boost/hana/detail/variadic/take.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::detail::variadic::take`.
   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_DETAIL_VARIADIC_TAKE_HPP
  11 #define BOOST_HANA_DETAIL_VARIADIC_TAKE_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/detail/variadic/split_at.hpp>
  15 #include <boost/hana/functional/always.hpp>
  16 #include <boost/hana/functional/reverse_partial.hpp>
  17
  18 #include <cstddef>
  19
  20
  21 BOOST_HANA_NAMESPACE_BEGIN namespace detail { namespace variadic {
  22     struct take_impl2 {
  23         template <typename F, typename ...Xs>
  24         constexpr decltype(auto) operator()(F&& f, Xs&& ...xs) const {
  25             return static_cast<F&&>(f)(static_cast<Xs&&>(xs)...);
  26         }
  27     };
  28
  29     struct take_impl1 {
  30         template <typename ...Xs>
  31         constexpr auto operator()(Xs&& ...xs) const {
  32             return hana::always(
  33                 reverse_partial(take_impl2{},
  34                     static_cast<Xs&&>(xs)...)
  35             );
  36         }
  37     };
  38
  39     template <std::size_t n>
  40     struct take_t {
  41         template <typename ...Xs>
  42         constexpr decltype(auto) operator()(Xs&& ...xs) const {
  43             return variadic::split_at<n>(static_cast<Xs&&>(xs)...)(take_impl1{});
  44         }
  45     };
  46
  47     template <std::size_t n>
  48     constexpr take_t<n> take{};
  49 }} BOOST_HANA_NAMESPACE_END
  50
  51 #endif // !BOOST_HANA_DETAIL_VARIADIC_TAKE_HPP