598bfd3f
 Hu Chunming
提交_GLIBCXX_USE_CX...
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/*!
@file
Defines `boost::hana::front`.
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_FRONT_HPP
#define BOOST_HANA_FRONT_HPP
#include <boost/hana/fwd/front.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/concept/iterable.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
namespace boost { namespace hana {
//! @cond
template <typename Xs>
constexpr decltype(auto) front_t::operator()(Xs&& xs) const {
using It = typename hana::tag_of<Xs>::type;
using Front = BOOST_HANA_DISPATCH_IF(front_impl<It>,
hana::Iterable<It>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Iterable<It>::value,
"hana::front(xs) requires 'xs' to be an Iterable");
#endif
return Front::apply(static_cast<Xs&&>(xs));
}
//! @endcond
template <typename It, bool condition>
struct front_impl<It, when<condition>> : default_ {
template <typename Xs>
static constexpr decltype(auto) apply(Xs&& xs)
{ return hana::at_c<0>(static_cast<Xs&&>(xs)); }
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_FRONT_HPP
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