What is virtual class in oops?

In object-oriented programming, a virtual class is a nested inner class whose functions and member variables can be overridden and redefined by subclasses of the outer class. Virtual classes are analogous to virtual functions. Virtual classes solve the extensibility problem of extending data abstraction with new functions and representations. Like virtual functions, virtual classes follow the same rules of definition, overriding, and reference. When a derived class inherits from a base class, it must define or override the virtual inner classes it inherited from the base class. An object of the child class may be referred to by a reference or pointer of the parent class type or the child class type. When the reference or pointer invoke the virtual inner classes, the derived class's implementation will be called if the object is of the derived class type. The type of the outer class determines the run time of the inner virtual class. A method with an object argument has access to the object's virtual classes. The method can use the virtual classes of its arguments to create instances and declare variables. Virtual classes of different instances are not compatible. For example, a base class Machine could have a virtual class Parts. Subclass Car would implement Parts differently than the subclass Bicycle, but the programmer can call any methods in the virtual inner class Parts on any class Machine object, and get the Parts implementation of that specific derived class. class Machine { void run () {} virtual class Parts { }; }; // The inner class "Parts" of the class "Machine" may return the number of wheels the machine has. class Car: public Machine { void run() { cout << "The car is running." << endl; } class Parts { int get_Wheels () { cout << "A car has 4 wheels." << endl; return 4; } string get_Fuel_Type () { cout << "A car uses gasoline for fuel." << endl; return "gasoline"; } }; }; read less

C++ supports multiple inheritance. Suppose a class is inherited by two classes which in turn are inherited by a single class. If this last child wants to access a data member of its grandparent there would be ambiguity as there would be two copies of that grandparent in the memory. To solve this problem we use virtual base class concept. A virtual keyword instructs compiler to have only one instance of the class. So now there would be no ambiguity. read less