C++ Virtual Functions

What is a virtual function?

• Virtual function is a polymorphism technique.
• Refers to performing the same operation in a hierarchy of classes. Typically used in scenarios where the base class pointer is used to hold derived class objects and perform the same operation. Refer example below.
• When a virtual functions is called on a base class pointer the compiler decides to defer the decision on which function to call until the program is running thereby doing late binding.
• The actual function called at run-time depends on the contents of the pointer and not the type.
• Internally the compiler creates a VTABLE for each which has virtual functions.
• Addresses of virtual functions are placed in the VTABLE. If a virtual function is not redefined in the derived class, the base class function address is used in VTABLE
• When objects are created compiler also places a VPTR pointing to starting address of VTABLE using which the correct function is invoked.

EXAMPLE: Demonstrate the late binding using virtual functions

#include <iostream>

using namespace std;

// Base class with virtual function
class Base {
    int data1;

    public:
        Base ( int d1) { data1 = d1; }
        virtual void print() { cout << "Base" << endl; }

};

// Derived class 1
class Derived1 : public Base {
    int data2;

    public:
        Derived1(int d1, int d2) : Base (d1) { data2 = d2; }
        void print() { cout << "Derived1" << endl; }

};

// Derived class 2
class Derived2 : public Base {
    public:
        Derived2(int d1) : Base (d1) { }
        void print() { cout << "Derived2" << endl; }

};

void main()
{
    Base b(40);
    Derived1 d1(10, 20);
    Derived2 d2(30);

    Base* ptr;
    ptr=&d1;
    ptr->print();

    ptr=&d2;
    ptr->print();
}

OUTPUT:

Derived1
Derived2