Re: linker error in virtual function

From:

Michael DOUBEZ <michael.doubez@free.fr>

Newsgroups:

comp.lang.c++

Date:

Mon, 14 Apr 2008 10:06:38 +0200

Message-ID:

<48030fbe$0$19237$426a74cc@news.free.fr>

Rahul a ?crit :

Hi Everyone,

I have the following polymorphic classes,

class Shape
{
public : virtual void draw()
         {
         }
       virtual void sample();
};

class Circle : public Shape
{
public : virtual void draw()
         {
             cout<<"Circle::draw"<<endl;
         }
};

int main()
{
    Shape *ptr = new Circle;
    ptr->draw();
        return(0);
}

and i get a linker error saying undefined external sumbol __sample...
But i haven't really invoked the function sample(). Note that the code
works fine when i change sample() to a non-virtual function prototype,
why is this the case?

If you make it virtual, 10.3-8 of the standard requires that:
"A virtual function declared in a class shall be defined or declared
pure or both; but no diagnostic is required."

Here you don't have the diagnostic of missing definition but the linker
requires it (for its virtual table).

But the standard doesn't require the definition of a function that is
not used (non-virtual sample() in your case). See 3.2 of the standard
for the ins and outs of when a function is *used*.

Michael