How to obtain a pointer out of a C++ vtable?

Question

Say you have a C++ class like:

class Foo {
 public:
  virtual ~Foo() {}
  virtual DoSomething() = 0;
};

The C++ compiler translates a call

Answer 1

If you call derived->DoSomething(), and DoSomething() is not virtual in the derived class, the compiler should generate a straight call already.

If you call base->DoSomething(), the compiler has to check one way or another which version of DoSomething() to call, and a vtable is as efficient method as any. If you could guarantee it to always be an instance of the base class, you wouldn't need to make the method virtual in the first place.

Under select circumstances, it might make sense to do a static_cast before calling a bunch of non-virtual derived methods that are virtual in the base class, but as vtable lookups are common, accounted for, and relatively inexpensive, this definitely falls under the category of premature optimization.

Templates are another standard C++ means of reusing code without causing a vtable lookup.