ceph/src/boost/libs/hana/include/boost/hana/back.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::back`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_BACK_HPP
  11 #define BOOST_HANA_BACK_HPP
  12
  13 #include <boost/hana/fwd/back.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concept/iterable.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/length.hpp>
  20
  21 #include <cstddef>
  22
  23
  24 BOOST_HANA_NAMESPACE_BEGIN
  25     //! @cond
  26     template <typename Xs>
  27     constexpr decltype(auto) back_t::operator()(Xs&& xs) const {
  28         using It = typename hana::tag_of<Xs>::type;
  29         using Back = BOOST_HANA_DISPATCH_IF(back_impl<It>,
  30             hana::Iterable<It>::value
  31         );
  32
  33     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  34         static_assert(hana::Iterable<It>::value,
  35         "hana::back(xs) requires 'xs' to be an Iterable");
  36     #endif
  37
  38         return Back::apply(static_cast<Xs&&>(xs));
  39     }
  40     //! @endcond
  41
  42     template <typename It, bool condition>
  43     struct back_impl<It, when<condition>> : default_ {
  44         template <typename Xs>
  45         static constexpr decltype(auto) apply(Xs&& xs) {
  46             constexpr std::size_t len = decltype(hana::length(xs))::value;
  47             static_assert(len > 0, "hana::back(xs) requires 'xs' to be non-empty");
  48             return hana::at_c<len - 1>(static_cast<Xs&&>(xs));
  49         }
  50     };
  51 BOOST_HANA_NAMESPACE_END
  52
  53 #endif // !BOOST_HANA_BACK_HPP