C++11之this_thread namespace

C++11中对于多线程并发编程提供了强大的支持，在this_thread名字空间中提供了几个常用函数get_id, sleep_for, sleep_until和yield。

yield

其中std::this_thread::yield对操作系统的调度器发出请求，主动让出执行权给其他的可运行线程。它的具体行为跟具体的实现相关，例如跟调度器算法相关。

下面简单的代码示例中循环100万次，每到一万的整数倍时主动让出执行权。

// demonstration of std::this_thread::yield
// by Bruce Jia 
 
#include 
#include 
#include 
 
class HighResStopwatch
{
private:
    std::chrono::time_point start;
 
public:
    HighResStopwatch() noexcept: start(std::chrono::high_resolution_clock::now()) {
    }
 
    ~HighResStopwatch() noexcept {
        
        auto elapsed = std::chrono::high_resolution_clock::now() - start;
        std::cout << std::chrono::duration_cast(elapsed).count()
              << " microseconds\n";
    }
 
    HighResStopwatch(HighResStopwatch &) = delete;
    void operator = (HighResStopwatch &) = delete;
};
 
int main()
{
    long maxCount = 1000000L;
    int j = 1;
 
    for (long i=1; i<=maxCount; i++) {
        if (i % 10000 == 0) {
            std::cout << "yield " << j++ << std::endl;
            HighResStopwatch watch;
            std::this_thread::yield();
        }
    }    
}

这里一个简单的类HighResStopwatch在析构的时候会打印出它的存活时间，近似等于出让执行权的时间。输出类似如下

yield 1
1 microseconds
yield 2
260 microseconds
yield 3
0 microseconds
yield 4
0 microseconds
yield 5
0 microseconds
yield 6
96 microseconds
yield 7
99 microseconds
yield 8
93 microseconds
yield 9
92 microseconds
yield 10
99 microseconds
yield 11
2 microseconds
yield 12
0 microseconds
yield 13
0 microseconds
yield 14
0 microseconds
yield 15
41 microseconds
yield 16
1 microseconds
yield 17
0 microseconds
yield 18
36 microseconds
yield 19
0 microseconds
yield 20
0 microseconds
yield 21
0 microseconds
yield 22
14 microseconds
yield 23
14 microseconds
yield 24
0 microseconds
yield 25
14 microseconds
yield 26
14 microseconds
yield 27
0 microseconds
yield 28
14 microseconds
yield 29
0 microseconds
yield 30
0 microseconds
yield 31
14 microseconds
yield 32
13 microseconds
yield 33
0 microseconds
yield 34
14 microseconds
yield 35
14 microseconds
yield 36
14 microseconds
yield 37
0 microseconds
yield 38
14 microseconds
yield 39
1 microseconds
yield 40
0 microseconds
yield 41
14 microseconds
yield 42
1 microseconds
yield 43
0 microseconds
yield 44
13 microseconds
yield 45
14 microseconds
yield 46
0 microseconds
yield 47
14 microseconds
yield 48
0 microseconds
yield 49
0 microseconds
yield 50
0 microseconds
yield 51
14 microseconds
yield 52
1 microseconds
yield 53
0 microseconds
yield 54
13 microseconds
yield 55
1 microseconds
yield 56
0 microseconds
yield 57
13 microseconds
yield 58
0 microseconds
yield 59
0 microseconds
yield 60
13 microseconds
yield 61
0 microseconds
yield 62
0 microseconds
yield 63
14 microseconds
yield 64
26 microseconds
yield 65
0 microseconds
yield 66
0 microseconds
yield 67
14 microseconds
yield 68
14 microseconds
yield 69
0 microseconds
yield 70
14 microseconds
yield 71
0 microseconds
yield 72
0 microseconds
yield 73
0 microseconds
yield 74
14 microseconds
yield 75
0 microseconds
yield 76
0 microseconds
yield 77
13 microseconds
yield 78
0 microseconds
yield 79
0 microseconds
yield 80
0 microseconds
yield 81
0 microseconds
yield 82
14 microseconds
yield 83
14 microseconds
yield 84
52 microseconds
yield 85
1 microseconds
yield 86
0 microseconds
yield 87
14 microseconds
yield 88
14 microseconds
yield 89
1 microseconds
yield 90
0 microseconds
yield 91
24 microseconds
yield 92
14 microseconds
yield 93
13 microseconds
yield 94
0 microseconds
yield 95
14 microseconds
yield 96
13 microseconds
yield 97
1 microseconds
yield 98
0 microseconds
yield 99
14 microseconds
yield 100
14 microseconds

 输出中记录的时间差长短不一，长的有数百微秒，短的则是0。对于0的情况，是因为调度器没有找到（例如相同优先级的可运行线程队列为空）其他合适的线程执行，所以马上再次执行当前线程。

get_id

std::this_thread::get_id的使用非常简单，它返回当前线程的id。

int main()
{
    auto tid1 = std::this_thread::get_id();
    std::cout << "tid1 : " << tid1 << std::endl;
 
    auto tid2 = std::this_thread::get_id();
    std::cout << "tid2 : " << tid2 << std::endl;
}

例如上面代码都运行在main thread中，所以tid1和tid2一定是相同的。