Choose Best Available Function Through Tag Inheritance

Question

Assume the user defines some subset of the following functions:

void f(int) {}
void g(int) {}
void h(int) {}
// ...

Your task is to write a

Answer 1

First, we define a priority class.

template struct priority : priority {};
template<> struct priority<0> {};

It can be used to give a total order on the functions as follows:

template auto call_best(Int i, priority<2>) -> decltype(f(i)) { return f(i); }
template auto call_best(Int i, priority<1>) -> decltype(g(i)) { return g(i); }
template auto call_best(Int i, priority<0>) -> decltype(h(i)) { return h(i); }

void call_best(int i) { call_best(i, priority<2>{}); }

The Int template parameter and the decltype() make sure that only the defined functions compete for being called (keyword SFINAE). The priority tag class allows us to pick the best one among them.

Note that the SFINAE part only works if you have at least one parameter on which you can template. If anyone has an idea on how to avoid this, please tell me.