When is VTable in C++ created?

Question

I would like to know when is a vtable created? Whether its in the startup code before main() or is it at some other point of time??

Answer 1

A vtable isn't a C++ concept so if they are used and when they are created if they are used will depend on the implementation.

Typically, vtables are structures created at compile time (because they can be determined at compile time). When objects of a particular type are created at runtime they will have a vptr which will be initialized to point at a static vtable at construction time.

Answer 2

The vtable is created at compile time. When a new object is created during run time, the hidden vtable pointer is set to point to the vtable.

Keep in mind, though, that you can't make reliable use if the virtual functions until the object is fully constructed. (No calling virtual functions in the constructor.)

EDIT I thought I'd address the questions in the comments.

As has been pointed out, the exact details of how the vtable is created and used is left up to the implementation. The c++ specification only provides specific behaviors that must be guaranteed, so there's plenty of wiggle room for the implementation. It doesn't have to use vtables at all (though most do). Generally, you don't need to know those details. You just need to know that when you call a virtual function, it do what you expect regardless of how it does it.

That said, I'll clarify a couple points about a typical implementation. A class with virtual functions has a hidden pointer (we'll call vptr) that points to the vtable for that class. Assume we have an employee class:

class Employee {
public:
  virtual work();
};

This class will have a vptr in it's structure, so it actually may look like this:

class Employee {
public:
  vtble *vptr;  // hidden pointer
  virtual work();
};

When we derive from this class, it will also have a vptr, and it must be in the same spot (in this case, at the beginning). That way when a function is called, regardless of the type of derived class, it always uses the vptr at the beginning to find the right vtable.