朋友访问受保护的嵌套类 [英] Friend access to protected nested class

问题描述

我有以下C ++代码：

I have the following C++ code:

class A {
 protected:
  struct Nested {
    int x;
  };
};

class B: public A {
  friend class C;
};

class C {
  void m1() {
    B::Nested n; // or A::Nested
  }
};

使用g ++ 4.4编译此代码段，无论我使用B :: Nested还是A ::嵌套在m1中。 Clang接受 B :: Nested ，但是如果我 A :: Nested ，则不会编译。

Compiling this snippet with g++ 4.4, it does not make a difference whether I use B::Nested or A::Nested in m1. Clang accepts B::Nested, but does not compile if I A::Nested. Is that a bug in g++ or in clang?

推荐答案

根据标准，GCC是正确的，Clang是错误的。它表示在11.2 / 4

According to the Standard, GCC is correct and Clang is wrong. It says at 11.2/4

成员m在N类中命名时可访问，如果

A member m is accessible when named in class N if

• m被保护，并且引用发生在N类的成员或朋友中，或在从N派生的类P的成员或朋友中，其中m P的成员是私人或受保护的

这是Clang bugreport的主题，从建立Qt： http://llvm.org/bugs/show_bug.cgi?id=6840 。一个ang人说

This is subject of this Clang bugreport, which prevents Clang from building Qt: http://llvm.org/bugs/show_bug.cgi?id=6840 . One Clang guy says

其实，我有意没有实现这个规则。这是一个
起草错误或一个可怕的错误。它中断了整个protected
说明符，它使代码的良好形式依赖于
存在完全不相关的类，它对
实现了高成本，并且它是正式不可判定的在存在模板的情况下。

Actually, I intentionally haven't implemented this rule yet. It is either a drafting error or a horrible mistake. It neuters the entire 'protected' specifier, it makes the well-formedness of code dependent on the existence of completely unrelated classes, it imposes high costs on the implementation, and it's formally undecidable in the presence of templates.

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