SFINAE

sfinae.cpp

#include <type_traits>

namespace sfinae {

template<bool b> using where = typename std::enable_if<b, std::nullptr_t>::type;

template<class A, class = std::nullptr_t>
struct case_t {
  static char const* label() { return "cond-default"; }
};

template<>
struct case_t<bool> {
  static char const* label() { return "bool"; }
};

template<class A>
struct case_t<A, where<std::is_integral<A>{}>> {
  static char const* label() { return "cond-integral"; }
};

template<class A>
struct case_t<A, where<std::is_floating_point<A>{}>> {
  static char const* label() { return "cond-floating point"; }
};

template<class A, where<!std::is_integral<A>{} && !std::is_floating_point<A>{}> = nullptr>
char const* label() { return "default"; }

template<class A, where<std::is_same<A, bool>{}> = nullptr>
char const* label() { return "bool"; }

template<class A, where<std::is_integral<A>{} && !std::is_same<A, bool>{}> = nullptr>
char const* label() { return "integral"; }

template<class A, where<std::is_floating_point<A>{}> = nullptr>
char const* label() { return "floating point"; }

} // namespace

#include <iostream>
using namespace std;

int main() {
  cout << sfinae::case_t<nullptr_t>::label() << endl;
  cout << sfinae::case_t<bool     >::label() << endl;
  cout << sfinae::case_t<int      >::label() << endl;
  cout << sfinae::case_t<float    >::label() << endl;

  cout << sfinae::label<nullptr_t>() << endl;
  cout << sfinae::label<bool     >() << endl;
  cout << sfinae::label<int      >() << endl;
  cout << sfinae::label<float    >() << endl;
  return 0;
}