ceph/src/boost/libs/hana/include/boost/hana/fwd/plus.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::plus`.
   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_FWD_PLUS_HPP
  11 #define BOOST_HANA_FWD_PLUS_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Associative binary operation on a `Monoid`.
  19     //! @ingroup group-Monoid
  20     //!
  21     //! @param x, y
  22     //! Two objects to combine with the `Monoid`'s binary operation.
  23     //!
  24     //!
  25     //! Cross-type version of the method
  26     //! --------------------------------
  27     //! The `plus` method is "overloaded" to handle distinct data types
  28     //! with certain properties. Specifically, `plus` is defined for
  29     //! _distinct_ data types `A` and `B` such that
  30     //! 1. `A` and `B` share a common data type `C`, as determined by the
  31     //!    `common` metafunction
  32     //! 2. `A`, `B` and `C` are all `Monoid`s when taken individually
  33     //! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Monoid`-embeddings, as
  34     //!    determined by the `is_embedding` metafunction.
  35     //!
  36     //! The definition of `plus` for data types satisfying the above
  37     //! properties is obtained by setting
  38     //! @code
  39     //!     plus(x, y) = plus(to<C>(x), to<C>(y))
  40     //! @endcode
  41     //!
  42     //!
  43     //! Example
  44     //! -------
  45     //! @include example/plus.cpp
  46 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  47     constexpr auto plus = [](auto&& x, auto&& y) -> decltype(auto) {
  48         return tag-dispatched;
  49     };
  50 #else
  51     template <typename T, typename U, typename = void>
  52     struct plus_impl : plus_impl<T, U, when<true>> { };
  53
  54     struct plus_t {
  55         template <typename X, typename Y>
  56         constexpr decltype(auto) operator()(X&& x, Y&& y) const;
  57     };
  58
  59     constexpr plus_t plus{};
  60 #endif
  61 BOOST_HANA_NAMESPACE_END
  62
  63 #endif // !BOOST_HANA_FWD_PLUS_HPP