50 lines
1.5 KiB
C++
50 lines
1.5 KiB
C++
/*!
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@file
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Forward declares `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_FWD_FIRST_HPP
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#define BOOST_HANA_FWD_FIRST_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 pair.
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//! @ingroup group-Product
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//!
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//! Note that if the `Product` actually stores the elements it contains,
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//! `hana::first` is required to return a lvalue reference, a lvalue
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//! reference to const or a rvalue reference to the first element, where
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//! the type of reference must match that of the pair passed to `first`.
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//! If the `Product` does not store the elements it contains (i.e. it
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//! generates them on demand), this requirement is dropped.
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//!
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//!
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//! Example
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//! -------
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//! @include example/first.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto first = [](auto&& product) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename P, typename = void>
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struct first_impl : first_impl<P, when<true>> { };
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struct first_t {
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template <typename Pair>
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constexpr decltype(auto) operator()(Pair&& pair) const;
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};
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constexpr first_t first{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_FIRST_HPP
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