1 /*! 2 @file 3 Defines `boost::hana::drop_front_exactly`. 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_EXACTLY_HPP 11 #define BOOST_HANA_DROP_FRONT_EXACTLY_HPP 12 13 #include <boost/hana/fwd/drop_front_exactly.hpp> 14 15 #include <boost/hana/bool.hpp> 16 #include <boost/hana/concept/integral_constant.hpp> 17 #include <boost/hana/concept/iterable.hpp> 18 #include <boost/hana/config.hpp> 19 #include <boost/hana/core/dispatch.hpp> 20 #include <boost/hana/drop_front.hpp> 21 #include <boost/hana/integral_constant.hpp> 22 #include <boost/hana/is_empty.hpp> 23 24 25 BOOST_HANA_NAMESPACE_BEGIN 26 //! @cond 27 template <typename Xs, typename N> operator ()(Xs && xs,N const & n) const28 constexpr auto drop_front_exactly_t::operator()(Xs&& xs, N const& n) const { 29 using It = typename hana::tag_of<Xs>::type; 30 using DropFrontExactly = BOOST_HANA_DISPATCH_IF(drop_front_exactly_impl<It>, 31 hana::Iterable<It>::value && 32 hana::IntegralConstant<N>::value 33 ); 34 35 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS 36 static_assert(hana::Iterable<It>::value, 37 "hana::drop_front_exactly(xs, n) requires 'xs' to be an Iterable"); 38 39 static_assert(hana::IntegralConstant<N>::value, 40 "hana::drop_front_exactly(xs, n) requires 'n' to be an IntegralConstant"); 41 #endif 42 43 static_assert(N::value >= 0, 44 "hana::drop_front_exactly(xs, n) requires 'n' to be non-negative"); 45 46 return DropFrontExactly::apply(static_cast<Xs&&>(xs), n); 47 } 48 49 template <typename Xs> operator ()(Xs && xs) const50 constexpr auto drop_front_exactly_t::operator()(Xs&& xs) const { 51 return (*this)(static_cast<Xs&&>(xs), hana::size_c<1>); 52 } 53 //! @endcond 54 55 namespace detail { 56 template <typename Xs, typename N> check_dfe_overflow(Xs const & xs,N const &,hana::true_)57 constexpr void check_dfe_overflow(Xs const& xs, N const&, hana::true_) { 58 constexpr bool n_overflew_length = decltype( 59 hana::is_empty(hana::drop_front(xs, hana::size_c<N::value - 1>)) 60 )::value; 61 static_assert(!n_overflew_length, 62 "hana::drop_front_exactly(xs, n) requires 'n' to be less than or " 63 "equal to the number of elements in 'xs'"); 64 } 65 66 template <typename Xs, typename N> check_dfe_overflow(Xs const &,N const &,hana::false_)67 constexpr void check_dfe_overflow(Xs const&, N const&, hana::false_) { } 68 } 69 70 template <typename It, bool condition> 71 struct drop_front_exactly_impl<It, when<condition>> : default_ { 72 template <typename Xs, typename N> applydrop_front_exactly_impl73 static constexpr auto apply(Xs&& xs, N const& n) { 74 auto result = hana::drop_front(static_cast<Xs&&>(xs), n); 75 constexpr bool check_for_overflow = 76 decltype(hana::is_empty(result))::value && N::value != 0; 77 78 detail::check_dfe_overflow(xs, n, hana::bool_c<check_for_overflow>); 79 80 return result; // NRVO applied 81 } 82 }; 83 BOOST_HANA_NAMESPACE_END 84 85 #endif // !BOOST_HANA_DROP_FRONT_EXACTLY_HPP 86