compile_time_compare_string.cc

#include <cstddef>
#include <iostream>

namespace cts { // compile time string
  using std::size_t;
  template <size_t From, typename charT, size_t N1, size_t N2>
  struct eq_helper {
    static constexpr int exec(const charT (&s1)[N1], const charT (&s2)[N2]) {
      return s1[From]<s2[From] ? -1 : s2[From]<s1[From] ? 1 : eq_helper<From+1, charT, N1, N2>::exec(s1, s2);
    }
  };
  template <typename charT, size_t N>
  struct eq_helper<N, charT, N, N> {
    static constexpr int exec(const charT (&)[N], const charT (&s2)[N]) { return 0; }
  };
  template <typename charT, size_t N1, size_t N2>
  struct eq_helper<N1, charT, N1, N2> {
    static constexpr int exec(const charT (&)[N1], const charT (&s2)[N2]) { return 1; }
  };
  template <typename charT, size_t N1, size_t N2>
  struct eq_helper<N2, charT, N1, N2> {
    static constexpr int exec(const charT (&)[N1], const charT (&s2)[N2]) { return -1; }
  };
  template <typename charT, size_t N1, size_t N2>
  constexpr int compare(const charT (&s1)[N1], const charT (&s2)[N2]) {
    return eq_helper<0, charT, N1, N2>::exec(s1, s2);
  }
}

int main()
{
  std::cout << std::integral_constant<int, cts::compare("aaa", "aaa")>::value << '\n';
  std::cout << std::integral_constant<int, cts::compare("aaa", "bbb")>::value << '\n';
  std::cout << std::integral_constant<int, cts::compare("aaa", "aa")>::value << '\n';
  std::cout << std::integral_constant<int, cts::compare("aaa", "aaaa")>::value << '\n';
  std::cout << std::integral_constant<int, cts::compare("ccc", "bbb")>::value << '\n';
}