new classes. 
</P>
<P>This is a very powerful language feature, since it allows one to give
some classes further abilities than the others (or to restrict
classes). This way large program structures, like certain design
pattern parts, may be instantiated. Meta-classes hold the common
abstract parts of the structures. They allow one to form libraries of
such structures very easily. 
</P>
<H3>Example 1: Overloading the info method of classes
</H3>
<P>
As a simple example we can derive a new meta-class
<tt>NoClassInfo</tt> from <tt>Class</tt>. Later we override the
<tt>info</tt> method of <tt>Class</tt>. Thus the classes created with
<tt>NoClassInfo</tt>, have an <tt>info</tt> option that only produces
an error message. All classes created with <tt>NoClassInfo</tt>, like
<tt>Agent</tt> in the example below, are not capable of accessing the class
<tt>info</tt> method anymore:
</P>
<pre CLASS="code">
  <tt>Class</tt> NoClassInfo <tt>-superclass</tt> <tt>Class</tt>
  <it># redefine info ability</it>
  NoClassInfo <tt>instproc</tt> <tt>info</tt> args {
    error &quot;No class info lookup&quot;
  }
  <it># derive agent class from meta-class, which</it>
  <it># can not access class info</it>
  NoClassInfo Agent
</pre>
Now a call like: 
<pre CLASS="code">
  Agent <tt>info</tt> superclass
</pre><P>
triggers the error message. 
</P>

<H3>Example 2: Defining Classes that Count Their Instances
</H3>
<p>Meta-classes are frequently used to define some bookkeeping
about the number of instances on the class level. In the following
example we define a meta-class named <tt>CountedClass</tt> which
defines classes that count their instances:
<pre CLASS="code">
  <tt>Class</tt> CountedClass <tt>-superclass</tt> <tt>Class</tt> <tt>-parameter</tt> {{counter 0}}
  CountedClass <tt>instproc</tt> <tt>create</tt> args {
    <tt>my</tt> <tt>incr</tt> counter
    <tt>next</tt>
  }
  CountedClass <tt>instproc</tt> <tt>dealloc</tt> args {
    <tt>my</tt> <tt>incr</tt> counter -1
    <tt>next</tt>
  }
  CountedClass Dog

  Dog piffie
  Dog idefix
  <tt>puts</tt> "nr of dogs: [Dog counter]"

  piffie <tt>destroy</tt>
  <tt>puts</tt> "nr of dogs: [Dog counter]"
</pre>
Note that the behavior introduced by meta-classes
can be orthogonal to the behavior of the classes. One can
define <tt>Dog</tt> as a specialization of <tt>Animal</tt>
or defines a special kind of dog such as <tt>Poodle</tt> using
the method <tt>superclass</tt> as usual.
</p>

<H3>Example 3: The Singleton Meta-Class
</H3>
<p>Finally, a small example, which is more practical. Some
applications have the requirement that only one instance of a class
might be defined at a certain time. Such a behavior is frequently
called a "Singleton". In XOTcl we can define a class singleton by
overloading the <tt>create</tt> method of <tt>Class</tt>: when
<tt>create</tt> is called and there exists already an instance of the
singleton it is returned instead of a new instance.
<pre CLASS="code">
  <tt>Class</tt> Singleton <tt>-superclass</tt> <tt>Class</tt>
  Singleton <tt>instproc</tt> <tt>create</tt> args {
    <tt>expr</tt> {[<tt>my</tt> exists instance] ? [<tt>my</tt> <tt>set</tt> instance] : [<tt>my</tt> <tt>set</tt> instance [<tt>next</tt>]]}
  }
</pre>
 If someone wants to have a class e.g. <tt>Manager</tt> to be a
 singleton, you can create it by e.g.
<pre CLASS="code">
  Singleton Manager <tt>-superclass</tt> FOO
</pre>
</p>


<H2>
<A NAME="destroy-logic"></A>Create, Destroy, and Recreate Methods
</H2>
<P>
<P>
XOTcl allows since version 0.84 for a flexible destroy and recreate scheme.
<tt>create</tt> and <tt>alloc</tt> are both Class instprocs 
handling creation for their instances. I.e.:
</P>
<pre CLASS="code">
 <em>className</em> <tt>alloc</tt> [<tt>self</tt>]
</pre>
and
<pre CLASS="code">
 <em>className</em> <tt>create</tt> [<tt>self</tt>]
</pre>
<P>
are used for creating an instance. A similar method <tt>dealloc</tt>
exists on Class that handles physical destruction of an object. The
method <tt>destroy</tt> on Object which lets an object destroy itself in fact
has the following behavior:
</P>
<pre CLASS="code">
  <tt>Object</tt> <tt>instproc</tt> <tt>destroy</tt> args {
   [<tt>my</tt> <tt>info</tt> class] <tt>dealloc</tt> [<tt>self</tt>]
  }
</pre>
<P>
However, this behavior is not implemented in XOTcl, but in C.
<tt>create</tt>  distinguishes between the following situations:
</P>
<ul>
<li> <em>Create a new object:</em> 
    By default <tt>create</tt> calls <tt>alloc</tt> and then
   <tt>doInitializations</tt>.
<li> <em>Recreate an existing object:</em> 
    When the specified object exists, it is recreated through the
    <tt>recreate</tt> method:
<pre CLASS="code">
  <em>givenClass</em> <tt>recreate</tt> [<tt>self</tt>]
</pre>
    <P>
    The method <tt>recreate</tt> can be customized like all other
  XOTcl methods (e.g. by overloading or interception). 
    By default <tt>recreate</tt> calls <tt>cleanup</tt> followed by
  <tt>doInitializations</tt>.
  </p>
  <p>
  Note that <tt>recreate</tt> is not called, when a someone tries
  to recreate a class as an object or an object as a class. In these
  cases, <tt>destroy</tt> + <tt>create</tt> are used.
<pre CLASS="code">
    Class c
    Object c ;# here, "c" is downgraded to an object, no "recreate" is called
</pre>
</ul>
<p>
For <tt>create</tt> and <tt>recreate</tt>, the method <tt>doInitializations</tt> 
is called automatically from C and has the following default behavior: 
</p>
<ul>
<li> Search for parameter default values,
<li> Call parameter initialization methods,
<li> Call the constructor <tt>init</tt>.
</ul>
<P>
Each step has a method call that can be changed, intercepted, etc.  Of
course, <tt>cleanup</tt>, <tt>recreate</tt>, <tt>dealloc</tt>, 
etc. can also be overloaded or intercepted.
</P>
<P>
Consider a typical case for overloading <tt>recreate</tt>: a structure
preserving <tt>recreate</tt> that cleans up the class but 
preserves the existing class hierarchy (subclass and instance relationships):
</P>
<pre CLASS="code">
  <tt>Class</tt> StructurePreservingRecreate
  StructurePreservingRecreate <tt>instproc</tt> <tt>recreate</tt> {cl args} {
    <tt>if</tt> {[<tt>my</tt> <tt>isclass</tt> $cl]} {
      <tt>set</tt> subclass [$cl <tt>info</tt> subclass]
      <tt>set</tt> instances [$cl <tt>info</tt> instances]
    }
    <tt>next</tt>
    <tt>if</tt> {[<tt>my</tt> <tt>isclass</tt> $cl]} {
      <tt>foreach</tt> sc $subclass {
        $sc <tt>superclass</tt> $cl
      }
      <tt>foreach</tt> i $instances {
        $i <tt>class</tt> $cl
      }
    }
  }
  <tt>Object</tt> instmixin add StructurePreservingRecreate
</pre>
<p>
Now the following code does not change the superclass or instance
relationships of C:
</p>
<pre CLASS="code">
  <tt>Class</tt> A
  <tt>Class</tt> B
  <tt>Class</tt> C <tt>-superclass</tt> {A B}
  <tt>Class</tt> D
  <tt>Class</tt> E <tt>-superclass</tt> {C D}
  C c1
  C c2

  <it># recreate -> is structure preserving</it>
  <tt>Class</tt> C <tt>-superclass</tt> {A B}
  C c2

  <it># test</it>
  <tt>puts</tt> superclass=[C <tt>info</tt> superclass]
  <tt>puts</tt> subclass=[C <tt>info</tt> subclass]
  <tt>puts</tt> instances=[C <tt>info</tt> instances]
  <tt>puts</tt> class=[c1 <tt>info</tt> class]
  <tt>puts</tt> class=[c2 <tt>info</tt> class]
</pre>

Starting with XOTcl 1.4.0, xotcl provides also a user-friendly
way for a structure-prevering recreate implemented in C. Since this version, one
can configure "softrecreate" as follow.
<pre CLASS="code">
<tt>::xotcl::configure softrecreate</tt> true
</pre>
This command causes that recreates are structure-conservative.

<H2>
<A NAME="non-pos-args"></A>Methods with Non-Positional Arguments
</H2>
<P>
<P>
So far we have introduced methods only with positional arguments: that
is, the position of an argument in the argument list determines to
which local variable the argument is bound, when the method is
invoked. Sometimes non-positional arguments -- arguments that carry
both a name and a value when the method is invoked -- are
useful. Before a non-positional argument can be used, it must be
defined in the method definition using the following syntax:
</P>
<pre CLASS="code">
 <em>className</em> <tt>instproc</tt> methodName <em>?non-pos-args? args</em> body ?assertions
 <em>objName</em> <tt>proc</tt> methodName <em>?non-pos-args?</em> args body ?assertions
</pre>
<P>
The non-positional arguments are defined with the following syntax:
</P>
<pre CLASS="code">
 {-name?:checkoption1, checkoption2, ...? default value} \
     {-name?:checkoption1, checkoption2, ...? ?default value?} ...
</pre>
<p>
Only the name of the non-positional argument is really required, all
other parts of this syntax are optional.
</p>
<p>
Let's consider a simple example, where a method with two
non-positional args is defined; one has only a name ("a"), and one has
a name and a default value (b):
</p>
<pre CLASS="code">
 <tt>Object</tt> o
 o <tt>proc</tt> someproc {-a {-b {1 2 3}} x y} {
     puts "$a $b $x $y"
 }
</pre><P>
We can invoke this method as follows:
</p>
<pre CLASS="code">
 o someproc -b {4 5} -a 1 3 4
</pre><P>

Here, the order of <tt>a</tt> and <tt>b</tt> can be changed; hence the name
non-positional arguments.  As <tt>b</tt> has a default value, we do not need
to provide a value for it. In the following invocation <tt>b</tt> has the
value "1 2 3":
</p>
<pre CLASS="code">
 o someproc -a 1 3 4
</pre><P>
The ordinary arguments start after the last non-positional argument
(here: "3 4").  We can explicitly end the non-positional arguments by
using "--". This is useful if we want to provide arguments that contain 
dashes ("-"), e.g.:
</p>
<pre CLASS="code">
 o someproc -a 1 -- -b -c
</pre><P>

Sometimes we want to check or control the non-positional
arguments. For instance, in the above invocation, we might want to
check that <tt>a</tt> is not forgotten, because otherwise the method cannot
execute properly. This is the role of the checkoptions. There are three
predefined checkoptions: <tt>required</tt>, <tt>boolean</tt> and 
<tt>switch</tt>. <tt>required</tt> checks
whether a non-positional argument is given, <tt>boolean</tt> checks that a
non-positional argument is of boolean type. For instance:
</p>
<pre CLASS="code">
 <tt>Class</tt> P
 P <tt>instproc</tt> someproc {-a:required {-b:boolean true}} {
     puts "$a $b"
 }
 P p
</pre><P>

This method requires <tt>a</tt>, and <tt>b</tt> needs to be 
of type boolean (is has
the default value <tt>true</tt>). This invocation is valid:
</p>
<pre CLASS="code">
 p someproc -a 1 -b 0
</pre><P>

This invocation is invalid, because <tt>a</tt> is missing, 
and <tt>b</tt> is not a Tcl boolean type:
</p>
<pre CLASS="code">
 p someproc -b " a b v"
</pre><P>
The checkoption <tt>switch</tt> is similar to <tt>boolean</tt> except
it does not require an additional argument. If the default value is 
<tt>false</tt>, the switch can be turned on, if the default is <tt>true</tt>
it can be switched off.
<p>
The checkoptions are extensible. In fact, they are defined in an
object <tt>::xotcl::nonposArgs</tt>. We can extend this object with new
methods.
<!-- and we can provide other checkobjects. -->
A check option method has the following syntax:
</P>
<pre CLASS="code">
 <em>someobject|someclass</em> <tt>proc</tt>|<tt>instproc</tt> methodName {?optional nonpositional arguments? argName arg} {
  ...
 }
</pre>
<p>

<tt>argName</tt> is here used to denote the name of the argument, 
and <tt>arg</tt> is the provided value.
</p>


<!--
<p>
In seldom cases, more flexibility might be required. Using the
checkoption "checkobj" we can switch the object that is used for
checking the options. This can be done at any place in the checkoption
list. The following example defines new checkoptions in a separate
object and switches to this object for checking. Then the check is
switched back to "xotcl::nonposArgs" to invoke the predefined option
"required":

</P>
<pre CLASS="code">
Object colorchecker
colorchecker proc color {a b c argName args} {
    puts "$a $b $c"
}
colorchecker proc reddish {argName args} {
    puts "reddish"
}
o proc color {{{-color:checkobj colorchecker,color must be red,reddish,checkobj xotcl::nonposArgs, required} red}} {} {
    puts "$b $arg"
}
</pre></p>
-->

Of course, the non-positional arguments can also be introspected. The
following <tt>info</tt> options return the non-positional arguments of a
method:

</P>
<pre CLASS="code">
 <em>objName</em> <tt>info</tt> <em>nonposargs</em> methodName
 <em>className</em> <tt>info</tt> <em>instnonposargs</em> methodName
</pre></p>

<!-- PAGE BREAK -->

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="interceptors"></A><FONT
COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Message
Interception Techniques
			</FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic7" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>

<P>Even though object-orientation orders program structures around
data, objects are characterized primarily by their behavior.
Object-oriented programming style encourages the access of
encapsulated data only through the methods of an object, since this
enables data abstractions. A method invocation can be interpreted as a
message exchange between the calling and the called object.
Therefore, objects are at runtime only traceable through their message
exchanges. At this point the message interceptors can be applied to
catch and manipulate all incoming and outgoing messages of an
object. 
<P>
</P>Generally interceptors can be applied to attach additional or
extrinsic concerns to an object or a class or a class hierarchy. For
instance roles or aspects can be implemented this way on various
levels of scale.
</P>

<P>We have already discussed some interception techniques
implicitly. E.g., the <tt>unknown</tt> mechanism intercepts messages
that have not be found on the object. It can be used as a very useful
programming technique, e.g., the define a default behavior for an
object. The interceptors presented in this section have a different
character: They are applied before/after the original method <em>even
if the method is defined for the target object</em>. Thus these
interception techniques may be applied 
</P>

<P>We will discuss the message interceptors in this section in
detail. The table below gives an impression, when which interceptor
may be applied.

