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::front`.
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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_FRONT_HPP
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#define BOOST_HANA_FWD_FRONT_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 first element of a non-empty iterable.
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//! @ingroup group-Iterable
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//!
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//! Given a non-empty Iterable `xs` with a linearization of `[x1, ..., xN]`,
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//! `front(xs)` is equal to `x1`. If `xs` is empty, it is an error to
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//! use this function. Equivalently, `front(xs)` must be equivalent to
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//! `at_c<0>(xs)`, and that regardless of the value category of `xs`
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//! (`front` must respect the reference semantics 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/front.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto front = [](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 front_impl : front_impl<It, when<true>> { };
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struct front_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 front_t front{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_FRONT_HPP
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