1 /*! 2 @file 3 Defines `boost::hana::first`. 4 5 @copyright Louis Dionne 2013-2017 6 Distributed under the Boost Software License, Version 1.0. 7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) 8 */ 9 10 #ifndef BOOST_HANA_FIRST_HPP 11 #define BOOST_HANA_FIRST_HPP 12 13 #include <boost/hana/fwd/first.hpp> 14 15 #include <boost/hana/concept/product.hpp> 16 #include <boost/hana/config.hpp> 17 #include <boost/hana/core/dispatch.hpp> 18 19 20 BOOST_HANA_NAMESPACE_BEGIN 21 //! @cond 22 template <typename Pair> operator ()(Pair && pair) const23 constexpr decltype(auto) first_t::operator()(Pair&& pair) const { 24 using P = typename hana::tag_of<Pair>::type; 25 using First = BOOST_HANA_DISPATCH_IF(first_impl<P>, 26 hana::Product<P>::value 27 ); 28 29 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS 30 static_assert(hana::Product<P>::value, 31 "hana::first(pair) requires 'pair' to be a Product"); 32 #endif 33 34 return First::apply(static_cast<Pair&&>(pair)); 35 } 36 //! @endcond 37 38 template <typename P, bool condition> 39 struct first_impl<P, when<condition>> : default_ { 40 template <typename ...Args> 41 static constexpr auto apply(Args&& ...) = delete; 42 }; 43 BOOST_HANA_NAMESPACE_END 44 45 #endif // !BOOST_HANA_FIRST_HPP 46