Differences between branching and virtual methods

Question

I was trying to find more info on the matter but could only find this: In C++ why and how are virtual functions slower?

The answer says that the virtual call "Get[s] the right function address from the vtable into a register (the index where the correct function address is stored is decided at compile-time).".

But how does it know which is the correct function (address) in the first place? Doesn't it have to check which type the underlying object has in order to do so, thus being similar to a branching switch statement?

Answer 1

The C++ compiler creates the vtable for each class. If classes A and B both are sub-classes of class Base, and override methods in base, then the vtable for A and the vtable for B are constructed with pointers to the respective overridden methods.

For the code where the virtual method is called, the entry in the vtable for the object is loaded with the address of the correct method for the sub-class.

Assuming that class Base has four virtual methods, m1, m2, m3, and m4, the vtable will have 4 entries and m3 will be the third entry in the table. Sub-class A overrides methods m1 and m4, while sub-class B overrides them all. The vtable for A will have four entries, with pointers to A::m1, Base::m2, Base:m3, and A::m4, while B will also have four entries, with pointers to B::m1, B::m2, B::m3, and B::m4. At the call site where m3 is being invoked, the third vtable entry is loaded with the correct method pointer because the compiler constructed the tables this way.

(Of course, there is much more to vtable construction and layout than I have presented, so your mileage may vary.)