Download - Multiple Inheritance Diamond Problem in C++

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C++ Article

Diamond Problem

10/12/2011

Saket Kr. Pathak

Saket kr. Pathak

C++ Article

Saket Kr. Pathak

What is Diamond problem ... ?

Two friends taking burger to the common.

Hello all my mate... few days back, one of my friend close to my heart because of this Qs.

:) asked me ... in a tea stall (the gathering place near my home, at evening after

returning back from Office) ...

Yup, unfortunately after days ... today I got time to read a bit more and trying to

represent my view ...

Let's see a code-snippet ...

//Base class

class Burger

{

public:

Burger(int); //Simple parameterized constructor

virtual ~Burger(); //Normal Override Destruct-or to provide memory leak

virtual void McVeggie(); //General Virtual Function to override in Derived Classes

virtual void cafeMocha(); //General Virtual Function to override in Derived Classes

};

//Derived class - 1st

class Guy1 : public Burger

{

public:

Guy1(int); //Simple parameterized constructor

Saket kr. Pathak

C++ Article

Saket Kr. Pathak

virtual ~Guy1(); //Normal Override Destruct-or to provide memory leak



};

//Derived class - 2nd

class Guy2 : public Burger

{

public:

Guy2(int); //Simple parameterized constructor

virtual ~Guy2(); //Normal Override Destruct-or to provide memory leak



};

//Derived class – 3rd

class CommonFriend : public Guy1, public Guy2

{

public:

CommonFriend(); //Simple parameterized constructor

virtual ~CommonFriend(); //Normal Override Destruct-or to provide memory leak

};

Now when we call a burger of either type for the common friend like; int main()

{

CommonFriend needBurger;

needBurger.McVeggie(); //Error type Ambiguous

needBurger.cafeMocha(); //Error type Ambiguous

return 0;

}

This will create Compiler Error for "Ambiguous Type". Now why ... ???

As we all know through the concept of overriding in inheritance we simple put a new

definition for the function having same signature from base class, these are maintained

by compiler by providing virtual function table (vtable) for classes Guy1 and Guy2.

Saket kr. Pathak

C++ Article

Saket Kr. Pathak

But here in 3rd level of inheritance that is indeed multiple inheritances, the provided

virtual function table has the pointer for both Guy1 and Guy2 classes which contains

their respective pointer of Burger.

As the result when we are calling for Mc.Veggie and cafeMocha, the compiler gets

ambiguous to make it happen. To avoid this we can do it ... we need to do ...

//Derived class - 1st

class Guy1 : public virtual Burger

{

public:

//Same as above

};

//Derived class - 2nd

class Guy2 : public virtual Burger

{

public:

//Same as above

}; Now when the virtual function table for CommonFriend will be created then Compiler

will mind the above structure and keep single instance of all the above base classes

i.e Bueger, Guy1, Guy2.

That's all I think we need to do with multiple inheritance and Diamond problem.

Yes people may surprise with the naming conventions of class, functions and instances.

It's my choice I had tried to represent the problem in layman terms that will be easily

imaginable as well as understandable instead of tricky words or random Letters ... :)

that's my way ... yup I will expect comments from you C++ buddies ... so welcome ... :)

References:

http://en.wikipedia.org/wiki/Multiple_inheritance

http://www.cprogramming.com/tutorial/virtual_inheritance.html

http://www.cs.cmu.edu/~donna/public/malayeri.TR08-169.pdf

Saket kr. Pathak

http://en.wikipedia.org/wiki/Multiple_inheritance

http://www.cprogramming.com/tutorial/virtual_inheritance.html

http://www.cs.cmu.edu/~donna/public/malayeri.TR08-169.pdf