polymorphism - C++: Why does a structclass need a virtual method in order to be polymorphic?

Question

Following this question, I'm wondering why a structclass in C++ has to have a virtual method in order to be polymorphic.

Forcing a virtual destructor makes sense, but if there's no destructor at all, why is it mandatory to have a virtual method?

Answer 1

Because the type of a polymorphic object in C++ is, basically, determined from the pointer to its vtable, which is the table of virtual functions. The vtable is, however, only created if there's at least one virtual method. Why? Because in C++, you never get what you didn't explicitly ask for. They call it "you don't have to pay for something you don't need". Don't need polymorphism? You just saved a vtable.