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58 lines
1.5 KiB
C++
58 lines
1.5 KiB
C++
/*!
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@file
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Defines `boost::hana::length`.
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@copyright Louis Dionne 2013-2017
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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_LENGTH_HPP
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#define BOOST_HANA_LENGTH_HPP
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#include <boost/hana/fwd/length.hpp>
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#include <boost/hana/concept/foldable.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/integral_constant.hpp>
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#include <boost/hana/unpack.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 auto length_t::operator()(Xs const& xs) const {
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using S = typename hana::tag_of<Xs>::type;
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using Length = BOOST_HANA_DISPATCH_IF(length_impl<S>,
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hana::Foldable<S>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Foldable<S>::value,
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"hana::length(xs) requires 'xs' to be Foldable");
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#endif
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return Length::apply(xs);
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}
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//! @endcond
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namespace detail {
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struct argn {
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template <typename ...Xs>
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constexpr hana::size_t<sizeof...(Xs)> operator()(Xs const& ...) const
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{ return {}; }
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};
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}
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template <typename T, bool condition>
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struct length_impl<T, when<condition>> : default_ {
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template <typename Xs>
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static constexpr auto apply(Xs const& xs) {
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return hana::unpack(xs, detail::argn{});
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_LENGTH_HPP
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