ceph/src/boost/libs/hana/test/tuple/usability_of_types.cpp

   1 // Copyright Louis Dionne 2013-2017
   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/at.hpp>
   6 #include <boost/hana/back.hpp>
   7 #include <boost/hana/front.hpp>
   8 #include <boost/hana/integral_constant.hpp>
   9 #include <boost/hana/tuple.hpp>
  10 #include <boost/hana/type.hpp>
  11
  12 #include <type_traits>
  13 namespace hana = boost::hana;
  14
  15
  16 // The fact that a reference to a `type<...>` is returned from `front(types)`
  17 // and friends used to break the `decltype(front(types))::type` pattern,
  18 // because we would be trying to fetch `::type` inside a reference. To work
  19 // around this, a unary `operator+` turning a lvalue `type` into a rvalue
  20 // `type` was added.
  21
  22 struct T; struct U; struct V;
  23
  24 int main() {
  25     auto check = [](auto types) {
  26         static_assert(std::is_same<
  27             typename decltype(+hana::front(types))::type, T
  28         >{}, "");
  29
  30         static_assert(std::is_same<
  31             typename decltype(+hana::at(types, hana::size_c<1>))::type, U
  32         >{}, "");
  33
  34         static_assert(std::is_same<
  35             typename decltype(+hana::back(types))::type, V
  36         >{}, "");
  37     };
  38
  39     check(hana::make_tuple(hana::type_c<T>, hana::type_c<U>, hana::type_c<V>));
  40     check(hana::tuple_t<T, U, V>);
  41 }