    <P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Message
	Interceptors Overview</STRONG></P>

    <CENTER>
      <TABLE BORDER=1 CELLPADDING=2>
	<TR>
	  <TD></TD>
	  <TD><P ALIGN=LEFT><strong>Applied When</strong></P></TD>
	  <TD><P ALIGN=LEFT><strong>Primary Target Structure</strong></P></TD>
	  <TD><P ALIGN=LEFT><strong>Coverage</strong></P></TD>
	</TR>
	<TR>
	  <TD><P ALIGN=LEFT><em> Per-Object Filter</em></P></TD>
	  <TD><P ALIGN=LEFT>before/after a call</P></TD>
	  <TD><P ALIGN=LEFT> object hierarchies</P></TD>
	  <TD><P ALIGN=LEFT>all methods</P></TD>
	</TR>
	<TR>
	  <TD><P ALIGN=LEFT><em> Per-Class Filter</em></P></TD>
	  <TD><P ALIGN=LEFT>before/after a call</P></TD>
	  <TD><P ALIGN=LEFT> class and class hierarchies</P></TD>
	  <TD><P ALIGN=LEFT>all methods</P></TD>
	</TR>
	<TR>
	  <TD><P ALIGN=LEFT><em> Per-Object Mixin</em></P></TD>
	  <TD><P ALIGN=LEFT> before/after a call</P></TD>
	  <TD><P ALIGN=LEFT> object</P></TD>
	  <TD><P ALIGN=LEFT> specific methods</P></TD>
	</TR>
	<TR>
	  <TD><P ALIGN=LEFT><em> Per-Class Mixin</em></P></TD>
	  <TD><P ALIGN=LEFT> before/after a call</P></TD>
	  <TD><P ALIGN=LEFT> class and class hierarchies</P></TD>
	  <TD><P ALIGN=LEFT> specific methods</P></TD>
	</TR>
	<TR>
	  <TD><P ALIGN=LEFT><em> Unknown Mechanism</em></P></TD>
	  <TD><P ALIGN=LEFT> after method was not found</P></TD>
	  <TD><P ALIGN=LEFT> object</P></TD>
	  <TD><P ALIGN=LEFT> all unknown calls</P></TD>
	</TR>
      </TABLE>
    </CENTER>

<br>

<H2><A NAME="filter"></A>Filter
</H2>

<P>The filter (see <a href="#xotcl-filter">[Neumann and Zdun
1999a]</a> for more details) is a language construct to implement
broader extensional concerns either for a single object or for several
classes or class hierarchies. This way large program structures at the
scale of several classes or class hierarchies can be managed.  It is a
very general interception mechanism which can be used in various
application areas. E.g. a very powerful programming language support
for certain design patterns is easily achievable, but there are also
several other domains which are covered, like tracing of program
structures, self-documentation at run-time, re-interpretation of the
running program, etc.
</P>
<P>A <I>per-class filter</I> is a special instance method that is
registered for a class <I>C</I>. A <I>per-object filter</I> is a
special instance method that is registered for a object
<I>o</I>. Every time an object of class, <I>C</I> or the object
<I>o</I> respectively, receives a message,
the <I>filter</I> method is invoked automatically.
</P>
<H3><A NAME="filter_usage"></A>Usage of Filters 
</H3>
<P>All messages to a filtered object must go through the filter before
they reach their destination object. A simple example would be a sole
filter on the class of the object. To define such a filter two steps
are necessary. Firstly a filter method has to be defined, then the
filter has to be registered. The filter method consists of three parts
which are all optional. A filter method has the following form:
</P>
<pre CLASS="code">
  <EM>className</em> <tt>instproc</tt> <em>FilterName args</em> {
    <em>pre-part</em>
    <tt>next</tt>
    <em>post-part</em>
  }
</pre>
<P>
When a filter comes to execution at first the actions in the <EM>pre-part</EM>
are processed.  The filter is free in what it does with the
message. Especially it can (a) pass the message, which was perhaps
modified in the <EM>pre-part</EM>, to other filters and finally to
the object. It can (b) redirect it to another destination. Or it can
(c) decide to handle the message on its own. The forward passing of
messages is implemented through the <tt>next</tt> primitive of XOTcl.
After the filter has passed its pre-part, the actual called method is
invoked through <tt>next</tt>. 
</P>
<P>After the call of <tt>next</tt> is processed, the execution returns
to the point in the filter, where the <tt>next</tt> call is located
and resumes execution with the actions of the <EM>post-part</EM>.
These may contain arbitrary statements, but especially may take the
result of the actual called method (which is returned by the
next-call) and modify it. The caller then receives the result of the
filter, instead of the result of the actual called method.
</P>
<P>The pre- and post-part may be filled with any ordinary
XOTcl-statements. The distinction between the three parts is just a
naming convention for explanation purposes. 
</P>
<P>The filter uses the <tt>args</tt> argument which lets us use a list of
variable length as arguments, since it must filter a lot of different
calls, which may have different argument lists. Furthermore, it may
pass through arguments to other filters and the preceding filters may
change the argument list. 
</P>
<P>Since any proc/instproc may be a filter, a registration of the
filter is necessary, in order to tell XOTcl, which instprocs are
filters on which classes. The <tt>filter</tt> and <tt>instfilter</tt>
instance methods are able to handle this task for per-object filters
and per-class filters respectively. Similar to the XOTcl language
introduced so far, the filter registration is dynamic at run-time. By
supplying a new list of filters to
<tt>filter</tt>/<tt>instfilter</tt>, the programmer can change the
filters registered on a class at arbitrary times. The filter instance
method has the syntax:
</P>
<pre CLASS="code">
  <em>className</em> <tt>instfilter</tt> <em>filterList</em>
</pre>
for per-class filters and:
<pre CLASS="code">
  <em>objName</em> <tt>filter</tt> <em>filterList</em>
</pre>
for per-object filters.

<P>
Now a simple example should show the filter's usage. In the preceding
examples we have defined several rooms. Every time a room action
occurs it is likely that the graphical sub-system has to change
something on the output of that particular room. Therefore, at first
we need a facility to be informed every time an action on a room
happens. This is quite easily done using filters: 
</P>
<pre CLASS="code">
  <tt>Class</tt> Room
  Room r1; Room r2;       <it># just two test objects</it>

  Room <tt>instproc</tt> roomObservationFilter args {
    <tt>puts</tt> "now a room action begins"
    <tt>set</tt> result [<tt>next</tt>]
    <tt>puts</tt> "now a room action ends - Result: $result"
    <tt>return</tt> $result
  }

  Room <tt>instfilter</tt> roomObservationFilter
</pre><P>
Now every action performed on room objects is notified with a pre-
and a post-message to the standard output stream. We return the
result of the actual called method, since we don't want to change the
program behavior at all. E.g. we can set an instance variable on both
of the two room objects: 
</P>
<pre CLASS="code">
  r1 <tt>set</tt> name &quot;room 1&quot;
  r2 <tt>set</tt> name &quot;room 2&quot;
</pre><P>
The output would be: 
</P>
<pre CLASS="code">
  now a room action begins
  now a room action ends - Result: room 1
  now a room action begins
  now a room action ends - Result: room 2
</pre><P STYLE="margin-bottom: 0in">
<BR>
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><A NAME="oneFilter"></A><A NAME="718"></A>
&#160;&#160; 
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 4:</STRONG>
Cascaded Message Filtering</P>
<CENTER>
	<TABLE WIDTH=480 BORDER=0 CELLPADDING=2 CELLSPACING=0>
		<COL WIDTH=476>
		<TR>
			<TD WIDTH=476>
				<P><FONT SIZE=1 STYLE="font-size: 2pt"><IMG SRC="cascaded-message-filter.gif" NAME="Graphic14" ALIGN=BOTTOM WIDTH=474 HEIGHT=281 BORDER=0></FONT></P>
			</TD>
		</TR>
	</TABLE>
</CENTER>
<P><BR><BR>
</P>
<P>All classes may have more than one filter. In fact they may have a
whole filter chain, where the filters are cascaded through <tt><tt>next</tt></tt>.
The <tt>next</tt> method is responsible for the forwarding of
messages to the remaining filters in the chain one by one till all
pre-parts are executed. Then the actual method is executed and
then the post-parts come to turn. If one next-call is omitted the
chain ends in this filter method. As an example for an additional
filter we may register a filter that just counts the calls to
rooms. 
</P>
<pre CLASS="code">
  Room <tt>set</tt> callCounter 0;  <it># set class variable</it>
  Room <tt>instproc</tt> counterFilter args {
    [<tt>self</tt> class] <tt>instvar</tt> callCounter
    <tt>incr</tt> callCounter
    <tt>puts</tt> "the call number callCounter to a room object"
    <tt>next</tt>
  }
  Room <tt>instfilter</tt> {roomObservationFilter counterFilter}
</pre><P>
Filters are invoked in registration order. The order may be changed
by removing them and adding them in new order. Filters are inherited
by subclasses. E.g. in the preceding example for the next path, an
<tt>OvalOffice</tt> was derived from the <tt>Room</tt> class. Without
a change to the program each <tt>OvalOffice</tt> object automatically
produces the same filter output as rooms. 
</P>
<P STYLE="margin-bottom: 0in"><BR>
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><A NAME="filterInheritance"></A><A NAME="734"></A>
&#160;&#160; 
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 5:</STRONG>
Filter Inheritance</P>
<CENTER>
	<TABLE WIDTH=2 BORDER=0 CELLPADDING=2 CELLSPACING=0>
		<COL WIDTH=0>
		<TR>
			<TD>
				<P><FONT SIZE=1 STYLE="font-size: 2pt"><IMG SRC="filter-inheritance.gif" NAME="Graphic15" ALIGN=BOTTOM WIDTH=508 HEIGHT=350 BORDER=0></FONT></P>
			</TD>
		</TR>
	</TABLE>
</CENTER>
<P><BR>Filter chains can also be combined through (multiple)
inheritance using the <tt>next</tt> method. When the filter chain of
the object's class is passed, the filter chains of the superclasses
are invoked using the same precedence order as for inheritance. Since
on the subclass there may also be a another filter chain, without
sophisticated computing in the pre- and post-parts one can produce
easily a powerful tracing facility. E.g. if we want to distinguish an
<tt>OvalOffice</tt> from other rooms we may want to add a filter
solely for rooms of the type <tt>OvalOffice</tt>: 
</P>
<pre CLASS="code">
  <tt>Class</tt> OvalOffice <tt>-superclass</tt> Room
  OvalOffice o1;  <it># test object</it>
  OvalOffice <tt>instproc</tt> ovalOfficeObservationFilter args {
    <tt>puts</tt> &quot;actions in an oval office&quot;
    <tt>next</tt>
  }
  OvalOffice <tt>instfilter</tt> ovalOfficeObservationFilter
</pre><P>
A simple call to the <tt>o1</tt> object, like: 
</P>
<pre CLASS="code">
  o1 <tt>set</tt> location "Washington"
</pre><P>
produces the following output: 
</P>
<pre CLASS="code">
  actions in an oval office
  now a room action begins
  the call number 3 to a room object
  now a room action ends - Result: Washington
</pre><P>
As seen already, filter registrations can be added dynamically at
runtime. But they may also be removed. Perhaps the counting on rooms
should stop after a while, then a simple call of the <tt>instfilter</tt>
method is sufficient: 
</P>
<pre CLASS="code">
  Room <tt>instfilter</tt> roomObservationFilter
</pre>
<P>Filters can be removed completely by giving an empty list to the
registration method:</P>
<pre CLASS="code">
  Room <tt>instfilter</tt> {}
</pre>


<P> Per-object filters operate on a single object. E.g. if we only
want to observe a single Room object room1, we can use the filter
method to register the roomObservationFilter only for this particular
instance:</P>
<pre CLASS="code">
  room1 <tt>filter</tt> roomObservationFilter
</pre>
<P> As a filter we can register any method in the precedence order of
the class or object. Thus we can also register procs as per-object
filters. Additionally, meta-class methods may be registered as
per-class filters. Filters are linearized so that each filter is only
executed once, even if it is registered multiple times. 
</P>
<P><BR><BR>
</P>
<H3><A NAME="filter_info"></A>Introspection on Filters
</H3>
In order to gain information about the currently registered filters on
a certain object/class, the class-object info option <tt>filters </tt> and
the  class info option <tt>instfilters </tt> may be
queried. It returns a list of the currently registered filters:
</P>
<pre CLASS="code">
  <em>className</em> <tt>info</tt> instfilter
  <em>objName</em> <tt>info</tt> filter
</pre>
<p>

A special call-stack info option for filters is <tt>self
filterreg</tt>. It returns the name of the object or class on which
the filter is registered. Since the filter may be registered on other
objects/classes than the one on which it is defined, this may vary from
<tt>self class</tt> in the filter.

The command returns a list of the form:
<pre CLASS="code">
  <em>objName</em> <tt>filter</tt> <em>filterName</em>
</pre>
or:
<pre CLASS="code">
  <em>className</em> <tt>instfilter</tt> <em>filterName</em>
</pre>
respectively.
</P>

<P><BR><BR>
</P>
<H3><A NAME="filter_trace"></A>Example: A Simple Trace Filter 
</H3>
<P>The trace example primarily demonstrates the inheritance of filter
chains. Since all classes inherit from <tt>Object</tt>, a filter on
this class is applied on all messages to objects. The <tt>Trace</tt>
object encapsulates methods for managing the tracing: 
</P>
<pre CLASS="code">
  <tt>Object</tt> Trace
  Trace <tt>set</tt> traceStream stdout

  Trace <tt>proc</tt> openTraceFile name {
    <tt>my</tt> <tt>set</tt> traceStream [open $name w]
  }

  Trace <tt>proc</tt> closeTraceFile {} {
    <tt>close</tt> $Trace::traceStream
    <tt>my</tt> <tt>set</tt> traceStream stdout
  }

  Trace <tt>proc</tt> <tt>puts</tt> line {
    <tt>puts</tt> $Trace::traceStream $line
  }

  Trace <tt>proc</tt> add className {
    $className <tt>instfilter</tt> [concat [$className <tt>info</tt> filter] traceFilter]
  }
</pre><P>
First we define the object and set a variable for the stream to which
we send the trace outputs (here: stdout). With a method for opening
and a method for closing a file we can redirect the trace stream to a
file. <tt>puts</tt> is helper method for the filter to print an
output to the selected output stream. In <tt>add</tt> the <tt>traceFilter</tt>
is appended to the existing filters of a specified class. The actual
filter method (see below) displays the calls and exits of methods
with an according message. The calls are supplied with the arguments,
the exit traces contain the result values. We have to avoid the
tracing of the trace methods explicitly. 
</P>
<pre CLASS="code">
  <tt>Object</tt> <tt>instproc</tt> traceFilter args {
    <it># don't trace the Trace object</it>
    <tt>if</tt> {[string equal [<tt>self</tt>] ::Trace]} {return [<tt>next</tt>]}
    <tt>set</tt> context &quot;[<tt>self</tt> class]-&gt;[<tt>self</tt> callingproc]&quot;
    <tt>set</tt> method [<tt>self</tt> calledproc]
    switch -- $method {
      proc -
      instproc {::set dargs [<tt>list</tt> [lindex $args 0] [lindex $args 1] ...] }
      default  {::set dargs $args }
    }
    Trace::puts &quot;CALL $context&gt;  [<tt>self</tt>]-&gt;$method $dargs&quot;
    <tt>set</tt> result [<tt>next</tt>]
    Trace::puts &quot;EXIT $context&gt;  [<tt>self</tt>]-&gt;$method ($result)&quot;
    <tt>return</tt> $result
  }
</pre><P>
As trace message we write the callee&acute;s context (class and
proc), the invoked method (using <tt>calledproc</tt>), and the given
arguments. In the switch statement we avoid to print whole method
bodies. 
</P>
<P>With 
</P>
<pre CLASS="code">
  Trace add Room
</pre><P>
messages to all rooms, including all instances of <tt>Room</tt>&acute;s
subclasses, are surrounded with a CALL and an EXIT output. With
</P>
<pre CLASS="code">
  Trace add Object
</pre><P>
messages to all objects in an XOTcl environment are surrounded with a
CALL and an EXIT output. In general, it is possible to restrict the
trace to instances of certain classes, or to produce trace output for
only certain methods. This requires registration methods and a more
sophisticated implementation of the filter method. 
</P>
<P><BR><BR>
</P>

<H2><A NAME="mixins"></A>Mixin Classes
</H2>

<P>Per-object and per-class mixins (see <a
href="#xotcl-mixin">[Neumann and Zdun 1999c]</a> for more details) are
another interception technique of XOTcl to handle complex
data-structures dynamically. Here, we use mixin as a short form for
mixin class. All methods which are mixed into the execution of the
current method, by method chaining or through a mixin class, are
called <I>mixin methods</I>.  Mixin classes resembles the filter
presented in the preceding section. While the filters work on all
calls to all methods of an object/class hierarchy, the mixin classes
are applied on specific methods. The filter is defined in a single
method, while the mixin is composes several method in a class.
 
