C++ OO Virtual Inheritance, Diamond Problem

Virtual inheritance is a mechanism in C++ that allows a derived class to inherit from a base class such that only one instance of the inherited class is present in the memory, even if multiple paths exist in the inheritance hierarchy.

In a regular inheritance relationship, when a derived class inherits from a base class, a new instance of the base class is created for each derived class. However, in some cases, this can lead to ambiguity, for example, when a class inherits from multiple base classes that themselves have a common base class. Virtual inheritance solves this problem by creating a single shared base class subobject. 

	A

B		C

	D

The diamond problem is a common problem that arises in C++ when multiple inheritance is used with virtual inheritance. It occurs when a class inherits from two classes that have a common base class. The result is that the derived class has two copies of the common base class, leading to ambiguities in the code.

For example, consider the following class hierarchy:

      Animal
     /      \
   Dog    Cat
     \      /
      Pet

 

Here, Dog and Cat inherit from Animal, and Pet inherits from both Dog and Cat. If Pet were to call a method inherited from Animal, there would be two copies of that method available: one from Dog and one from Cat. This ambiguity can lead to errors and is known as the diamond problem.

To solve the diamond problem, virtual inheritance is used. Virtual inheritance allows a class to inherit from a base class only once, even if it appears multiple times in the class hierarchy. In the example above, Pet would inherit virtually from Animal, resulting in only one copy of the Animal class in the Pet class hierarchy. This ensures that there are no ambiguities when calling methods inherited from the common base class. 

#include <iostream>

using namespace std;

class Prim_A{
    public:
        void printStuff() {
            cout << "I am from A Class" << endl;
        }
};

class Sec_B : virtual public Prim_A{

};

class Sec_C : virtual public Prim_A{

};

class Ter_D : public Sec_B, public Sec_C{

};

int main() {

    Ter_D Object;
    Object.printStuff();

    return 0;
}

This code demonstrates the concept of virtual inheritance in C++.

In this example, there are three classes: Prim_A, Sec_B, and Sec_C, and a derived class Ter_D. The Prim_A class has a method printStuff(), which simply outputs a message to the console.

Both Sec_B and Sec_C inherit from Prim_A using virtual inheritance. This means that when the Ter_D class inherits from both Sec_B and Sec_C, there will be only one instance of Prim_A shared by both Sec_B and Sec_C.

This avoids the diamond problem and ensures that there is only one copy of the Prim_A base class in Ter_D. The main() function creates an object of Ter_D class and calls the printStuff() method which is inherited from Prim_A through virtual inheritance.