Use protected static function of the base class from the friend of the derived class

Question

Different compiler seems to have different opinion on the subject. The following code compiles fine with gcc, but fails with clang:

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

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

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

clang's error is:

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.

possible duplicate? http://stackoverflow.com/questions/491000/friend-class-inherited-classes-are-not-friend-as-well — Blake McConnell, Jun 03 '15 at 17:04
@BlakeMcConnell I don't think so. In the question you are pointing to, "friend arrow" has a different direction: class hierarchy is a friend of a single class. — anxieux, Jun 03 '15 at 17:08
agreed, but have a look at this one: http://stackoverflow.com/questions/9878723/does-a-friend-see-base-classes — Blake McConnell, Jun 03 '15 at 17:13
@BlakeMcConnell yes, this looks similar, but we wouldn't see T.C.'s exhaustive answer if I'd find this question before posting mine :) — anxieux, Jun 03 '15 at 17:17
@BlakeMcConnell That one's for non-static members, for which there is an extra rule. — T.C., Jun 03 '15 at 17:19

score 2 · Accepted Answer · answered Jun 03 '15 at 17:08

This is CWG issue 1873, which changed the rules for this case ([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

[...]

Here, N is Base, P is Derived, m is f(), and R occurs in a member of DerivedFriend; pre-CWG1873 this would be allowed, but CWG1873 removed the "friend of a derived class" case and makes this ill-formed.

The fix is to refer to f as a member of Derived rather than Base.

@anxieux It was reported by a Clang dev, so I'm not surprised. — T.C., Jun 03 '15 at 17:16

Use protected static function of the base class from the friend of the derived class

1 Answers1