3 Defines `boost::hana::first`.
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)
10 #ifndef BOOST_HANA_FIRST_HPP
11 #define BOOST_HANA_FIRST_HPP
13 #include <boost/hana/fwd/first.hpp>
15 #include <boost/hana/concept/product.hpp>
16 #include <boost/hana/config.hpp>
17 #include <boost/hana/core/dispatch.hpp>
20 BOOST_HANA_NAMESPACE_BEGIN
22 template <typename Pair>
23 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
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");
34 return First::apply(static_cast<Pair&&>(pair));
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;
43 BOOST_HANA_NAMESPACE_END
45 #endif // !BOOST_HANA_FIRST_HPP