folly::overload and folly::variant_match

[folly.git] / folly / Overload.h

diff --git a/folly/Overload.h b/folly/Overload.h
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+/*
+ * Copyright 2017 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 details {
+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();
+};
+} // details
+
+/*
+ * Combine multiple `Cases` in one function object
+ */
+template <typename... Cases>
+decltype(auto) overload(Cases&&... cases) {
+  return details::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));
+}
+
+} // folly