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<HTML
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>Classes and Objects</TITLE
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><DIV
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><H1
><A
NAME="oop"
>Chapter 13. Classes and Objects</A
></H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
><A
HREF="oop.html#keyword.class"
><TT
CLASS="literal"
>class</TT
></A
></DT
></DL
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><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="keyword.class"
><TT
CLASS="literal"
>class</TT
></A
></H1
><P
>&#13;    A class is a collection of variables and functions working with
    these variables.  A class is defined using the following syntax:
 
    <DIV
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><PRE
CLASS="programlisting"
>  1&nbsp;
  2&nbsp;&#60;?php
  3&nbsp;class Cart {
  4&nbsp;    var $items;  // Items in our shopping cart
  5&nbsp;   
  6&nbsp;    // Add $num articles of $artnr to the cart
  7&nbsp; 
  8&nbsp;    function add_item ($artnr, $num) {
  9&nbsp;        $this-&#62;items[$artnr] += $num;
 10&nbsp;    }
 11&nbsp;   
 12&nbsp;    // Take $num articles of $artnr out of the cart
 13&nbsp; 
 14&nbsp;    function remove_item ($artnr, $num) {
 15&nbsp;        if ($this-&#62;items[$artnr] &#62; $num) {
 16&nbsp;            $this-&#62;items[$artnr] -= $num;
 17&nbsp;            return true;
 18&nbsp;        } else {
 19&nbsp;            return false;
 20&nbsp;        }   
 21&nbsp;    }
 22&nbsp;}
 23&nbsp;?&#62;
 24&nbsp;     </PRE
></TD
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><P
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>
   </P
><P
>&#13;    This defines a class named Cart that consists of an associative
    array of articles in the cart and two functions to add and remove
    items from this cart.
   </P
><P
>&#13;    Classes are types, that is, they are blueprints for actual
    variables. You have to create a variable of the desired type with
    the new operator.
   </P
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>  1&nbsp;
  2&nbsp; $cart = new Cart;
  3&nbsp; $cart-&#62;add_item("10", 1);
  4&nbsp;    </PRE
></TD
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><P
></P
></DIV
><P
>&#13;    This creates an object $cart of the class Cart. The function
    add_item() of that object is being called to add 1 item of article
    number 10 to the cart.  </P
><P
> Classes can be extensions of
    other classes. The extended or derived class has all variables and
    functions of the base class and what you add in the extended
    definition.  This is done using the extends keyword. Multiple
    inheritance is not supported.
   </P
><DIV
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><P
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><PRE
CLASS="programlisting"
>  1&nbsp;
  2&nbsp;class Named_Cart extends Cart {
  3&nbsp;    var $owner;
  4&nbsp;  
  5&nbsp;    function set_owner ($name) {
  6&nbsp;        $this-&#62;owner = $name;
  7&nbsp;    }
  8&nbsp;}
  9&nbsp;    </PRE
></TD
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></TABLE
><P
></P
></DIV
><P
>&#13;    This defines a class Named_Cart that has all variables and
    functions of Cart plus an additional variable $owner and an
    additional function set_owner(). You create a named cart the usual
    way and can now set and get the carts owner. You can still use
    normal cart functions on named carts:
   </P
><DIV
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><P
></P
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><PRE
CLASS="programlisting"
>  1&nbsp;
  2&nbsp;$ncart = new Named_Cart;    // Create a named cart
  3&nbsp;$ncart-&#62;set_owner ("kris"); // Name that cart
  4&nbsp;print $ncart-&#62;owner;        // print the cart owners name
  5&nbsp;$ncart-&#62;add_item ("10", 1); // (inherited functionality from cart)
  6&nbsp;    </PRE
></TD
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><P
></P
></DIV
><P
>&#13;    Within functions of a class the variable $this means this
    object. You have to use $this-&#62;something to access any variable or
    function named something within your current object.
   </P
><P
>&#13;    Constructors are functions in a class that are automatically
    called when you create a new instance of a class. A function
    becomes a constructor when it has the same name as the class.
   </P
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><P
></P
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><PRE
CLASS="programlisting"
>  1&nbsp;
  2&nbsp;class Auto_Cart extends Cart {
  3&nbsp;    function Auto_Cart () {
  4&nbsp;        $this-&#62;add_item ("10", 1);
  5&nbsp;    }
  6&nbsp;}
  7&nbsp;    </PRE
></TD
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><P
></P
></DIV
><P
>&#13;    This defines a class Auto_Cart that is a Cart plus a constructor
    which initializes the cart with one item of article number "10"
    each time a new Auto_Cart is being made with "new". Constructors
    can also take arguments and these arguments can be optional, which
    makes them much more useful.
   </P
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><P
></P
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><TD
><PRE
CLASS="programlisting"
>  1&nbsp;
  2&nbsp;class Constructor_Cart extends Cart {
  3&nbsp;    function Constructor_Cart ($item = "10", $num = 1) {
  4&nbsp;        $this-&#62;add_item ($item, $num);
  5&nbsp;    }
  6&nbsp;}
  7&nbsp; 
  8&nbsp;// Shop the same old boring stuff.
  9&nbsp; 
 10&nbsp;$default_cart   = new Constructor_Cart;
 11&nbsp; 
 12&nbsp;// Shop for real...
 13&nbsp; 
 14&nbsp;$different_cart = new Constructor_Cart ("20", 17);
 15&nbsp;    </PRE
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>Caution</B
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>&#13;     For derived classes, the constructor of the parent class is not
     automatically called when the derived class's constructor is
     called.
    </P
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