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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::permutations`.
   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_PERMUTATIONS_HPP
  11 #define BOOST_HANA_FWD_PERMUTATIONS_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Return a sequence of all the permutations of the given sequence.
  19     //! @ingroup group-Sequence
  20     //!
  21     //! Specifically, `permutations(xs)` is a sequence whose elements are
  22     //! permutations of the original sequence `xs`. The permutations are not
  23     //! guaranteed to be in any specific order. Also note that the number
  24     //! of permutations grows very rapidly as the length of the original
  25     //! sequence increases. The growth rate is `O(length(xs)!)`; with a
  26     //! sequence `xs` of length only 8, `permutations(xs)` contains over
  27     //! 40 000 elements!
  28     //!
  29     //!
  30     //! Example
  31     //! -------
  32     //! @include example/permutations.cpp
  33 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  34     constexpr auto permutations = [](auto&& xs) {
  35         return tag-dispatched;
  36     };
  37 #else
  38     template <typename S, typename = void>
  39     struct permutations_impl : permutations_impl<S, when<true>> { };
  40
  41     struct permutations_t {
  42         template <typename Xs>
  43         constexpr auto operator()(Xs&& xs) const;
  44     };
  45
  46     constexpr permutations_t permutations{};
  47 #endif
  48 BOOST_HANA_NAMESPACE_END
  49
  50 #endif // !BOOST_HANA_FWD_PERMUTATIONS_HPP