多线程基本用法

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;

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：原子的比较 *this 和 expect的值，若它们逐位相等，则以 desired 替换前者（进行读修改写操作）。否则，将 *this 中的实际值加载进 expected （进行加载操作）。

 1 #include <atomic>
 2 #include <iostream>
 3  
 4 std::atomic<int>  ai;
 5  
 6 int  tst_val= 4;
 7 int  new_val= 5;
 8 bool exchanged= false;
 9  
10 void valsout()
11 {
12     std::cout << "ai= " << ai
13           << "  tst_val= " << tst_val
14           << "  new_val= " << new_val
15           << "  exchanged= " << std::boolalpha << exchanged
16           << "
";
17 }
18  
19 int main()
20 {
21     ai= 3;
22     valsout();
23  
24     // tst_val != ai   ==>  tst_val is modified
25     exchanged= ai.compare_exchange_strong( tst_val, new_val );
26     valsout();
27  
28     // tst_val == ai   ==>  ai is modified
29     exchanged= ai.compare_exchange_strong( tst_val, new_val );
30     valsout();
31 }

分析：

1. 当ai = 3时，ai不等于 tst_val, 将tst_val 的值设为3，返回false。

2. 当ai= 3时，ai 等于tst_val, 将tst_val 的值设为new_val, 即将5赋值给ai，返回true。

参考资料

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