virtual function = 0

[finchCZ]

Hi everybody

i have a question. Why is in this code

class Body {
  public:
    virtual double Volume() = 0 {
    }
    virtual double Surface() = 0 {
    }
}

equating to zero?

Thanks

[coder777]

It is called pure virtual or abstract function and requires to be overwritten in an derived class.

[Bazzy]

The syntax above is wrong, To have pure virtual functions you should have this:

class Body {
  public:
    virtual double Volume() = 0;
    virtual double Surface() = 0;
};

( No function bodies )

[finchCZ]

But it translates VS2011 without errors...
So, everywhere, when i can use virtual/abstract function i must get the equating to zero, why is that? What be done, when i won't use this equating? I mean just use

virtual type Function();

without function bodies and the equating to zero

[filipe]

A pure virtual function makes its class abstract, because it cannot be instantiated.

[guestgulkan]

To finchCZ
You need to learn up on Classes and polymorphism

[finchCZ]

yes i know, so i asked a question, because i want to learn it

thanks for help anyway

[firedraco]

FYI everyone, you CAN give a pure virtual function a definition, it just won't be used.

http://www.parashift.com/c++-faq-lite/abcs.html#faq-22.4

[hanst99]

Not sure if I understood that, so you can make a function purely virtual which prevents the instantiation of the base class, but you can still call it from a derived class or what? Otherwise I wouldn't see any point in providing a definition for an abstract function.

[guestgulkan]

You can still call it though.

[firedraco]

If you give a definition, you cannot instantiate the class, as per a normal pure virtual function. Basically all it does is let you possibly define some common functionality that derived classes might want.

[guestgulkan]

This is what I mean (although I have not sen it in practice)

class Base
{
public:
    virtual void VFuncA()=0;
    int x;

};

//Give the pure virtual function a body
void Base::VFuncA()
{
    x =3;

}

class Derived: public Base
{

public:
    void VFuncA()
    {

        Base::VFuncA();//calls bas class version

    }

};


int _tmain(int argc, _TCHAR* argv[])
{
    
     //Base *pb = new Base;//Error base class is abstract
    
    Base *pb = new Derived;
    
    pb->VFuncA();
    
   delete pb;

    return 0;
}