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<html><head>
<title>Smalltalk Object Model</title>
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<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
<hr>
<h1>The Smalltalk Object Model</h1>
<hr>
<p>This page is based on a document written by Glenn Hollowell:</p>
<ul>
<li><nobr><a href="http://www.objs.com/x3h7/smalltalk.htm"
>http://www.objs.com/x3h7/smalltalk.htm</a></nobr></li>
</ul>
<p>The original document has been adapted for use with the <nobr>XLISP
2.0</nobr> object system.</p>
<hr>
<h2>1 Basic Concepts</h2>
<hr>
<p>Within the context of Smalltalk, objects are implemented in code through
encapsulation and polymorphism.</p>
<p>Encapsulation is the approach used in Smalltalk to bundle everything
needed into an object to preserve the integrity of the enclosed data.
<nobr>The code</nobr> within an object performs operations on the objects
internal data. Operations in Smalltalk are performed as the result of a
messages being sent to an object to execute a specific portion of its code,
usually called <nobr>a 'method'</nobr>.</p>
<p>Polymorphism is the way that the Smalltalk language allows methods of the
same name to have predictable and meaningful results in related instances,
yet perform the operations differently to achieve the results.</p>
<p>Polymorphism is typically implemented in Smalltalk through the
abstraction of the common properties of a group of objects into classes and
hierarchically subclassing shared properties using inheritance, along with
specialization of the subclass to define the differences.</p>
<p>Classes serve as templates because they define the instance variables for
all the class instance variables and methods. <nobr>The instance</nobr> of a
class is created by sending a
<a href="../reference/keyword-new.htm">:new</a> message to the class which
uniquely identifies the object instance and allocates space for its
variables.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2 Objects</h2>
<hr>
<p>An object is an encapsulated software component made up of memory and
operations that creates a coherent programming entity. All objects are an
instance of a class. Objects have public and private properties. An object's
implementation details are private and are hidden from other objects.
<ul>
<li><p>An object's public properties are its messages that make up its
interface.</p></li>
<li><p>The object's private properties are its variables.</p></li>
</ul>
<p>Interaction with an object only occurs via these messages to its
interface. <nobr>All object</nobr> instances of a given class have a common
message interface.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.1 Operations</h2>
<hr>
<p>An operation is the action taken by an object instance as result of a
message being received through the object's interface. Messages requesting
operations are addressed to specific recipient object instances, thus
Smalltalk uses the 'classical' object model and the method to execute in
response to a message is determined by searching the object's class
hierarchy.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.2 Requests</h2>
<hr>
<p>A request is act of sending a message to an object's interface with the
expectation that the object will perform a known operation that is
associated with the message.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.3 Messages</h2>
<hr>
<p>A message is a request to an object instance to perform an operation. A
message expression consists of a receiver, selector [message name], and
potentially some arguments. <nobr>The selector</nobr> must be mapped to a
method of the same name, encapsulated in the receiver object's class
hierarchy. <nobr>The arguments</nobr>, if any, are used by the method to
perform its operation.</p>
<pre class="example">
(send <font color="#0000CC">receiver :selector</font> [<font color="#0000CC">arguments</font>])
</pre>
<p>In Smalltalk there exist several message types, like unary, binary, and
keyword messages. <nobr>In XLISP</nobr> there is only one message type, so
the description of the Smalltalk message types has been omitted from this
document.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.4 Specification of Behavioral Semantics</h2>
<hr>
<p>Behavior is defined by methods specified in classes which are implemented
as class instances and execution is triggered in those instances by message
requests.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.5 Methods</h2>
<hr>
<p>A method is the executable code rendered in the class and executed in the
context of an object instance. It defines how to perform an operation in the
object instance. It is made up of a message pattern that is used to match
against incoming messages, temporary variables, and a sequence of
instructions. A method execution is triggered when message is received that
matches the methods' message pattern.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.6 State</h2>
<hr>
<p>The state of an instance is represented by the values of its instance
variables.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.7 Object Lifetime</h2>
<hr>
<p>Object instances are created with the
<a href="../reference/keyword-new.htm">:new</a> method. Each object
instance is given a unique identifier called an object pointer or object
reference.</p>
<p><div class="box">
<p>New classes are created by using the "subclass" method. The "new" and
"subclass" methods are inheritedby almost all classes. Every instance object
pointer is also associated with an object pointer of a class. Unlike the
"new" and "subclass" methods, there are no specific methods that remove an
object that is no longer useful. Smalltalk objects are deleted when they are
no longer reachable (garbage collected).</p>
</div></p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.8 Behavior/State Grouping</h2>
<hr>
<p>Smalltalk uses the 'classical' object model.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.9 Communication Model</h2>
<hr>
<p>All communications within Smalltalk occur through messages between
objects and most Smalltalk implementations support a form of concurrency.
