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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::overload`. | |
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_OVERLOAD_HPP | |
11 | #define BOOST_HANA_FUNCTIONAL_OVERLOAD_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/detail/decay.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! @ingroup group-functional | |
19 | //! Pick one of several functions to call based on overload resolution. | |
20 | //! | |
21 | //! Specifically, `overload(f1, f2, ..., fn)` is a function object such | |
22 | //! that | |
23 | //! @code | |
24 | //! overload(f1, f2, ..., fn)(x...) == fk(x...) | |
25 | //! @endcode | |
26 | //! | |
27 | //! where `fk` is the function of `f1, ..., fn` that would be called if | |
28 | //! overload resolution was performed amongst that set of functions only. | |
29 | //! If more than one function `fk` would be picked by overload resolution, | |
30 | //! then the call is ambiguous. | |
31 | //! | |
32 | //! ### Example | |
33 | //! @include example/functional/overload.cpp | |
34 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
35 | constexpr auto overload = [](auto&& f1, auto&& f2, ..., auto&& fn) { | |
36 | return [perfect-capture](auto&& ...x) -> decltype(auto) { | |
37 | return forwarded(fk)(forwarded(x)...); | |
38 | }; | |
39 | }; | |
40 | #else | |
41 | template <typename F, typename ...G> | |
42 | struct overload_t | |
43 | : overload_t<F>::type | |
44 | , overload_t<G...>::type | |
45 | { | |
46 | using type = overload_t; | |
47 | using overload_t<F>::type::operator(); | |
48 | using overload_t<G...>::type::operator(); | |
49 | ||
50 | template <typename F_, typename ...G_> | |
51 | constexpr explicit overload_t(F_&& f, G_&& ...g) | |
52 | : overload_t<F>::type(static_cast<F_&&>(f)) | |
53 | , overload_t<G...>::type(static_cast<G_&&>(g)...) | |
54 | { } | |
55 | }; | |
56 | ||
57 | template <typename F> | |
58 | struct overload_t<F> { using type = F; }; | |
59 | ||
60 | template <typename R, typename ...Args> | |
61 | struct overload_t<R(*)(Args...)> { | |
62 | using type = overload_t; | |
63 | R (*fptr_)(Args...); | |
64 | ||
65 | explicit constexpr overload_t(R (*fp)(Args...)) | |
66 | : fptr_(fp) | |
67 | { } | |
68 | ||
69 | constexpr R operator()(Args ...args) const | |
70 | { return fptr_(static_cast<Args&&>(args)...); } | |
71 | }; | |
72 | ||
73 | struct make_overload_t { | |
74 | template <typename ...F, | |
75 | typename Overload = typename overload_t< | |
76 | typename detail::decay<F>::type... | |
77 | >::type | |
78 | > | |
79 | constexpr Overload operator()(F&& ...f) const { | |
80 | return Overload(static_cast<F&&>(f)...); | |
81 | } | |
82 | }; | |
83 | ||
84 | constexpr make_overload_t overload{}; | |
85 | #endif | |
86 | BOOST_HANA_NAMESPACE_END | |
87 | ||
88 | #endif // !BOOST_HANA_FUNCTIONAL_OVERLOAD_HPP |