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>章 19. 类与对象（PHP 5）</TD
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> 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.
</P
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> 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)
<A
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>visibillity</A
>. For example,
if the abstract method is defined as protected, the function implementation
must be defined as either protected or public.
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>例 19-18. Abstract class example</B
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<font color="#0000BB"><?php<br />abstract </font><font color="#007700">class </font><font color="#0000BB">AbstractClass<br /></font><font color="#007700">{<br /> </font><font color="#FF8000">// Force Extending class to define this method<br /> </font><font color="#0000BB">abstract protected </font><font color="#007700">function </font><font color="#0000BB">getValue</font><font color="#007700">();<br /> </font><font color="#0000BB">abstract protected </font><font color="#007700">function </font><font color="#0000BB">prefixValue</font><font color="#007700">(</font><font color="#0000BB">$prefix</font><font color="#007700">);<br /><br /> </font><font color="#FF8000">// Common method<br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">printOut</font><font color="#007700">() {<br /> print </font><font color="#0000BB">$this</font><font color="#007700">-></font><font color="#0000BB">getValue</font><font color="#007700">() . </font><font color="#DD0000">"\n"</font><font color="#007700">;<br /> }<br />}<br /><br />class </font><font color="#0000BB">ConcreteClass1 </font><font color="#007700">extends </font><font color="#0000BB">AbstractClass<br /></font><font color="#007700">{<br /> </font><font color="#0000BB">protected </font><font color="#007700">function </font><font color="#0000BB">getValue</font><font color="#007700">() {<br /> return </font><font color="#DD0000">"ConcreteClass1"</font><font color="#007700">;<br /> }<br /><br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">prefixValue</font><font color="#007700">(</font><font color="#0000BB">$prefix</font><font color="#007700">) {<br /> return </font><font color="#DD0000">"</font><font color="#007700">{</font><font color="#DD0000">$prefix</font><font color="#007700">}</font><font color="#DD0000">ConcreteClass1"</font><font color="#007700">;<br /> }<br />}<br /><br />class </font><font color="#0000BB">ConcreteClass2 </font><font color="#007700">extends </font><font color="#0000BB">AbstractClass<br /></font><font color="#007700">{<br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">getValue</font><font color="#007700">() {<br /> return </font><font color="#DD0000">"ConcreteClass2"</font><font color="#007700">;<br /> }<br /><br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">prefixValue</font><font color="#007700">(</font><font color="#0000BB">$prefix</font><font color="#007700">) {<br /> return </font><font color="#DD0000">"</font><font color="#007700">{</font><font color="#DD0000">$prefix</font><font color="#007700">}</font><font color="#DD0000">ConcreteClass2"</font><font color="#007700">;<br /> }<br />}<br /><br /></font><font color="#0000BB">$class1 </font><font color="#007700">= new </font><font color="#0000BB">ConcreteClass1</font><font color="#007700">;<br /></font><font color="#0000BB">$class1</font><font color="#007700">-></font><font color="#0000BB">printOut</font><font color="#007700">();<br />echo </font><font color="#0000BB">$class1</font><font color="#007700">-></font><font color="#0000BB">prefixValue</font><font color="#007700">(</font><font color="#DD0000">'FOO_'</font><font color="#007700">) .</font><font color="#DD0000">"\n"</font><font color="#007700">;<br /><br /></font><font color="#0000BB">$class2 </font><font color="#007700">= new </font><font color="#0000BB">ConcreteClass2</font><font color="#007700">;<br /></font><font color="#0000BB">$class2</font><font color="#007700">-></font><font color="#0000BB">printOut</font><font color="#007700">();<br />echo </font><font color="#0000BB">$class2</font><font color="#007700">-></font><font color="#0000BB">prefixValue</font><font color="#007700">(</font><font color="#DD0000">'FOO_'</font><font color="#007700">) .</font><font color="#DD0000">"\n"</font><font color="#007700">;<br /></font><font color="#0000BB">?></font>
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>上例将输出：</P
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>ConcreteClass1
FOO_ConcreteClass1
ConcreteClass2
FOO_ConcreteClass2</PRE
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> Old code that has no user-defined classes or functions named
'abstract' should run without modifications.
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