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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::at` and `boost::hana::at_c`. | |
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_AT_HPP | |
11 | #define BOOST_HANA_FWD_AT_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | #include <cstddef> | |
17 | ||
18 | ||
19 | BOOST_HANA_NAMESPACE_BEGIN | |
20 | //! Returns the `n`th element of an iterable. | |
21 | //! @ingroup group-Iterable | |
22 | //! | |
23 | //! Given an `Iterable` and an `IntegralConstant` index, `at` returns the | |
24 | //! element located at the index in the linearization of the iterable. | |
25 | //! Specifically, given an iterable `xs` with a linearization of | |
26 | //! `[x1, ..., xN]`, `at(xs, k)` is equivalent to `xk`. | |
27 | //! | |
28 | //! If the `Iterable` actually stores the elements it contains, `at` is | |
29 | //! required to return a lvalue reference, a lvalue reference to const | |
30 | //! or a rvalue reference to the matching element, where the type of | |
31 | //! reference must match that of the iterable passed to `at`. If the | |
32 | //! `Iterable` does not store the elements it contains (i.e. it generates | |
33 | //! them on demand), this requirement is dropped. | |
34 | //! | |
35 | //! | |
36 | //! @param xs | |
37 | //! The iterable in which an element is retrieved. The iterable must | |
38 | //! contain at least `n + 1` elements. | |
39 | //! | |
40 | //! @param n | |
41 | //! A non-negative `IntegralConstant` representing the 0-based index of | |
42 | //! the element to return. It is an error to call `at` with an index that | |
43 | //! out of bounds of the iterable. | |
44 | //! | |
45 | //! | |
46 | //! Example | |
47 | //! ------- | |
48 | //! @include example/at.cpp | |
49 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
50 | constexpr auto at = [](auto&& xs, auto const& n) -> decltype(auto) { | |
51 | return tag-dispatched; | |
52 | }; | |
53 | #else | |
54 | template <typename It, typename = void> | |
55 | struct at_impl : at_impl<It, when<true>> { }; | |
56 | ||
57 | struct at_t { | |
58 | template <typename Xs, typename N> | |
59 | constexpr decltype(auto) operator()(Xs&& xs, N const& n) const; | |
60 | }; | |
61 | ||
62 | constexpr at_t at{}; | |
63 | #endif | |
64 | ||
65 | //! Equivalent to `at`; provided for convenience. | |
66 | //! @ingroup group-Iterable | |
67 | //! | |
68 | //! | |
69 | //! @note | |
70 | //! `hana::at_c<n>` is an overloaded function, not a function object. | |
71 | //! Hence, it can't be passed to higher-order algorithms. This is done | |
72 | //! for compile-time performance reasons. | |
73 | //! | |
74 | //! | |
75 | //! Example | |
76 | //! ------- | |
77 | //! @include example/at_c.cpp | |
78 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
79 | template <std::size_t n> | |
80 | constexpr auto at_c = [](auto&& xs) { | |
81 | return hana::at(forwarded(xs), hana::size_c<n>); | |
82 | }; | |
83 | #else | |
84 | template <std::size_t n, typename Xs> | |
85 | constexpr decltype(auto) at_c(Xs&& xs); | |
86 | #endif | |
87 | BOOST_HANA_NAMESPACE_END | |
88 | ||
89 | #endif // !BOOST_HANA_FWD_AT_HPP |