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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::drop_front_exactly`.
   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_FRONT_EXACTLY_HPP
  11 #define BOOST_HANA_FWD_DROP_FRONT_EXACTLY_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 first `n` elements of an iterable, and return the rest.
  19     //! @ingroup group-Iterable
  20     //!
  21     //! Given an `Iterable` `xs` with a linearization of `[x1, x2, ...]` and
  22     //! a non-negative `IntegralConstant` `n`, `drop_front_exactly(xs, n)` is
  23     //! an iterable with the same tag as `xs` whose linearization is
  24     //! `[xn+1, xn+2, ...]`. In particular, note that this function does not
  25     //! mutate the original iterable in any way. If `n` is not given, it
  26     //! defaults to an `IntegralConstant` with a value equal to `1`.
  27     //!
  28     //! It is an error to use `drop_front_exactly` with `n > length(xs)`. This
  29     //! additional guarantee allows `drop_front_exactly` to be better optimized
  30     //! than the `drop_front` function, which allows `n > length(xs)`.
  31     //!
  32     //!
  33     //! @param xs
  34     //! The iterable from which elements are dropped.
  35     //!
  36     //! @param n
  37     //! A non-negative `IntegralConstant` representing the number of elements
  38     //! to be dropped from the iterable. In addition to being non-negative,
  39     //! `n` must be less than or equal to the number of elements in `xs`.
  40     //! If `n` is not given, it defaults to an `IntegralConstant` with a value
  41     //! equal to `1`.
  42     //!
  43     //!
  44     //! Example
  45     //! -------
  46     //! @include example/drop_front_exactly.cpp
  47 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  48     constexpr auto drop_front_exactly = [](auto&& xs[, auto const& n]) {
  49         return tag-dispatched;
  50     };
  51 #else
  52     template <typename It, typename = void>
  53     struct drop_front_exactly_impl : drop_front_exactly_impl<It, when<true>> { };
  54
  55     struct drop_front_exactly_t {
  56         template <typename Xs, typename N>
  57         constexpr auto operator()(Xs&& xs, N const& n) const;
  58
  59         template <typename Xs>
  60         constexpr auto operator()(Xs&& xs) const;
  61     };
  62
  63     constexpr drop_front_exactly_t drop_front_exactly{};
  64 #endif
  65 BOOST_HANA_NAMESPACE_END
  66
  67 #endif // !BOOST_HANA_FWD_DROP_FRONT_EXACTLY_HPP