ambiguous access error in c++

Question

I got an error ambiguous access mg. but mg is only protected and inherited using public access. why is it so? throw some light please. Thanks for your time !

class A{    
    protected:    
        int mg; 
        static int ms; 
};    

class B : public A{    
    protected:    
        using A::ms;  
};    

class C : public A, public B{    
    public:    
        void fn(){  
            cout << mg; 
            cout << ms;
        }  
};  

int A::ms = 0;

int main(){    
    C c; 
    c .fn(); 
}

Why do you need to explicitly derive C from A? Perhaps this example is too trimmed-down to be of use. — Matthew Lundberg, Dec 23 '12 at 17:50

score 2 · Answer 1 · answered Dec 23 '12 at 17:53

In this example, you have two copies of the base class A in class C, as B already derives from A. This also gives a base class of A in C:

class C : public B{    
   public:    
    void fn(){  
    cout << mg; 
    cout << ms;
    }  
 };

This problem is known as the "diamond of death" and is used to explain the danger of multiple inheritance. Except here it has been reduced to a "triangle of death."

score 2 · Answer 2 · edited May 23 '17 at 11:48

2

in C, A::mg can refer to the one inherited from A, or to the one inherited from B because B inherits A. So the call is ambiguous : which one do you really refer to ?

Using virtual inheritance can solve this problem. You can see this answer which is on a problem really similar than yours : the diamond inheritance.

edited May 23 '17 at 11:48

Community

1
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answered Dec 23 '12 at 17:53

Synxis

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You've identified the problem, but virtual inheritance is most likely not an appropriate solution. – Ben Voigt Dec 23 '12 at 17:56
Agree, but I just cited virtual inheritance as a possible solution, not the only one solution. – Synxis Dec 23 '12 at 17:57

ambiguous access error in c++

2 Answers2