Non-type variadic function templates in C++11

Question

I saw a blog post which used non-type variadic templates (currently not supported by gcc, only by clang).

template 
stru         


        

Answer 1

Luc Danton's solution doesn't behave correctly with parameters which are not of type int, but can be implicitly converted to an int. Here's one which does:

template struct first_type { typedef T type; };
template int max_checked(T n) { return n; }
template
int max_checked(T1 n1, T2 n2, Ts ...ns)
{
  int maxRest = max_checked(n2, ns...);
  return n1 > maxRest ? n1 : maxRest;
}
template auto max(T &&...t) ->
  decltype(max_checked::type...>(t...))
{
  return max_checked::type...>(t...);
}

struct S { operator int() { return 3; } };
int v = max(1, 2.0, S()); // v = 3.

Here, max forwards all arguments unchanged to max_checked, which takes the same number of arguments of type int (provided by performing a pack-expansion on the first_type template). The decltype(...) return type is used to apply SFINAE if any argument can't be converted to int.