using-declaration in derived class does not hide same function derived from base class

Question

Have a look at the following code:

struct A { public:     virtual void f(){std::cout << "in A";}; };  struct B : A{ public:    virtual void f(){std::cout << "in B";};    int a; };  struct C : B{     using A::f;     void test(){f();} };   int main()  {     C c;     c.f(); // calls B::f, the final overrider     c.C::f(); // calls A::f because of the using-declaration     c.test(); //calls B::f     return 0; } 

Per my understanding, the B::f() in C should hide the A::f() which is brought to C by using-declaration; if so, then why does c.C::f() still call A::f()?

If c.C::f() calls A::f(), that should mean that in the scope of C, f() should be always refer to A::f(), this is the function of the using-declaration. Then why in the C::test(), call to f() is still evaluated to B::f()?

Answer 1

Very nice question, a complicated case of name lookup.

Basically, when the name f is looked up in the scope of C, it always finds A::f due to the using-declaration. So all the calls c.f(), c.C::f(), and f() in C::test(), resolve the name f to A::f.

Next comes virtual dispatch. If a virtual function is called by an unqualified name, dynamic dispatch happens and the final overrider is called. This covers c.f() and the f() call in C::test(), since these are unqualified.

The call c.C::f() uses a qualified name for f, which suppresses dynamic dispatch and the function to which the name resolved is called directly. Since that function is A::f (thanks to the using-declaration), A::f is called non-virtually. The relevant rules follow (quoting C++14 final draft N4140, emphasis mine):

§10.3/15

Explicit qualification with the scope operator (5.1) suppresses the virtual call mechanism.

§5.2.2/1

... If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called. Otherwise, its final overrider (10.3) in the dynamic type of the object expression is called; such a call is referred to as a virtual function call.