598bfd3f
 Hu Chunming
提交_GLIBCXX_USE_CX...
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/*!
@file
Defines `boost::hana::length`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_LENGTH_HPP
#define BOOST_HANA_LENGTH_HPP
#include <boost/hana/fwd/length.hpp>
#include <boost/hana/concept/foldable.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/unpack.hpp>
namespace boost { namespace hana {
//! @cond
template <typename Xs>
constexpr auto length_t::operator()(Xs const& xs) const {
using S = typename hana::tag_of<Xs>::type;
using Length = BOOST_HANA_DISPATCH_IF(length_impl<S>,
hana::Foldable<S>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Foldable<S>::value,
"hana::length(xs) requires 'xs' to be Foldable");
#endif
return Length::apply(xs);
}
//! @endcond
namespace detail {
struct argn {
template <typename ...Xs>
constexpr hana::size_t<sizeof...(Xs)> operator()(Xs const& ...) const
{ return {}; }
};
}
template <typename T, bool condition>
struct length_impl<T, when<condition>> : default_ {
template <typename Xs>
static constexpr auto apply(Xs const& xs) {
return hana::unpack(xs, detail::argn{});
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_LENGTH_HPP
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