</P>
<H3><A NAME="mixin_supplemental"></A>Supplemental Classes 
</H3>
<P>Mixin classes cover a problem which is not solvable elegantly just
by the method chaining, introduced so far. To bring in an addition to
a class, the normal XOTcl way is to define a mixin method and chain
the methods through <tt>next</tt>, e.g.:
</P>
<pre CLASS="code">
  <tt>Class</tt> Basic
  Basic <tt>instproc</tt> someProc  {
    <it># do the basic computations</it>
  }
  <tt>Class</tt> Addition
  Addition <tt>instproc</tt> someProc {
    <it># do the additional computations</it>
    <tt>next</tt>
  }
</pre><P>
In order to mix-in the additional functionality of the <EM>supplemental</EM>
class <tt>Addition</tt> a new helper class (sometimes called
intersection class) has to be defined, like: 
</P>
<pre CLASS="code">
  Basic+Addition <tt>-superclass</tt> {Addition Basic}
</pre><P>
This is even applicable in a dynamical manner, every object of the
class <tt>Basic</tt> may be changed to class <tt>Basic+Addition</tt>
at arbitrary times, e.g.: 
</P>
<pre CLASS="code">
  Basic basicObj
  ...
  basicObj class Basic+Addition
</pre><P>
Now consider a situation with two addition classes. Then following
set of classes has to be defined to cover all possible combinations: 
</P>
<pre CLASS="code">
  <tt>Class</tt> Basic
  <tt>Class</tt> Addition1
  <tt>Class</tt> Addition2
  <tt>Class</tt> Basic+Addition1 <tt>-superclass</tt> {Addition1 Basic}
  <tt>Class</tt> Basic+Addition2 <tt>-superclass</tt> {Addition2 Basic}
  <tt>Class</tt> Basic+Addition1+Addition2 <tt>-superclass</tt> {Addition2 Addition1 Basic}
</pre><P>
The number of necessary helper classes rises exponential. For <I>n</I>
additions, 2<I><SUP>n-1</SUP></I> (or their permutations if order
matters) artificially constructed helper-classes are needed to
provide all combinations of additional mix-in functionality.
Furthermore it is possible that the number of additions is unlimited,
since the additions may produce other additions as side-effects. This
demonstrates clearly that the subclass mechanism provides only a
poor mechanism for mix-in of orthogonal functionality. Therefore we
provide an extension in the form of class-object mixin classes, which are
added in front of the search precedence of classes. 
</P>
<H3><A NAME="mixin-usage"></A>Per-Object Mixins 
</H3>
<P>The mix-ins methods extend the next-path of shadowed methods.
Therefore, per-object mix-in methods use the <tt>next</tt> primitive
to access the next shadowed method. Consider the following example: 
</P>
<pre CLASS="code">
  <tt>Class</tt> Agent
  Agent <tt>instproc</tt> move {x y} { 
    <it># do the movement</it>
  }
  <tt>Class</tt> InteractiveAgent <tt>-superclass</tt> Agent
  <it># Addition-Classes</it>
  <tt>Class</tt> MovementLog
  MovementLog <tt>instproc</tt> move {x y} { 
    <it># movement logging</it>
    <tt>next</tt>
  }
  <tt>Class</tt> MovementTest
  MovementTest <tt>instproc</tt> move {x y} {
    <it># movement testing</it>
    <tt>next</tt>
  }
</pre><P>
An agent class is defined, which allows agents to move around. Some
of the agents may need logging of the movements, some need a testing
of the movements, and some both (perhaps only for a while). These
functionalities are achieved through the additional classes, which we
will apply through per-object mixins. 
</P>
<P>Before we can use the per-object mix-ins on a particular object,
we must register the mixins on it with the <tt>mixin</tt> instance
method. It has the syntax: 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>mixin</tt> <em>mixinList</em>
</pre><P>

For example we may create two interactive agents, where one is logged
and one is tested: 
</P>
<pre CLASS="code">
  InteractiveAgent i1; InteractiveAgent i2
  i1 <tt>mixin</tt> MovementLog
  i2 <tt>mixin</tt> MovementTest
</pre><P>
At arbitrary times the mixins can be changed dynamically. For example
<tt>i2</tt>'s movements can also be logged: 
</P>
<pre CLASS="code">
  i2 <tt>mixin</tt> MovementTest MovementLog
</pre><P STYLE="margin-bottom: 0in">
<BR>
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><A NAME="per-obj-mixin"></A><A NAME="662"></A>
&#160;&#160; 
</P>
<P ALIGN=CENTER STYLE="margin-bottom: 0in"><STRONG>Figure 6:</STRONG>
Per-Object Mix-ins: Next-Path for the Example</P>
<CENTER>
	<TABLE WIDTH=315 BORDER=0 CELLPADDING=2 CELLSPACING=0>
		<COL WIDTH=311>
		<TR>
			<TD WIDTH=311>
				<P><FONT SIZE=1 STYLE="font-size: 2pt"><IMG SRC="next-path-mixin-movement.gif" NAME="Graphic16" ALIGN=BOTTOM WIDTH=307 HEIGHT=187 BORDER=0></FONT></P>
			</TD>
		</TR>
	</TABLE>
</CENTER>
<P><BR><BR>
</P>
<P>The <tt>mixin</tt> option of the <tt>info</tt> instance method
allows us to introspect the per-object mixins. It has the syntax: 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>info</tt> mixin <em>?pattern?</em>
</pre>
<P STYLE="margin-bottom: 0in">
It returns the list of all mix-ins of the object, if <tt>pattern</tt>
is not specified, otherwise it returns the matching per class-object mixin classes.
</P>
	
The inverse operation of <tt>info mixin</tt> is <tt>mixinof</tt> finds
out, into which objects a per-object mixin class is mixed into.
	
<pre CLASS="code">
  <em>clsName</em> <tt>info</tt> mixinof <em>?pattern?</em>
</pre>
	
<P> Note that the constructors (init methods) of per-object mixins (and per-class mixins)
are only called, if the mixin is registered already during object
initialization (when <tt>init</tt> is called). For per-object mixins, one can 
achieve the initialization of a mixin via an idiom like 
<pre CLASS="code">
  <tt>Object</tt> o <tt>-mixin</tt> M <tt>-init</tt>
</pre> 
that registers the mixin before <tt>init</tt> is called. When a mixin is registered
after object creation and it needs initializations, it is necessary to
define special methods for this.  Note that the behavior described
here is introduced in version 0.84 to ensure consistent behavior of
intrinsic classes, per-object and per-class mixins, and to achieve
predictable behavior for dynamic registration for all kind of mixins,
and as well during recreations of objects having mixins
registered. Older versions used heuristics for the initialization of
per-object mixins.
</P>

<H3><A NAME="per-class-mixins"></A>Per-Class Mixins
</H3>

<P>Per-class mixins are exactly identical in their behavior to
per-object mixins, but they operate on classes. Thus they are the
class-specific variant of the per-object mixins, like instprocs are a
class-specific variant of procs. Therefore, in the language the
per-class mixins are called instmixins.
</p>

<P>
In general a per-class mixin is a class which is mixed into the
precedence order of all instances of the class and all its subclasses
it is registered for. It is also searched before the object's class
itself is searched, but after per-object mixins. 
</p>
<P>
Per-class mixins are <em>linearized</em> according to the 
<a href='#precedence order'>precedence order</a> 
like classes on the superclass hierarchy.  I.e. from the full
list of per-object mixins, per-class mixins, and intrinsic classes
(and all the superclasses of all these classes) always the last
occurrence is used.
</p>

<P>
From the point of view of language expressibility instmixins are not
required, because they cannot express anything that per-object mixins
cannot express already (like procs can express any instproc
feature). As alternative to instmixins, we could simply register the
per-object mixins in the constructor of the class.
</p>

<P>
But there at least the following reasons for instmixins as an
additional language construct:
<OL>
<LI> we can at runtime determine with <tt>info mixin</tt>
and <tt>info instmixin</tt> whether it is a class- or object-specific
mixin. Thus we get a better structuring at runtime. 
<LI> We have not to 'pollute' the constructors with per-class mixin
registrations. Therefore, the constructors get more understandable.
<LI>If it is required to add (and remove) dynamically interceptors
to a set of objects, which are instances of a certain type, per-class
mixins are much easier to handle (e.g. add an instmixin to Object
to intercept e.g. all calls to certain predefined methods).
<LI>The language is more 'symmetrical', since any object-specific
feature in XOTcl has a class-specific variant. 
</OL>
<P>

<P>The mix-ins methods of per-class mixins extend the next-path of
shadowed methods in the same way as per-object mixin methods. Before
we can use a per-class mix-in on a particular class, we must
register the mixin on it with the <tt>instmixin</tt> instance method. It
has the syntax:
</P>
<pre CLASS="code">
  <em>className</em> <tt>instmixin</tt> <em>mixinList</em>
</pre>
The inverse operation of <tt>info inmixin</tt> is <tt>instmixinof</tt> finds
out, into which objects a per-object mixin class is mixed into.
	
<pre CLASS="code">
  <em>className</em> <tt>info</tt> instmixinof <em>?-closure? ?pattern?</em>
</pre>
<P>
Now consider that in the given per-object mixin example all
interactive agents should be tested. We could either build a subclass
<tt>TestedInteractiveAgent</tt> or register the per-object mixin in
the constructor of the interactive agent class. The subclass solution
leads to the same combinatorial explosion of intersection classes as
discussed in the previous section, if more supplemental classes are
added. The per-object mixin solution pollutes the constructor and does
not prevail the structural semantics that the 'tested' property
belongs to the interactive agent class at runtime
</P>
<P>
Here, we can use a per-class mixin:
</P>
<pre CLASS="code">
  <tt>Class</tt> Agent
  Agent <tt>instproc</tt> move {x y} {<it># do the movement</it>}
  <tt>Class</tt> InteractiveAgent <tt>-superclass</tt> Agent
  <tt>Class</tt> MovementTest
  MovementTest <tt>instproc</tt> move {x y} {
    <it># movement testing</it>
    <tt>next</tt>
  }

  <it># now register the instmixin</it>
  InteractiveAgent <tt>instmixin</tt> MovementTest

</pre>

<P> The per-class mixin now operates on all interactive agent
including the instances of subclasses. E.g. for interactive agents
<tt>i1</tt> and <tt>i2</tt> we automatically have movement
testing. <tt>i2 </tt> is also logged, since it has the logging class
as object-specific mixin:
</P>
<pre CLASS="code">
  InteractiveAgent i1
  InteractiveAgent i2 <tt>-mixin</tt> MovementLog

  i1 move 3 4
  i2 move 1 2 
</pre>

<P>
At arbitrary times the instmixins can be changed dynamically.
</P>
<P>The <tt>instmixin</tt> option of the class <tt>info</tt> instance
method allows us to introspect the per-class mixins. It has the
syntax:
</P>
<pre CLASS="code">
  <em>className</em> <tt>info</tt> instmixin <em>?className2?</em>
</pre>
<P STYLE="margin-bottom: 0in">
It returns the list of all instmixins of the class, if <em>className2</em>
is not specified, otherwise it returns <tt>1</tt>, if <em>className2</em>
is a mixin of the object, or <tt>0</tt> if not. 
</P>

<p>Per-class mixins are applied transitively. That means the per-class
mixin A of a per-class mixin B is also applied for an objectin B's
scope. This is exactly the same as how superclasses are applied for
instances. Consider the following example</p>

<pre CLASS="code">
  <tt>Class</tt> X11 \
     <tt>-instproc</tt> test args {
	<tt>puts</tt> [<tt>self</tt> class]
	<tt>next</tt>
     }
  <tt>Class</tt> X12 \
    <tt>-instproc</tt> test args {
	<tt>puts</tt> [<tt>self</tt> class]
	<tt>next</tt>
    }
  <tt>Class</tt> X \
    <tt>-instmixin</tt> {X11 X12} \
    <tt>-instproc</tt> test args {
	<tt>puts</tt> [<tt>self</tt> class]
	<tt>next</tt>
    }

  <tt>Class</tt> Y \
    <tt>-instmixin</tt> X

  Y <tt>create</tt> y -test
  X <tt>create</tt> x -test
</pre>
<p> Here the application as a superclass (for x) yields the same
result as the application as an instmixin (for y): 
<pre CLASS="code">
  ::X11 
  ::X12 
  ::X
</pre>
<H2><A NAME="precedence order"></A>Precedence Order
</H2>

<P>The precedence order is composed by the precedence order of the
superclass hierarchy (as explained earlier) and the message
interceptors. In general, filters precede mixins and the superclass
hierarchy. They are applied in the order of the next path of the
object. Thus per-object filters are ordered before per-class
filters.</p>

<p>Mixins are processed after the filters. Again, they are applied in
the order of the next path of the object. Thus per-object mixins are
ordered before per-class mixins.</p>

<p>Finally, the object's own heritage order comes in the order: object,
class, superclasses.</p>

<p>The three precedence order lists (filters, mixins, and classes) are
pre-calculated and cached.</p>

<p>Filters as well as classes (mixins and ordinary classes) are
linearized. That means, each filter and each class can be only once on
a precedence order list. If a filter or class can be reached more than
once, than the last occurrence is used.</p>

<p>For instance, consider a class A is superclass, per-class mixin,
and per-object mixin. On the precedence order lists only the last
occurrence as a superclass is used after linearization.</P>


<H2><A NAME="guards"></A>Guards for Filters and Mixins
</H2>

Message interceptors, such as filters and mixins, are applied for
potentially huge number of messages. In many cases it is possible to
reduce the effective number of cases in which interceptors are
applied. Interceptor guards offer this functionality: they are boolean
conditions with which you can specify in which cases a registered
interceptor should be applied.

<H3><A NAME="filter_guards"></A> Filter Guards</H3>
<P>
A filter guard is a set of conditions that determine whether a filter
is to be executed upon a certain invocation or not. Syntactically we can
append a filter guard to the filter registration, or it can be
registered using the methods <tt>filterguard</tt> for filters and
<tt>instfilterguard</tt> for instfilters.

</P><P> Each filter guard is an ordinary condition. A filter guard is
executed in the call frame of the filter to be executed, if the filter
guard returns 1. Thus, the call-stack information are already set to
the values of the targeted filter - and these values can be used in
the filter guard.

