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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 |