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

Your code will work if you follow the convention of giving your functions a deduced return type (i.e auto/decltype(auto)). This can only be done in C++14 wherein a function with a deduced return type cannot be used until it is defined (even in an unevaluated operand), causing a substitution failure otherwise. Here's your example that works in clang 3.5 but unfortunately not in g++ 4.9.

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

// ********
auto f(int);
auto g(int);
auto h(int) { std::cout << "h()"; }
// ********

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>{}); }

int main()
{
    call_best(0); // calls h()
}

clang 3.5 g++ 4.9