【php类与对象】对象和引用

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对象和引用

PHP 的引用是别名，就是两个不同的变量名字指向相同的内容。
在 PHP 5，一个对象变量已经不再保存整个对象的值。只是保存一个标识符来访问真正的对象内容。 当对象作为参数传递，作为结果返回，或者赋值给另外一个变量，另外一个变量跟原来的不是引用的关系，只是他们都保存着同一个标识符的拷贝，这个标识符指向同一个对象的真正内容。

Example #1 引用和对象<?phpclass A {
    public $foo = 1;
}  

$a = new A;$b = $a;     // $a ,$b都是同一个标识符的拷贝
             // ($a) = ($b) = <id>$b->foo = 2;echo $a->foo."\n";$c = new A;$d = &$c;    // $c ,$d是引用
             // ($c,$d) = <id>$d->foo = 2;echo $c->foo."\n";$e = new A;function foo($obj) {
    // ($obj) = ($e) = <id>
    $obj->foo = 2;
}

foo($e);echo $e->foo."\n";?>

---

User Contributed Notes

/*
Notes on reference:
A reference is not a pointer. However, an object handle IS a pointer. Example:
*/<?phpclass Foo {
  private static $used;  private $id;  public function __construct() {
    $id = $used++;
  }  public function __clone() {
    $id = $used++;
  }
}$a = new Foo;
 // $a is a pointer pointing to Foo object 0$b = $a;
 // $b is a pointer pointing to Foo object 0, however, $b is a copy of $a$c = &$a; 
 // $c and $a are now references of a pointer pointing to Foo object 0$a = new Foo; 
 // $a and $c are now references of a pointer pointing to Foo object 1, $b is still a pointer pointing to Foo object 0unset($a); 
 // A reference with reference count 1 is automatically converted back to a value. Now $c is a pointer to Foo object 1$a = &$b; 
 // $a and $b are now references of a pointer pointing to Foo object 0$a = NULL; 
 // $a and $b now become a reference to NULL. Foo object 0 can be garbage collected nowunset($b); 
 // $b no longer exists and $a is now NULL$a = clone $c; 
 // $a is now a pointer to Foo object 2, $c remains a pointer to Foo object 1unset($c); 
 // Foo object 1 can be garbage collected now.$c = $a; 
 // $c and $a are pointers pointing to Foo object 2unset($a); 
 // Foo object 2 is still pointed by $c$a = &$c; 
 // Foo object 2 has 1 pointers pointing to it only, that pointer has 2 references: $a and $c;const ABC = TRUE;
 if(ABC) {  
 $a = NULL; 
 // Foo object 2 can be garbage collected now because $a and $c are now a reference to the same NULL value
 } else {  
 unset($a); 
 // Foo object 2 is still pointed to $c
 }

---

<?php
//The two are meant to be the same$a = "Clark Kent"; 
//a==Clark Kent$b = &$a; 
//The two will now share the same fate.$b="Superman"; 
// $a=="Superman" too.echo $a; 
echo $a="Clark Kent"; 
// $b=="Clark Kent" too.unset($b); 
// $b porced from $a$b="Bizarro";
echo $a; 
// $a=="Clark Kent" still, since $b is a free agent pointer now.
//The two are NOT meant to be the same.$c="King";
$d="Pretender to the Throne";echo $c."\n"; 
// $c=="King"echo $d."\n"; 
// $d=="Pretender to the Throne"swapByValue($c, $d);
echo $c."\n"; 
// $c=="King"echo $d."\n"; 
// $d=="Pretender to the Throne"swapByRef($c, $d);
echo $c."\n"; 
// $c=="Pretender to the Throne"echo $d."\n"; 
// $d=="King"function swapByValue($x, $y)
{
$temp=$x;$x=$y;$y=$temp;
//All this beautiful work will disappear
//because it was done on COPIES of pointers.
//The originals pointers still point as they did.}function swapByRef(&$x, &$y)
{
$temp=$x;$x=$y;
$y=$temp;
//Note the parameter list: now we switched &#39;em REAL good.
}
?>

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