Question

When I have something like this:

class A
{
  virtual void rat();
};

class B : public A
{
  virtual void rat() { ; } //implemented!
};

int main(int argc, char **argv)
{
  A *a = new B;
  delete a;
}

I get linker errors:

unless I make the base rat a pure virtual.

However, when I have this:

class A
{
  public:
  void rat();
};

int main(int argc, char **argv)
{
  A a;
}

This compiles fine and doesn’t give me an undefined reference link error unless I explicitly try to call the rat function in my main (a.rat();). What is the rule on unimplemented base class virtual functions which are, however, implemented in a derived class as in the first failing code snippet?

Answer 1

When both classes define virtual functions, C++ compiler needs to build vtables for both classes A and B. To build A‘s vtable, the compiler needs A::rat() – that is where the reference is coming from.

When A has no virtual functions, there are no reference to A::rat from anywhere, hence you do not get a compile error.

As I’m sure you know, you can fix this error by making A::rat a pure virtual, thus providing the needed value for the vtable (in this case, the value is zero).