classInstanceSize and vtable

[Etienne Cimon via Digitalmars-d-learn]

I'm trying to figure out the size difference between a final class and a 
class (which carries a vtable pointer).

import std.stdio;

class A { void print(){} }

final class B { void print(){} }

void main(){
         writeln(__traits(classInstanceSize, A));
         writeln(__traits(classInstanceSize, B));
}


Returns:
8
8

I'm not sure, why does a final class carry a vtable pointer?