</P><P>
Let us consider a simple program:
</P>
<pre CLASS="code">
<tt>Class</tt> Room
Room <tt>instproc</tt> enter {name} {<tt>puts</tt> [<tt>self proc</tt>]}
Room <tt>instproc</tt> leave {name} {<tt>puts</tt> [<tt>self proc</tt>]}
Room <tt>instproc</tt> loggingFilter args {
    <tt>puts</tt> [<tt>self calledproc</tt>]
    <tt>next</tt>
}
Room <tt>instfilter</tt> loggingFilter
</pre>

<P>
Now consider we only want to apply the logging filter for enter and
leave, not for any other message sent to Room instances. In the
following example, for instance, we do not want to log the <tt>set</tt>
message:
</p>

<pre CLASS="code">
Room r 
r enter Uwe
r leave Uwe
r <tt>set</tt> roomName "Office"
</pre>

<P>
In this example a filterguard can be applied to restrict the
application of the filter to those two methods:
</p>

<pre CLASS="code">
Room <tt>instfilterguard</tt> loggingFilter {
  <tt>[self calledproc</tt>] == "enter" || 
  [<tt>self calledproc</tt>] == "leave"}
</pre>

<P>
Here we limit the filter application of the logging filter on rooms to
calls to enter and leave. All other calls are not filtered at
all. Note that the same syntax can also be applied for
<tt>filterguard</tt>. Also, there is a short form to register filter
guards directly during filter registration. The following code has the
same semantics as the filter and filter guard definitions above:
</P>

<pre CLASS="code">
Room <tt>instfilter</tt> {{loggingFilter <tt>-guard</tt> {
    [<tt>self calledproc</tt>] == "enter" || 
    [<tt>self calledproc</tt>] == "leave"}}}
</pre><P>

The filter guard language construct is registration centric. It only
applies for the class or object on which a filter is registered, not
for all applications of the filter method. That is, if we use
loggingFilter on another class we may give no or completely
different filter guards.
</P><P>
If no filter guard is given for a filter, we assume that it is to be
applied on all methods (equivalent to the filter guard '1' which is
always true).
</P>

<P>
There are introspection options for filter guards. In particular, we
can use <tt>info filterguard</tt> and <tt>info instfilterguard</tt>
for getting the filter guards for a particular filter or instfilter
respectively. For instance:
</P>
<pre CLASS="code">
<tt>puts</tt> [Room <tt>info instfilterguard </tt>loggingFilter]
</pre><P>
This prints out the content of the above guard definition. 
We can also append <tt>-guard</tt> to <tt>info filter</tt> or 
<tt>info instfilter</tt> to obtain a filter definition with guards:
</P>
<pre CLASS="code">
<tt>puts</tt> [Room <tt>info instfilter -guards</tt>]
</pre><P>

<P>
If we call a method from within a filter guard, as for instance
callsMethod, we might require some parameters from the guard's
context, such as <tt>calledproc</tt>. These parameters can be passed
as references, as in the following example:
</P>

<pre CLASS="code">
  Room <tt>instfilter</tt> loggingFilter
  Room <tt>instfilterguard</tt> loggingFilter {[<tt>my</tt> callsMethod openURL [<tt>self calledproc</tt>]]}
</pre><P>
This example works because the filterguard is already set to the scope of the guard. Now we can use this dynamic  <tt>calledproc</tt> context in the called method:
</P>
<pre CLASS="code">
  Room <tt>instproc</tt> callsMethod {method calledproc} {
    <tt>return</tt>[<tt>string</tt> <tt>match</tt> $calledproc $method]
  }
</pre><P>
We simply check whether the called method
matches the given method name or not.

<H3><A NAME="mixin_guards"></A> Mixin Guards</H3>
<P>
Similar to filters, there are mixin guards, defined with
<tt>mixinguard</tt> and <tt>instmixinguard</tt>, or with
<tt>-guard</tt> during mixin registration. Consider a simple example:
there are a number of birds who have two mixins: Fly and Sing. For Fly
there are limitations: a bird can only fly if it is at least two years
old and is not a Penguin. Such problems are  be solved with
mixin guards:
</P>

<pre CLASS="code">
  <tt>Class</tt> Fly
  Fly <tt>instproc</tt> fly {} {<tt>puts</tt> "[<tt>my</tt> signature]: yippee, fly like an eagle!"}

  <tt>Class</tt> Sing
  Sing <tt>instproc</tt> sing {} {<tt>puts</tt> "[<tt>my</tt> signature]: what a difference a day makes"}

  <tt>Class</tt> Animal -parameter age
  Animal <tt>instproc</tt> unknown args { <tt>puts</tt> "[<tt>my</tt> signature]: how should I $args?"}
  Animal <tt>instproc</tt> signature {} {
    <tt>return</tt> "[<tt>self</tt>] [<tt>my info class</tt>] ([<tt>my</tt> age] years)"
  }

  <tt>Class</tt> Bird -superclass Animal
  <tt>Class</tt> Penguine -superclass Bird
  <tt>Class</tt> Parrot -superclass Bird
  <tt>Class</tt> Duck -superclass Bird

  Parrot tweedy -age 1
  Penguine pingo -age 5
  Duck donald -age 4
  Parrot lora -age 6

  Bird <tt>instmixin</tt> {{Fly <tt>-guard</tt> {[my age] > 2 && ![<tt>my istype</tt> Penguine]}} Sing}
</pre><P>
An invocation like:
</P>

<pre CLASS="code">
<tt>foreach</tt> bird {tweedy pingo donald lora} { $bird fly }
</pre><P>
yields the following result:
</P>
<pre CLASS="code">
::tweedy ::Parrot (1 years): how should I fly?
::pingo ::Penguine (5 years): how should I fly?
::donald ::Duck (4 years): yippee, fly like an eagle!
::lora ::Parrot (6 years): yippee, fly like an eagle!
</pre>

<P>
There are similar introspection options for mixin guards as those for
filter guards. In particular, we can use <tt>info mixinguard</tt> and
<tt>info instmixinguard</tt> for getting the mixin guards for a
particular mixin or instmixin respectively.
</P>

<H2><A NAME="updateinterceptors"></A>Querying, Setting, Altering Filter and Mixin Lists
</H2>

The methods <tt>mixin</tt>, <tt>instmixin</tt>, <tt>filter</tt> and
<tt>instfilter</tt> are <a href='#system-slots'>system slots</a>
having the same query and update interface. 
<UL>
<LI>If one of those methods is called without argument, it returns the current
setting. </LI>
<LI>If it is called with one argument, the argument is used to
set the specified list as indicated in the above examples. </LI>
<LI>If these methods are called with more than one argument, the first argument
is used to specify the action. Possible values for the action are
<tt>set</tt>, <tt>get</tt>, <tt>add</tt> and <tt>delete</tt>. See below for 
commonly used examples. 
</UL>
<P>
<CENTER>	
<TABLE BORDER='1' width='90%'>
<TR><TD nowrap='1'><tt>obj mixin</tt></TD>            <TD>same as: <tt>obj info mixin</tt></TD></TR>
<TR><TD nowrap='1'><tt>obj mixin {C1 C2}</tt></TD>    <TD>same as: <tt>obj mixin assign {C1 C2}</tt></TD></TR>
<TR><TD nowrap='1'><tt>obj mixin assign {C1 C2}</tt></TD><TD>sets the mixins for <tt>obj</tt></TD></TR>
<TR><TD nowrap='1'><tt>obj mixin add C3</tt></TD>     <TD>adds the mixin <tt>C3</tt> on front of the mixin list</TD></TR>
<TR><TD nowrap='1'><tt>obj mixin add C3 end</tt></TD> <TD>adds the mixin <tt>C3</tt> at the end the mixin list</TD></TR>
<TR><TD nowrap='1'><tt>obj mixin add C3 3</tt></TD>   <TD>adds the mixin <tt>C3</tt> at the 3rd position</TD></TR>
<TR><TD nowrap='1'><tt>obj mixin delete ::C3</tt></TD><TD>removes the mixin <tt>C3</tt> from the mixin list.
Use absolute class names. <tt>delete</tt> supports an optional flag <tt>-nocomplain</tt> that does not produce an error, when the specified class is not in the list. 
</TD></TR>
</TABLE>
</CENTER>	
</P>
<P>
Note that the list of possible actions can be
extended by extending the class <tt>::xotcl::Relations</tt>.
</P>

<H2>
<A NAME="callstack_info"></A>Querying Call-stack Information
</H2>
<P STYLE="margin-bottom: 0in">Since the presented interceptors are
normal XOTcl instprocs they can access all XOTcl introspection
abilities introduced so far. In instprocs all recent information is
accessible within their scope. But the interceptors are mechanisms,
which cover more then their sole scope. The meaningful usage of the
meta-programming abilities often requires to go further and to get
information from the caller's and the callee's scope (e.g. for
delegation decisions). Therefore, we introduced rich call-stack
information for the interceptors. Note that these are also available
for ordinary methods, but the "called..." info options return empty
strings.
</P>
<P> All call-stack information are packed compactly into the
<tt>self</tt> primitive as additional options. Note, before XOTcl
version 0.84 these were implemented as a part of the <tt>info</tt>
method. They are part of the <tt>self</tt> command for conceptual
integrity: introspection options in <tt>info</tt> can be expected to
produce the same result, when they are not explicitly changed. In
contrast, all information provided by <tt>self</tt> are
call-stack dependent.
</P>

<CENTER>
      <P><STRONG>Querying Call-stack Information via <tt>self</tt> </STRONG></P>
      <TABLE BORDER=1>
	<TR>
	  <TD><tt>self activelevel</tt>
	  </TD>
	  <TD  VALIGN=TOP>
	    <P ALIGN=LEFT>Returns the stack level from where the
	    current command was invoked from, or where the last next
	    was called (whatever is closer to the invocation).
	      If the current command was invoked from
	    an XOTcl method the absolute level is returned (e.g. #4)
	    which can be used in the <tt>uplevel</tt> or
	    <tt>upvar</tt> Tcl command or XOTcl method. If the current
	    command was not invoked from an XOTcl method, the value 1
	    is returned.
	  </TD>
	</TR>

	<TR>
	  <TD width=25%><tt>self calledproc</tt>
	  </TD>
	  <TD  VALIGN=TOP>
	    <P ALIGN=LEFT>Returns the name of the method which was invoked in
	      the original call.
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self calledclass</tt>
	  </TD>
	  <TD  VALIGN=TOP>
	    <P ALIGN=LEFT>Returns the name
	      of the class which presumably (if no dynamic class change occurs
	      afterwards) is invoked in
	      the original call.
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self callingclass</tt>
	  </TD>
	  <TD  VALIGN=TOP>
	    <P ALIGN=LEFT>Returns the name of the class from which the
	      call was invoked (if one exists, otherwise an empty
	      string).
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self callinglevel</tt>
	  </TD>
	  <TD  VALIGN=TOP>
	    <P ALIGN=LEFT>Returns the stack level from where the
	    current command was invoked from. In contrary to
	    <tt>activelevel</tt> next-calls are ignored in the
	    computation. If the current command was invoked from an
	    XOTcl method the absolute level is returned (e.g. #4)
	    which can be used in the <tt>uplevel</tt> or
	    <tt>upvar</tt> Tcl command or XOTcl method. If the current
	    command was not invoked from an XOTcl method, the value 1 is
	    returned.
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self callingproc</tt>
	  </TD>
	  <TD  VALIGN=TOP>
	    <P ALIGN=LEFT>Returns the name of the method from which the
	      call was invoked (if one exists, otherwise an empty
	      string).
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self callingobject</tt>
	  </TD>
	  <TD  VALIGN=TOP>
	    <P ALIGN=LEFT>Returns the name of the object from which the
	      call was invoked (if one exists, otherwise an empty
	      string).
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self filterreg</tt>
	  </TD>
	  <TD VALIGN=TOP>
	    <P ALIGN=LEFT>In a filter:  returns the name
	      of the object/class on which the filter is registered. Returns either 
	      '<tt><em>objName</em> <tt>filter</tt> <em>filterName</em></tt>' or 
	      '<tt><em>className</em> instfilter <em>filterName</em></tt>'.
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self isnextcall</tt>
	  </TD>
	  <TD VALIGN=TOP>
	    <P ALIGN=LEFT>Return 1 if this method
              was invoked via next, otherwise 0
	  </TD>
	</TR>
	<TR>
	  <TD><tt>self next</tt>
	  </TD>
	  <TD VALIGN=TOP>
	    <P ALIGN=LEFT>Return the
	      "next" method on the path as a string, i.e. the method which will be
	      called by [next].
	  </TD>
	</TR>
      </TABLE>
    </CENTER>
</P>
<p><br></p>
<P>Note that three options with the prefix <tt>calling</tt>
represent the values of <tt>self</tt>, <tt>self proc</tt>, and <tt>self
class</tt> in the scope where the original call was invoked. In the
following section we will show a simple program in which all of the
<tt>info</tt> options have different values.

<H3><A NAME="filter_info_example"></A><BR>Filter Call-stack Information Example 
</H3>
<P>Now we discuss a simple example that shows that all filter
introspection options may have different values: 
</P>
<pre CLASS="code">
  <tt>Class</tt> InfoTrace
  InfoTrace <tt>instproc</tt> infoTraceFilter args { 
    <tt>puts</tt> &quot;SELF:                [<tt>self</tt>]&quot;
    <tt>puts</tt> &quot;SELF PROC:           [<tt>self</tt> proc]&quot;
    <tt>puts</tt> &quot;SELF CLASS:          [<tt>self</tt> class]&quot;
    <tt>puts</tt> &quot;INFO CLASS:          [<tt>my</tt> <tt>info</tt> class]&quot;
    <tt>puts</tt> &quot;CALLED PROC:         [<tt>self</tt> calledproc]&quot;
    <tt>puts</tt> &quot;CALLING PROC:        [<tt>self</tt> callingproc]&quot;
    <tt>puts</tt> &quot;CALLING OBJECT:      [<tt>self</tt> callingobject]&quot;
    <tt>puts</tt> &quot;CALLING CLASS:       [<tt>self</tt> callingclass]&quot;
    <tt>puts</tt> &quot;REGISTRATION CLASS:  [<tt>self</tt> filterreg]&quot;
    <tt>puts</tt> &quot;CALLING LEVEL:       [<tt>self</tt> callinglevel]&quot;
    <tt>puts</tt> &quot;ACTIVE LEVEL:        [<tt>self</tt> activelevel]&quot;
    <tt>next</tt>
  }

  <tt>Class</tt> CallingObjectsClass
  CallingObjectsClass callingObject

  <tt>Class</tt> FilterRegClass <tt>-superclass</tt> InfoTrace
  <tt>Class</tt> FilteredObjectsClass <tt>-superclass</tt> FilterRegClass 
  FilteredObjectsClass  filteredObject 

  CallingObjectsClass <tt>instproc</tt> callingProc args {
     filteredObject <tt>set</tt> someVar 0
  }    
  FilterRegClass <tt>instfilter</tt> infoTraceFilter</pre><P>
The invocation of <tt>callingObject callingProc</tt> produces the
following output: 
</P>
<pre CLASS="code">
  SELF:                ::filteredObject
  SELF PROC:           infoTraceFilter
  SELF CLASS:          ::InfoTrace
  INFO CLASS:          ::FilteredObjectsClass
  CALLED PROC:         set
  CALLING PROC:        callingProc
  CALLING OBJECT:      ::callingObject
  CALLING CLASS:       ::CallingObjectsClass
  REGISTRATION CLASS:  ::FilterRegClass instfilter infoTraceFilter
  CALLING LEVEL:       #1
  ACTIVE LEVEL:        #1</pre><P>
The filter reports for <tt>self</tt> the value <tt>filteredObject</tt>,
since this is the object on which the <tt>set</tt> call is invoked;
<tt>infoTraceFilter</tt> is the method of the filter, and therefore,
the actual proc, while the actual class is <tt>InfoTrace</tt>, the
filter's class. The class of the actual object is
<tt>FilteredObjectsClass</tt>. 
</P>
<P>The called procedure is <tt>set</tt>. While the program stays in a
XOTcl-instproc all calling-info-options are set, the calling
procedure is <tt>callingProc</tt>, the calling class is the class,
where the method is defined (namely <tt>CallingObjectsClass</tt>),
and the object from which the call invoked is <tt>callingObject</tt>.
</P>
<P>In this example, the calling level is equal to the active level, both
are #1.
</P>

