【C++编程】函数compare_exchange_strong

 compare_exchange_strong

函数原型：

bool compare_exchange_weak(T &expected, T desired, std::memory_order success, std::memory_order failure);
bool compare_exchange_weak(T &expected, T desired, std::memory_order success, std::memory_order failure) volatile;
bool compare_exchange_weak(T &expected, T desired, std::memory_order order = std::memory_order_seq_cst);
bool compare_exchange_weak(T &expected, T desired, std::memory_order order = std::memory_order_seq_cst) volatile;
bool compare_exchange_strong(T &expected, T desired, std::memory_order success = std::memory_order failure);
bool compare_exchange_strong(T &expected, T desired, std::memory_order success = std::memory_order failure) volatile;
bool compare_exchange_strong(T &expected, T desired, std::memory_order order = std::memory_order_seq_cst);
bool compare_exchange_strong(T &expected, T desired, std::memory_order order = std::memory_order_seq_cst) volatile;

该函数会将当前原子变量的值与 expected 进行比较：

• 如果相等，则用 desired 替换原子变量的值，并返回 true。
• 如果不相等：
  • 原子变量的值不会被修改。
  • expected 会被更新为原子变量当前的值。
  • 返回 false。

 

#include <iostream>
#include <atomic>

int main() {
    std::atomic<int> atomic_val(42);

    int expected = 42;
    int desired = 100;

    if (atomic_val.compare_exchange_strong(expected, desired)) {
        std::cout << "Update succeeded, new value: " << atomic_val.load() << "\n";
    } else {
        std::cout << "Update failed, expected was changed to: " << expected << "\n";
    }

    return 0;
}

输出：

Update succeeded, new value: 100

说明：如果 atomic_val 原来不是 42，那么会输出 “Update failed”。

 

compare_exchange_weak vs strong：

• compare_exchange_strong: 不容忍伪失败（spurious failure），适用于要求一致性的场景。
• compare_exchange_weak: 可能出现伪失败，适用于循环重试的情况（如实现自旋锁），性能更高。

参考资料

1 . std::atomic<T>::compare_exchange_weak, std::atomic<T>::compare_exchange_strong的用法