48 lines
1.3 KiB
C++
48 lines
1.3 KiB
C++
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/*!
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@file
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Defines `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_FRONT_HPP
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#define BOOST_HANA_FRONT_HPP
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#include <boost/hana/fwd/front.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/concept/iterable.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Xs>
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constexpr decltype(auto) front_t::operator()(Xs&& xs) const {
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using It = typename hana::tag_of<Xs>::type;
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using Front = BOOST_HANA_DISPATCH_IF(front_impl<It>,
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hana::Iterable<It>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Iterable<It>::value,
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"hana::front(xs) requires 'xs' to be an Iterable");
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#endif
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return Front::apply(static_cast<Xs&&>(xs));
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}
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//! @endcond
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template <typename It, bool condition>
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struct front_impl<It, when<condition>> : default_ {
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template <typename Xs>
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static constexpr decltype(auto) apply(Xs&& xs)
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{ return hana::at_c<0>(static_cast<Xs&&>(xs)); }
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FRONT_HPP
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