c++并发编程之thread::join()和thread::detach()

thread::join(): 阻塞当前线程，直至 *this 所标识的线程完成其执行。*this 所标识的线程的完成同步于从 join() 的成功返回。

该方法简单暴力，主线程等待子进程期间什么都不能做。thread::join()会清理子线程相关的内存空间，此后thread object将不再和这个子线程相关了，即thread object不再joinable了，所以join对于一个子线程来说只可以被调用一次，为了实现更精细的线程等待机制，可以使用条件变量等机制。

#include <iostream>
#include <thread>
#include <chrono>
 
void foo()
{
    std::cout << "foo is started\n";
    // 模拟昂贵操作
    std::this_thread::sleep_for(std::chrono::seconds(1));
    std::cout << "foo is done\n";
}
 
void bar()
{
    std::cout << "bar is started\n";
    // 模拟昂贵操作
    std::this_thread::sleep_for(std::chrono::seconds(1));
    std::cout << "bar is done\n";
}
 
int main()
{
    std::cout << "starting first helper...\n";
    std::thread helper1(foo);
 
    std::cout << "starting second helper...\n";
    std::thread helper2(bar);
 
    std::cout << "waiting for helpers to