virtual functions, why ?

[Johan]

Hi,

I am new to C++. Why use virtual functions

class A
{
public :
A() {}
virtual ~A() {}

virtual void paint() {......}
};

class B : public A
{
public :
B() {}
virtual ~B() {}

virtual void paint() {......}
};

Why do paint have to be virtual if it has its own member function paint. Why
do you want to use virtual functions ?

John

 

[Ioannis Vranos]

Hi,

I am new to C++. Why use virtual functions


Why do paint have to be virtual if it has its own member function paint. Why
do you want to use virtual functions ?

Consider the following program:


#include <iostream>

class Drawing
{
public :

virtual ~Drawing() {}

virtual void paint()
{
std::cout<<"Drawing:aint() was called!\n";
}
};

class Rectangular: public Drawing
{
public :

virtual ~Rectangular() {}

virtual void paint()
{
std::cout<<"Rectangular:ain() was called!\n";
}
};


void somefunc(Drawing *p)
{
p->paint();
}


int main()
{
Rectangular r;

somefunc(&r);
}





somefunc() processes a pointer to a Drawing object, and the appropriate
paint() specialisation of a Drawing object is called.


Had paint() been non-virtual, Drawing:aint() would have been called.

 

[Rolf Magnus]

Hi,

I am new to C++. Why use virtual functions

class A
{
public :
A() {}
virtual ~A() {}

virtual void paint() {......}
};

class B : public A
{
public :
B() {}
virtual ~B() {}

virtual void paint() {......}
};

Why do paint have to be virtual if it has its own member function paint.
Why do you want to use virtual functions ?

The keyword is polymorphism. This is a very important concept of C++ and
object oriented programming in general. The idea is that you can e.g. do
something like:

A* objects[2];
objects[0] = new B;
objects[1] = new OtherClassDerivedFromA;

So you can store objects of different types. But now there is a problem.
What happens if you do:

objects[0]->paint();
objects[1]->paint();

objects[0] and objects[1] are not the same type, but they are both stored
using a pointer to A. If paint() isn't virtual, A:aint() gets called. If
you make paint() virtual in A, the runtime system automatically finds out
which derived class the object is really of and calls the paint() of that
class.

 

[EventHelix.com]

The following article about the "open closed principle" explains this:

http://www.eventhelix.com/RealtimeMantra/Object_Oriented/open_closed_principle.htm

Deepa