1/*
2 * Copyright 2017-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#pragma once
18
19#include <type_traits>
20#include <utility>
21
22/**
23 * folly implementation of `std::overload` like functionality
24 *
25 * Example:
26 * struct One {};
27 * struct Two {};
28 * boost::variant<One, Two> value;
29 *
30 * variant_match(value,
31 * [] (const One& one) { ... },
32 * [] (const Two& two) { ... });
33 */
34
35namespace folly {
36
37namespace detail {
38template <typename...>
39struct Overload;
40
41template <typename Case, typename... Cases>
42struct Overload<Case, Cases...> : Overload<Cases...>, Case {
43 Overload(Case c, Cases... cs)
44 : Overload<Cases...>(std::move(cs)...), Case(std::move(c)) {}
45
46 using Case::operator();
47 using Overload<Cases...>::operator();
48};
49
50template <typename Case>
51struct Overload<Case> : Case {
52 explicit Overload(Case c) : Case(std::move(c)) {}
53
54 using Case::operator();
55};
56} // namespace detail
57
58/*
59 * Combine multiple `Cases` in one function object
60 */
61template <typename... Cases>
62decltype(auto) overload(Cases&&... cases) {
63 return detail::Overload<typename std::decay<Cases>::type...>{
64 std::forward<Cases>(cases)...};
65}
66
67/*
68 * Match `Variant` with one of the `Cases`
69 *
70 * Note: you can also use `[] (const auto&) {...}` as default case
71 *
72 */
73template <typename Variant, typename... Cases>
74decltype(auto) variant_match(Variant&& variant, Cases&&... cases) {
75 return apply_visitor(
76 overload(std::forward<Cases>(cases)...), std::forward<Variant>(variant));
77}
78
79} // namespace folly
80