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49 lines
1.4 KiB
C++
49 lines
1.4 KiB
C++
/*!
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@file
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Forward declares `boost::hana::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_BACK_HPP
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#define BOOST_HANA_FWD_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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//! Returns the last element of a non-empty and finite iterable.
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//! @ingroup group-Iterable
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//!
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//! Given a non-empty and finite iterable `xs` with a linearization of
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//! `[x1, ..., xN]`, `back(xs)` is equal to `xN`. Equivalently, `back(xs)`
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//! must be equivalent to `at_c<N-1>(xs)`, and that regardless of the
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//! value category of `xs` (`back` must respect the reference semantics
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//! of `at`).
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//!
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//!
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//! Example
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//! -------
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//! @include example/back.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto back = [](auto&& xs) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename It, typename = void>
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struct back_impl : back_impl<It, when<true>> { };
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struct back_t {
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template <typename Xs>
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constexpr decltype(auto) operator()(Xs&& xs) const;
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};
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constexpr back_t back{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_BACK_HPP
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