什么决定什么类对象在PHP中销毁？ [英] What determines when a class object is destroyed in PHP?

问题描述

假设我们有 CFoo 类。在下面的示例中， CFoo :: __ destruct（）被调用？

  function MyPHPFunc（）
 {
 $ foo = new CFoo（）; 
 
。 。 。 
 
 //何时/在哪里/ $ foo如何销毁/删除？ 
} 
  

在这个例子中，当脚本退出 MyPHPFunc 因为 $ foo 将无法访问？

解决方案

在PHP中，所有的值都保存在所谓的 zval 中。那些 zval 包含实际数据，类型信息和 - 这对您的问题很重要 - 引用计数。请查看以下代码段：

  $ a = new B; // $ a指向zval（new B），refcount = 1 
 $ b = $ a; // $ a，$ b指向zval（new B），refcount = 2（+1）
 $ c = $ b; // $ a，$ b，$ c指向zval（new B），refcount = 3（+1）
 unset（$ a）; // $ b，$ c指向zval（new B），refcount = 2（-1）
  

refcount 到达 0 后， zval refcount 达到

0 ：

• unset 变量：

    $ a = new B; // refcount = 1 
   unset（$ a）; // refcount = 0 => __destruct！ 
    

  但：

   code> $ a = new B; // refcount = 1 
   $ b = $ a; // refcount = 2 
   unset（$ a）; // refcount = 1 => no destruct as refcount> 0，即使unset（）被调用！ 
    




• 离开功能（或方法）范围

    function a（）{
   $ a = new B; // refcount = 1 
  } // refcount = 0 => __destruct！ （因为$ a不再存在）
    




• 脚本执行结束

    $ a = new B; // refcount = 1 
   die（）; // refcount = 0 => __destruct！ （在脚本执行结束所有vars被释放）
   //不需要是die（），可以正常执行结束
    

这些显然不是导致减少 refcount 的所有条件，

此外，我应该提及，因为PHP 5.3循环引用也将被检测到。因此，如果对象 $ a 引用对象 $ b 和 $ b 参考 $ a ，并且没有任何进一步引用 $ a 或 $ b refcount 将会是 1 ，但它们仍然会被释放 __ destruct ed）。在这种情况下，虽然销毁的顺序是未定义的行为。

Let's say that we have class CFoo. In the following example when is CFoo::__destruct() called?

function MyPHPFunc()
{
  $foo = new CFoo();

  . . .

  // When/where/how does $foo get destroyed/deleted?
}

In this example would the destructor be called when the script exits the scope of MyPHPFunc because $foo would no longer be accessible?

解决方案

In PHP all values are saved in so called zvals. Those zvals contain the actual data, type information and - this is important for your question - a reference count. Have a look at the following snippet:

$a = new B; // $a         points to zval(new B) with refcount=1
$b = $a;    // $a, $b     point to  zval(new B) with refcount=2 (+1)
$c = $b;    // $a, $b, $c point to  zval(new B) with refcount=3 (+1)
unset($a);  //     $b, $c point to  zval(new B) with refcount=2 (-1)

As soon as the refcount reaches 0 the zval is freed and the object destructor is called.

Here are some examples of the refcount reaching 0:

• unseting a variable:

  $a = new B; // refcount=1
  unset($a);  // refcount=0 => __destruct!
  

  But:

  $a = new B; // refcount=1
  $b = $a;    // refcount=2
  unset($a);  // refcount=1 => no destruct as refcount > 0, even though unset() was called!
  

• leaving function (or method) scope

  function a() {
      $a = new B; // refcount=1
  }               // refcount=0 => __destruct! (as $a does not exist anymore)
  

• script execution end

  $a = new B; // refcount=1
  die();      // refcount=0 => __destruct! (on script execution end all vars are freed)
  // doesn't need to be die(), can be just normal execution end
  

These obviously are not all conditions leading to a reduction of refcount, but the ones you will most commonly meet.

Also I should mention that since PHP 5.3 circular references will be detected, too. So if object $a references object $b and $b references $a and there aren't any further references to $a or $b the refcounts of both will be 1, but they still will be freed (and __destructed). In this case though the order of destruction is undefined behavior.

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