ceph/src/boost/boost/hana/drop_back.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::drop_back`.
   4
   5 @copyright Louis Dionne 2013-2017
   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_DROP_BACK_HPP
  11 #define BOOST_HANA_DROP_BACK_HPP
  12
  13 #include <boost/hana/fwd/drop_back.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concept/integral_constant.hpp>
  17 #include <boost/hana/concept/sequence.hpp>
  18 #include <boost/hana/config.hpp>
  19 #include <boost/hana/core/dispatch.hpp>
  20 #include <boost/hana/core/make.hpp>
  21 #include <boost/hana/integral_constant.hpp>
  22 #include <boost/hana/length.hpp>
  23
  24 #include <cstddef>
  25 #include <utility>
  26
  27
  28 BOOST_HANA_NAMESPACE_BEGIN
  29     //! @cond
  30     template <typename Xs, typename N>
  31     constexpr auto drop_back_t::operator()(Xs&& xs, N const& n) const {
  32         using S = typename hana::tag_of<Xs>::type;
  33         using DropBack = BOOST_HANA_DISPATCH_IF(drop_back_impl<S>,
  34             hana::Sequence<S>::value &&
  35             hana::IntegralConstant<N>::value
  36         );
  37
  38     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  39         static_assert(hana::Sequence<S>::value,
  40         "hana::drop_back(xs, n) requires 'xs' to be a Sequence");
  41
  42         static_assert(hana::IntegralConstant<N>::value,
  43         "hana::drop_back(xs, n) requires 'n' to be an IntegralConstant");
  44     #endif
  45
  46         static_assert(N::value >= 0,
  47         "hana::drop_back(xs, n) requires 'n' to be non-negative");
  48
  49         return DropBack::apply(static_cast<Xs&&>(xs), n);
  50     }
  51
  52     template <typename Xs>
  53     constexpr auto drop_back_t::operator()(Xs&& xs) const {
  54         return (*this)(static_cast<Xs&&>(xs), hana::size_c<1>);
  55     }
  56     //! @endcond
  57
  58     template <typename S, bool condition>
  59     struct drop_back_impl<S, when<condition>> : default_ {
  60         template <typename Xs, std::size_t ...n>
  61         static constexpr auto drop_back_helper(Xs&& xs, std::index_sequence<n...>) {
  62             return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs))...);
  63         }
  64
  65         template <typename Xs, typename N>
  66         static constexpr auto apply(Xs&& xs, N const&) {
  67             constexpr std::size_t n = N::value;
  68             constexpr std::size_t len = decltype(hana::length(xs))::value;
  69             return drop_back_helper(static_cast<Xs&&>(xs),
  70                                     std::make_index_sequence<(n > len ? 0 : len - n)>{});
  71         }
  72     };
  73 BOOST_HANA_NAMESPACE_END
  74
  75 #endif // !BOOST_HANA_DROP_BACK_HPP