C++ Virtual Functions

When two or more objects are derived from a common base class, we can prevent multiple copies of the base class being present in an object derived from those objects by declaring the base class as virtual when it is being inherited. Such a base class is known as virtual base class. This can be achieved by preceding the base class’ name with the word virtual.

Consider following example:

class A

{

public:

int i;

};

class B : virtual public A

{

public:

int j;

};

class C: virtual public A

{

public:

int k;

};

class D: public B, public C

{

public:

int sum;

};

int main()

{

D ob;

ob.i = 10; //unambiguous since only one copy of i is inherited.

ob.j = 20;

ob.k = 30;

ob.sum = ob.i + ob.j + ob.k;

cout << “Value of i is : ”<< ob.i<<”n”;

cout << “Value of j is : ”<< ob.j<<”n”; cout << “Value of k is :”<< ob.k<<”n”;

cout << “Sum is : ”<< ob.sum <<”n”;

return 0;

}.

Overriding of functions occurs in Inheritance. A derived class may override a base class member function. In overriding, the function names and parameter list are same in both the functions. Depending upon the caller object, proper function is invoked.

Consider following sample code:

class A

{

int a;

public:

A()

{

a = 10;

}

void show()

{

cout << a;

}

};

class B: public A

{

int b;

public:

B()

{

b = 20;

}

void show()

{

cout << b;

}

};

int main()

{

A ob1;

B ob2;

ob2.show(); // calls derived class show() function. o/p is 20

return 0;

}

As seen above, the derived class functions override base class functions. The problem with this is, the derived class objects can not access base class member functions which are overridden in derived class.

Base class pointer can point to derived class objects; but it has access only to base members of that derived class.

Therefore, pA = &ob2; is allowed. But pa->show() will call Base class show() function and o/p would be 10.

A virtual function is a member function that is declared within a base class and redefined by a derived class. To create virtual function, precede the function’s declaration in the base class with the keyword virtual. When a class containing virtual function is inherited, the derived class redefines the virtual function to suit its own needs.

Base class pointer can point to derived class object. In this case, using base class pointer if we call some function which is in both classes, then base class function is invoked. But if we want to invoke derived class function using base class pointer, it can be achieved by defining the function as virtual in base class, this is how virtual functions support runtime polymorphism.

Consider following program code:

Class A

{

int a;

public:

A()

{

a = 1;

}

virtual void show()

{

cout <<a;

}

};

Class B: public A

{

int b;

public:

B()

{

b = 2;

}

virtual void show()

{

cout <<b;

}

};

int main()

{

A *pA;

B oB;

pA = &oB;

pA->show();

return 0;

}

Output is 2 since pA points to object of B and show() is virtual in base class A.

When a Derived Class object is assigned to Base class, the base class' contents in the derived object are copied to the base class leaving behind the derived class specific contents. This is referred as Object Slicing. That is, the base class object can access only the base class members. This also implies the separation of base class members from derived class members has happened.

class base

{

public:

int i, j;

};

class derived : public base

{

public:

int k;

};

int main()

{

base b;

derived d;

b=d;

return 0;

}

here b contains i and j where as d contains i, j& k. On assignment only i and j of the d get copied into i and j of b. k does not get copied. on the effect object d got sliced.

If the destructor in the base class is not made virtual, then an object that might have been declared of type base class and instance of child class would simply call the base class destructor without calling the derived class destructor.

Hence, by making the destructor in the base class virtual, we ensure that the derived class destructor gets called before the base class destructor.

class a

{

public:

a(){printf("nBase Constructorn");}

~a(){printf("nBase Destructorn");}

};

class b : public a

{

public:

b(){printf("nDerived Constructorn");}

~b(){printf("nDerived Destructorn");}

};

int main()

{

a* obj=new b;

delete obj;

return 0;

}

Output:

Base Constructor

Derived Constructor

Base Destructor

By Changing

~a(){printf("nBase Destructorn");}

to

virtual ~a(){printf("nBase Destructorn");}

Output:

Base Constructor

Derived Constructor

Derived Destructor

Base Destructor.

A pure virtual function is a function which has no definition in the base class. Its definition lies only in the derived class ie it is compulsory for the derived class to provide definition of a pure virtual function. Since there is no definition in the base class, these functions can be equated to zero.

The general form of pure virtual function is:

virtual type func-name(parameter-list) = 0;

Consider following example of base class Shape and classes derived from it viz Circle, Rectangle, Triangle etc.

class Shape

{

int x, y;

public:

virtual void draw() = 0;

};

class Circle: public Shape

{

public:

draw()

{

//Code for drawing a circle

}

};

class Rectangle: public Shape

{

Public:

void draw()

{

//Code for drawing a rectangle

}

};

class Triangle: public Shape

{

Public:

void draw()

{

//Code for drawing a triangle

}

};

Thus, base class Shape has pure virtual function draw(); which is overridden by all the derived classes.

When two or more objects are derived from a common base class, we can prevent multiple copies of the base class being present in an object derived from those objects by declaring the base class as virtual when it is being inherited. Such a base class is known as virtual base class. This can be achieved by preceding the base class' name with the word virtual.

Consider following example:

class A

{

public:

int i;

};

class B : virtual public A

{

public:

int j;

};

class C: virtual public A

{

public:

int k;

};

class D: public B, public C

{

public:

int sum;

};

int main()

{

D ob;

ob.i = 10; //unambiguous since only one copy of i is inherited.

ob.j = 20;

ob.k = 30;

ob.sum = ob.i + ob.j + ob.k;

cout << "Value of i is : "<< ob.i<<"n";

cout << "Value of j is : "<< ob.j<<"n"; cout << "Value of k is :"<< ob.k<<"n";

cout << "Sum is : "<< ob.sum <<"n";

return 0;

}.

A pure virtual function is a function which has no definition in the base class. Its definition lies only in the derived class ie it is compulsory for the derived class to provide definition of a pure virtual function. Since there is no definition in the base class, these functions can be equated to zero.

The general form of pure virtual function is:

virtual type func-name(parameter-list) = 0;

Consider following example of base class Shape and classes derived from it viz Circle, Rectangle, Triangle etc.

class Shape

{

int x, y;

public:

virtual void draw() = 0;

};

class Circle: public Shape

{

public:

draw()

{

//Code for drawing a circle

}

};

class Rectangle: public Shape

{

Public:

void draw()

{

//Code for drawing a rectangle

}

};

class Triangle: public Shape

{

Public:

void draw()

{

//Code for drawing a triangle

}

};

Thus, base class Shape has pure virtual function draw(); which is overridden by all the derived classes.

Static Binding:

By default, matching of function call with the correct function definition happens at compile time. This is called static binding or early binding or compile-time binding. Static binding is achieved using function overloading and operator overloading. Even though there are two or more functions with same name, compiler uniquely identifies each function depending on the parameters passed to those functions.

Dynamic Binding:

C++ provides facility to specify that the compiler should match function calls with the correct definition at the run time; this is called dynamic binding or late binding or run-time binding. Dynamic binding is achieved using virtual functions. Base class pointer points to derived class object. And a function is declared virtual in base class, then the matching function is identified at run-time using virtual table entry.