嵌套类：从嵌套的受保护类访问封闭类的受保护成员 [英] Nested classes: Access to protected member of enclosing class from a nested protected class

问题描述

此代码在msvc / g ++上编译：

This code compiles on msvc/g++:

class A{
protected:
    int i;
    class B{
    public:
        A* a;
        B(A* a_)
        :a(a_){
        }
        void doSomething(){
            if (a)
                a->i = 0;//<---- this part
        }       
    };
public:
    A()
    :i(0){
    }
};

正如你所看到的，B可以访问封闭类的protected部分， t声明为朋友。

As you can see, B gets access to "protected" section of enclosing class, although it isn't declared as friend.

这是一个标准（符合标准）行为吗？

Is this a standard(standard-conforming) behavior?

我有时使用这个功能，但我不记得一个规则，说嵌套的受保护类应该自动获得对封闭类的所有受保护数据的访问。

I use this feature sometimes, but I don't remember a rule saying that nested protected class should automatically get access to all protected data of enclosing class.

推荐答案

在C ++ 03标准中，11.8p1说：

In the C++03 standard, 11.8p1 says:

但是，缺陷报告45 的分辨率（标准）说明了相反的情况，因此定义了你看到的行为：

However, the resolution for defect report 45 (to the standard) states the opposite, and hence defines the behavior you see:

嵌套类是一个成员，

在C ++ 0x中，11.8的文本已更改以反映此事实，因此这对于C ++ 03和C ++ 0x符合的编译器都是有效的行为。

In C++0x the text of 11.8 has been changed to reflect this fact, so this is valid behavior for both C++03 and C++0x conforming compilers.

另请参见 cprogramming的> />这个线程中的com / cplusplus-programming / 110207-can-nested-class-access-parent-classs-private- .com 论坛。

See also this thread from the cprogramming.com forums.

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