【C++編程】函數compare_exchange_strong

 compare_exchange_strong

函數原型：

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;

用法： 原子的比較 *this  和 expect的值，若它們逐位相等，則以 desired 替換前者（進行讀修改寫操作）。否則，將 *this 中的實際值加載進 expected （進行加載操作）。

#include <atomic>
#include <iostream>

std::atomic<int>  ai;

int  tst_val= 4;
int  new_val= 5;
bool exchanged= false;

void valsout()
{
    std::cout << "ai= " << ai
          << "  tst_val= " << tst_val
          << "  new_val= " << new_val
          << "  exchanged= " << std::boolalpha << exchanged
          << "\n";
}

int main()
{
    ai= 3;
    valsout();

    // tst_val != ai   ==>  tst_val is modified
    exchanged= ai.compare_exchange_strong( tst_val, new_val );
    valsout();

    // tst_val == ai   ==>  ai is modified
    exchanged= ai.compare_exchange_strong( tst_val, new_val );
    valsout();
}

 

分析：

1. 當ai = 3時，ai不等於 tst_val, 將tst_val 的值設為3，返回false。

2. 當ai= 3時，ai 等於tst_val, 將tst_val 的值設為new_val, 即將5賦值給ai，返回true。

參考資料

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