ceph/src/boost/libs/hana/include/boost/hana/first.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::first`.
   4
   5 @copyright Louis Dionne 2013-2016
   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>
  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
  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