Virtual Base Class in C++

In C++, a virtual base class is a class that acts as a base for other classes and contains virtual functions. It is used to avoid multiple copies of the same base class in a hierarchy of classes.

 

When a class inherits from a virtual base class, it shares the same instance of the virtual base class with other classes in the hierarchy. This means that only one copy of the virtual base class is created, which reduces memory usage and avoids ambiguity in function calls.

 

To declare a virtual base class, the keyword "virtual" is used before the base class name in the derived class declaration. For example, consider the following code:

 

class A {

  int x;

};

 

class B : virtual public A {

  int y;

};

 

class C : virtual public A {

  int z;

};

 

class D : public B, public C {

  int w;

};

 

In this code, classes B and C both inherit virtually from class A. Class D then inherits from both classes B and C. Because B and C inherit virtually from A, class D only contains one instance of class A.

 

It is important to note that virtual base classes should be used with caution, as they can increase the complexity of the class hierarchy and make code more difficult to understand and maintain. However, in some cases, they can be very useful for reducing memory usage and avoiding ambiguity in function calls.