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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::zero`.
   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_ZERO_HPP
  11 #define BOOST_HANA_FWD_ZERO_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Identity of `plus`.
  19     //! @ingroup group-Monoid
  20     //!
  21     //! @tparam M
  22     //! The tag (a `Monoid`) of the returned identity.
  23     //!
  24     //!
  25     //! Example
  26     //! -------
  27     //! @include example/zero.cpp
  28 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  29     template <typename M>
  30     constexpr auto zero = []() -> decltype(auto) {
  31         return tag-dispatched;
  32     };
  33 #else
  34     template <typename M, typename = void>
  35     struct zero_impl : zero_impl<M, when<true>> { };
  36
  37     template <typename M>
  38     struct zero_t;
  39
  40     template <typename M>
  41     constexpr zero_t<M> zero{};
  42 #endif
  43 BOOST_HANA_NAMESPACE_END
  44
  45 #endif // !BOOST_HANA_FWD_ZERO_HPP