How to use "using" for a function?

Question

How to use "using" for a function? For example

class A;
void f(int);

struct B
{
   using BA = A;
   using Bf = f; ???
};

Answer 1

You can do

struct B
{
   using BA = A;
   constexpr static auto Bf = f;
}

That way you don't have to worry about specifying the type, which can be annoying.

You don't want to declare a non-static variable, otherwise every single copy of your object will be carrying around a function pointer. You also don't want it to be mutable, because then you can reassign it. You also don't want it to be potentially determined at runtime, because then the compiler will have to prove to itself in a given context that a call to Bf is really calling f, or else pay function indirection costs. The constexpr handles these last two points.

Answer 2

If f is just a function, you could either add a member function pointer:

void (*Bf)(int) = f;

or just wrap it in a function:

void Bf(int i) { f(i); }

Probably prefer the latter though, as it avoids adding extra members to B.

---

If f is a function template, you can't just alias, you'd need to forward to it:

template <class... Args>
auto Bf(Args&&... args)
    -> decltype(::f(std::forward<Args>(args)...))
    noexcept(noexcept(::f(std::forward<Args>(args...))))
{
    return ::f(std::forward<Args>(args)...);
}

which is... yeah.