boost/hana/second.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::second`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_SECOND_HPP
0011 #define BOOST_HANA_SECOND_HPP
0012
0013 #include <boost/hana/fwd/second.hpp>
0014
0015 #include <boost/hana/concept/product.hpp>
0016 #include <boost/hana/config.hpp>
0017 #include <boost/hana/core/dispatch.hpp>
0018
0019
0020 namespace boost { namespace hana {
0021     //! @cond
0022     template <typename Pair>
0023     constexpr decltype(auto) second_t::operator()(Pair&& pair) const {
0024         using P = typename hana::tag_of<Pair>::type;
0025         using Second = BOOST_HANA_DISPATCH_IF(second_impl<P>,
0026             hana::Product<P>::value
0027         );
0028
0029     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0030         static_assert(hana::Product<P>::value,
0031         "hana::second(pair) requires 'pair' to be a Product");
0032     #endif
0033
0034         return Second::apply(static_cast<Pair&&>(pair));
0035     }
0036     //! @endcond
0037
0038     template <typename P, bool condition>
0039     struct second_impl<P, when<condition>> : default_ {
0040         template <typename ...Args>
0041         static constexpr auto apply(Args&& ...) = delete;
0042     };
0043 }} // end namespace boost::hana
0044
0045 #endif // !BOOST_HANA_SECOND_HPP