ceph/src/boost/libs/hana/include/boost/hana/detail/nested_to_fwd.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::detail::nested_to`.
   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_DETAIL_NESTED_TO_FWD_HPP
  11 #define BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP
  12
  13 #include <boost/hana/config.hpp>
  14
  15
  16 BOOST_HANA_NAMESPACE_BEGIN namespace detail {
  17     template <typename Algorithm>
  18     struct nested_to_t {
  19         template <typename X>
  20         constexpr decltype(auto) operator()(X&& x) const;
  21     };
  22
  23     //! @ingroup group-details
  24     //! Provides a `.to` static constexpr function object.
  25     //!
  26     //! When creating a binary function object of type `Algo` whose signature
  27     //! is `Object x Object -> Return`, `nested_to<Algo>` can be used as a base
  28     //! class of `Algo`. Doing so will provide a static constexpr member called
  29     //! `to`, which has the following signature:
  30     //! @code
  31     //!     Object -> Object -> Return
  32     //! @endcode
  33     //!
  34     //! Note that the function object `Algo` must be default-constructible,
  35     //! since the algorithm will be called as `Algo{}(arguments...)`.
  36     //!
  37     //! @note
  38     //! This function object is especially useful because it takes care of
  39     //! avoiding ODR violations caused by the nested static constexpr member.
  40     template <typename Algorithm>
  41     struct nested_to { static constexpr nested_to_t<Algorithm> to{}; };
  42
  43     template <typename Algorithm>
  44     constexpr nested_to_t<Algorithm> nested_to<Algorithm>::to;
  45 } BOOST_HANA_NAMESPACE_END
  46
  47 #endif // !BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP