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Diffstat (limited to 'boost/hana/fwd/drop_front_exactly.hpp')
-rw-r--r-- | boost/hana/fwd/drop_front_exactly.hpp | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/boost/hana/fwd/drop_front_exactly.hpp b/boost/hana/fwd/drop_front_exactly.hpp new file mode 100644 index 0000000000..3cb11d76a7 --- /dev/null +++ b/boost/hana/fwd/drop_front_exactly.hpp @@ -0,0 +1,67 @@ +/*! +@file +Forward declares `boost::hana::drop_front_exactly`. + +@copyright Louis Dionne 2013-2016 +Distributed under the Boost Software License, Version 1.0. +(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) + */ + +#ifndef BOOST_HANA_FWD_DROP_FRONT_EXACTLY_HPP +#define BOOST_HANA_FWD_DROP_FRONT_EXACTLY_HPP + +#include <boost/hana/config.hpp> +#include <boost/hana/core/when.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! Drop the first `n` elements of an iterable, and return the rest. + //! @ingroup group-Iterable + //! + //! Given an `Iterable` `xs` with a linearization of `[x1, x2, ...]` and + //! a non-negative `IntegralConstant` `n`, `drop_front_exactly(xs, n)` is + //! an iterable with the same tag as `xs` whose linearization is + //! `[xn+1, xn+2, ...]`. In particular, note that this function does not + //! mutate the original iterable in any way. If `n` is not given, it + //! defaults to an `IntegralConstant` with a value equal to `1`. + //! + //! It is an error to use `drop_front_exactly` with `n > length(xs)`. This + //! additional guarantee allows `drop_front_exactly` to be better optimized + //! than the `drop_front` function, which allows `n > length(xs)`. + //! + //! + //! @param xs + //! The iterable from which elements are dropped. + //! + //! @param n + //! A non-negative `IntegralConstant` representing the number of elements + //! to be dropped from the iterable. In addition to being non-negative, + //! `n` must be less than or equal to the number of elements in `xs`. + //! If `n` is not given, it defaults to an `IntegralConstant` with a value + //! equal to `1`. + //! + //! + //! Example + //! ------- + //! @include example/drop_front_exactly.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + constexpr auto drop_front_exactly = [](auto&& xs[, auto const& n]) { + return tag-dispatched; + }; +#else + template <typename It, typename = void> + struct drop_front_exactly_impl : drop_front_exactly_impl<It, when<true>> { }; + + struct drop_front_exactly_t { + template <typename Xs, typename N> + constexpr auto operator()(Xs&& xs, N const& n) const; + + template <typename Xs> + constexpr auto operator()(Xs&& xs) const; + }; + + constexpr drop_front_exactly_t drop_front_exactly{}; +#endif +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_DROP_FRONT_EXACTLY_HPP |