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