在类定义上的构造函数 VS 上设置变量 [英] Setting variables on Constructor VS on the class definition

问题描述

最近我一直在想，在类定义上的构造函数 VS 上初始化具有默认值的变量是否有区别.

Lately I've been wondering if there's a difference between initializing the variables that have a default value on the Constructor VS on the class definition.

考虑到优化，哪个更好:

Which one is better, taking into consideration the optimization:

class TestClass
{
 private $test_var = 'Default Value';
 function __construct()
 {
 }
}

class TestClass2
{
 private $test_var;
 function __construct()
 {
  $this->test_var = 'Default Value';
 }
}

推荐答案

在构造函数之外初始化属性的好处是，有人阅读你的代码会立即知道它的默认值.

The advantage of initializing properties outside of the constructor is that someone reading your code will immediatly know its a default value.

不便之处在于您不能以这种方式使用所有类型的数据——例如，不适用于对象实例化，或者我记得的 heredoc 语法.

Inconvenient is you cannot use every kind of data this way -- will not work with object instanciations, for instance, or with heredoc syntax, from what I recall.

我认为在性能方面没有太大区别 - 无论如何，在您的应用程序中可能有很多更重要的事情;-)

I don't think there is much of a difference when it comes to performance -- anyway, there are probably lots of things that matter a lot more, in your application ;-)

不过，纯粹是为了好玩，使用 Vulcan Logic Disassembler :

Still, purely for fun, using the Vulcan Logic Disassembler :

以第一个代码示例(temp-2.php):

With the first example of code (temp-2.php) :

<?php
class TestClass {
    private $test_var = 'Default Value';
    function __construct() {
    }
}
$a = new TestClass();

你得到这些操作码:

$ php -d extension=vld.so -d vld.active=1 temp-2.php
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-2.php
function name:  (null)
number of ops:  11
compiled vars:  !0 = $a
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   2     0  EXT_STMT
         1  NOP
   7     2  EXT_STMT
         3  ZEND_FETCH_CLASS                                 :1      'TestClass'
         4  EXT_FCALL_BEGIN
         5  NEW                                              $2      :1
         6  DO_FCALL_BY_NAME                              0
         7  EXT_FCALL_END
         8  ASSIGN                                                   !0, $2
         9  RETURN                                                   1
        10* ZEND_HANDLE_EXCEPTION

Class TestClass:
Function __construct:
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-2.php
function name:  __construct
number of ops:  4
compiled vars:  none
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   4     0  EXT_NOP
   5     1  EXT_STMT
         2  RETURN                                                   null
         3* ZEND_HANDLE_EXCEPTION

End of function __construct.

End of class TestClass.

虽然，使用第二个代码示例(temp-3.php):

While, with the second example of code (temp-3.php) :

<?php
class TestClass2 {
    private $test_var;
    function __construct() {
        $this->test_var = 'Default Value';
    }
}
$a = new TestClass2();

你得到这些操作码:

$ php -d extension=vld.so -d vld.active=1 temp-3.php
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-3.php
function name:  (null)
number of ops:  11
compiled vars:  !0 = $a
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   2     0  EXT_STMT
         1  NOP
   8     2  EXT_STMT
         3  ZEND_FETCH_CLASS                                 :1      'TestClass2'
         4  EXT_FCALL_BEGIN
         5  NEW                                              $2      :1
         6  DO_FCALL_BY_NAME                              0
         7  EXT_FCALL_END
         8  ASSIGN                                                   !0, $2
   9     9  RETURN                                                   1
        10* ZEND_HANDLE_EXCEPTION

Class TestClass2:
Function __construct:
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-3.php
function name:  __construct
number of ops:  7
compiled vars:  none
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   4     0  EXT_NOP
   5     1  EXT_STMT
         2  ZEND_ASSIGN_OBJ                                          'test_var'
         3  ZEND_OP_DATA                                             'Default+Value'
   6     4  EXT_STMT
         5  RETURN                                                   null
         6* ZEND_HANDLE_EXCEPTION

End of function __construct.

End of class TestClass2.

所以，我猜有一点不同......但不是那么重要^^

So, I'm guessing there is a bit of a difference... But not that important ^^

由你来解释操作码——但有趣的是在第一次转储中没有默认值"的痕迹......有趣，实际上^^
似乎 VLD 不能(或只是不能)转储所有内容:-(

Up to you to interpret the opcodes -- but the funny thing is there is no trace of 'Default Value' in the first dump... interesting, actually ^^
Seems VLD cannot (or just doesn't) dump everything :-(

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