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<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.13 $ -->
<sect1 id="language.oop5.abstract">
<title>Class Abstraction</title>
<para>
PHP 5 introduces abstract classes and methods. It is not allowed to create
an instance of a class that has been defined as abstract. Any class that
contains at least one abstract method must also be abstract. Methods
defined as abstract simply declare the method's signature they cannot
define the implementation.
</para>
<para>
When inheriting from an abstract class, all methods marked abstract in
the parent's class declaration must be defined by the child; additionally,
these methods must be defined with the same (or weaker)
<link linkend="language.oop5.visibility">visibillity</link>. For example,
if the abstract method is defined as protected, the function implementation
must be defined as either protected or public.
</para>
<example>
<title>Abstract class example</title>
<programlisting role="php">
<![CDATA[
<?php
abstract class AbstractClass
{
// Force Extending class to define this method
abstract protected function getValue();
abstract protected function prefixValue($prefix);
// Common method
public function printOut() {
print $this->getValue() . "\n";
}
}
class ConcreteClass1 extends AbstractClass
{
protected function getValue() {
return "ConcreteClass1";
}
public function prefixValue($prefix) {
return "{$prefix}ConcreteClass1";
}
}
class ConcreteClass2 extends AbstractClass
{
public function getValue() {
return "ConcreteClass2";
}
public function prefixValue($prefix) {
return "{$prefix}ConcreteClass2";
}
}
$class1 = new ConcreteClass1;
$class1->printOut();
echo $class1->prefixValue('FOO_') ."\n";
$class2 = new ConcreteClass2;
$class2->printOut();
echo $class2->prefixValue('FOO_') ."\n";
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
ConcreteClass1
FOO_ConcreteClass1
ConcreteClass2
FOO_ConcreteClass2
]]>
</screen>
</example>
<para>
Old code that has no user-defined classes or functions named
'abstract' should run without modifications.
</para>
</sect1>
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