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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");
//
// One of the lambda may specify overload for several types via generic lambda.
//
// absl::variant<std::string, int32_t, int64_t> v(int32_t{1});
// assert(std::visit(absl::Overload(
// [](const std::string& s) { return s.size(); },
// [](const auto& s) { return sizeof(s); }), v) == 4);
//
// Note: absl::Overload 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_
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