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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::minus`.
   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_MINUS_HPP
  11 #define BOOST_HANA_FWD_MINUS_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Subtract two elements of a group.
  19     //! @ingroup group-Group
  20     //!
  21     //! Specifically, this performs the `Monoid` operation on the first
  22     //! argument and on the inverse of the second argument, thus being
  23     //! equivalent to:
  24     //! @code
  25     //!     minus(x, y) == plus(x, negate(y))
  26     //! @endcode
  27     //!
  28     //!
  29     //! Cross-type version of the method
  30     //! --------------------------------
  31     //! The `minus` method is "overloaded" to handle distinct data types
  32     //! with certain properties. Specifically, `minus` is defined for
  33     //! _distinct_ data types `A` and `B` such that
  34     //! 1. `A` and `B` share a common data type `C`, as determined by the
  35     //!    `common` metafunction
  36     //! 2. `A`, `B` and `C` are all `Group`s when taken individually
  37     //! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Group`-embeddings, as
  38     //!    determined by the `is_embedding` metafunction.
  39     //!
  40     //! The definition of `minus` for data types satisfying the above
  41     //! properties is obtained by setting
  42     //! @code
  43     //!     minus(x, y) = minus(to<C>(x), to<C>(y))
  44     //! @endcode
  45     //!
  46     //!
  47     //! Example
  48     //! -------
  49     //! @include example/minus.cpp
  50 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  51     constexpr auto minus = [](auto&& x, auto&& y) -> decltype(auto) {
  52         return tag-dispatched;
  53     };
  54 #else
  55     template <typename T, typename U, typename = void>
  56     struct minus_impl : minus_impl<T, U, when<true>> { };
  57
  58     struct minus_t {
  59         template <typename X, typename Y>
  60         constexpr decltype(auto) operator()(X&& x, Y&& y) const;
  61     };
  62
  63     constexpr minus_t minus{};
  64 #endif
  65 BOOST_HANA_NAMESPACE_END
  66
  67 #endif // !BOOST_HANA_FWD_MINUS_HPP