ceph/src/boost/libs/hana/example/misc/nth.cpp

   1 // Copyright Louis Dionne 2013-2016
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/functional/arg.hpp>
   6
   7 #include <cstddef>
   8 #include <utility>
   9 namespace hana = boost::hana;
  10
  11
  12 constexpr int to_int(char c)
  13 { return static_cast<int>(c) - 48; }
  14
  15 template <std::size_t N>
  16 constexpr long long parse(const char (&arr)[N]) {
  17     long long number = 0, base = 1;
  18     for (std::size_t i = 0; i < N; ++i) {
  19         number += to_int(arr[N - 1 - i]) * base;
  20         base *= 10;
  21     }
  22     return number;
  23 }
  24
  25 template <char ...c>
  26 struct pick {
  27     static constexpr unsigned long n = parse<sizeof...(c)>({c...});
  28
  29     template <typename ...T>
  30     constexpr auto operator()(T&& ...args) const
  31     { return hana::arg<n>(std::forward<T>(args)...); }
  32 };
  33
  34 template <char ...c> constexpr pick<c...> operator"" _st() { return {}; }
  35 template <char ...c> constexpr pick<c...> operator"" _nd() { return {}; }
  36 template <char ...c> constexpr pick<c...> operator"" _rd() { return {}; }
  37 template <char ...c> constexpr pick<c...> operator"" _th() { return {}; }
  38
  39
  40 static_assert(1_st(1, '2', 3.3, 444) == 1, "");
  41 static_assert(2_nd(1, '2', 3.3, 444) == '2', "");
  42 static_assert(3_rd(1, '2', 3.3, 444) == 3.3, "");
  43 static_assert(4_th(1, '2', 3.3, 444) == 444, "");
  44
  45 int main() { }