| // 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. | |
| // The IfConstexpr and IfConstexprElse utilities in this file are meant to be | |
| // used to emulate `if constexpr` in pre-C++17 mode in library implementation. | |
| // The motivation is to allow for avoiding complex SFINAE. | |
| // | |
| // The functions passed in must depend on the type(s) of the object(s) that | |
| // require SFINAE. For example: | |
| // template<typename T> | |
| // int MaybeFoo(T& t) { | |
| // if constexpr (HasFoo<T>::value) return t.foo(); | |
| // return 0; | |
| // } | |
| // | |
| // can be written in pre-C++17 as: | |
| // | |
| // template<typename T> | |
| // int MaybeFoo(T& t) { | |
| // int i = 0; | |
| // absl::utility_internal::IfConstexpr<HasFoo<T>::value>( | |
| // [&](const auto& fooer) { i = fooer.foo(); }, t); | |
| // return i; | |
| // } | |
| namespace absl { | |
| ABSL_NAMESPACE_BEGIN | |
| namespace utility_internal { | |
| template <bool condition, typename TrueFunc, typename FalseFunc, | |
| typename... Args> | |
| auto IfConstexprElse(TrueFunc&& true_func, FalseFunc&& false_func, | |
| Args&&... args) { | |
| return std::get<condition>(std::forward_as_tuple( | |
| std::forward<FalseFunc>(false_func), std::forward<TrueFunc>(true_func)))( | |
| std::forward<Args>(args)...); | |
| } | |
| template <bool condition, typename Func, typename... Args> | |
| void IfConstexpr(Func&& func, Args&&... args) { | |
| IfConstexprElse<condition>(std::forward<Func>(func), [](auto&&...){}, | |
| std::forward<Args>(args)...); | |
| } | |
| } // namespace utility_internal | |
| ABSL_NAMESPACE_END | |
| } // namespace absl | |