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// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <atomic>
namespace vespalib::datastore {
/**
* Copyable atomic wrapper for a primitive value that offers value store and load
* functionality with explicit memory ordering constraints. Intended to be used for
* non-EntryRef values where atomicity and transitive visibility is a requirement.
*
* Copying always happens with relaxed ordering, as it expects that the copier has
* already loaded the source AtomicValueWrapper with an ordering that is appropriate
* for observing any transitive memory dependencies.
*
* This wrapper is intentionally not implicitly convertible to/from values of the
* underlying primitive type.
*
* Note: use AtomicEntryRef instead if you're wrapping an EntryRef directly.
*/
template <typename T>
class AtomicValueWrapper {
static_assert(std::atomic<T>::is_always_lock_free);
std::atomic<T> _value;
public:
constexpr AtomicValueWrapper() noexcept : _value() {}
constexpr explicit AtomicValueWrapper(T value) noexcept : _value(value) {}
AtomicValueWrapper(const AtomicValueWrapper& rhs) noexcept
: _value(rhs._value.load(std::memory_order_relaxed))
{}
AtomicValueWrapper(AtomicValueWrapper&& rhs) noexcept
: _value(rhs._value.load(std::memory_order_relaxed))
{}
AtomicValueWrapper& operator=(const AtomicValueWrapper& rhs) noexcept {
_value.store(rhs._value.load(std::memory_order_relaxed),
std::memory_order_relaxed);
return *this;
}
void store_release(T value) noexcept {
_value.store(value, std::memory_order_release);
}
void store_relaxed(T value) noexcept {
_value.store(value, std::memory_order_relaxed);
}
[[nodiscard]] T load_acquire() const noexcept {
return _value.load(std::memory_order_acquire);
}
[[nodiscard]] T load_relaxed() const noexcept {
return _value.load(std::memory_order_relaxed);
}
[[nodiscard]] bool operator==(const AtomicValueWrapper& rhs) const noexcept {
return (load_relaxed() == rhs.load_relaxed());
}
[[nodiscard]] bool operator!=(const AtomicValueWrapper& rhs) const noexcept {
return !(*this == rhs);
}
};
}
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