cost of Virtual-and-final function at base class (vtable/virtual-cost)

Question

Is there any vtable/virtual cost for a virtual final function (final at base class)?

class B{
    public: virtual void fFinal() final{ .... do something  ..... }
    public: void fCheap() { .... do something ..... }
    public: virtual void fVirtual() { .... do something ..... }
};
class C : public B{
    public: virtual void fVirtual() { .... do something ..... }
};

Will the cost of fFinal() = cost of fCheap() or fVirtual()?

I am going to use final+virtual to prevent human-error ("Don't override me").
fCheap() is not so safe, because I can still hide the parent's function.

These below links don't provide answer.

• What's the point of a final virtual function?
• Does the cost of virtual functions increase with the number of classes in the inheritance tree?
• http://en.cppreference.com/w/cpp/language/final

Answer 1

As far as I can tell, it is unspecified how virtual functions mechanism is implemented by any particular compiler. It is most likely that they put a pointer to vtable into the class if it has at least one virtual function even if this virtual function is marked as final.

I've tested a simplifed example based on your code snippet:

class Base {
    public:
        virtual void foo() final {}
};

static_assert(sizeof(Base) > 1, "No pointer to vtable");

Seems that both gcc 6.3 and clang 4.0 add a pointer to vtable.

It means that the rules which govern usual virtual functions apply for a function which marked both virtual and final in the base class. As a consequence, Base class grows in size and, and when you call foo via the pointer/reference to Base class or some class derived from Base, you pay some additional cost for being redirected through the vtable.