`absl::Overload()` which returns a functor that provides overloads based on the functors passed to it.

PiperOrigin-RevId: 568476251
Change-Id: Ic625c9b5300d1db496979c178ca1e655581f9276

1 files changed, 75 insertions, 0 deletions

diff --git a/absl/functional/overload.h b/absl/functional/overload.h
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+// Copyright 2023 The Abseil Authors.
+//
+// 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
+//
+//      https://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.
+//
+// -----------------------------------------------------------------------------
+// File: overload.h
+// -----------------------------------------------------------------------------
+//
+// `absl::Overload()` returns a functor that provides overloads based on the
+// functors passed to it.
+// Before using this function, consider whether named function overloads would
+// be a better design.
+// One use case for this is locally defining visitors for `std::visit` inside a
+// function using lambdas.
+
+// Example: Using  `absl::Overload` to define a visitor for `std::variant`.
+//
+// std::variant<int, std::string, double> v(int{1});
+//
+// assert(std::visit(absl::Overload(
+//                        [](int) -> absl::string_view { return "int"; },
+//                        [](const std::string&) -> absl::string_view {
+//                          return "string";
+//                        },
+//                        [](double) -> absl::string_view { return "double"; }),
+//                     v) == "int");
+//
+// Note: This requires C++17.
+
+#ifndef ABSL_FUNCTIONAL_OVERLOAD_H_
+#define ABSL_FUNCTIONAL_OVERLOAD_H_
+
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+
+template <int&... ExplicitArgumentBarrier, typename... T>
+auto Overload(T&&... ts) {
+  struct OverloadImpl : absl::remove_cvref_t<T>... {
+    using absl::remove_cvref_t<T>::operator()...;
+  };
+  return OverloadImpl{std::forward<T>(ts)...};
+}
+#else
+namespace functional_internal {
+template <typename T>
+constexpr bool kDependentFalse = false;
+}
+
+template <typename Dependent = int, typename... T>
+auto Overload(T&&...) {
+  static_assert(functional_internal::kDependentFalse<Dependent>,
+                "Overload is only usable with C++17 or above.");
+}
+
+#endif
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_OVERLOAD_H_