template.cc

#include <cstdlib>
#include <iostream>
#include <type_traits>
#include <string>

template <char... xs>
struct FooIntLiteral {
  using result_type = int;

  static int result() {
    const char str[] = {xs..., '\0'};
    return atoi(str);
  }
};

template <char... xs>
struct FooStringLiteral {
  using result_type = std::string;

  static std::string result() {
    const char str[] = {xs..., '\0'};
    return str;
  }
};

template <typename Char, Char x, Char... xs>
struct FooLiteral;

template <char x, char... xs>
struct FooLiteral<char, x, xs...> 
  : public std::conditional<'0' <= x && x <= '9',
      FooIntLiteral<x, xs...>,
      FooStringLiteral<x, xs...>>::type {
};

template <typename Char, Char... xs>
typename FooLiteral<Char, xs...>::result_type operator""_foo() {
  return FooLiteral<Char, xs...>::result();
}


int main() {
  std::cout << "1234"_foo * 2 << '\n' << "abcdef"_foo << std::endl;
}