76 lines
2.5 KiB
C++
76 lines
2.5 KiB
C++
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/*!
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@file
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Defines `boost::hana::take_front` and `boost::hana::take_front_c`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_TAKE_FRONT_HPP
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#define BOOST_HANA_TAKE_FRONT_HPP
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#include <boost/hana/fwd/take_front.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/concept/integral_constant.hpp>
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#include <boost/hana/concept/sequence.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/core/make.hpp>
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#include <boost/hana/integral_constant.hpp>
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#include <boost/hana/length.hpp>
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#include <cstddef>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Xs, typename N>
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constexpr auto take_front_t::operator()(Xs&& xs, N const& n) const {
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using S = typename hana::tag_of<Xs>::type;
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using TakeFront = BOOST_HANA_DISPATCH_IF(take_front_impl<S>,
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hana::Sequence<S>::value &&
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hana::IntegralConstant<N>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Sequence<S>::value,
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"hana::take_front(xs, n) requires 'xs' to be a Sequence");
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static_assert(hana::IntegralConstant<N>::value,
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"hana::take_front(xs, n) requires 'n' to be an IntegralConstant");
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#endif
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return TakeFront::apply(static_cast<Xs&&>(xs), n);
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}
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//! @endcond
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template <typename S, bool condition>
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struct take_front_impl<S, when<condition>> : default_ {
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template <typename Xs, std::size_t ...n>
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static constexpr auto take_front_helper(Xs&& xs, std::index_sequence<n...>) {
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return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs))...);
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}
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template <typename Xs, typename N>
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static constexpr auto apply(Xs&& xs, N const&) {
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constexpr std::size_t n = N::value;
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constexpr std::size_t size = decltype(hana::length(xs))::value;
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return take_front_helper(static_cast<Xs&&>(xs),
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std::make_index_sequence<(n < size ? n : size)>{});
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}
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};
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template <std::size_t n>
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struct take_front_c_t {
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template <typename Xs>
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constexpr auto operator()(Xs&& xs) const {
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return hana::take_front(static_cast<Xs&&>(xs), hana::size_c<n>);
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_TAKE_FRONT_HPP
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