How can I determine if a compiler uses early or late binding on a virtual function?

Question

I have the following code:

class Pet {
public:
  virtual string speak() const { return ""; }
};

class Dog : public Pet {
public:
  string speak() const { return "Bark!"; }
};

int main() {
  Dog ralph;
  Pet* p1 = &ralph;
  Pet& p2 = ralph;
  Pet p3;

  // Late binding for both:
  cout << "p1->speak() = " << p1->speak() <<endl;
  cout << "p2.speak() = " << p2.speak() << endl;

  // Early binding (probably):
  cout << "p3.speak() = " << p3.speak() << endl;
}

I have been asked to determine whether the compiler uses early or late binding for the final function call. I have searched online but have found nothing to help me. Can someone tell me how I would carry out this task?

Answer 1

You can look at the disassembly, to see whether it appears to be redirecting through a vtable.

The clue is whether it calls directly to the address of the function (early binding) or calls a computed address (late binding). The other possibility is that the function is inlined, which you can consider to be early binding.

Of course the standard doesn't dictate the implementation details, there may be other possibilities, but that covers "normal" implementations.

Answer 2

You can always use hack :D

//...
Pet p3;
memset(&p3, 0, sizeof(p3));
//...

If compiler does use vtbl pointer, guess what will gonna happen :>

p3.speak()  // here