c++ - 当你是 friend 时，为什么 GCC 不允许从私有(private)嵌套类继承？

Question

问同样的问题:为什么 GCC 允许从私有(private)嵌套类继承？对于非模板类，它允许从私有(private)嵌套类继承，如果它是一个 friend ，但不是模板类。是错误吗？

template<class Base>
class InheritFromBaseMember : public Base::MemberPrivate // error
{
    using PrivateMember = typename Base::MemberPrivate; // works fine
};

class MyBase{
    friend class InheritFromBaseMember<MyBase>;

    // another try to declare it friend
    template<class T>
    friend class InheritFromBaseMember;

    friend class AnotherClass;

    class MemberPrivate{};
};

class AnotherClass : public MyBase::MemberPrivate{}; // works fine

int main() {
    InheritFromBaseMember<MyBase>{};
}

来自 g++ 5.3.0 的错误消息:

error: 'class MyBase::MemberPrivate' is private
     class MemberPrivate{};
           ^
error: within this context
 class InheritFromBaseMember : public Base::MemberPrivate // error
       ^

Answer 1

这绝对是一个 gcc 错误。 gcc 有 lots of issues与友元和模板。这个例子几乎完全出现在标准中，在 [class.friend] 下，强调我的:

Declaring a class to be a friend implies that the names of private and protected members from the class granting friendship can be accessed in the base-specifiers and member declarations of the befriended class.
[ Example:

class A {
class B { };
    friend class X;
};

struct X : A::B { // OK: A::B accessible to friend
    A::B mx;      // OK: A::B accessible to member of friend
    class Y {
        A::B my;  // OK: A::B accessible to nested member of friend
    };
};

—end example ]