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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::drop_back`.
   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_DROP_BACK_HPP
  11 #define BOOST_HANA_FWD_DROP_BACK_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Drop the last `n` elements of a finite sequence, and return the rest.
  19     //! @ingroup group-Sequence
  20     //!
  21     //! Given a finite `Sequence` `xs` with a linearization of `[x1, ..., xm]`
  22     //! and a non-negative `IntegralConstant` `n`, `drop_back(xs, n)` is a
  23     //! sequence with the same tag as `xs` whose linearization is
  24     //! `[x1, ..., xm-n]`. If `n` is not given, it defaults to an
  25     //! `IntegralConstant` with a value equal to `1`.
  26     //!
  27     //! In case `length(xs) <= n`, `drop_back` will simply drop the whole
  28     //! sequence without failing, thus returning an empty sequence.
  29     //!
  30     //!
  31     //! @param xs
  32     //! The sequence from which elements are dropped.
  33     //!
  34     //! @param n
  35     //! A non-negative `IntegralConstant` representing the number of elements
  36     //! to be dropped from the end of the sequence. If `n` is not given, it
  37     //! defaults to an `IntegralConstant` with a value equal to `1`.
  38     //!
  39     //!
  40     //! Example
  41     //! -------
  42     //! @include example/drop_back.cpp
  43 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  44     constexpr auto drop_back = [](auto&& xs[, auto const& n]) {
  45         return tag-dispatched;
  46     };
  47 #else
  48     template <typename S, typename = void>
  49     struct drop_back_impl : drop_back_impl<S, when<true>> { };
  50
  51     struct drop_back_t {
  52         template <typename Xs, typename N>
  53         constexpr auto operator()(Xs&& xs, N const& n) const;
  54
  55         template <typename Xs>
  56         constexpr auto operator()(Xs&& xs) const;
  57     };
  58
  59     constexpr drop_back_t drop_back{};
  60 #endif
  61 BOOST_HANA_NAMESPACE_END
  62
  63 #endif // !BOOST_HANA_FWD_DROP_BACK_HPP