Is final used for optimization in C++?

c++ c++11 virtual final
tmlen picture tmlen · May 24, 2016 · Viewed 7.5k times · Source 
class A {
public:
    virtual void f() = 0;
};

class B : public A {
public:
    void f() final override { };
};

int main() {
    B* b = new B();
    b->f();
}

In this case, is the compiler required to still do the v-table lookup for b->f();, or can it call B::f() directly because it was marked final?

Answer

Niall picture Niall · May 24, 2016

Is final used for optimization in C++?

It can be, and is.

As noted, it is being used already; see here and here showing the generated code for the override with and without final.

An optimisation along these lines would relate to the "de-virtualization" of the virtual calls. This is not always immediately affected by the final of the class nor method. Albeit they offer help to determine this, the normal rules of the virtual functions and class hierarchy apply.

If the compiler can determine that at runtime a particular method will always be called (e.g. given the OP example, with an automatic object), it could apply such an optimisation anyway, irrespective of whether the method is final or not.

Optimisations fall under the as-if rule, that allow the compiler to apply any transformation so long as the observable behaviour is as-if the original code had been executed.

Related questions

override on non-virtual functions

The C++11 FDIS it says If a virtual function is marked with the virt-specifier override and does not override a member function of a base class, the program is ill-formed. [ Example: struct B { virtual void f(int); }; struct D : B { …

Read More
Requiring virtual function overrides to use override keyword

C++11 added override to ensure that member functions you write that you intend to override base-class virtual functions actually do (or won't compile). But in a large object hierarchy, sometimes you could accidentally end up writing a member function that …

Read More
Pure virtual functions in C++11

In C++98, the null pointer was represented by the literal 0 (or in fact any constant expression whose value was zero). In C++11, we prefer nullptr instead. But this doesn't work for pure virtual functions: struct X { virtual void foo() = nullptr; }; …

Read More