Inheritance and virtual functions

**Cryptomega** · 02-26-2003

I am writing some graphic software and I've run into a problem that I cannot resolve. Neithor text nor online reference has provided me with a solution.

Say I have a class called Circle. This class has a member function called Draw(). Inside this function it makes a call to another virtual member function called Behave(). The reason I have it set up like this is so when I derive a class from Circle, say Circle2, I can add more functionality to Draw() by inserting code into Behave(), without actually redefining Draw().

Of course to call Behave() from Draw I have to write Circle::Behave();. The result is the original Behave() is called, and not the one redefined in Circle2, even though it is a virtual function.

Can anyone help me with this? I know this is not how virtual functions are designed to be used, but it could greatly help the effieciency of my code. I will admit that I am a bit new to C++ (and programming in general), so any help from the C++ gurus here will be greatly appreciated.

**beginner** · 02-26-2003

The virtual functions should behave the way you wanted it to if your derived class, Circle in this situation inherits the base class that defines the virtual function behave().
thats all i know..

**nextus** · 02-27-2003

well in Circle you have a virtual function Behave...so i am guessing it is declared like

Code:

//inside the class definition
virtual void Behave();

if you have a derive class from this..then you should also have another function name Behave() in the derive class...let say Circle2..

so now..

Code:

Circle2 aCircle2;
Circle aCircle1;
Circle* pCircle;

pCircle = aCircle2;
pCircle->Behave(); //should call the dervive Behave();

pCircle = aCircle;
pCircle->Behave(); //should call the base class Behave();

hehe..you are getting into some polymorphism stuff here

**Polymorphic OOP** · 02-27-2003

Originally posted by Cryptomega
Of course to call Behave() from Draw I have to write Circle::Behave();

No you don't. That will force static binding (so it will always call the Cirvle version). Just write

Behave()

Also, if you aren't going to be redefining Draw (IE if you're just going to be redefining Behave), then the draw function does not have to be made virtual.

**Cryptomega** · 02-27-2003

Polymorphic, your right. I did not know that you could call a member function that way from another member function. (Like I said I'm new at this).

Thanks for the help. I appreciate all the replies

Terrance11 · 02-28-2003

Originally posted by Cryptomega
Polymorphic, your right. I did not know that you could call a member function that way from another member function. (Like I said I'm new at this).

Thanks for the help. I appreciate all the replies

there's a implicit this pointer, so it really looks like this:

this->behave();

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