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>章 19. 类与对象（PHP 5）</TD
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> Creating a copy of an object with fully replicated properties is not
always the wanted behavior. A good example of the need for copy
constructors, is if you have an object which represents a GTK window and the
object holds the resource of this GTK window, when you create a duplicate
you might want to create a new window with the same properties and have the
new object hold the resource of the new window. Another example is if your
object holds a reference to another object which it uses and when you
replicate the parent object you want to create a new instance of this other
object so that the replica has its own separate copy.
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> An object copy is created by using the clone keyword (which calls the
object's __clone() method if possible). An object's __clone() method
cannot be called directly.
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>$copy_of_object = clone $object;</PRE
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> When an object is cloned, PHP 5 will perform a shallow copy of all of the
object's properties. Any properties that are references to other variables,
will remain references. If a __clone() method is defined, then the newly
created object's __clone() method will be called, to allow any necessary
properties that need to be changed.
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>例 19-31. Cloning an object</B
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<font color="#0000BB"><?php<br /></font><font color="#007700">class </font><font color="#0000BB">SubObject<br /></font><font color="#007700">{<br /> static </font><font color="#0000BB">$instances </font><font color="#007700">= </font><font color="#0000BB">0</font><font color="#007700">;<br /> </font><font color="#0000BB">public $instance</font><font color="#007700">;<br /><br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">__construct</font><font color="#007700">() {<br /> </font><font color="#0000BB">$this</font><font color="#007700">-></font><font color="#0000BB">instance </font><font color="#007700">= ++</font><font color="#0000BB">self</font><font color="#007700">::</font><font color="#0000BB">$instances</font><font color="#007700">;<br /> }<br /><br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">__clone</font><font color="#007700">() {<br /> </font><font color="#0000BB">$this</font><font color="#007700">-></font><font color="#0000BB">instance </font><font color="#007700">= ++</font><font color="#0000BB">self</font><font color="#007700">::</font><font color="#0000BB">$instances</font><font color="#007700">;<br /> }<br />}<br /><br />class </font><font color="#0000BB">MyCloneable<br /></font><font color="#007700">{<br /> </font><font color="#0000BB">public $object1</font><font color="#007700">;<br /> </font><font color="#0000BB">public $object2</font><font color="#007700">;<br /><br /> function </font><font color="#0000BB">__clone</font><font color="#007700">()<br /> {<br /> </font><font color="#FF8000">// Force a copy of this->object, otherwise<br /> // it will point to same object.<br /> </font><font color="#0000BB">$this</font><font color="#007700">-></font><font color="#0000BB">object1 </font><font color="#007700">= </font><font color="#0000BB">clone $this</font><font color="#007700">-></font><font color="#0000BB">object1</font><font color="#007700">;<br /> }<br />}<br /><br /></font><font color="#0000BB">$obj </font><font color="#007700">= new </font><font color="#0000BB">MyCloneable</font><font color="#007700">();<br /><br /></font><font color="#0000BB">$obj</font><font color="#007700">-></font><font color="#0000BB">object1 </font><font color="#007700">= new </font><font color="#0000BB">SubObject</font><font color="#007700">();<br /></font><font color="#0000BB">$obj</font><font color="#007700">-></font><font color="#0000BB">object2 </font><font color="#007700">= new </font><font color="#0000BB">SubObject</font><font color="#007700">();<br /><br /></font><font color="#0000BB">$obj2 </font><font color="#007700">= </font><font color="#0000BB">clone $obj</font><font color="#007700">;<br /><br /><br />print(</font><font color="#DD0000">"Original Object:\n"</font><font color="#007700">);<br /></font><font color="#0000BB">print_r</font><font color="#007700">(</font><font color="#0000BB">$obj</font><font color="#007700">);<br /><br />print(</font><font color="#DD0000">"Cloned Object:\n"</font><font color="#007700">);<br /></font><font color="#0000BB">print_r</font><font color="#007700">(</font><font color="#0000BB">$obj2</font><font color="#007700">);<br /><br /></font><font color="#0000BB">?></font>
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>上例将输出：</P
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Original Object:<br />MyCloneable Object<br />(<br /> [object1] => SubObject Object<br /> (<br /> [instance] => 1<br /> )<br /><br /> [object2] => SubObject Object<br /> (<br /> [instance] => 2<br /> )<br /><br />)<br />Cloned Object:<br />MyCloneable Object<br />(<br /> [object1] => SubObject Object<br /> (<br /> [instance] => 3<br /> )<br /><br /> [object2] => SubObject Object<br /> (<br /> [instance] => 2<br /> )<br /><br />)</font>
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