【C++编程】条件变量condition_variable

std::condition_variable 条件变量

 

void wait( std::unique_lock<std::mutex>& lock ); #1
template< class Predicate >
void wait( std::unique_lock<std::mutex>& lock, Predicate stop_waiting ); #2

多线程轮流打印ABC

#include <iostream>
#include <thread>
#include <condition_variable>
#include <vector>
#include <algorithm>
#include <functional>
#include <thread>
#include <mutex>

std::mutex mtx;
std::condition_variable variable;
char g_ch = 0;

void print_fun(char ch) {
  int cyle_cnt = 10;
  char ch_ = ch - 'A';

  for (int i = 0; i < cyle_cnt; i++) {
    std::unique_lock<std::mutex> ulk(mtx);
    variable.wait(ulk, [ch_] { return ch_ == g_ch; });
    std::cout << (char)(ch_ + 'A') << std::endl;
    g_ch = (ch_ + 1) % 3;
    ulk.unlock();

    variable.notify_all();
  }
}

int main()
{
  std::vector<std::thread> threads;
  threads.push_back(std::thread(print_fun, 'A'));
  threads.push_back(std::thread(print_fun, 'B'));
  threads.push_back(std::thread(print_fun, 'C'));

  std::for_each(threads.begin(), threads.end(), std::mem_fn(&std::thread::join));

  std::cout << std::endl;
  return 0;
}

例1 ：

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;

void worker_thread()
{
	std::unique_lock lk(m);
	cv.wait(lk, [] { return ready; });

	std::cout << "Worker thread is processing data\n";
	data += " after processing";

	processed = true;
	std::cout << "Worker thread signals data processing completed\n";

	lk.unlock();
	cv.notify_one();
}

int main()
{
	std::thread worker(worker_thread);

	data = "Example data";
	{
		std::lock_guard lk(m);
		ready = true;
		std::cout << "main() signals data ready for processing\n";
	}
	cv.notify_one();

	{
		std::unique_lock lk(m);
		cv.wait(lk, [] { return processed; });
	}
	std::cout << "Back in main(), data = " << data << '\n';

	worker.join();
}