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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::always`. | |
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_FUNCTIONAL_ALWAYS_HPP | |
11 | #define BOOST_HANA_FUNCTIONAL_ALWAYS_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/detail/create.hpp> | |
15 | ||
16 | #include <utility> | |
17 | ||
18 | ||
19 | BOOST_HANA_NAMESPACE_BEGIN | |
20 | //! @ingroup group-functional | |
21 | //! Return a constant function returning `x` regardless of the | |
22 | //! argument(s) it is invoked with. | |
23 | //! | |
24 | //! Specifically, `always(x)` is a function such that | |
25 | //! @code | |
26 | //! always(x)(y...) == x | |
27 | //! @endcode | |
28 | //! for any `y...`. A copy of `x` is made and it is owned by the | |
29 | //! `always(x)` function. When `always(x)` is called, it will return | |
30 | //! a reference to the `x` it owns. This reference is valid as long | |
31 | //! as `always(x)` is in scope. | |
32 | //! | |
33 | //! | |
34 | //! ### Example | |
35 | //! @include example/functional/always.cpp | |
36 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
37 | constexpr auto always = [](auto&& x) { | |
38 | return [perfect-capture](auto const& ...y) -> decltype(auto) { | |
39 | return forwarded(x); | |
40 | }; | |
41 | }; | |
42 | #else | |
43 | template <typename T> | |
44 | struct _always { | |
45 | T val_; | |
46 | ||
47 | template <typename ...Args> | |
48 | constexpr T const& operator()(Args const& ...) const& | |
49 | { return val_; } | |
50 | ||
51 | template <typename ...Args> | |
52 | constexpr T& operator()(Args const& ...) & | |
53 | { return val_; } | |
54 | ||
55 | template <typename ...Args> | |
56 | constexpr T operator()(Args const& ...) && | |
57 | { return std::move(val_); } | |
58 | }; | |
59 | ||
60 | constexpr detail::create<_always> always{}; | |
61 | #endif | |
62 | BOOST_HANA_NAMESPACE_END | |
63 | ||
64 | #endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP |