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

   1 /*!
   2 @file
   3 Defines `boost::hana::front`.
   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_FRONT_HPP
  11 #define BOOST_HANA_FRONT_HPP
  12
  13 #include <boost/hana/fwd/front.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concept/iterable.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19
  20
  21 BOOST_HANA_NAMESPACE_BEGIN
  22     //! @cond
  23     template <typename Xs>
  24     constexpr decltype(auto) front_t::operator()(Xs&& xs) const {
  25         using It = typename hana::tag_of<Xs>::type;
  26         using Front = BOOST_HANA_DISPATCH_IF(front_impl<It>,
  27             hana::Iterable<It>::value
  28         );
  29
  30     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  31         static_assert(hana::Iterable<It>::value,
  32         "hana::front(xs) requires 'xs' to be an Iterable");
  33     #endif
  34
  35         return Front::apply(static_cast<Xs&&>(xs));
  36     }
  37     //! @endcond
  38
  39     template <typename It, bool condition>
  40     struct front_impl<It, when<condition>> : default_ {
  41         template <typename Xs>
  42         static constexpr decltype(auto) apply(Xs&& xs)
  43         { return hana::at_c<0>(static_cast<Xs&&>(xs)); }
  44     };
  45 BOOST_HANA_NAMESPACE_END
  46
  47 #endif // !BOOST_HANA_FRONT_HPP