For example, Smalltalk-80 provides for multiple independent processes, and
semaphores provide the common mechanism for synchronizing the independent
processes.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>2.10 Events</h2>
<hr>
<p>No specific mechanisms for handling events are built into the
Smalltalk language, but "event handling" logic is commonly implemented in
Smalltalk applications through its normal messaging techniques.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>4 Polymorphism</h2>
<hr>
<p>Polymorphism is the way that the Smalltalk language allows methods of
same name to have predictable and meaningful results in related instances,
yet perform the operations differently to achieve similar results.</p>
<p>Smalltalk supports polymorphism by allowing methods defined in classes to
be overridden with methods of the same name, but different logic, in a
subsequent subclass. In addition, methods of the same name can be defined in
a total different subclass hierarchy.</p>
<p>In the class hierarchy below, all of the lowest level classes are defined
to accept the message <nobr>'moveForward' (mFabstract)</nobr>, but several
different versions are implemented as indicated by (MFn) in the diagram.
<nobr>The message</nobr> sent to the instances of 'Bus' or 'Auto' uses the
method defined in the 'Motorized' class. <nobr>The same</nobr> method is
inherited by 'Train' and 'Motorcycle', but both have overridden the
inherited method by defining a method with different logic using the same
message pattern. <nobr>All the</nobr> others instances in the diagram have
the 'moveForward' message pattern in their interface, but are not in the
'Motorized' inheritance chain. <nobr>All of</nobr> the <nobr>'moveForward'
(mFabstract)</nobr> methods function as you intuitively expect.</p>
<pre class="example">
Transport Mechanism (mFabstract)
/--------------- | -------------------\
Animal Powered Human Powered Motorized
/ (mF1) \ / \ / (mF5) \
Buggy Wagon Land-based Water-based Public Private
/ \ / (mF4) \ / \ / \
Bike Skates Row Boat Canoe Bus Train Auto Motorcycle
(mF2) (mF3) (mF6) (mF7)
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>5 Encapsulation</h2>
<hr>
<p>The approach used in Smalltalk is to encapsulate all objects, whether a
complex as a sorted list or as simple as a string, into a single programming
entity that includes data and logic. Further, other objects can not invoke
the encapsulated data or logic. In order to interact, other objects must
send messages to an object's interface that will cause the object to perform
a function that will have a known effect. See also entry under
<nobr><a href="#2-3">2.3 Messages</a></nobr>.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>6 Identity, Equality, Copy</h2>
<hr>
<p><div class="box">
<p>Smalltalk provides unique identity for all objects. Objects can be tested
for equivalence (Are the objects being compared the same object?) and
equality (every object may implement its own definition of equality).</p>
<p>Copying of objects can be performed as a deep copy (object's structure
and the objects pointed to by its variables are copied) or shallow copy
(objects pointed to by variables, their variables are not copied, but are
shared with the original object).</p>
</div></p>
<p>XLISP does not provide <nobr>build-in</nobr> functions or methods to
compare or to copy objects.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>7 Types and Classes</h2>
<hr>
<p>Smalltalk does not have a separate notion of 'type' [message protocol
shared by a group of objects] apart from 'class'.</p>
<p>A class is a group of objects that represent the same type of entity and
share the same methods. A class describes the implementation of a set of
similar objects. Classes are themselves objects. All objects belong to some
class and an object is an instance of the class of which it belongs.</p>
<p><b>XLISP:</b> If an object is a member of a class can be tested by
sending an <a href="../reference/keyword-isa.htm">:isa</a> message.</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name=""></a>
<hr>
<h2>8 Inheritance and Delegation</h2>
<hr>
<p>Smalltalk class relationships are defined in terms of inheritance. The
properties of one class are be inherited from another class in a
hierarchical structure beginning with the <nobr>upper-most</nobr>
<nobr>class <a href="../reference/object.htm">object</a></nobr>. <nobr>In
inheritance</nobr>, the inheriting class is called the 'subclass' and the
class being inherited from is call the 'superclass'. <nobr>A subclass</nobr>
inherits all of its superclass' variables and methods.</p>
<p>Abstract classes are classes from which other classes are inherited
(subclassed) only. Instances can be implemented any non-abstract class.</p>
<p>Subclasses are specialization of their superclasses. A subclass may add
variables, but it cannot delete those inherited from the superclass. A
subclass may accept an inherited method or it may override the method by
providing an new method with the same name (or message selector).</p>
<p>Object instances are instances of a class. Smalltalk does not provide for
delegation (defined as object instances being created from other object
instances and not a class).</p>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<hr>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
</body></html>
|
