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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::comparing`.
   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_COMPARING_HPP
  11 #define BOOST_HANA_FWD_COMPARING_HPP
  12
  13 #include <boost/hana/config.hpp>
  14
  15
  16 namespace boost { namespace hana {
  17     //! Returns a function performing `equal` after applying a transformation
  18     //! to both arguments.
  19     //! @ingroup group-Comparable
  20     //!
  21     //! `comparing` creates an equivalence relation based on the result of
  22     //! applying a function to some objects, which is especially useful in
  23     //! conjunction with algorithms that accept a custom predicate that must
  24     //! represent an equivalence relation.
  25     //!
  26     //! Specifically, `comparing` is such that
  27     //! @code
  28     //!     comparing(f) == equal ^on^ f
  29     //! @endcode
  30     //! or, equivalently,
  31     //! @code
  32     //!     comparing(f)(x, y) == equal(f(x), f(y))
  33     //! @endcode
  34     //!
  35     //! @note
  36     //! This is not a tag-dispatched method (hence it can't be customized),
  37     //! but just a convenience function provided with the `Comparable` concept.
  38     //!
  39     //!
  40     //! Signature
  41     //! ---------
  42     //! Given a Logical `Bool` and a Comparable `B`, the signature is
  43     //! @f$ \mathtt{comparing} : (A \to B) \to (A \times A \to Bool) @f$.
  44     //!
  45     //!
  46     //! Example
  47     //! -------
  48     //! @include example/comparing.cpp
  49 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  50     constexpr auto comparing = [](auto&& f) {
  51         return [perfect-capture](auto&& x, auto&& y) {
  52             return equal(f(forwarded(x)), f(forwarded(y)));
  53         };
  54     };
  55 #else
  56     struct comparing_t {
  57         template <typename F>
  58         constexpr auto operator()(F&& f) const;
  59     };
  60
  61     BOOST_HANA_INLINE_VARIABLE constexpr comparing_t comparing{};
  62 #endif
  63 }} // end namespace boost::hana
  64
  65 #endif // !BOOST_HANA_FWD_COMPARING_HPP