<!-- PAGE BREAK -->

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
    <TR>
      <TD WIDTH=75%>
	<P><A NAME="slots"></A><FONT COLOR="#ffffff" FACE="Arial, Helvetica" SIZE=6>Slots</FONT>
	</P>
      </TD>
      <TD>
	<IMG SRC="logo-100.jpg" NAME="Graphic9" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
    </TR>
</TABLE>

<p>A slot is a meta-object that manages property-changes of objects. A
property is either an attribute or a role in an relation.  In a
nutshell, a slot has among other attributes:</p>

<ul>
<li>a <em>name</em> (which it used to access it), 
<li>a <em>domain</em> (object or class on which it can be used) , and
<li>can be <em>multivalued</em> or not.
</ul>

<p> We distinguish between <em>system slots</em> (predefined slots
  like <tt>class</tt>, <tt>superclass</tt>, <tt>mixin</tt>,
  <tt>instmixin</tt>, <tt>filter</tt>, <tt>instfilter</tt>) and
  <em>attribute slots</em> (e.g. attributes of classes).  </p>

<h2><a name='system-slots'></a>System Slots</h2>
<p>
System slots are predefined slots defining e.g. some relations
between classes, or between objects and classes. The predefined system slots
are:
<ul>
  
  <li> <tt>superclass</tt>: every class in XOTcl has one or more
  superclasses.  The name of this slot is <tt>superclass</tt>, the
  domain is <tt>::xotcl::Class</tt>, the slot is multivalued, since
  one object might have multiple superclasses.<p>

  <li> <tt>class</tt>: every object has a class; therefore, the domain
  of the slot is <tt>::xotcl::Class</tt>, the property is not multivalued.<p>

  <li> <tt>mixin</tt>: every object in XOTcl can have one or more
  mixin classes.  The name of this slot is <tt>mixin</tt>, the domain
  is <tt>::xotcl::Object</tt> , the slot is multivalued.<p>

  <li> <tt>instmixin</tt>: same as above, but the domain is
  <tt>::xotcl::Class</tt>.<p>

  <li> <tt>filter</tt>, <tt>instfilter</tt>: similar to <tt>mixin</tt>
  and <tt>instmixin</tt>.

</ul> <p>The system slots were introduced earlier with their
semantics.  Here we just point out, that they have all the same
interfaces for querying, setting, adding and removing of slot
values.</p>

<p>Every slot can be used set and query the property from its domain.
The syntax for setting values is
<pre CLASS="code">
  <em>object property newValue</em>
</pre>
	 and for getting its values is
<pre CLASS="code">
   <tt>set</tt> x [<em>object property</em>]
</pre>
where <em>property</em> denotes the slot name.
Every multivalued slot provides the methods <tt>add</tt> and
<tt>delete</tt>.

Here are a few examples for using the system slot <tt>mixin</tt> which we have introduced already in the section of the <a href='#mixins'>mixins</a>
<pre CLASS="code">
  <tt>Object</tt> o; <tt>Class</tt> M; <tt>Class</tt> N
  o <tt>mixin</tt> ::M      <em>;# replacing the per-object mixins of o with M</em>
  o <tt>mixin reset</tt> ::M  <em>;# same as before</em>
  o <tt>mixin add</tt> ::N   <em>;# add N to the front of the mixin list</em>
  o <tt>mixin delete</tt> ::M <em>;# delete M from the mixin list</em>
  puts [o <tt>mixin</tt>]   <em>;# query the current mixin list</em>
</pre>         

Every system slot (e.g. superclass) has the exact same interface.

<h2><a name='attribute-slots'></a>Attribute Slots</h2>

<p> Attribute slots are used to manage the setting and querying of
instance variables. We define now a person with three attributes
<tt>name</tt>, <tt>salary</tt> and <tt>projects</tt>. </p>

<pre CLASS="code">
  <tt>Class</tt> Person <tt>-slots</tt> {
    <tt>Attribute</tt> name
    <tt>Attribute</tt> salary <tt>-default</tt> 0
    <tt>Attribute</tt> projects <tt>-default</tt> {} <tt>-multivalued</tt> true
  }
</pre>

<p>These attributes might have a default value or they might be 
      multivalued. When an instance of class Person is created, the
      slot names can be used for specifying values for the slots.</p>

<pre CLASS="code">
  Person p1 -name "Joe"	
</pre>

<p>Object p1 has three instance variables, namely <tt>name</tt>,
<tt>salary</tt> and <tt>projects</tt>. Since slot <tt>projects</tt> is
multivalued, we can add a value to the list of values the <tt>add</tt>
subcommand.</p>

<pre CLASS="code">
  Project project1 -name XOTcl \
     -description "A highly flexible OO scripting language"

  p1 projects <tt>add</tt> ::project1
  p1 projects <tt>add</tt> some-other-value
</pre>

<p>The value of the instance variable <tt>project</tt> of Person
<tt>p1</tt> is now the list <tt>{some-other-value ::project1}</tt>.
</p>

<p>Attribute slots are implemented via dynamic object aggregations
(see <a href='#nesting'>below</a>), where the Class objects contain
the slot objects with the information like default etc. In order to
prevent name clashes between the slot objects and the methods of a
class (like e.g. <tt>create</tt>), an intermediary object named
<tt>slot</tt> is used as a container of the slot objects. In the example above
we create an object structure of the following form:</p>
<pre CLASS="code">
  Person
  Person <tt>slot</tt> name
  Person <tt>slot</tt> salary
  Person <tt>slot</tt> projects
</pre>

<p> This object structure can be used to query and modify the slot
properties or to add additional methods to the slot objects. One
application is for example to used slot-specific methods for checking
slot values, as shown in the next section.

<pre CLASS="code">
  Person info vars  <it>;# results in the list of variables of ::Person</it>
  Person <tt>slot</tt> name info vars <it>;# list of variables of the slot object ::Person::slot::name</it>
</pre>
Since slot objects are ordinary XOTcl objects, they can have their own slots as well (such as <tt>default</tt>, <tt>name</tt> etc.).

The following example
sets and queries the default of the slot <tt>name</tt> of <tt>Person</tt>:
</p>

<pre CLASS="code">
  Person <tt>slot</tt> name <tt>default</tt> "gustaf"
  ? {Person <tt>slot</tt> name <tt>default</tt>} gustaf
</pre>

<p>However, due to the current implementation, it is necessary to
re-init the slot object when the slot properties (such as
e.g. <tt>default</tt>) are changed. This can be achieved by calling
the method <tt>init</tt> of the slot object.
</p>

<p>Note that a statement for creating a slot object like</p>
<pre CLASS="code">
  <em>...</em> {
    <tt>Attribute</tt> name
    <em>...</em>
  }
</pre>
<p>is a short hand notation for </p>
<pre CLASS="code">
  <em>...</em> {
    <tt>Attribute</tt> <tt>create</tt> name
    <em>...</em>
  }
</pre>
<p>This is exactly the same situation like every where else in XOTcl,
when an object/class is created. One has to use <tt>create</tt> explicitly,
when a name of a slot object conflicts with a method of the class
(e.g. one has to use "<tt>Attribute create class</tt>" if a slot named
<tt>class</tt> is created).</p>

<p>One cannot define on a meta-class an attribute named <tt>slot</tt>
or <tt>slots</tt> and use then "<tt>... MetaClass Foo -slots {
::xotcl::Attribute x}...</tt> to create the slot objects. To handle
this naming conflict, one has to create the slot objects outside of the
aggregation and to provide the values for the properties of Attribute
(domain, manager, .... ) by hand. </p>


<H3><A NAME="setter"></A>Setter and Getter Methods for Slots</H3>

<p>When a slot is called via its name, the call is delegated to the
slot object. Per default, the slot value is read via the <tt>get</tt>
method of the slot and it is set the <tt>assign</tt> method. By
redefining these methods, it is possible to provide custom setter and
getter methods. The following example redefines the setter methods
<tt>assign</tt> to check, whether an attribute value is within the
range between 1 and 99.</p>

<pre CLASS="code">
  Class <tt>create</tt> A <tt>-slots</tt> {
    Attribute foo <tt>-default</tt> 1 <tt>-proc</tt> assign {domain var value} {
      <tt>if</tt> {$value < 0 || $value > 99} {
        <tt>error</tt> "$value is not in the range of 0 .. 99"
      }  
      $domain <tt>set</tt> $var $value
    }
  }

  A <tt>create</tt> a1
  ? {a1 foo 10} 10
  ? {a1 foo} 10
  ? {catch {a1 foo -1}} 1
</pre>

<p>For the most common simple cases with single valued attributes,
where neither setter or getter are redefined, XOTcl optimizes the slot
access function and replaces the delegation to the slot object by the C-level implementation of <tt>instparametercmd</tt>.

<p>Note that it is possible to subclass <tt>Attribute</tt> (e.g. in
order to store more properties for attributes, like when attributes
are stored in a relational database) or to register mixin-classes or
filters.</p>


<H3><A NAME="parameter"></A>Backward-compatible Short-Hand Notation
for Attribute Slots</H3>

<p>XOTcl provides a short-hand notation for creating attribute slots,
which is backward compatible for the most important options of XOTcl
version prior to 1.5.0. Instead of writing</p>

<pre CLASS="code">
  <tt>Class</tt> Car <tt>-slots</tt> {
    Attribute owner
    Attribute doors -default 4
  }
</pre>
<p>one can use as well </p>
<pre CLASS="code">
  <tt>Class</tt> Car <tt>-parameter</tt> {
    owner
    {doors 4}
  }
</pre>
<p>The various features of the prior implementation of <tt>parameter</tt> are
deprecated and will be removed in future versions.
<P>

<h2><a name='slot-experimental'></a>Experimental Slot Features</h2>
<h3><a name='value-checking'></a>Value Checking</h3>

<p>Attribute slots can have types assigned which are tested whenever
the instance variable is altered.  The slot <tt>salary</tt> is defined
as integer whereas <tt>projects</tt> is defined to be a list of
instances of the class <tt>::Project</tt> (a list of instances, since
<tt>projects</tt> is defined as multivalued).  </p>

<pre CLASS="code">
  <tt>Class</tt> Person <tt>-slots</tt> {
    <tt>Attribute</tt> name
    <tt>Attribute</tt> salary <tt>-default</tt> 0 <tt>-type</tt> integer
    <tt>Attribute</tt> projects <tt>-default</tt> {} <tt>-multivalued</tt> true <tt>-type</tt> ::Project
    <em>...</em>
  }

  Person p2 -name "Sue"	-salary 1000
</pre>

<p>It is as well possible to define custom value checkers and to
normalize the input values.  We extend the previous example and define
"<tt>my sex</tt>" as value for <tt>type</tt>.  If the value checker
consists of multiple words, the type check compiler assumes that the
value is a Tcl command, to which the actual value is appended as
additional argument before invocation. <tt>my</tt> refers to the 
slot object. In the example below, we define for the slot object an
object specific method that returns 1 or 0 depending on the success of the
check. This method (a) checks the values via <tt>switch</tt>
and (b) normalizes and resets the value via <tt>uplevel</tt>. 
</p>

<pre CLASS="code">
  <tt>Class</tt> Person <tt>-slots</tt> {
    <em>...</em>
    <tt>Attribute</tt> sex <tt>-type</tt> "my sex" <tt>-proc</tt> sex {value} {
      <tt>switch -glob</tt> $value {
        m* {<tt>my uplevel</tt> {$obj <tt>set</tt> $var m}; <tt>return</tt> 1}
        f* {<tt>my uplevel</tt> {$obj <tt>set</tt> $var f}; <tt>return</tt> 1}
        <tt>default</tt> {<tt>return</tt> 0}
      }
    }
  }
</pre>


<p>The slot values are actually checked via Tcl variable traces whenever the
associated variable gets a new value assigned. This means that the values are
enforced now matter how the variables are set. Therefore, the
checks are performed in the following two commands as well, although
the slot values are not accessed via the slot names. The checks will
throw an error in the second command, since <tt>1100x</tt> is not an
integer.  </p>

<pre CLASS="code">
  p2 <tt>incr</tt> salary 100
  p2 <tt>append</tt> salary x
</pre>     

<p> Similarly the second command below will throw an error, since
<tt>some-other-value</tt> is not an instance of <tt>::Project</tt>.

<pre CLASS="code">
  p2 projects <tt>add</tt> ::project1
  p2 projects <tt>add</tt> some-other-value     
</pre>

<p> When a check throws an error, the instance variables 
     are reset to the previous value. To restore the original
     value, an associative array <tt>__oldvalue()</tt> is kept as 
     instance variable in the object.
</p>

**** NOCHECK is removed ****

<p>  In general, checking of variables can be turned off globally
     by
<pre CLASS="code">
  ::xotcl::Slot <tt>instmixin add</tt> ::xotcl::Slot::Nocheck
</pre>
<p>
     This mixin replaces the methods <tt>check</tt> and
     <tt>checkall</tt> as well as <tt>mk_type_checker</tt> by
     no-ops. When the mixin is active and the Attribute definitions
     are loaded, the specified <tt>type</tt> has no effect.
</p>
<p>
     Value checking can be turned off also selectively for each slot via
     using <tt>::xotcl::Slot::Nocheck</tt> as per-object-mixin; if
     attributes are subclassed, 
     it is possible to register the <tt>Nocheck</tt> mixin on a
     subclass of <tt>Attribute</tt>. 
</p>

<h3><a name='trace-commands'></a>Init Commands and Value Commands for Slot Values</h3>

<p>An init command (<tt>initcmd</tt>) of a slot is similar to a
default and is a command to be executed when the value of the
associated variable is read the first time. That means that when an
object is created the associated variable has no value. On the
contrary, when a default is used, the variable is set to the default
value, when the object is created. </p>

The primary advantage of slot init commands is <em>Lacy
initialization:</em> When an object has many slots and the
initialization of all slots is costly (e.g. the value of each slot is
fetched from a relational database), and not all of the values are
needed for each instance, only the relevant variables of the object
are initialized on demand.

<pre CLASS="code">
  Class C -slots {
    Attribute x -initcmd {puts {init}; set _ 101}
  }

  C c1
  c1 <tt>info</tt> vars  <it>;# ==&gt; returns ""</it>
  c1 <tt>set </tt> x     <it>;# ==&gt; puts init, returns 101</it>
  c1 <tt>info</tt> vars  <it>;# ==&gt; returns "x"</it>
</p>
</pre>

<p>The initcmd is executed only once, when the variable is read the first
time. For later reads of the variable contents, the values are returned.

