c++ - 从派生类的 friend 那里使用基类的 protected 静态函数

Question

不同的编译器似乎对这个问题有不同的看法。以下代码在 gcc 下编译良好，但在 clang 下编译失败:

class Base {
protected:
    static void f() {}
};

class Derived : public Base {
    friend class DerivedFriend;
};

class DerivedFriend {
public:
    void g() {
        Base::f();
    }
};

clang 的错误是:

main.cpp:13:15: error: 'f' is a protected member of 'Base'
        Base::f();
              ^
main.cpp:3:17: note: declared protected here
    static void f() {}
                ^
1 error generated.

Answer 1

这是 CWG issue 1873 ，这改变了这种情况的规则([class.access.base]/p5):

A member m is accessible at the point R when named in class N if

• [...]
• m as a member of N is protected, and R occurs in a member or friend of class N, or in a member or friend of a class P derived from N, where m as a member of P is public, private, or protected, or
• [...]

这里，N是Base，P是Derived，m是f()，R出现在DerivedFriend的成员中；在 CWG1873 之前，这是允许的，但 CWG1873 删除了“派生类的友元”案例，并使其格式错误。

解决方法是将 f 引用为 Derived 的成员，而不是 Base。