hana/fwd/drop_back.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::drop_back`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_DROP_BACK_HPP
0011 #define BOOST_HANA_FWD_DROP_BACK_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Drop the last `n` elements of a finite sequence, and return the rest.
0019     //! @ingroup group-Sequence
0020     //!
0021     //! Given a finite `Sequence` `xs` with a linearization of `[x1, ..., xm]`
0022     //! and a non-negative `IntegralConstant` `n`, `drop_back(xs, n)` is a
0023     //! sequence with the same tag as `xs` whose linearization is
0024     //! `[x1, ..., xm-n]`. If `n` is not given, it defaults to an
0025     //! `IntegralConstant` with a value equal to `1`.
0026     //!
0027     //! In case `length(xs) <= n`, `drop_back` will simply drop the whole
0028     //! sequence without failing, thus returning an empty sequence.
0029     //!
0030     //!
0031     //! @param xs
0032     //! The sequence from which elements are dropped.
0033     //!
0034     //! @param n
0035     //! A non-negative `IntegralConstant` representing the number of elements
0036     //! to be dropped from the end of the sequence. If `n` is not given, it
0037     //! defaults to an `IntegralConstant` with a value equal to `1`.
0038     //!
0039     //!
0040     //! Example
0041     //! -------
0042     //! @include example/drop_back.cpp
0043 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0044     constexpr auto drop_back = [](auto&& xs[, auto const& n]) {
0045         return tag-dispatched;
0046     };
0047 #else
0048     template <typename S, typename = void>
0049     struct drop_back_impl : drop_back_impl<S, when<true>> { };
0050
0051     struct drop_back_t {
0052         template <typename Xs, typename N>
0053         constexpr auto operator()(Xs&& xs, N const& n) const;
0054
0055         template <typename Xs>
0056         constexpr auto operator()(Xs&& xs) const;
0057     };
0058
0059     BOOST_HANA_INLINE_VARIABLE constexpr drop_back_t drop_back{};
0060 #endif
0061 }} // end namespace boost::hana
0062
0063 #endif // !BOOST_HANA_FWD_DROP_BACK_HPP