64 lines
2.0 KiB
C++
64 lines
2.0 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::drop_back`.
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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_FWD_DROP_BACK_HPP
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#define BOOST_HANA_FWD_DROP_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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BOOST_HANA_NAMESPACE_BEGIN
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//! Drop the last `n` elements of a finite sequence, and return the rest.
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//! @ingroup group-Sequence
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//!
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//! Given a finite `Sequence` `xs` with a linearization of `[x1, ..., xm]`
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//! and a non-negative `IntegralConstant` `n`, `drop_back(xs, n)` is a
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//! sequence with the same tag as `xs` whose linearization is
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//! `[x1, ..., xm-n]`. If `n` is not given, it defaults to an
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//! `IntegralConstant` with a value equal to `1`.
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//!
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//! In case `length(xs) <= n`, `drop_back` will simply drop the whole
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//! sequence without failing, thus returning an empty sequence.
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//!
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//!
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//! @param xs
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//! The sequence from which elements are dropped.
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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 be dropped from the end of the sequence. If `n` is not given, it
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//! defaults to an `IntegralConstant` with a value equal to `1`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/drop_back.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto drop_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 drop_back_impl : drop_back_impl<S, when<true>> { };
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struct drop_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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template <typename Xs>
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constexpr auto operator()(Xs&& xs) const;
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};
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constexpr drop_back_t drop_back{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_DROP_BACK_HPP
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