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>
operator ()(Xs && xs,N const & n) const31     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>
operator ()(Xs && xs) const53     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>
drop_back_helperdrop_back_impl61         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>
applydrop_back_impl66         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
76