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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::drop_front`. | |
4 | ||
b32b8144 | 5 | @copyright Louis Dionne 2013-2017 |
7c673cae FG |
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_HPP | |
11 | #define BOOST_HANA_FWD_DROP_FRONT_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(xs, n)` is an | |
23 | //! 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 | //! In case `length(xs) <= n`, `drop_front` will simply drop the whole | |
29 | //! iterable without failing, thus returning an empty iterable. This is | |
30 | //! different from `drop_front_exactly`, which expects `n <= length(xs)` | |
31 | //! but can be better optimized because of this additional guarantee. | |
32 | //! | |
33 | //! | |
34 | //! @param xs | |
35 | //! The iterable from which elements are dropped. | |
36 | //! | |
37 | //! @param n | |
38 | //! A non-negative `IntegralConstant` representing the number of elements | |
39 | //! to be dropped from the iterable. If `n` is not given, it defaults to | |
40 | //! an `IntegralConstant` with a value equal to `1`. | |
41 | //! | |
42 | //! | |
43 | //! Example | |
44 | //! ------- | |
45 | //! @include example/drop_front.cpp | |
46 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
47 | constexpr auto drop_front = [](auto&& xs[, auto const& n]) { | |
48 | return tag-dispatched; | |
49 | }; | |
50 | #else | |
51 | template <typename It, typename = void> | |
52 | struct drop_front_impl : drop_front_impl<It, when<true>> { }; | |
53 | ||
54 | struct drop_front_t { | |
55 | template <typename Xs, typename N> | |
56 | constexpr auto operator()(Xs&& xs, N const& n) const; | |
57 | ||
58 | template <typename Xs> | |
59 | constexpr auto operator()(Xs&& xs) const; | |
60 | }; | |
61 | ||
62 | constexpr drop_front_t drop_front{}; | |
63 | #endif | |
64 | BOOST_HANA_NAMESPACE_END | |
65 | ||
66 | #endif // !BOOST_HANA_FWD_DROP_FRONT_HPP |