C++ Multipath Inheritance : Why the access using Base class scope is non-ambiguous?

Question

I am studying C++ and while studying virtual inheritance, I came across following doubt:

class A {
public:
    int x;
    A() { x = 677; }
    A(int a) { 
        cout << "A con , x= " << a << endl;
        x = a;
    }
};
class B : public A {
public:
    B(int a) : A(a) { }
};
class C :public A {
public:
    C(int a) : A(a) { }
};
class D : public B, public C {
public:
    D(int a,int b) : B(a),C(b) { }
};
int main()
{
    D d(5,6);
    cout << d.A::x;  //prints 5 
 }

In line cout << d.A::x; It prints 5. Shouldn't this call be ambiguous and if not why it prints 5?

How many different compilers did you try? Check if GCC, Clang and MSVC all agree or not. If not, you're probably flirting with undefined behaviour. — Louis Cloete
– Louis Cloete, Commented Nov 15, 2021 at 13:44
Latest GCC and Clang gives compile errors on Godbolt default settings. MSVC is happy to compile, but I don't read MASM well enough to be able to tell you what it would output and Godbolt doesn't have the option to execute code compiled with MSVC at the moment. — Louis Cloete
– Louis Cloete, Commented Nov 15, 2021 at 13:50
@LouisCloete different compilers not agreeing is a rather weak indicator for UB. Very often different compilers don't disagree even though there is UB. And sometimes they disagree without any UB in sight — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Nov 15, 2021 at 13:56

463035818_is_not_an_ai · Accepted Answer · 2021-11-15 14:08:47Z

d.A::x; is indeed ambiguous. GCC and Clang report it as error, only MSCV fails to do so: https://godbolt.org/z/1zhjdE6a8.

There is a note in [class.mi] with an example of multiple inheritance stating that:

In such lattices, explicit qualification can be used to specify which subobject is meant. The body of function C::f can refer to the member next of each L subobject:
void C::f() { A::next = B::next; }      // well-formed
Without the A:: or B:: qualifiers, the definition of C::f above would be ill-formed because of ambiguity ([class.member.lookup]). — end note]

It is just a note, because it follows from [class.member.lookup] (which is a little more contrived to read and understand) that without the qualifier the member access is ambiguous. "ill-formed" implies that a conforming compiler must issue either an error or warning.

Louis Cloete · Accepted Answer · 2021-11-15 14:31:59Z

1

This is ambiguous according to both GCC and Clang, but MSVC compiles it (Godbolt)

You can solve it by using virtual inheritance for B and C if it makes sense to only have one instance of A in D or you can specify via which parent the call must access A::x if you need two distinct instances of A in D.

std::cout << d.B::x; // go through B to its A
std::cout << d.C::x; // go through C to its A

The above compiles on all 3 compilers without error, because it is unambiguous. It specifies through which intermediate class to access A::x.

edited Nov 15, 2021 at 14:31

answered Nov 15, 2021 at 14:09

Louis Cloete

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2 Comments

Pete Becker Over a year ago

Be careful about recommending virtual inheritance. It is not a coding solution to ambiguities; it's a design decision. Yes, it could remove the ambiguity, but if there are supposed to be two x members, changing to virtual inheritance breaks the design.

Louis Cloete Over a year ago

@PeteBecker I added a part about the tradeoffs with virtual inheritance and gave an alternative if you need two unique instances. Do you agree with it?

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