<p>A value command (<tt>valuecmd</tt>) of a slot is similar to a
init command, except that it is executed whenever the value of the
variable is read. A value command can be used e.g. to implement live
updates for variables or for abstracting from sql sequences or the
like.  </p>

<p>Finally the value changed command (<tt>valuechangedcmd</tt>) can be
used to specify the behavior, whenever the value of the variable is
altered. This option is used to implement the value checking described
in the last section.</p>

<p> The slot parameters <tt>default</tt>, <tt>initcmd</tt> and
<tt>valuecmd</tt> have to be used mutually exclusively.
</p>



<!-- PAGE BREAK -->

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="nesting"></A><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Nested Classes
			and Dynamic Object Aggregations </FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic9" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>
<P>Most object-oriented analysis and design methods are based on the
concepts of generalization and aggregation. Generalization is
achieved through class hierarchies and inheritance, while static
aggregation is provided through embedding. Since version 8.0 Tcl
offers a namespace concept which can be used as a mechanism to
provide dynamic aggregations. 
</P>
<P>A <EM>namespace</EM> provides an encapsulation of variable and
procedure names in order to prevent unwanted name collisions with
other system components. Each namespace has a unique identifier which
becomes part of the fully qualified variable and procedure names.
Namespaces are therefore already object-based in the terminology of
Wegner. OTcl is object-oriented since it offers classes and class
inheritance. Its objects are also namespaces, but an object is more
than only a namespace. Therefore, two incompatible namespace concepts
have existed in OTcl in parallel. 
</P>
<P>In XOTcl every object and every class is
logically implemented as a separate Tcl namespace. The biggest benefit of this
design decision aside from performance advantages is the ability to
aggregate objects and nest classes. Contrary in OTcl every object has
a global identifier. Through the introspection abilities of
namespaces nested classes are also traceable at runtime and can be
changed dynamically. In XOTcl objects are allowed to contain nested
objects, which are dynamically changeable aggregates of the
containing object. 
</P>


<H3><A NAME="nested-classes"></A>Nested Classes</H3>

<P>The notation for nested classes follows the syntax of Tcl
namespaces by using ``::'' as a delimiter. For example the
description of a oval carpet and a desk can nest inside of the
<tt>OvalOffice</tt> class: 
</P>
<pre CLASS="code">
  <tt>Class</tt> OvalOffice
  <it># general carpet</it>
  <tt>Class</tt> Carpet
  <tt>Class</tt> OvalOffice::Desk
  <it># special oval carpet - no name collision</it>
  <tt>Class</tt> OvalOffice::Carpet <tt>-superclass</tt> ::Carpet
</pre><P>
Nested classes can be used exactly like ordinary classes, a user can
subclass it, derive instances, etc. The information about the
nesting structure of classes is available through the <tt>info</tt>
instance method: 
</P>
<pre CLASS="code">
  <em>className</em> <tt>info</tt> classchildren <em>?pattern?</em>
  <em>className</em> <tt>info</tt> classparent
</pre><P>
The <tt>classchildren</tt> option returns a list of children, if one
or more exist, otherwise it returns an empty string. <tt>classparent</tt>
results in the name of the parent class, if the class is nested.
Since nested classes are realized through namespaces, all
functionality offered by Tcl's <tt>namespace</tt> command is usable
from XOTcl as well. 
</P>
<H3><A NAME="obj-agg"></A>Dynamic Object Aggregations </H3>

<P>The nested classes only provide an aggregation of the descriptive
not of the runtime properties of an object. We have pointed out the
difference of object and class in XOTcl. Because of the splitting of a
class into class and class-object it is possible to give each object
its own namespace. The internal implementation of objects enable them
to contain nested objects, which are aggregates of the containing
object. In XOTcl these can be changed dynamically and introspected
through the language support of dynamic object aggregations <a
href="#xotcl-aggregation">[Neumann and Zdun 2000b]</a>. Suppose an
object of the class <tt>Agent</tt> should aggregate some property
objects of an agent, such as head and body:
</P>
<pre CLASS="code">
  <tt>Class</tt>Agent
  Agent myAgent

  <tt>Class</tt> Agent::Head
  <tt>Class</tt> Agent::Body

  Agent::Head ::myAgent::myHead
  Agent::Body ::myAgent::myBody
</pre><P>
Now the objects <tt>myHead</tt> and <tt>myBody</tt> are part of the
<tt>myAgent</tt> object and they are accessible through a
qualification using ``::'' (or through Tcl's namespace command). But
in the common case they will be accessed, as introduced so far: the
explicit full qualification is not necessary when such variables are
being accessed from within XOTcl methods, since the object changes to
its namespace. 
</P>
<P>The information about the part-of relationship of objects can be
obtained exactly the same way as for classes through the 
<tt>info</tt> interface: 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>info</tt> children <em>?pattern?</em>
  <em>objName</em> <tt>info</tt> parent
</pre>

<H3><A NAME="nest-agg"></A>Relationship between Class Nesting and Object Aggregation</H3>

<P>The classes <tt>Head</tt> and <tt>Body</tt> are children of the
<tt>Agent</tt> class. It is likely that all agents, interactive or
not, have properties for head and body. This implies a static or
predetermined relationship between class nesting and object
aggregation. Such predetermination do not exist in XOTcl, but are
simply build, when specifying the relationship in the constructor,
e.g.: 
</P>
<pre CLASS="code">
  Agent <tt>instproc</tt> <tt>init</tt> args {
    ::Agent::Head [<tt>self</tt>]::myHead
    ::Agent::Body [<tt>self</tt>]::myBody
  }
</pre><P>
Now all agents derived from the class have the two property objects
aggregated after creation. But still they are changeable in a
dynamical manner, e.g. with: 
</P>
<pre CLASS="code">
  Agent myAgent
  myAgent::myHead <tt>destroy</tt>
</pre><P>
The agent turns into a headless agent. In companion of the
introspection mechanisms such constructions could be very useful.
Suppose, that in the virtual world the agents heads may be slashed
from their bodies. The graphical system simply needs to ask with <tt>info
children</tt> on the agent's object, whether it has a head or not and
can choose the appropriate graphical representation. 
</P>

<h3><A NAME="contains"></A>Simplified Syntax for Creating Nested Object Structures</h3>

To ease the generation of nested structures, one can use the
predefined method <tt>contains</tt>. In essence, <tt>contains</tt>
changes the namespace, where objects are created to the object,
on which it is executed. In the example below, we create three
nested rectangles, where two of these contain two more points.
The outer rectangle is <tt>r0</tt> containing rectangle <tt>r1</tt>
and <tt>r2</tt>.
 
<pre CLASS="code">
  <tt>Class</tt> Point <tt>-parameter</tt> {{x 100} {y 300}}
  <tt>Class</tt> Rectangle <tt>-parameter</tt> {color}

  Rectangle r0 -color pink <tt>-contains</tt> {
    Rectangle r1 -color red <tt>-contains</tt> {
      Point x1 -x 1 -y 2
      Point x2 -x 1 -y 2
    }
    Rectangle r2 -color green <tt>-contains</tt> {
      Point x1
      Point x2
    }
  }

  ? {r0 color} pink
  ? {r0 r1 color} red
  ? {r0 r1 x1 x} 1
  ? {r0 r1 x2 y} 2
  ? {r0 r2 color} green
</pre>

<p>Every object in XOTcl is realized as a Tcl command. If nested
objects are created, these commands are available as object specific
methods.  So, instead of calling the contained rectangle r1 via the
fully qualified name <tt>::r0::r1</tt>, one can use <tt>r0
r1</tt>. This is exactly the same situation as it arises, when e.g. a
global Tcl proc <tt>proc o1 {} {...}</tt> and an XOTcl object o1
(created via <tt>Object o1</tt>) is created. Both commands cannot
coexist in the same namespace.

</p>

<H3><A NAME="copy-move"></A>Copy/Move</H3>

Often an object has to be copied/moved. This is a very useful
functionality when XOTcl should be used as a prototyping language.
The XOTcl method <tt>move</tt> provides this functionality. Another
common behavior is implemented by the <tt>copy</tt> method which
clones the actual object to a destination object via a deep copy operation.
The two methods have the syntax:

<pre CLASS="code">
  <em>objName</em> <tt>move</tt> <em>destination</em>
  <em>objName</em> <tt>copy</tt> <em>destination</em>
</pre><P>

Copy and move operations work with all object/class information, i.e.,
information on filters, mixins, parameters, etc. are
automatically copied. Copy and move are integrated with class nesting
and object aggregations. All copy/move operations are deep copy
operations: all nested objects/classes are automatically copied/moved,
too.

E.g. if we want to reuse an imperial march object of star wars for
star wars 2, we can just copy the object:
<pre CLASS="code">
  starWars::imperialMarch <tt>copy</tt> starWars2::imperialMarch
</pre>

Note that move is implemented in current versions of xotcl
as a copy plus subsequent destroy operation.
<p>






<!-- PAGE BREAK -->

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
    <TR>
      <TD WIDTH=75%>
	<P><A NAME="forwarding"></A><FONT COLOR="#ffffff" FACE="Arial, Helvetica" SIZE=6>Method Forwarding</FONT>
	</P>
      </TD>
      <TD>
	<IMG SRC="logo-100.jpg" NAME="Graphic9" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
    </TR>
</TABLE>
<P>As you have seen from many previous examples, XOTcl's primary command for
method forwarding is the <tt>next</tt> primitive.  <tt>next</tt> calls
the same-named method of the current object, usually with the same
argument list. However, frequently method forwarding is required
between different objects as well, commonly referred to as
<em>delegation</em>.</P>

<P>In general, delegation can be achieved in XOTcl without any special construct
using simple methods containing a few lines. However, In
several situations, forwarding is as well needed to plain
Tcl commands, for example, if object oriented stubs are implemented on
base of non-oo function calls. These functions might access instance
variables of the objects. XOTcl uses this functionality in various
situations, such as for instance in the implementation of the
<tt>set</tt>, <tt>unset</tt>, <tt>append</tt>, <tt>array</tt> methods among others.</p>

<p>The forwarding functionality is supported by XOTcl be the methods
<tt>forward</tt> and <tt>instforward</tt> that address these requirements
and provide an efficient implementation for these tasks.  </p>

<p>The forwarding command specifies that whenever <em>methodName</em> is called,
this invocation is delegated to <em>callee</em>, where the actual argument list
(from the invocation) is appended to the argument list specified in the forwarding command.
Like for procs and instprocs, we can distinguish between <tt>forward</tt> and
<tt>instforward</tt>, depending on we want to the method available for a single object
of for the instances of a class.</p>

<p>The general form of the forwarding commands is:
<pre CLASS="code">
  <em>obj</em> <tt> forward</tt> <em>methodName ?options? callee ?arglist?</em> 
  <em>cls</em> <tt> instforward</tt> <em>methodName ?options? callee ?arglist?</em> 
</pre>

where valid options are <tt>-objscope</tt>, <tt>-methodprefix</tt>,
<tt>-earlybinding</tt> and <tt>-default</tt>. The option
<tt>-objscope</tt> is used to specify that the command should be
executed in the scope of the calling object (i.e. instance variables apprear
as local variables), <tt>-methodprefix</tt> means that the called
method should be prefixed with the specified string (to avoid name
clashes), <tt>-earlybinding</tt> means that the function pointer of
the specified command (callee) is take at invocation time (should only
be done for (built-in) commands implemented in C), and
<tt>-default</tt> provides a means for providing default methods when
none are specified.</p>

<p>Each of the arguments after the method name (including
<tt>callee</tt>) can be be substituted an invocation time, or they are
taken literally. The arguments to be substituted are starting always
with a percent sign.  These arguments can be <tt>%self</tt>,
<tt>%proc</tt>, <tt>%1</tt>, <tt>%argclindex</tt>, or <tt>%</tt>
followed by a Tcl command, and it can be prefixed with a positional
prefix <tt>%@</tt>.  We will introduce the usage of these options and
argument substitutions based on examples.</P>

<p>In our first example we define an object <tt>dog</tt> and an object
<tt>tail</tt>. If the <tt>dog</tt> receives the call <tt>wag</tt> it
delegates this call to the <tt>tail</tt> and returns its result.  In
this introductory example, the method <tt>tail</tt> simply returns its
arguments. </p>

<p>In this example, forwarding is achieved through the method
<tt>forward</tt> that creates a forwarder command. This method
receives as first argument the name, under which the forwarder is
registered, followed by the object that receives the delegation (the
"callee"), followed my the (optional) method name and optional
arguments. More about this later. Here we register the forwarder under
the name <tt>wag</tt>, the callee is <tt>tail</tt>, and the method is
defined to have the name of the forwarder. We could have written here
<tt>dog forward wag tail wag</tt> as well, be we use <tt>%proc</tt>
which refers to the name of the forwarder. Using <tt>%proc</tt> is
slightly more general in cases the forwarder is renamed.
</p>

<pre CLASS="code">
  <it>###########################################</it>
  <it># trivial object delegation</it>
  <it>###########################################</it>
  <tt>Object</tt> dog
  <tt>Object</tt> tail
  tail <tt>proc</tt> wag args { <tt>return</tt> $args }
  dog <tt>forward</tt> wag tail %proc
</pre>

<p> With these definitions a call to "<tt>dog wag 100</tt>" calls
actually "<tt>tail wag 100</tt>" which returns the result of
<tt>100</tt>.</p>

<p>The following command shows the delegation to a Tcl command
(instead of delegation to an object). We define a simple forwarder
that forwards a call to the Tcl command <tt>expr</tt> with some
arguments.</p>

<pre CLASS="code">
  <it>###########################################</it>
  <it># adding </it>
  <it>###########################################</it>
  <tt>Object</tt> obj
  obj <tt>forward</tt> addOne <tt>expr</tt> 1 +
</pre>
The invocation <tt>obj addOne 5</tt> returns 6 as value.<p>


<p>In our next example we want additionally that the Tcl command
should to be evaluated in the context of the current object. This
means that the method can easily access instance variables of the
delegating object. We define a forwarder for the class <tt>X</tt> with
the name <tt>Incr</tt> (to avoid confusion with the already defined
method <tt>incr</tt>), we use the <tt>-objscope</tt> option and
specify <tt>incr</tt> as the callee.  Since the forwarder is defined
via <tt>instforward</tt> the forwarder is available to all instances
of the class.
<pre CLASS="code">
  <it>###########################################</it>
  <it># evaluating in scope </it>
  <it>###########################################</it>
  <tt>Class</tt> X <tt>-parameter</tt> {{x 1}}
  X <tt>instforward</tt> Incr <tt>-objscope</tt> incr
  
  X x1 -x 100
  x1 Incr x
  x1 Incr x
  x1 Incr x
</pre>
After the three calls to <tt>Incr</tt> the call <tt>x1 x</tt>
returns the value 103.</p>

<p>In our next example, we show the usage of the
<tt>%</tt>-substitution more advanced argument handling. This example
sketches the implementation of the <tt>mixin add</tt>, <tt>mixin
set</tt> methods as shown above. In order to obtain extensible
subcommands (such as <tt>mixin add</tt>, <tt>mixin delete</tt>, etc.), we
define an object for which the subcommands are defined as methods. We
will use this object as callee for the appropriate methods. So, we
define an object named <tt>mixin</tt> and define a forwarder with the
name <tt>Mixin</tt> (again we capitalize <tt>Mixin</tt> to avoid name clashes
with the already defined method<tt>mixin</tt> ).
<pre CLASS="code">
  <it>###########################################</it>
  <it># mixin example</it>
  <it>###########################################</it>
  <tt>Object create</tt> mixin
  mixin <tt>proc</tt> unknown {m args} {<tt>return</tt> [concat [self] $m $args]}
  obj <tt>forward</tt> Mixin mixin %1 %self
</pre>
We define here the method <tt>unknown</tt> to see what arguments are
passed. The following invocation will lead to the call in noted in the comment.
<pre CLASS="code">
  obj Mixin add M1       <it>;# calls ::mixin add ::obj M1</it>
</pre>
You see that <tt>%1</tt> was substituted by the first argument of the
invocation (here <tt>add</tt>) and <tt>%self</tt> was substituted by
the name of the current object (here <tt>::obj</tt>). The second
argument of the invocation (here <tt>M1</tt>) was appended as
usual. However, in calls like
<pre CLASS="code">
  obj Mixin
</pre>
we have to deal with cases, where the used argument (<tt>%1</tt>) is 
not given at the invocation. In this case we get either an
error message, or we can specify a default argument via the 
option <tt>-default</tt>:
<pre CLASS="code">
  obj <tt>forward</tt> Mixin <tt>-default</tt> {getter setter} mixin %1 %self
</pre>
This definition means that if no argument is specified in the
invocation we call the method <tt>getter</tt>, if one argument is
given the method <tt>setter</tt>, in other cases we use the specified
arguments. Therefore the following three invocations are delegated as
indicated in the comments.
<pre CLASS="code">
  obj Mixin              <it>;# calls ::mixin getter ::obj</it>
  obj Mixin M1           <it>;# calls ::mixin setter ::obj M1</it>
  obj Mixin add M1       <it>;# calls ::mixin add ::obj M1</it>
</pre>

