std::atomic::compare_exchange_strong
Defined in header <atomic> | ? | ? |
|---|---|---|
? | (1) | (since C++11) |
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; | ? | |
? | (2) | (since C++11) |
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; | ? | |
? | (3) | (since C++11) |
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; | ? | |
? | (4) | (since C++11) |
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; | ? |
原子比较对象表示成*this的对象表示形式expected，好像std::memcmp，如果这些是按位相等的，则将前者替换为desired%28执行读-修改-写操作%29。否则，加载存储在*this进expected%28执行加载操作%29。复制就像std::memcpy...
读-修改-写入和加载操作的内存模型包括success和failure分别。在%282%29和%284%29版本中order用于读-修改-写入和加载操作，但std::memory_order_acquire和std::memory_order_relaxed用于加载操作，如果order ==std::memory_order_acq_rel，或order ==std::memory_order_release分别。
参数
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。
例外
noexcept规格：
noexcept
注记
弱窗体%281-2%29的函数被允许伪造地失败，也就是说，表现为*this != expected即使他们是平等的。当比较和交换处于循环状态时，弱版本将在某些平台上获得更好的性能。
当弱比较和交换需要一个循环，而强比较和交换不需要循环时，强比较和交换就更好了，除非T可以包括填充位、陷阱位，或者为相同值%28例如提供多个对象表示。浮点南%29.。在这些情况下，弱比较和交换通常是有效的，因为它能快速地收敛于某些稳定的对象表示上。
例
比较和交换操作通常用作无锁数据结构的基本构建块。
二次
#include <atomic>
template<typename T>
struct node
{
T data;
node* next;
node(const T& data) : data(data), next(nullptr) {}
};
template<typename T>
class stack
{
std::atomic<node<T>*> head;
public:
void push(const T& data)
{
node<T>* new_node = new node<T>(data);
// put the current value of head into new_node->next
new_node->next = head.load(std::memory_order_relaxed);
// now make new_node the new head, but if the head
// is no longer what's stored in new_node->next
// (some other thread must have inserted a node just now)
// then put that new head into new_node->next and try again
while(!head.compare_exchange_weak(new_node->next, new_node,
std::memory_order_release,
std::memory_order_relaxed))
; // the body of the loop is empty
// Note: the above use is not thread-safe in at least
// GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
// MSVC prior to 2014-03-17 (bug 819819). The following is a workaround:
// node<T>* old_head = head.load(std::memory_order_relaxed);
// do {
// new_node->next = old_head;
// } while(!head.compare_exchange_weak(old_head, new_node,
// std::memory_order_release,
// std::memory_order_relaxed));
}
};
int main()
{
stack<int> s;
s.push(1);
s.push(2);
s.push(3);
}二次
演示如何比较[医]交换[医]强可以更改原子变量或用于比较的变量的值。
二次
#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();
}二次
产出：
二次
ai= 3 tst_val= 4 new_val= 5 exchanged= false
ai= 3 tst_val= 3 new_val= 5 exchanged= false
ai= 5 tst_val= 3 new_val= 5 exchanged= true二次
另见
atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) | atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (function template) |
|---|
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