Virtual Definitions

[Stopher]

can anyone explain what exactly this means?

virtual void myfunction(void);

 

[LazyMe]

It's a bit tricky to understand if you don't know about OO.

Suppose you have a class A with that virtual function:
class A {
...
...
virtual void myfunction(void);
}

And a derived class B:
class B : public A
...
...
void myfunction(void);
}

And a derived class C:
class C : public A
...
...
void myfunction(void);
}


Now if you do this:

void test(void) {
B clsB;
C clsC;

A* ptrA = NULL;

ptrA = &clsB;
ptrA->myfunction; //Method in class B will be executed.

ptrA = &clsC;
ptrA->myfunction; //Method in class C will be executed.

ptrA = new A;
ptrA->myfunction; //Method in class A will be executed.

delete ptrA;
}


Now this might seem a bit strange (especially in such a simple example) but actually this is one of the strongest points of OOP. Imagine for example having a collection of different types of employees (i.e. manager, secretary etc.) all derived from a base class of employee having a virtual function which calculates for example their salary. You can loop through the collection with a base class pointer and rest assured that the correct salary calculation will be done (a more classic example...).
Greetings,
Rick

 

[palbano]

virtual functions are the mechanism in C++ that implements Polymorphism

google Polymorphism

-pete