Avoid multiple inheritance induced ambiguity by using scope resolution

问题

Here is an example of multiple inheritance. I used the scope resolution operator to resolve the ambiguity instead of a virtual class.

struct A 
{
    int i;
};

struct B : A
{};

struct C : A
{};

struct D: B, C 
{
    void f()
    {
        B::i = 10;
    }
    void g()
    {
        std::cout << B::i <<std::endl;
    }
};

int main() 
{
    D d1;
    d1.f();
    d1.g();
    return 0;
}

Is B::i well-formed?

回答1:

Is B::i well-formed?

Yes, it is. The most pertinent reference is [class.qual]/1:

If the nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-name-specifier is looked up in the scope of the class, except for the cases listed below. The name shall represent one or more members of that class or of one of its base classes.

Which specifies you can name i on account of it being a member of B's base. The accessibility is only checked afterwards, and in your case it's public.

[class.access.base]/5

... The access to a member is affected by the class in which the member is named. This naming class is the class in which the member name was looked up and found... A member m is accessible at the point R when named in class N if

• there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B.

回答2:

Yes. These are extract of the c++ standard grammar rules:

id-expression:
  unqualified-id
  qualified-id

postfix-expression:
  [...]
  postfix-expression . template[opt] id-expression
  [...]

In [class.mcft.non-static]:

When an id-expression (8.1) that is not part of a class member access syntax (8.2.5) and not used to form a pointer to member (8.3.1) is used in a member of class X in a context where this can be used (8.1.2), if name lookup (6.4) resolves the name in the id-expression to a non-static non-type member of some class C, and if either the id-expression is potentially evaluated or C is X or a base class of X, the id-expression is transformed into a class member access expression (8.2.5) using (*this) (12.2.2.1) as the postfix-expression to the left of the . operator.

来源：https://stackoverflow.com/questions/46646773/avoid-multiple-inheritance-induced-ambiguity-by-using-scope-resolution