Virtual Functions in C++

Virtual Function is a function in base class, which is overrided in the derived class, and which tells the compiler to perform Late Binding on this function.

Virtual Keyword is used to make a member function of the base class Virtual.

Late Binding in C++

In Late Binding function call is resolved at runtime. Hence, now compiler determines the type of object at runtime, and then binds the function call. Late Binding is also called Dynamic Binding or Runtime Binding.

Problem without Virtual Keyword

Let's try to understand what is the issue that virtual keyword fixes,

class Base
{
    public:
    void show()
    {
        cout << "Base class";
    }
};

class Derived:public Base
{
    public:
    void show()
    {
        cout << "Derived Class";
    }
}

int main()
{
    Base* b;       //Base class pointer
    Derived d;     //Derived class object
    b = &d;
    b->show();     //Early Binding Ocuurs
}

Base class

When we use Base class's pointer to hold Derived class's object, base class pointer or reference will always call the base version of the function

Using Virtual Keyword in C++

We can make base class's methods virtual by using virtual keyword while declaring them. Virtual keyword will lead to Late Binding of that method.

class Base
{
    public:
    virtual void show()
    {
        cout << "Base class\n";
    }
};

class Derived:public Base
{
    public:
    void show()
    {
        cout << "Derived Class";
    }
}

int main()
{
    Base* b;       //Base class pointer
    Derived d;     //Derived class object
    b = &d;
    b->show();     //Late Binding Ocuurs
}

Derived class

On using Virtual keyword with Base class's function, Late Binding takes place and the derived version of function will be called, because base class pointer pointes to Derived class object.

Using Virtual Keyword and Accessing Private Method of Derived class

We can call private function of derived class from the base class pointer with the help of virtual keyword. Compiler checks for access specifier only at compile time. So at run time when late binding occurs it does not check whether we are calling the private function or public function.

#include <iostream>
using namespace std;

class A
{
    public:
    virtual void show()
    {
        cout << "Base class\n";
    }
};

class B: public A
{
    private:
    virtual void show()
    {
        cout << "Derived class\n";
    }
};

int main()
{
    A *a;
    B b;
    a = &b;
    a->show(); 
}

Derived class

Mechanism of Late Binding in C++

To accomplich late binding, Compiler creates VTABLEs, for each class with virtual function. The address of virtual functions is inserted into these tables. Whenever an object of such class is created the compiler secretly inserts a pointer called vpointer, pointing to VTABLE for that object. Hence when function is called, compiler is able to resovle the call by binding the correct function using the vpointer.

Important Points to Remember

1. Only the Base class Method's declaration needs the Virtual Keyword, not the definition.
2. If a function is declared as virtual in the base class, it will be virtual in all its derived classes.
3. The address of the virtual Function is placed in the VTABLE and the copiler uses VPTR(vpointer) to point to the Virtual Function.