为什么可以通过引用从外部类访问私有变量? [英] Why is it possible to access private variables from outside class by reference?

问题描述

如果我有一个公共类方法正在返回对不可见(私有或受保护)属性的引用，则可以使用该引用来获得直接访问:

If I have a public class method that is returning a reference to a non-visible (private or protected) property, I can use that reference to gain direct access:

PHP代码

class A
{
    private $property = 'orange';

    public function &ExposeProperty()
    {
        return $this->property;
    }

    public function Output()
    {
        echo $this->property;
    }
}

$obj = new A();

# prints 'orange'
$obj->Output();

$var = &$obj->ExposeProperty();
$var = 'apple';

# prints 'apple'
$obj->Output();

PHP中此功能的背后是否有原因?还是仅仅是设计监督，无法通过引用来跟踪访问冲突?

Is there a reasoning behind this functionality in PHP? Or is it just a design oversight, failing to keep track of access violations through references?

当您想要实现以下目标时，它显然很方便:

It obviously comes in handy when you want to achieve something like:

PHP代码

$this->load->resource();

其中load是修改$this的给定属性的对象.但是除了这个捷径之外，我看不到有很多可能的用途，否则使用有效的OOP模式是不可能的.

Where load is an object that modifies given properties of $this. But apart from this shortcut, I don't see many possible uses which wouldn't be possible with valid OOP patterns otherwise.

推荐答案

好吧，您正在显式返回对值的引用.您正在锁定前门，但随后将打开侧门.您正在故意瞄准并在这里射出自己的脚.如果$property是一个对象，并且无论是否带有&引用都将返回该对象，对该对象的任何修改也将反映在$property上.这就是 reference 的工作方式，它总是修改该引用指向的唯一值.

Well, you are explicitly returning a reference to a value. You're locking the front door, but are then opening a side entrance. You are very deliberately taking aim and shooting your own foot here. If $property was an object, and you'd return this object with or without & reference, any modifications to this object would be reflected on $property as well. That's how a reference works, it always modifies the one and only existing value that the reference points to.

可见性修饰符不是魔术般的铁皮保护".有多种方法可以绕过private可见性来访问和修改属性.它们主要是作为您自己和其他开发人员的标志，该标志不应直接访问此属性，仅供内部使用，而不是公开认可的API.如果您愿意，PHP会让您耳目一新算了没什么，没什么.

Visibility modifiers aren't magic iron clad "protections". There are any number of ways how you can circumvent a private visibility to access and modify the property. They're mostly there as a flag to yourself and other developers that this property should not be accessed directly, it's for internal use and not a publicly sanctioned API. And PHP will slap you on the wrist should you forget that. Nothing more, nothing less.

此外，这里没有任何侵犯.外部代码绝对不能访问或修改$obj->property.这是private唯一应禁止的事情.本质上，您是在对象上公开一个公共API，该公共API修改了private属性.通常，这是通过getter和setter函数完成的，但是按引用的API显然也可以工作.

Also, nothing is really being violated here. Outside code is at no point accessing or modifying $obj->property. That's the only thing private is supposed to prohibit. You're essentially exposing a public API on your object which modifies a private property. Usually this is done with getter and setter functions, but a by-reference API obviously works as well.

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