PHP5成员可见度 [英] PHP5 member visibility

问题描述

有人可以向我解释一下，为什么可以在PHP中执行以下操作，但是，例如在C＃或Java中却不能：

Could someone explain me, why is it possible to do the following in PHP, but, for example, not in C# or Java:

Class A {
    protected $a = 'Howdy!';
}

Class B extends A {
    public function howdy() {
        $created = new A();
        echo $created->a; <----- This is legal due to per-class visibility
    }
}

$b = new B();
echo $b->howdy();  <----- Hence, no fatal error here

此行为似乎已指定 此处 ，但我不能了解造成这种情况的根本原因 （在我看来，不能简单地实现每类可见性而不是每个实例，而没有充分的理由）。

This behavior seems to be specified here, but I can't understand the fundamental reason behind this (to my mind, one can't simply implement the per-class visibility instead of the per-instance one without having a strong reason for that).

推荐答案

正如您所指定的那样，它不起作用的原因是PHP在类级别上实现了访问控制，而其他语言则使用实例级别的方法。

The reason it doesn't work is, as you specified, PHP implements access control on a class level, where other languages use an instance level approach.

为什么有用？它允许您的类在自身的其他实例上进行操作，而不会暴露其私有数据。让我们举一个简单的值对象示例：

Why is it useful? It allows your classes to operate on other instances of itself without exposing its private data. Let's take a simple value-object example:

class Int {
    protected $value = 0;
    public function __construct($value) {
        $this->value = (int) $value;
    }
    public function add(Int $new) {
        return new Int($new->value + $this->value);
    }
}

$a = new Int(1);
$b = new Int(2);
$c = $a->add($b);

这使您可以保护受保护的信息，但仍可以在各个实例中使用它。

This lets you keep protected info encapsulated, yet still work with it across instances...

两种方法各有利弊...

There are pros and cons to both approaches...

这篇关于PHP5成员可见度的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！