47 lines
1.2 KiB
C++
47 lines
1.2 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::none`.
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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_NONE_HPP
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#define BOOST_HANA_FWD_NONE_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 whether all of the keys of the structure are false-valued.
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//! @ingroup group-Searchable
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//!
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//! The keys of the structure must be `Logical`s. If the structure is not
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//! finite, a true-valued key must appear at a finite "index" in order
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//! for this method to finish.
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//!
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//!
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//! Example
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//! -------
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//! @include example/none.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto none = [](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 S, typename = void>
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struct none_impl : none_impl<S, when<true>> { };
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struct none_t {
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template <typename Xs>
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constexpr auto operator()(Xs&& xs) const;
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};
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constexpr none_t none{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_NONE_HPP
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