Ambiguity in a fully qualified static member variable

Question

In this sample code, there is two sentences showing the same static variable. The first one gives no ambiguity, but the second one does, why?

#include <iostream>

using namespace std;

struct A { static const char a = 'a'; };
struct B : public A { };
struct C : public A { };
struct G : public B, public C { };

int main()
{
    G v;

    cout << G::B::A::a << endl;
    cout << v.B::A::a << endl;
}

GCC error (according to some comments, there's no ambiguity in clang):

main.cpp:15:18: error: 'A' is an ambiguous base of 'G'
  cout << v.B::A::a << endl;

Code on coliru

I know I saw something like this a month or so ago. I believe it does not know which parent to use in the `v` case. Trying to find it. — NathanOliver, May 25 '16 at 18:39
@barakmanos Precisely, I'm researching the case when I don't need virtual inheritance. — ABu, May 25 '16 at 18:40
@Peregring-lk, http://coliru.stacked-crooked.com/a/9d43d2604247b3c0 is about 20 times easier to read and understand. Most of your example is irrelevant noise that just obscures things. — Jonathan Wakely, May 25 '16 at 18:42
EDG accepts it too. Please report a GCC bug (if you can't find an existing one), with the minimal example (not your verbose original). Thanks. — Jonathan Wakely, May 25 '16 at 18:44
@JonathanWakely, I think, your clarified example is worthy an answer. — SergeyA, May 25 '16 at 18:50
It's not an answer, it's just the MCVE that should have been in the original question. — Jonathan Wakely, May 25 '16 at 19:00

score 5 · Accepted Answer · answered May 25 '16 at 19:08

This is clearly a bug in GCC, as a GCC maintainer recommends you report it. However, until it's fixed, you can use a nasty workaround like this:

std::cout << static_cast<B &>(v).A::a;

The advantage is this will help disambiguate if in a (complex) scenario that there are variables with the same name in one of the base classes.

Ambiguity in a fully qualified static member variable

1 Answers1