68 lines
2.4 KiB
C++
68 lines
2.4 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::drop_front_exactly`.
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@copyright Louis Dionne 2013-2017
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_DROP_FRONT_EXACTLY_HPP
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#define BOOST_HANA_FWD_DROP_FRONT_EXACTLY_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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namespace boost { namespace hana {
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//! Drop the first `n` elements of an iterable, and return the rest.
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//! @ingroup group-Iterable
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//!
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//! Given an `Iterable` `xs` with a linearization of `[x1, x2, ...]` and
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//! a non-negative `IntegralConstant` `n`, `drop_front_exactly(xs, n)` is
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//! an iterable with the same tag as `xs` whose linearization is
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//! `[xn+1, xn+2, ...]`. In particular, note that this function does not
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//! mutate the original iterable in any way. If `n` is not given, it
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//! defaults to an `IntegralConstant` with a value equal to `1`.
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//!
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//! It is an error to use `drop_front_exactly` with `n > length(xs)`. This
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//! additional guarantee allows `drop_front_exactly` to be better optimized
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//! than the `drop_front` function, which allows `n > length(xs)`.
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//!
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//!
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//! @param xs
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//! The iterable from which elements are dropped.
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//!
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//! @param n
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//! A non-negative `IntegralConstant` representing the number of elements
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//! to be dropped from the iterable. In addition to being non-negative,
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//! `n` must be less than or equal to the number of elements in `xs`.
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//! If `n` is not given, it defaults to an `IntegralConstant` with a value
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//! equal to `1`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/drop_front_exactly.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto drop_front_exactly = [](auto&& xs[, auto const& n]) {
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return tag-dispatched;
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};
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#else
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template <typename It, typename = void>
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struct drop_front_exactly_impl : drop_front_exactly_impl<It, when<true>> { };
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struct drop_front_exactly_t {
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template <typename Xs, typename N>
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constexpr auto operator()(Xs&& xs, N const& n) const;
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template <typename Xs>
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constexpr auto operator()(Xs&& xs) const;
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};
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BOOST_HANA_INLINE_VARIABLE constexpr drop_front_exactly_t drop_front_exactly{};
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#endif
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}} // end namespace boost::hana
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#endif // !BOOST_HANA_FWD_DROP_FRONT_EXACTLY_HPP
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