循环中的对象属性重新分配

Question

为什么：

$test_obj = new stdClass(); 
$array = [];
for ($i=0; $i < 5; $i++) { 
    $test_obj->num = $i;
    array_push($array, $test_obj);
}
var_dump($array);

生成：

array(5) {
  [0] =>
  class stdClass#1 (1) {
    public $num =>
    int(4)
  }
  [1] =>
  class stdClass#1 (1) {
    public $num =>
    int(4)
  }
  [2] =>
  class stdClass#1 (1) {
    public $num =>
    int(4)
  }
  [3] =>
  class stdClass#1 (1) {
    public $num =>
    int(4)
  }
  [4] =>
  class stdClass#1 (1) {
    public $num =>
    int(4)
  }
}

和：

$array = [];
for ($i=0; $i < 5; $i++) { 
    $test_obj = new stdClass(); 
    $test_obj->num = $i;
    array_push($array, $test_obj);
}
var_dump($array);

生成：

array(5) {
  [0] =>
  class stdClass#1 (1) {
    public $num =>
    int(0)
  }
  [1] =>
  class stdClass#2 (1) {
    public $num =>
    int(1)
  }
  [2] =>
  class stdClass#3 (1) {
    public $num =>
    int(2)
  }
  [3] =>
  class stdClass#4 (1) {
    public $num =>
    int(3)
  }
  [4] =>
  class stdClass#5 (1) {
    public $num =>
    int(4)
  }
}

但是：

$test_obj = new stdClass(); 
$array = [];
for ($i=0; $i < 5; $i++) { 

    $test_obj->num = $i;
    array_push($array, $test_obj);
    var_dump($test_obj);
}

生成：

class stdClass#1 (1) {
  public $num =>
  int(0)
}
class stdClass#1 (1) {
  public $num =>
  int(1)
}
class stdClass#1 (1) {
  public $num =>
  int(2)
}
class stdClass#1 (1) {
  public $num =>
  int(3)
}
class stdClass#1 (1) {
  public $num =>
  int(4)
}

有人可以向我解释为什么 var_dump 在循环中能够打印出不同的对象属性但是当它被推入数组时，对象属性变成了最后一个值？

是因为我推的是同一个物体吗？在处理变量而不是对象时，重新分配期间如何工作？

Answer 1

您每次都将相同的对象放入数组中：

// instance of the object
$test_obj = new stdClass(); 
$array = [];
for ($i=0; $i < 5; $i++) { 
    $test_obj->num = $i;
    array_push($array, $test_obj);
}

等同于：

$test_obj = new stdClass(); 
$array = [$test_obj, $test_obj, $test_obj, $test_obj, $test_obj];

因此，如果您更改对象的某些属性，它会为您的所有数组项执行此操作，因为它们引用相同的对象：

$test_obj = new stdClass(); 
$test_obj->num = 0;
// all items in the array now are `0`
$array = [$test_obj, $test_obj, $test_obj, $test_obj, $test_obj];
$test_obj->num = 1;
// all items in the array now are `1`

它适用于您的第二个示例的原因是您正在创建一个新对象并将其附加到数组。

你的第三个例子之所以这样做，是因为此时对象的值是：

$test_obj = new stdClass(); 
$test_obj->num = 0;
var_dump($test_obj); // ->num == 0
$test_obj->num = 1;
var_dump($test_obj); // ->num == 1
$test_obj->num = 2;
var_dump($test_obj); // ->num == 2

$test_obj = new stdClass(); 
$array = [];
for ($i=0; $i < 5; $i++) { 
    $test_obj->num = $i;
    array_push($array, $test_obj);
    // $test_obj->num == 0 on first iteration
    // $test_obj->num == 1 on first iteration
    // $test_obj->num == 3 on first iteration
    // $test_obj->num == 4 on first iteration
}

// $test_obj->num == 4 after the loop finished which is the same as your first example

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