virtual keyword in c++

[cassius]

I have been looking up the meaning of this keyword " virtual" and although theres no shortage of explanations I am too thick to understand any of them.

I know it can be used with classes or functions though.

 

Anyone willing to take a shot at explaining this keyword??

[tjheldna]

virtual is very important

 

I'm not too technical on this but it comes into play when you have inherited objects I'll try to use it in a game example:

 

 

Say I have the base class object with a function tostring() and declared as a virtual function.

 

class baseCamera

{

virtual string tostring()

}

 

now I have a derived classes which overrides the tostring method of the base class.

 

class fpsCamera: public baseCamera

{

string tostring();

}

 

class tpsCamera: public baseCamera

{

string tostring();

}

 

 

Say in my game world I always want to reference the current camera, I would use a baseCamera pointer as I can point to either the fps and tps camera (whichever is the current)

 

class GameWorld

{

baseCamera *cam;

}

 

now I want to call cam->tostring() I don't care if it's tps or fps and I don't want to call the baseCamera's tostring method I want to call the tpsCameras tostring() method or the fpsCamera's tostring() and that's what adding virtual will do for you. It will call the derived classes method first.

 

Hope I explained it good enough

[Guppy]

class animal{
virtual void makenoise() { std::cout << "??" << std::endl; }
}

class dog: public animal{
void makenoise() { std::cout << "wof!" << std::endl; }
}


std::vector<animal> animals;

 

 

 

 

 

now if you iter over your animals vector dogs says woof, with out the virtual keyword they would say ?? even though they define their own makenoise() command - you would have to downcast them to dogs ( and know to do this ) to make them say woof.

 

 

adding

 

class bigDog: public dog{
void makenoise() { std::cout << "big wof!" << std::endl; }
}

 

 

to the above example big doges will go "big wof!" even though dog is missing the virtual keyword - this is because virtual functions will always stay virtual. For this reason it's concidered poilite to type it out anyway when you overload a virtual function, even if you dont need to. It just avoids confusion

[cassius]

Thanks fellas I think its sinking in.