c++ 之 std::this_thread::yield 与std::this_thread::sleep_for

std::this_thread::yield：　当前线程放弃执行，操作系统调度另一线程继续执行。即当前线程将未使用完的“CPU时间片”让给其他线程使用，等其他线程使用完后再与其他线程一起竞争"CPU"。
std::this_thread::sleep_for：　表示当前线程休眠一段时间，休眠期间不与其他线程竞争CPU，根据线程需求，等待若干时间。

this_thread 包装了一组可以访问当前线程信息的函数

1、get_id()      获取当前线程的id。

// thread::get_id / this_thread::get_id
#include <iostream>       // std::cout
#include <thread>         // std::thread, std::thread::id, std::this_thread::get_id
#include <chrono>         // std::chrono::seconds
 
std::thread::id main_thread_id = std::this_thread::get_id();
 
void is_main_thread() {
  if ( main_thread_id == std::this_thread::get_id() )
    std::cout << "This is the main thread.\n";
  else
    std::cout << "This is not the main thread.\n";
}
 
int main() 
{
  is_main_thread();
  std::thread th (is_main_thread);
  th.join();
}

输出结果

This is the main thread.
This is not the main thread.

2、yield()
调用线程放弃执行，回到准备状态，重新分配cpu资源。所以调用该方法后，可能执行其他线程，也可能还是执行该线程

// this_thread::yield example
#include <iostream>       // std::cout
#include <thread>         // std::thread, std::this_thread::yield
#include <atomic>         // std::atomic
 
std::atomic<bool> ready (false);
 
void count1m(int id) {
  while (!ready) {             // wait until main() sets ready...
    std::this_thread::yield();
  }
  for (volatile int i=0; i<1000000; ++i) {}
  std::cout << id;
}
 
int main ()
{
  std::thread threads[10];
  std::cout << "race of 10 threads that count to 1 million:\n";
  for (int i=0; i<10; ++i) threads[i]=std::thread(count1m,i);
  ready = true;               // go!
  for (auto& th : threads) th.join();
  std::cout << '\n';
 
  return 0;
}

输出结果

race of 10 threads that count to 1 million...
6189370542

3、template <class Clock, class Duration>
       void sleep_until (const chrono::time_point<Clock,Duration>& abs_time);
阻塞调用线程，一直到指定事件

// this_thread::sleep_for example
#include <iostream>       // std::cout
#include <iomanip>        // std::put_time
#include <thread>         // std::this_thread::sleep_until
#include <chrono>         // std::chrono::system_clock
#include <ctime>          // std::time_t, std::tm, std::localtime, std::mktime
 
int main() 
{
  using std::chrono::system_clock;
  std::time_t tt = system_clock::to_time_t (system_clock::now());
 
  struct std::tm * ptm = std::localtime(&tt);
  std::cout << "Current time: " << std::put_time(ptm,"%X") << '\n';
 
  std::cout << "Waiting for the next minute to begin...\n";
  ++ptm->tm_min; ptm->tm_sec=0;
  std::this_thread::sleep_until (system_clock::from_time_t (mktime(ptm)));
 
  std::cout << std::put_time(ptm,"%X") << " reached!\n";
 
  return 0;
}

输出结果

Current time: 11:52:36
Waiting for the next minute to begin...
11:53:00 reached!

4、template <class Rep, class Period>
       void sleep_for (const chrono::duration<Rep,Period>& rel_time);
阻塞调用线程，一直到指定时间段后。

// this_thread::sleep_for example
#include <iostream>       // std::cout
#include <thread>         // std::this_thread::sleep_for
#include <chrono>         // std::chrono::seconds
 
int main() 
{
  std::cout << "countdown:\n";
  for (int i=10; i>0; --i) {
    std::cout << i << '\n';
    std::this_thread::sleep_for (std::chrono::seconds(1));
  }
  std::cout << "Lift off!\n";
 
  return 0;
}

输出结果

countdown:
10
9
8
7
6
5
4
3
2
1
Lift off!