79 lines
2.2 KiB
C++
79 lines
2.2 KiB
C++
/*!
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@file
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Forward declares `boost::hana::take_back`.
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@copyright Louis Dionne 2013-2017
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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_FWD_TAKE_BACK_HPP
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#define BOOST_HANA_FWD_TAKE_BACK_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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#include <cstddef>
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namespace boost { namespace hana {
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//! Returns the last `n` elements of a sequence, or the whole sequence
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//! if the sequence has less than `n` elements.
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//! @ingroup group-Sequence
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//!
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//! Given a `Sequence` `xs` and an `IntegralConstant` `n`, `take_back(xs, n)`
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//! is a new sequence containing the last `n` elements of `xs`, in the
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//! same order. If `length(xs) <= n`, the whole sequence is returned and
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//! no error is triggered.
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//!
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//!
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//! @param xs
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//! The sequence to take the elements from.
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//!
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//! @param n
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//! A non-negative `IntegralConstant` representing the number of elements
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//! to keep in the resulting sequence.
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//!
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//!
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//! Example
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//! -------
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//! @include example/take_back.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto take_back = [](auto&& xs, auto const& n) {
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return tag-dispatched;
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};
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#else
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template <typename S, typename = void>
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struct take_back_impl : take_back_impl<S, when<true>> { };
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struct take_back_t {
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template <typename Xs, typename N>
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constexpr auto operator()(Xs&& xs, N const& n) const;
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};
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BOOST_HANA_INLINE_VARIABLE constexpr take_back_t take_back{};
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#endif
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//! Equivalent to `take_back`; provided for convenience.
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//! @ingroup group-Sequence
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//!
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//!
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//! Example
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//! -------
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//! @include example/take_back_c.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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template <std::size_t n>
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constexpr auto take_back_c = [](auto&& xs) {
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return hana::take_back(forwarded(xs), hana::size_c<n>);
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};
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#else
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template <std::size_t n>
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struct take_back_c_t;
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template <std::size_t n>
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BOOST_HANA_INLINE_VARIABLE constexpr take_back_c_t<n> take_back_c{};
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#endif
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}} // end namespace boost::hana
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#endif // !BOOST_HANA_FWD_TAKE_BACK_HPP
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