<p>When we implement subcommands by delegating to other commands
(as shown in the last example), there can be situations where naming
conflicts might arise. For example, if we want to implement a
subcommand method <tt>class</tt> we might not want to implement a new
method <tt>class</tt> on the callee, since this would overwrite the
standard definition of <tt>class</tt>. To overcome such difficulties,
we provide the option <tt>-methodprefix</tt>. The following example
shows how to prefix every called method with the prefix <tt>@</tt>.
</p>
<pre CLASS="code">
  <it>###########################################</it>
  <it># sketching extensible info</it>
  <it>###########################################</it>
  <tt>Object</tt> Info
  Info <tt>proc</tt> @mixin {o} {
    $o <tt>info</tt> mixin
  }
  Info <tt>proc</tt> @class {o} { <it>;# without prefix, doing here a [Info class] would be wrong</it>
    $o <tt>info</tt> class
  }
  Info <tt>proc</tt> @help {o} { <it>;# define a new subcommand for info</it>
    <tt>foreach</tt> c [my <tt>info</tt> procs] {<tt>lappend</tt> result [<tt>string range</tt> $c 1 end]}
    <tt>return</tt> $result
  }
  Object <tt>instforward</tt> Info -methodprefix @ Info %1 %self 
</pre>
With this definitions, the following call is rewritten as indicated in the comment.
<pre CLASS="code">
  x1 Info class          <it>;# ::Info @class ::x1</it>
</pre>

<p>When a forwarder is defined, the callee (the target command) can be
omitted. When the callee is not specified, the method-name is used
instead. When the method-name has a namespace prefix, the method name
is the tail and the callee is the fully qualified name.
</p>
<pre CLASS="code">
  <it>###########################################</it>
  <it># optional callee</it>
  <it>###########################################</it>
  obj <tt>set</tt> x 2
  obj <tt>forward</tt> append -objscope
  <tt>Object</tt> n; <tt>Object</tt> n::x
  obj <tt>forward</tt> ::n::x
</pre>
With this definitions of the forwarder <tt>append</tt> and <tt>x</tt>,
the following calls are rewritten as indicated in the comment.
<pre CLASS="code">
  obj append x y z        <it>;# ::append x y z ... returning  2yz</it>
  obj x self              <it>;# ::n::x self    ... returning  ::n::x</it>
</pre>
<p>The forwarder <tt>append</tt> forwards the call to the Tcl command
<tt>append</tt>, which accesses the instance variable <tt>x</tt> and
appends the specified values.</p>

<p>The list of tokens executed by the forwarder might
contain Tcl commands executed during every invocations. This makes it
for instance possible to pass instances variables to the callee. In
the next example the object has the instvar named <tt>x</tt> which is
multiplied by a factor of 10 when the method <tt>x*</tt> is invoked.
<pre CLASS="code">
  <it>###########################################</it>
  <it># command substitution</it>
  <it>###########################################</it>
  obj <tt>set</tt> x 10
  obj <tt>forward</tt> x* <tt>expr</tt> {%my <tt>set</tt> x} *
</pre>
With this definitions, the following call is rewritten as indicated in the comment.
<pre CLASS="code">
  obj x* 10               <it>;# expr 10 * 10 ... returning  100</it>
</pre>

<p>In certain situations it is necessary to insert arguments always at
the same position (e.g. at the second to last position). The
positional addressing can be achieved by prefixing the arguments of
the forward specification by <tt>%@POS </tt>, where <tt>POS</tt> is
either a positive (argument positing from the beginning) or negative
integer (argument counting from the end) or the constant <tt>end</tt>
(denoting the last position). After <em>POS</em> a single space is
used as a delimiter for the rest of the argument, which might be
some other %-substitution or a constant. The positional arguments 
are evaluated from left to right and should be used in ascending order. 
</p>

<p>The following examples show a few usages of the positional arguments
in the forwarder. The forwarders f1 to f5 are created, followed by
one or more usages. The first argument of the usage is the call to forewarder, the second argument is the result.
</p>
<pre CLASS="code">
  <it>###########################################</it>
  <it># forwarding with positional arguments</it>
  <it>###########################################</it>
  <tt>Object</tt> obj
  obj <tt>forward</tt> f1 list {%@end 13}
  ? {obj f1 1 2 3 } [<tt>list</tt> 1 2 3 13]

  obj <tt>forward</tt> f2 list {%@-1 13}
  ? {obj f2 1 2 3 } [<tt>list</tt> 1 2 13 3]

  obj <tt>forward</tt> f3 list {%@1 13}
  ? {obj f3 1 2 3 } [<tt>list</tt> 13 1 2 3]
  ? {obj f3} [list 13]

  obj <tt>forward</tt> f4 list {%@2 13}
  ? {obj f4 1 2 3 } [<tt>list</tt> 1 13 2 3]

  obj <tt>forward</tt> f5 {%@end 99} {%@0 list} 10
  ? {obj f5} [<tt>list</tt> 10 99]
  ? {obj f5 a b c} [<tt>list</tt> 10 a b c 99]
</pre>

<p> The construct <tt>%argclindex LIST</tt> can be used to substitute an argument
depending on the number of arguments when the forwarder is
invoked. For example, it is possible to call forward to a different
method depending on how many arguments are specified. The number of arguments
is used as an index in the specified list. When the number of arguments is larger
than the number of elements in the specified list, an error is generated.</p>

<pre CLASS="code">
  <it>###############################################</it>
  <it># substitution depending on number of arguments</it>
  <it>###############################################</it>
  obj <tt>forward</tt> f %self [list %argclindex [list a b c]]
  obj <tt>proc</tt> a args {<tt>return</tt> [<tt>list</tt> [<tt>self proc</tt>] $args]}
  obj <tt>proc</tt> b args {<tt>return</tt> [<tt>list</tt> [<tt>self proc</tt>] $args]}
  obj <tt>proc</tt> c args {<tt>return</tt> [<tt>list</tt> [<tt>self proc</tt>] $args]}
  ? {obj f} [list a {}]
  ? {obj f 1 } [list b 1]
  ? {obj f 1 2} [list c {1 2}]
  ? {catch {obj f 1 2 3}} 1
</pre>

<p>Finally, the concluding example defines a class <tt>chan</tt> to use the
I/O-commands in an OO-manner. The proc open is used to create a
<tt>chan</tt> instance. For the channel object we provide the method
<tt>close</tt> (to close a channel and to destroy the channel object),
<tt>puts</tt> (to write on a stream), <tt>blocked</tt> (to check
whether last command exhausted all input), and <tt>fconfigure</tt> (to
configure the stream). Note that for <tt>puts</tt> we specified that
the actual stream should be inserted as the second to last argument.
</p>
<pre CLASS="code">
  <tt>Class</tt> chan <tt>-parameter</tt> stream
  <it># create stream and object</it>
  chan <tt>proc</tt> open args { 
    <tt>set</tt> stream [<tt>eval</tt> open $args]
    <tt>my create</tt> $stream -stream $stream  ;# make an object
  }
  <it># close stream and destroy object</it>
  chan <tt>instproc</tt> close {} {
    <tt>close</tt> [<tt>my</tt> stream]
    [<tt>self</tt>] <tt>destroy</tt>
  }
  <it># handle other subcommands (methods) via unknown</it>
  chan <tt>instproc</tt> unknown {m args} {
    <tt>set</tt> valid [<tt>lsort</tt> [chan <tt>info</tt> instcommands]]
    stderr puts "unknown chan method '$m' $args called; 
      	defined methods: $valid"
  }
  chan <tt>create</tt> stdout -stream stdout   <it>;# define standard stream</it>
  chan <tt>create</tt> stderr -stream stderr   <it>;# define standard stream</it>

  chan <tt>instforward</tt> puts puts {%@-1 %my stream}
  chan <tt>instforward</tt> blocked fblocked {%my stream}
  chan <tt>instforward</tt> fconfigure fconfigure {%my stream} 

  <tt>set</tt> c [chan open /tmp/junk w]
  $c puts -nonewline "hello"
  $c puts -nonewline " world"
  $c puts ""
  $c xxx                                       <it>;# trigger unknown</it>
  <it># The stream instances denote the currently open streams</it>
  stderr puts "currently open streams: [chan info instances]" 
  $c close
  stderr puts "currently open streams: [chan info instances]"
</pre>




<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="assertions"></A><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Assertions
			</FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic10" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>
<P>In order to improve reliability and self documentation we added
assertions to XOTcl. The implemented assertions are modeled after the
``design by contract'' concept of Bertrand Meyer. In XOTcl assertions
can be specified in form of formal and informal pre- and
post-conditions for each method. The conditions are defined as a list
of and-combined constraints. The formal conditions have the form of
normal Tcl conditions, while the informal conditions are defined as
comments (specified with a starting ``<tt>#</tt>''). The lists
containing the pre- and post-conditions are appended to the method
definition (see example below). 
</P>
<P>Since XOTcl offers per-object specialization it is desirable to
specify conditions within objects as well (this is different to the
concept of Meyer). Furthermore there may be conditions which must be
valid for the whole class or object at any visible state (that means
in every pre- and post-condition). These are called invariants and
may be defined with following syntax for class invariants: 
</P>
<pre CLASS="code">
  <em>className</em> instinvar <em>invariantList</em>
</pre><P>
or for objects invariants: 
</P>
<pre CLASS="code">
  <em>objName</em> invar <em>invariantList</em>
</pre><P>
Logically all invariants are appended to the pre- and post-conditions
with a logical ``and''. All assertions can be introspected. 
</P>
<P>Since assertions are contracts they need not to be tested if one
can be sure that the contracts are fulfilled by the partners. But for
example when a component has changed or a new one is developed the
assertions could be checked on demand. For this purpose the <tt>check</tt>
method can be used either to test the pre- or the post-conditions.
The syntax is: 
</P>
<pre CLASS="code">
  <em>objName</em> check <em>?all? ?instinvar? ?invar? ?pre? ?post?</em>
</pre><P>
Per default all options are turned off. <tt>check all</tt> turns all
assertion options for an object on, an arbitrary list (maybe empty)
can be used for the selection of certain options. Assertion options
are introspected by the <tt>info check</tt> option. The following
class is equipped with assertions: 
</P>
<pre CLASS="code">
  <tt>Class</tt> Sensor <tt>-parameter</tt> {{value 1}}
  Sensor instinvar {
    {[regexp {^[0-9]$} [<tt>my</tt> value]] == 1}
  }
  Sensor <tt>instproc</tt> incrValue {} {
    <tt>my</tt> <tt>incr</tt> value
  } {
    {# pre-condition:} 
    {[<tt>my</tt> value] &gt; 0}
  } {
    {# post-condition:} 
    {[<tt>my</tt> value] &gt; 1}
  }
</pre><P>
The <tt>parameter</tt> instance method defines an instance variable
<tt>value</tt> with value <tt>1</tt>. The invariant expresses the
condition (using the Tcl command <tt>regexp</tt>), that the value
must be a single decimal digit. The method definition expresses the
formal contract between the class and its clients that the method
<tt>incrValue</tt> only gets input-states in which the value of the
variable <tt>value</tt> is positive. If this contract is fulfilled by
the client, the class commits itself to supply a post-condition where
the variable's value is larger than 1. The formal conditions are
ordinary Tcl conditions. If checking is turned on for sensor <tt>s</tt>:
</P>
<pre CLASS="code">
  s check all
</pre><P>
the pre-conditions and invariants are tested at the beginning and the
post-condition and invariants are tested at the end of the method
execution automatically. A broken assertion, like calling <tt>incrValue</tt>
9 times (would break the invariant of being a single digit) results
in an error message. 
</P>
<p>
In assertions we do not check methods that modify or introspect
assertions. These are
<tt>check</tt>,<tt>info</tt>,<tt>proc</tt>,<tt>instproc</tt>,<tt>invar</tt>,
and <tt>instinvar</tt>. The reason for this is that we want to be able
to recover a malicious action in a <tt>catch</tt> error handler, like:
</P>
<pre CLASS="code">
  ...
  <tt>if</tt> {[<tt>catch</tt> {<tt>my</tt> assertionBreakingAction} errMsg]} {
    <tt>puts</tt> "CAUGHT ERROR: $errMsg"
    <it># remember checking options, for turning them on later again</it>
    <tt>set</tt> check [<tt>my</tt> <tt>info</tt> check]
    <tt>my</tt> check {}
    <it># recover from broken assertion</it>
    ...
    <it># turning checking on again </it>
    $fb check $check
  }
</pre>

<!-- PAGE BREAK -->

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="meta-data"></A><FONT
COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Meta-Data
and Automatic Documentation
			</FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic11" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>
<P>To enhance the understandability and the consistency between
documentation and program it is useful to have a facility to make the
documentation a part of the program. There are several kinds of
meta-data which are interesting for a class, e.g. the author, a
description, the version, etc. 
</P>
<P>
Older versions of XOTcl have contained a special meta-data command
<tt>metadata</tt>. This command is now (from version 0.83) deprecated
and replaced by an integrated solution with XOTcl's API documentation
functionality. The object <tt>@</tt> is used for documentation and
metadata issues. Per default it is not evaluated at all. Everything
that is send to <tt>@</tt> is simply ignored. That way we do not waste
memory/performance at runtime, if we do not require to parse the
metadata/documentation.
</P>
<P>
If we have to know the meta-data/documentation, as for instance in the
<tt>xoDoc</tt> component and the <tt>makeDoc</tt> tool, that handle
XOTcl's internal documentation, we have to re-define the documentation
object. Alternatively, we can partially parse the source code for
<tt>@</tt> commands.
</P>
<P>
With <tt>@</tt> the meta-data/documentation is handled by first class
XOTcl objects. By defining alternate @ implementations - as in
<tt>xoDoc</tt>/<tt>makeDoc</tt> - we can evaluate the
meta-data/documentation arbitrarily. <tt>xoDoc</tt>/<tt>makeDoc</tt>
are only an HTML back-end, but the basic idea is to provide support for
several other usages as well (e.g. XML, RDF, on-line help,
documentation of dynamic structures, etc).
</P>
<P>
The object<tt>@</tt> handles comments via its <tt>unknown</tt>
method. <tt>xoDoc</tt> adds the appropriate instprocs to t<tt>@</tt> to produce HTML
output. The appropriate command is:
</P>
<pre CLASS="code">
  tclsh src/lib/makeDoc.xotcl <em>DOCDIR DOCFILES</em>
</pre><P>

The source of a documentation is structurally very similar to the
XOTcl constructs being commented. E.g. one can copy an instproc and
add comments at the right places, like:
</P>
<pre CLASS="code">
    <tt>Class</tt> C
    C <tt>instproc</tt> m {a1 a2} {
       <tt>return</tt> [<tt>expr</tt> {$a1+$a2}]
    }
</pre><P>

    can be commented as follows
 </P>
<pre CLASS="code">
    @ <tt>Class</tt> C { description { "<tt>my</tt> sample class"} }
    @ C <tt>instproc</tt> m {a1 "first number" a2 "second number"} {
       description "add two numbers"
       <tt>return</tt> "sum of a1 and a2"
    }
</pre></P>
<P>
 One can do essentially a copy+paste of the source and add the
 comments via attribute value pairs.  Every basic language construct
 can have a "description". If you want to include other properties to
 the description, you can add them like:
  </P>
<pre CLASS="code">
    @ C <tt>instproc</tt> m {a1 "first number" a2 "second number"} {
       author "GN+UZ"
       date "Feb 31"
       description "add two numbers"
       <tt>return</tt> "sum of a1 and a2"
    }
</pre><P>
 
    This way, author and date are added automatically to the generated
    HTML file.
    
