71 lines
2.2 KiB
C
71 lines
2.2 KiB
C
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// Copyright 2023 The Abseil Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// The IfConstexpr and IfConstexprElse utilities in this file are meant to be
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// used to emulate `if constexpr` in pre-C++17 mode in library implementation.
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// The motivation is to allow for avoiding complex SFINAE.
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//
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// The functions passed in must depend on the type(s) of the object(s) that
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// require SFINAE. For example:
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// template<typename T>
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// int MaybeFoo(T& t) {
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// if constexpr (HasFoo<T>::value) return t.foo();
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// return 0;
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// }
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//
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// can be written in pre-C++17 as:
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//
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// template<typename T>
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// int MaybeFoo(T& t) {
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// int i = 0;
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// absl::utility_internal::IfConstexpr<HasFoo<T>::value>(
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// [&](const auto& fooer) { i = fooer.foo(); }, t);
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// return i;
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// }
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#ifndef ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
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#define ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
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#include <tuple>
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#include <utility>
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#include "absl/base/config.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace utility_internal {
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template <bool condition, typename TrueFunc, typename FalseFunc,
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typename... Args>
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auto IfConstexprElse(TrueFunc&& true_func, FalseFunc&& false_func,
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Args&&... args) {
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return std::get<condition>(std::forward_as_tuple(
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std::forward<FalseFunc>(false_func), std::forward<TrueFunc>(true_func)))(
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std::forward<Args>(args)...);
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}
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template <bool condition, typename Func, typename... Args>
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void IfConstexpr(Func&& func, Args&&... args) {
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IfConstexprElse<condition>(std::forward<Func>(func), [](auto&&...){},
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std::forward<Args>(args)...);
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}
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} // namespace utility_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
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