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// Copyright 2022 Huawei Cloud Computing Technology Co., Ltd.
//
// 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.
#ifndef INCLUDED_SRC_BUILDTOOL_MULTITHREADING_ATOMIC_VALUE_HPP
#define INCLUDED_SRC_BUILDTOOL_MULTITHREADING_ATOMIC_VALUE_HPP
#include <atomic>
#include <functional>
#include <memory>
#include "src/utils/cpp/atomic.hpp"
// Value that can be set and get atomically. Reset is not thread-safe.
template <class T>
class AtomicValue {
public:
AtomicValue() noexcept = default;
AtomicValue(AtomicValue const& other) noexcept = delete;
AtomicValue(AtomicValue&& other) noexcept : data_{other.data_.load()} {}
~AtomicValue() noexcept = default;
auto operator=(AtomicValue const& other) noexcept = delete;
auto operator=(AtomicValue&& other) noexcept = delete;
// Atomically set value once and return its reference. If this method is
// called multiple times concurrently, the setter is called only once. In
// any case, this method blocks until the value is ready.
[[nodiscard]] auto SetOnceAndGet(
std::function<T()> const& setter) const& noexcept -> T const& {
if (data_.load() == nullptr) {
if (not load_.exchange(true)) {
data_.store(std::make_shared<T>(setter()));
data_.notify_all();
}
else {
data_.wait(nullptr);
}
}
return *data_.load();
}
[[nodiscard]] auto SetOnceAndGet(std::function<T()> const& setter) && =
delete;
// Reset, not thread-safe!
void Reset() noexcept {
load_ = false;
data_ = nullptr;
}
private:
mutable std::atomic<bool> load_{false};
mutable atomic_shared_ptr<T> data_{nullptr};
};
#endif // INCLUDED_SRC_BUILDTOOL_MULTITHREADING_ATOMIC_VALUE_HPP
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