C++ multiple inheritance ambiguous

Question

I have a problem in C++ multiple inheritance. here is my code and when I call display() function it is giving me request for member `display' is ambiguous. But Class M display() function is private.

#include<iostream>
#include<conio.h>
#include<stdio.h>

using namespace std;
class M
{

    void display()
    {
        cout<<"Class M"<<endl;
    }
};

class N
{
    public:
        void display()
        {
            cout<<"Class N"<<endl;
        }
};

class P:public M,public N
{

};

int main()
{

    P *ob = new P();
    ob->display();    // Why its giving me ambiguity error? Since only one copy is there right!!
    getch();
    return 0;
}

Can any one tell why exactly this problem?

The ambiguity error is because despite the scope of the function in M, the call is ambiguous. Try using the scope resolution operator :: to help resolve where the call should be made: msdn.microsoft.com/en-us/library/b451xz31.aspx — Martin
– Martin, Commented Jul 29, 2015 at 9:16
You can resolve it by putting using N::display in the public interface of P. — JorenHeit
– JorenHeit, Commented Jul 29, 2015 at 9:25

JorenHeit · Accepted Answer · 2015-07-29 09:30:41Z

4

As was mentioned by many already, overload resolution does not include visibility (public, private and protected). Assuming you want only the public version visible in P, you should use a using declaration in the public interface of P:

class P: public M, public N
{
public:
    using N::display;
};

IMHO, this is far more elegant than having to provide the scope in every call (obj->N::display()).

answered Jul 29, 2015 at 9:30

JorenHeit

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1 Comment

Raj Shekar Over a year ago

Thanks Joren. This helps

DhruvJoshi · Accepted Answer · 2015-07-29 09:24:38Z

1

Basically in C++, in case of multiple inheritance the derived class gets a copy of all methods of all parents. So when you do new P(), the new objects gets two different methods, with same name display(). And therefore there is an ambiguity. Scope resolution will help you here.

//define function pointer
void (N::*pfn)() = &N::display;
//call function using pointer
(ob->*pfn)();

edited Jul 29, 2015 at 9:24

answered Jul 29, 2015 at 9:19

DhruvJoshi

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Arun Chandrasekhara Pillai · Accepted Answer · 2015-07-29 09:17:27Z

-1

Solution of ambiguity in multiple inheritance: The ambiguity can be resolved by using the scope resolution operator to specify the class in which the member function lies as given below:

obj.a :: abc();

This statement invoke the function name abc() which are lies in base class a.

answered Jul 29, 2015 at 9:17

Arun Chandrasekhara Pillai

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