1 /*
2 * Copyright 2022 The Android Open Source Project
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 <utility>
20 #include <variant>
21
22 #include <ftl/details/match.h>
23
24 namespace android::ftl {
25
26 // Concise alternative to std::visit that compiles to branches rather than a dispatch table. For
27 // std::variant<T0, ..., TN> where N is small, this is slightly faster since the branches can be
28 // inlined unlike the function pointers.
29 //
30 // using namespace std::chrono;
31 // std::variant<seconds, minutes, hours> duration = 119min;
32 //
33 // // Mutable match.
34 // ftl::match(duration, [](auto& d) { ++d; });
35 //
36 // // Immutable match. Exhaustive due to minutes being convertible to seconds.
37 // assert("2 hours"s ==
38 // ftl::match(duration,
39 // [](const seconds& s) {
40 // const auto h = duration_cast<hours>(s);
41 // return std::to_string(h.count()) + " hours"s;
42 // },
43 // [](const hours& h) { return std::to_string(h.count() / 24) + " days"s; }));
44 //
45 template <typename... Ts, typename... Ms>
decltype(auto)46 decltype(auto) match(std::variant<Ts...>& variant, Ms&&... matchers) {
47 const auto matcher = details::Matcher{std::forward<Ms>(matchers)...};
48 static_assert(details::is_exhaustive_match_v<decltype(matcher), Ts&...>, "Non-exhaustive match");
49
50 return details::Match<Ts...>::match(variant, matcher);
51 }
52
53 template <typename... Ts, typename... Ms>
decltype(auto)54 decltype(auto) match(const std::variant<Ts...>& variant, Ms&&... matchers) {
55 const auto matcher = details::Matcher{std::forward<Ms>(matchers)...};
56 static_assert(details::is_exhaustive_match_v<decltype(matcher), const Ts&...>,
57 "Non-exhaustive match");
58
59 return details::Match<Ts...>::match(variant, matcher);
60 }
61
62 } // namespace android::ftl
63