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