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46 lines
1.3 KiB
C++
46 lines
1.3 KiB
C++
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/*!
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@file
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Defines `boost::hana::first`.
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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_FIRST_HPP
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#define BOOST_HANA_FIRST_HPP
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#include <boost/hana/fwd/first.hpp>
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#include <boost/hana/concept/product.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 Pair>
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constexpr decltype(auto) first_t::operator()(Pair&& pair) const {
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using P = typename hana::tag_of<Pair>::type;
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using First = BOOST_HANA_DISPATCH_IF(first_impl<P>,
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hana::Product<P>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Product<P>::value,
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"hana::first(pair) requires 'pair' to be a Product");
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#endif
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return First::apply(static_cast<Pair&&>(pair));
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}
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//! @endcond
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template <typename P, bool condition>
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struct first_impl<P, when<condition>> : default_ {
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template <typename ...Args>
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static constexpr auto apply(Args&& ...) = delete;
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FIRST_HPP
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