Methods with same names in multiple inteheritance in C++ [duplicate]

Question

I have a program

#include <iostream>

struct A {
    virtual char f() = 0;
};

struct B {
    virtual char f() = 0;
};

struct Derived : A, B {
public:
    char A::f() override { return CA; }
    char B::f() override { return CB; }

    const char CA = 'A';
    const char CB = 'B';
};

int main(int argc, char** argv)
{
    Derived d;

    A& a = d;

    printf("%c", a.f());

    return 0;
}

It works fine. But when I define A::f outside of the declaration, I have an error.

struct Derived : A, B {
public:
    char A::f() override;
    char B::f() override { return CB; }

    const char CA = 'A';
    const char CB = 'B';
};

char Derived::A::f() { return CA; } //error C2509: 'f': member function not declared in 'Derived'

My question is how to refer correctly to A::f in the definition?

@Someprogrammerdude interestingly, unlike gcc and clang, MSVC accepts it. See demo. — wohlstad
– wohlstad, Commented Apr 11 at 12:14
Why #include <iostream> and then not use anything from it but instead use stuff declared in <cstdio>? — Ted Lyngmo
– Ted Lyngmo, Commented Apr 11 at 12:21
My question is how to refer correctly to A::f in the definition? If the functions need to be independent, the most expedient solution is to either change the name of A::f, or change the name of B::f. — Eljay
– Eljay, Commented Apr 11 at 12:46
Only MSVC is happy with first version of your code: godbolt.org/z/E1exoMYY1 — Marek R
– Marek R, Commented Apr 11 at 14:18

xryl669 · Accepted Answer · 2025-04-11 16:53:26Z

2

You can't do that:

char A::f() override { return CA; }     
char B::f() override { return CB; }

Because you are defining 2 methods with the exact same signature and name. Also, you can't have qualified id(like Class::memberName) in the definition of a method in your class (except for very old version of C++)

If your intend is to call both parent method, you'll need to define a new method for this, like this:

struct C : public A, B 
{
    // This method is implementing both A and B's f method 
    char f() override { return 'C'; }
    // If neither A::f or B::f was pure, you'd write this:
    char fA() { return A::f(); }
    char fB() { return B::f(); }
};

For external definition, you'll write the usual, since there's no "base" source of the method in the definition:

char C::f() { return 'C'; }

EDIT: If you intend is to have different implementation for f method depending upon this being a A* or a B*, then you can do this:

struct CState
{
    // Some members you had in C initially
    int someState = 0;
}; 

struct ChildA : A, virtual CState 
{
    char f() override { someState = 1; return 'A'; }
};

struct ChildB : B, virtual CState
{
    char f() override { someState = 2; return 'B'; }
};

struct BothChild : public ChildA, public ChildB
{
};

struct C
{
    operator A&() { return (ChildA&)m; }
    operator B&() { return (ChildB&)m; }
    BothChild m; 
};

// Usage
int main()
{
    C c;
    A & a = c;
    B & b = c;
    printf("A: %c, B: %c\n", a.f(), b.f());
    printf("State: %d (either 1 or 2 depending on argument evaluation order)\n", c.m.someState);
}

edited Apr 11 at 16:53

answered Apr 11 at 12:23

xryl669

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8 Comments

Ted Lyngmo Apr 11 at 12:38

The "exact same name" is usually not a problem as long as they take different parameters and/or have different cv-qualifications (in this case, they don't though). The main issue is the forbidden qualified-ids (A::f and B::f) in the declaration of the member functions - so even if the base class functions had different names, fA and fB, it's illegal to declare the overridden functions char A::fA(); and char B::fB();

Dmitry Apr 11 at 13:19

Now I've understood that my question had risen from a wrong intention. I was going to use the Derived object as an A instance in one place and as an instance of B in the other place. But the C++ standard doesn't know my intentions and someone may want to use it as an object of type Derived and then the call d.f() will be ambiguous.

xryl669 Apr 11 at 13:40

If you need A interface in one place and B interface in another, you'll need to add a A & getA() { return (A*)this; } (resp B) and use than instead. They'll both call the same (and unique f method). If you need different implementation for f, you can't do that with a multiple inheritance, you'll have to implement both derived class independently and create a C class with composition instead (with a ChildA a and a ChildB b) member.

xryl669 Apr 11 at 13:42

@TedLyngmo: You're right. I meant the exact same signature as you said. You can declare char C::A::f() {} outside the class, but you are actually declaring A::f pure virtual method instead when you write that and it's confusing.

xryl669 Apr 11 at 13:52

@Dmitry: See edit for example implementation or this code

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