Is the virtual method in a simple class really virtual?

Question

I have a simple class, which has no derived types and no inheritance relations with other classes.

class A
{
    virtual void f() {}
}

I am wondering if the "virtual" method in fact is not really virtual, so the class is the same as

class A
{
    void f() {}
}

Answer 1

YOU know there is no derived types. The COMPILER doesn't know that. Such types, if ever defined, would be defined at a later time after A is defined.

So, at the point wherever A is used, f() has to be treated as virtual UNLESS the compiler can prove otherwise, for example when calling f() on a statically-typed A object, eg:

A a;
a.f();

In this context, the compiler can call A::f() statically, because it knows it can't possibly call anything else.

But when calling f() through an A& reference or an A* pointer instead, the compiler will have to use a virtual dispatch, since the compiler can't know exactly what type of object is being used at runtime.