[C++知识库]c++多线程：原子操作

1、声明头文件

#include <atomic>         // std::atomic, std::atomic_flag, ATOMIC_FLAG_INIT

原子操作：
对一个变量的访问操作，原来会分为好几个指令，由于多线程程序中，可能会存在数据竞争，可能导致对变量的操作不是原子性，那么从编译实现角度，将原本几个指令的操作变成一个指令来执行，可以在无锁情况下实现原子操作，从而避免数据竞争。

std::atomic::atomic 
default (1)	
atomic() noexcept = default;
initialization (2)	
constexpr atomic (T val) noexcept;
copy [deleted] (3)	
atomic (const atomic&) = delete;
Construct atomic
Constructs an atomic object:

(1) default constructor
Leaves the atomic object in an uninitialized state.
An uninitialized atomic object may later be initialized by calling atomic_init.
(2) initialization constructor
Initializes the object with val.
(3) copy construction
Deleted (atomic objects cannot be copied/moved).

注意：
atomic不可拷贝和移动

2、函数

atomic::store

Modify contained value (public member function )
原子地修改容器中的值

atomic::load

Read contained value (public member function )
原子地读取容器中的值

std::atomic::compare_exchange_strong

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;

expected-reference to the value expected to be found in the atomic object

desired-the value to store in the atomic object if it is as expected

success-the memory synchronization ordering for the read-modify-write operation if the comparison succeeds. All values are permitted.

failure-the memory synchronization ordering for the load operation if the comparison fails. Cannot be std::memory_order_release or std::memory_order_acq_rel and cannot specify stronger ordering than success (until C++17)

order-the memory synchronization ordering for both operations

返回值：true如果成功地更改了基础原子值，false否则。

3、使用

std::atomic<int>  ai;
ai.store(3);
// tst_val != ai   ==>  tst_val is modified
exchanged= ai.compare_exchange_strong( tst_val, new_val );
// tst_val == ai   ==>  ai is modified
exchanged= ai.compare_exchange_strong( tst_val, new_val );  

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。