[php classes and objects] objects and references

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Objects and References

PHP references are aliases, that is, two different variable names point to the same content.
In PHP 5, an object variable no longer holds the value of the entire object. Just save an identifier to access the real object content. When an object is passed as a parameter, returned as a result, or assigned to another variable, the other variable has no reference relationship with the original one, but they both store a copy of the same identifier, which points to the real content of the same object. .

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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