boost/hana/drop_front.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::drop_front`.
   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_FRONT_HPP
  11 #define BOOST_HANA_DROP_FRONT_HPP
  12
  13 #include <boost/hana/fwd/drop_front.hpp>
  14
  15 #include <boost/hana/concept/integral_constant.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/core/tag_of.hpp>
  20 #include <boost/hana/integral_constant.hpp>
  21
  22
  23 BOOST_HANA_NAMESPACE_BEGIN
  24     //! @cond
  25     template <typename Xs, typename N>
  26     constexpr auto drop_front_t::operator()(Xs&& xs, N const& n) const {
  27         using It = typename hana::tag_of<Xs>::type;
  28         using DropFront = BOOST_HANA_DISPATCH_IF(drop_front_impl<It>,
  29             hana::Iterable<It>::value &&
  30             hana::IntegralConstant<N>::value
  31         );
  32
  33     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  34         static_assert(hana::Iterable<It>::value,
  35         "hana::drop_front(xs, n) requires 'xs' to be an Iterable");
  36
  37         static_assert(hana::IntegralConstant<N>::value,
  38         "hana::drop_front(xs, n) requires 'n' to be an IntegralConstant");
  39     #endif
  40
  41         return DropFront::apply(static_cast<Xs&&>(xs), n);
  42     }
  43
  44     template <typename Xs>
  45     constexpr auto drop_front_t::operator()(Xs&& xs) const {
  46         return (*this)(static_cast<Xs&&>(xs), hana::size_t<1>{});
  47     }
  48     //! @endcond
  49
  50     template <typename It, bool condition>
  51     struct drop_front_impl<It, when<condition>> : default_ {
  52         template <typename Xs, typename N>
  53         static constexpr auto apply(Xs&&, N const&) = delete;
  54     };
  55 BOOST_HANA_NAMESPACE_END
  56
  57 #endif // !BOOST_HANA_DROP_FRONT_HPP