/* * Copyright 2017-present Facebook, Inc. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ #pragma once #include <type_traits> #include <utility> /** * folly implementation of `std::overload` like functionality * * Example: * struct One {}; * struct Two {}; * boost::variant<One, Two> value; * * variant_match(value, * [] (const One& one) { ... }, * [] (const Two& two) { ... }); */ namespace folly { namespace detail { template <typename...> struct Overload; template <typename Case, typename... Cases> struct Overload<Case, Cases...> : Overload<Cases...>, Case { Overload(Case c, Cases... cs) : Overload<Cases...>(std::move(cs)...), Case(std::move(c)) {} using Case::operator(); using Overload<Cases...>::operator(); }; template <typename Case> struct Overload<Case> : Case { explicit Overload(Case c) : Case(std::move(c)) {} using Case::operator(); }; } // namespace detail /* * Combine multiple `Cases` in one function object */ template <typename... Cases> decltype(auto) overload(Cases&&... cases) { return detail::Overload<typename std::decay<Cases>::type...>{ std::forward<Cases>(cases)...}; } /* * Match `Variant` with one of the `Cases` * * Note: you can also use `[] (const auto&) {...}` as default case * */ template <typename Variant, typename... Cases> decltype(auto) variant_match(Variant&& variant, Cases&&... cases) { return apply_visitor( overload(std::forward<Cases>(cases)...), std::forward<Variant>(variant)); } } // namespace folly