Abstract classes

An interface describes the behavior or capabilities of a C++ class without committing to a particularimplementation of that class.

The C++ interfaces are implemented using abstract classes and these abstract classes should not beconfused with data abstraction which is a concept of keeping implementation details separate from associateddata.

A class is made abstract by declaring at least one of its functions as pure virtual function. A pure virtual functionis specified by placing "= 0" in its declaration as follows:

classBox

{

public:

// pure virtaul function

virtual double getVolume() = 0;

private:

doublelength; // Length of a box

doublebreadth; // Breadth of a box

doubleheight; // Height of a box

};

The purpose of an abstrac t c lass(often referred to as an ABC) is to provide an appropriate base class fromwhich other classes can inherit. Abstract classes cannot be used to instantiate objects and serves only as aninterface. Attempting to instantiate an object of an abstract class causes a compilation error.

Thus, if a subclass of an ABC needs to be instantiated, it has to implement each of the virtual functions, whichmeans that it supports the interface declared by the ABC. Failure to override a pure virtual function in a derivedclass, then attempting to instantiate objects of that class, is a compilation error.

Classes that can be used to instantiate objects are called concrete c lasses.

Abstract Class Example:

Consider the following example where parent class provides an interface to the base class to implement a function called g etArea():

#include <iostream>

using namespace std;

//Base class

classShape {

public:

//  pure virtual function providing interface framework.

virtualintgetArea() = 0;

voidsetWidth(intw)

{

width= w;

}

voidsetHeight(inth)

{

height= h;

}

protected: intwidth; intheight; };

//  Derived classes

classRectangle: public Shape

{

public: intgetArea()

{

return(width * height);

}

};

classTriangle: public Shape { public:

intgetArea()

{

return(width * height)/2;

}

}; intmain(void)

{

Rectangle Rect;

Triangle Tri; Rect.setWidth(5);

Rect.setHeight(7);

// Print the area of the object.

cout<<"Total Rectangle area: " <<Rect.getArea() <<endl;

Tri.setWidth(5);

Tri.setHeight(7);

// Print the area of the object.

cout<<"Total Triangle area: " <<Tri.getArea() <<endl;

return0;

}

When the above code is compiled and executed, it produces the following result:

Total Rectangle area: 35

Total Triangle area: 17