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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::drop_front_exactly`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
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> | |
28 | 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> | |
50 | 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> | |
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> | |
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> | |
73 | 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 |