    In addition, there is a <tt>@File</tt> hook for a per file
description, like:
  </P>
<pre CLASS="code">
@ @File {
  description {
    This is a file which provides a regression test
    for the features of the XOTcl - Language. 
  }
}
</pre><P>

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="additional-functionalities"></A><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Additional
			Functionalities </FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic12" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>
<H2><A NAME="abstract-classes"></A>Abstract Classes 
</H2>
<P>In XOTcl a class is defined abstract if at least one method of
this class is abstract. The instance method <tt>abstract</tt> defines
an abstract method and specifies its interface. Direct calls to
abstract methods produce an error message. E.g. a <tt>Storage</tt> class
provides an abstract interface for access to different storage forms:
</P>
<pre CLASS="code">
  <tt>Class</tt> Storage
  Storage abstract <tt>instproc</tt> open  {name}       
  Storage abstract <tt>instproc</tt> store {key value}
  Storage abstract <tt>instproc</tt> list  {}         
  Storage abstract <tt>instproc</tt> fetch key        
  Storage abstract <tt>instproc</tt> close {}         
  Storage abstract <tt>instproc</tt> delete {k} 
</pre><P>
All kinds of storage have to implement every method from the
interface. E.g. a GNU Database Access, a relational database access,
and several other storage forms may be derived by subclassing
(therefore, all conform to the same storage access interface). 
</P>

<H2><A NAME="cmdCheck"></A>Checking Commands for being Objects,
Classes, or Meta-Classes 
</H2>
<P>Since XOTcl is a hybrid language containing several Tcl commands,
sometimes its necessary for applications to distinguish between Tcl
commands and object commands for XOTcl. </tt>method of the
<tt>Object</tt> class looks up an <tt>objName</tt> and returns 1 if it
is an object and 0 if not:

<pre CLASS="code">
  <em>objName1</em> <tt>isobject</tt> <em>objName2</em>
</pre><P> 

If one can be sure that a command represents an
object, it might be unsure if the command is only an object or also
class or even meta-class. The two instance methods <tt>isclass</tt>
and <tt>ismetaclass</tt> check in the same manner, whether a class or
meta-class is given (since ever XOTcl class is an object, they also
return 0, when objName is not an XOTcl object).

<pre CLASS="code">
  <em>objName1</em> <tt>isclass</tt> <em>objName2</em>
  <em>objName1</em> <tt>ismetaclass</tt> </em>objName2</em>
</em></pre>

<H2>
<A NAME="Exit Handler"></A>Exit Handler 
</H2>
<P>A task for a programming language, sometimes of similar importance
as object creation, is the object destruction. XOTcl ensures that all
objects are destroyed and their destructors are invoked when XOTcl
applications terminate. For that reason objects and classes are
destroyed in the order objects, classes, meta-classes. Sometimes
further destruction order is of importance. For these cases, the XOTcl
language provides an exit handler, which is a user-defined proc, which
invokes user-defined exit handling just before the destruction of
objects, classes, meta-classes is invoked. For instance, the exit
handler lets the user specify objects which have to be destroyed
before all other objects.
</P>
<P> The exit handler is defined as a proc of <tt>Object</tt>, which is per default empty:
<pre CLASS="code">
  ::xotcl::Object <tt>proc</tt> __exitHandler {} {
    <it># clients should append exit handlers to this proc body</it>
    ;
  }
</pre>

<P> There are some procs of the <tt>Object</tt> class pre-defined,
which let us specify an exit handler conveniently:
</P>
<pre CLASS="code">
   <tt>Object</tt> setExitHandler body
   <tt>Object</tt> getExitHandler
   <tt>Object</tt> unsetExitHandler
</pre><P STYLE="margin-bottom: 0in">
<tt>setExitHandler</tt> lets us specify
a proc body that actually contains the user-defined exit handling:
<pre CLASS="code">
   <tt>Object</tt> setExitHandler {
     aObj <tt>destroy</tt>
     <tt>puts</tt> "exiting"
   }
</pre><P STYLE="margin-bottom: 0in">
destroys the object <tt>aObj</tt> before
all other objects and prints the message existing to the screen. With
<tt>getExitHandler</tt> the exit
handler can be introspected. E.g. if we just want to append the
destruction of object <tt>bObj</tt> to
an existing exit handler, we use <tt>getExitHandler</tt>:
</P>
<pre CLASS="code">
   <tt>Object</tt> setExitHandler &quot;[<tt>Object</tt> getExitHandler]; bObj <tt>destroy</tt>&quot;
</pre>
<P STYLE="margin-bottom: 0in">
<tt>unsetExitHandler</tt> deletes the exit handler. 
</P>
<pre STYLE="margin-top: 0.17in; margin-bottom: 0.2in; page-break-after: avoid">
</pre>
<H2><A NAME="autonames">Automatic Name Creation</A>
</H2>
The XOTcl <FONT SIZE=2>autoname</FONT>
instance method provides a simple way to take the task of
automatically creating names out of the responsibility of the
programmer. The example below shows how to create on each invocation
of method <FONT SIZE=2>new</FONT> an agent with a fresh name
(prefixed with <FONT SIZE=2>agent</FONT>): 
</P>
<pre CLASS="code">
  Agent <tt>proc</tt> new args {
    <tt>eval</tt> <tt>my</tt> [<tt>my</tt> <tt>autoname</tt> agent] $args
  }
</pre>
<p>
Autonames may have format strings as in the Tcl 'format' command.
E.g.: 
</P>
<pre CLASS="code">
  <em>objName</em> <tt>autoname</tt> a%06d
</pre>
<p>
produces 
<pre CLASS="code">
  a000000, a000001, a000002, ...
</pre>
</P>

<!-- PAGE BREAK -->

<TABLE COLS=2 WIDTH=100% BORDER=0 CELLPADDING=2 CELLSPACING=0 BGCOLOR="#000055">
	<TR>
		<TD WIDTH=75%>
			<P><A NAME="cext"></A><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>Integrating XOTcl Programs with C Extensions (such as TK)
			</FONT></FONT></FONT>
			</P>
		</TD>
		<TD>
			<IMG SRC="logo-100.jpg" NAME="Graphic2" ALIGN=RIGHT WIDTH=102 HEIGHT=42 BORDER=0></TD>
	</TR>
</TABLE>

<p>Because all XOTcl commands are in the ::xotcl namespace, it is
usually no problem to integrate XOTcl with other Tcl extensions. Most
often it works to import the XOTcl commands (like Object, Class) into
the current namespace because there are no name-clashes with the
commands defined by other extensions.</p>

<p>Consider you want to perform a deeper integration of another
extension and XOTcl because you want to benefit from XOTcl's object
system. For instance, you might want to introduce composite TK widgets
(sometimes called mega-widgets) as classes and inherit from these
classes. Here, you have two options: you can change or extend the C
code of that other extension to provide XOTcl classes or objects, or
you can write an XOTcl wrapper in Tcl.  For the first alternative,
there are some examples provided in the XOTcl distribution. XOTclGdbm
provides an OO Tcl interface to the GDBM database, for
instance. XOTclSdbm does the same for SDBM, and the TclExpat wrapper
provides a class-based interface to the TclExpat XML parser.</p>

<p>Consider you do not want to change the C code of a Tcl
extension. Then you can write an OO wrapper in XOTcl for the commands
of the other extension. For stateless commands, you can simply write
forwarder methods. If the extension maintains some state, you
typically associate the state handle with an XOTcl parameter, acquire
the state in the XOTcl constructor, and align the XOTcl destructor
with the stateful instance.</p>

<p>Consider you want to wrap the Tk button widget. You can acquire the
widget in the constructor, and maintain the widget ID in a
parameter. You now can forward invocations to this widget ID
(e.g. when using "pack"), or register command callbacks (like
buttonPressed). Note that we let the "self" command be replaced in the
scope of the current method so that TK receives the correct object ID
for the callback.  In the destructor we destroy the widget as well (we
use "catch" because sometimes widgets can destroyed by other means as
well (e.g. by their parent widget, when a widget/object hierarchy is
destroyed at once).</p>

<pre CLASS="code">
  <tt>Class</tt> MyButton <tt>-parameter</tt> {button}
  MyButton <tt>instproc</tt> buttonPressed args {
    <tt>puts</tt> "pressed [<tt>my</tt> button]"
  }
  MyButton <tt>instproc</tt> <tt>init</tt> args {
    <tt>set</tt> ID [<tt>namespace</tt> tail [<tt>self</tt>]]
    <tt>my</tt> <tt>instvar</tt> button
    <tt>set</tt> button [button .$ID \
      -text "My Button $ID" \
      -command [<tt>list</tt> [<tt>self</tt>] buttonPressed]] 
    pack $button
    <tt>next</tt>
  }
  MyButton <tt>instproc</tt> <tt>destroy</tt> args {
     <tt>catch</tt> {destroy [<tt>my</tt> button]}
     <tt>next</tt>
  }

  <it># a test -> 3 buttons, destroy one of them</it>
  <tt>foreach</tt> b {a b c} {
    MyButton $b
  }
  b <tt>destroy</tt>
</pre>

<p> The "trick" to substitute "self" within the current method scope
works for all kinds of command callbacks. Extensions such as TK,
however, often work with bindings to (global) variables as well. Using
global variables is frowned upon in the OO community. Instead you
should use instance variables of objects. As Tcl can only bind to
existing namespace variables (and XOTcl acquires the namespace of an
object on demand), you have to make sure that the namespace of an
object exists before binding a variable. That can be done with
"requireNamespace":</p>
<pre CLASS="code">
  GUIClass <tt>instproc</tt> buildEntry win {
    <tt>my</tt> <tt>requireNamespace</tt>
    <tt>entry</tt> $win -textvariable [<tt>self</tt>]::entryValue
    <tt>my</tt> <tt>set</tt> entryValue {Init Value}
  }
</pre>

<p>Note that in the above example we have used to tail of the object ID
as ID for the widget. Usually, it is a good idea to the object name,
if possible, for TK (and other extensions) IDs as well. Another option
is to use a autoname to get a unique name for the ID.</p>

<p>Sometimes you want to simply send all invocations, not implemented by
XOTcl, to the wrapped command. Here, it is tedious to write a wrapper
for each of these methods. Instead you can use "unknown" to handle
automatic forwarding. Consider you want to wrap TK commands like pack
and replace XOTcl object names with their TK widget ID, so that you can
use both IDs synonymously. You can rename the respective TK commands in
the following way:

<pre CLASS="code">
  <tt>foreach</tt> tkCommand {bell bind bindtags clipboard event 
    focus font grid image lower option pack place raise 
    selection send tk tkwait winfo wm} { 
   <tt> rename</tt> ::$tkCommand __tk_$tkCommand
    TkCommand ::$tkCommand
    ::$tkCommand <tt>set</tt> wrapped __tk_$tkCommand
  }
</pre>

<p>The XOTcl class handling the ID substitution for the TK command
might look as follows:</p>

<pre CLASS="code">
  <tt>Class</tt> TkCommand <tt>-parameter</tt> wrapped
  TkCommand <tt>instproc</tt> unknown args {
      <tt>my</tt> <tt>instvar</tt> wrapped
      <tt>set</tt> args [Widget replaceWithWidgetIDs $args]
      <it># now call the command</it>
      <tt>eval</tt> $wrapped $args
  }
</pre>

<p></p>

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			<P><A NAME="references"></A><FONT COLOR="#ffffff"><FONT FACE="Arial, Helvetica"><FONT SIZE=6>References
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<P></P> <STRONG>[Zdun, Strembeck, Neumann 2007]</STRONG> U. Zdun,
M. Strembeck, G. Neumann: Object-Based and Class-Based Composition of
Transitive Mixins, <em>Information and Software Technology</em>, 49(8) 2007 .

<P></P>
<A NAME="xotcl-filter"><STRONG>[Neumann and Zdun 1999a]</STRONG></A>
G. Neumann and U. Zdun.
Filters as a language support for design patterns in object-oriented
  scripting languages.
In <EM>Proceedings of COOTS'99, 5th Conference on Object-Oriented
  Technologies and Systems</EM>, San Diego, May 1999.

<P></P><A NAME="xotcl-objpattern"><STRONG>[Neumann and Zdun 1999b]</STRONG></A>
G. Neumann and U. Zdun.
Implementing object-specific design patterns using per-object mixins.
In <EM>Proc. of NOSA`99, Second Nordic Workshop on Software
  Architecture</EM>, Ronneby, Sweden, August 1999.

<P></P><A NAME="xotcl-mixin"><STRONG>[Neumann and Zdun 1999c]</STRONG></A>
G. Neumann and U. Zdun.
Enhancing object-based system composition through per-object mixins.
In <EM>Proceedings of Asia-Pacific Software Engineering Conference
  (APSEC)</EM>, Takamatsu, Japan, December 1999.

<P></P><A NAME="xotcl"><STRONG>[Neumann and Zdun 2000a]</STRONG></A>
G. Neumann and U. Zdun.
 XOT<SMALL>CL</SMALL>, an object-oriented scripting language.
In <EM>Proceedings of Tcl2k: The 7th USENIX Tcl/Tk Conference</EM>,
  Austin, Texas, February 2000.

<P></P><A NAME="xotcl-aggregation"><STRONG>[Neumann and Zdun 2000b]</STRONG></A>
G. Neumann and U. Zdun. Towards the Usage of Dynamic Object
                  Aggregations as a Form of Composition
In: <EM>Proceedings of Symposium of Applied Computing (SAC'00)</EM>, Como, Italy, Mar 19-21, 2000.

<P></P><A NAME="xotcl2"><strong>[Neumann and Sobernig 2009]</strong></A>
G. Neumann, S. Sobernig: XOTcl 2.0 - A Ten-Year Retrospective and Outlook, in: <em>Proceedings of the Sixteenth Annual Tcl/Tk Conference</em>, Portland, Oregon, October, 2009. 


<P></P><A NAME="tcl"><STRONG>[Ousterhout 1990]</STRONG></A>
J. K. Ousterhout.
Tcl: An embeddable command language.
In <EM>Proc. of the 1990 Winter USENIX Conference</EM>, January 1990.

<P></P><A NAME="ousterhout"><STRONG>[Ousterhout 1998]</STRONG></A>
J. K. Ousterhout.
Scripting: Higher Level Programming for the 21st Century, IEEE Computer 31(3), March 1998.

<P></P><A NAME="otcl"><STRONG>[Wetherall and Lindblad 1995]</STRONG></A>
D. Wetherall and C. J. Lindblad. Extending Tcl for Dynamic
    Object-Oriented Programming. Proc. of the Tcl/Tk Workshop '95, July 1995.
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