ceph/src/boost/boost/hana/fwd/second.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::second`.
   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_FWD_SECOND_HPP
  11 #define BOOST_HANA_FWD_SECOND_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 namespace boost { namespace hana {
  18     //! Returns the second element of a pair.
  19     //! @ingroup group-Product
  20     //!
  21     //! Note that if the `Product` actually stores the elements it contains,
  22     //! `hana::second` is required to return a lvalue reference, a lvalue
  23     //! reference to const or a rvalue reference to the second element, where
  24     //! the type of reference must match that of the pair passed to `second`.
  25     //! If the `Product` does not store the elements it contains (i.e. it
  26     //! generates them on demand), this requirement is dropped.
  27     //!
  28     //! Example
  29     //! -------
  30     //! @include example/second.cpp
  31 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  32     constexpr auto second = [](auto&& product) -> decltype(auto) {
  33         return tag-dispatched;
  34     };
  35 #else
  36     template <typename P, typename = void>
  37     struct second_impl : second_impl<P, when<true>> { };
  38
  39     struct second_t {
  40         template <typename Pair>
  41         constexpr decltype(auto) operator()(Pair&& pair) const;
  42     };
  43
  44     BOOST_HANA_INLINE_VARIABLE constexpr second_t second{};
  45 #endif
  46 }} // end namespace boost::hana
  47
  48 #endif // !BOOST_HANA_FWD_SECOND_HPP