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物件與引用
PHP 的引用是別名,就是兩個不同的變數名字指向相同的內容。
在 PHP 5,一個物件變數已經不再保存整個物件的值。只是保存一個標識符來存取真正的物件內容。當物件作為參數傳遞,作為結果返回,或者賦值給另一個變量,另外一個變量跟原來的不是引用的關係,只是他們都保存著同一個標識符的拷貝,這個標識符指向同一個對象的真正內容。
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 'em REAL good. } ?>
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