std::jthread与std::thread区别

​std::jthread是C++20新引入的线程类，与 std::thread 类似，或者说，jthread是对thread进一步的封装，功能更强大​。

​std::jthread的​j实际上是​joining的缩写​，​众所周知，std::thread在其生命周期结束时调用join()(让主线程等待该子线程完成，然后主线程再继续执行，对于不会停止的线程，不要使用join()，防止阻塞其他线程)，或调用detach()(调用detach()进行线程分离，使其不影响其他线程运行，比如一个线程中无限循环执行的场景下，需要detach()）。如果join()和detach()都没有被调用，析构函数​将立即导致程序异常终止。C++20引入的std::jthread得以解决这个问题，std::jthread对象被析构时，会自动调用join()，等待执行流结束。​

​此外，​std::jthread​​支持外部请求中止操作​，调用join()后可能需要等待很长时间，甚至是永远等待。std::jthread除了提供std::stop_token能够主动取消或停止正在执行的线程，还增加了std::stop_callback允许在停止线程操作时调用一组回调函数。​

来看看cpprefercence关于std::jthread::~jthread的解释：

std::jthread::~jthread

Destroys the ​​jthread​​ object.

If ​​*​this​ has an associated thread (​joinable​()​ ​==​ ​true​​), calls request_stop() and then join().

Notes

The request_stop() has no effect if the ​​jthread​​ was previously requested to stop.

A ​​jthread​​ object does not have an associated thread after

• it was default-constructed
• it was moved from
• join() has been called
• detach() has been called

If join() throws an exception (e.g. because deadlock is detected),​​ std::terminate()​​ may be called.

关于​std::jthread::join​的作用：​阻塞当前线程直至 ​*​this 所标识的线程结束其执行。​

看例程：

#include 
#include 
#include 
 
void foo()
{
    // 模拟耗费大量资源的操作
    std::this_thread::sleep_for(std::chrono::seconds(1));
}
 
void bar()
{
    // 模拟耗费大量资源的操作
    std::this_thread::sleep_for(std::chrono::seconds(1));
}
 
int main()
{
    std::cout << "starting first helper...\n";
    std::jthread helper1(foo);
 
    std::cout << "starting second helper...\n";
    std::jthread helper2(bar);
 
    std::cout << "waiting for helpers to finish..." << std::endl;
    helper1.join();
    helper2.join();
 
    std::cout << "done!\n";
}

输出结果如下：

starting first helper...
starting second helper...
waiting for helpers to finish...
done!

​std::jthread::joinable​，主要是用来检查 std::jthread 对象是否标识活跃的执行线程，直接看cpprefercence相关例程：

#include 
#include 
#include 
 
void foo()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
}
 
int main()
{
    std::jthread t;
    std::cout << "before starting, joinable: " << std::boolalpha << t.joinable()
              << '\n';
 
    t = std::thread(foo);
    std::cout << "after starting, joinable: " << t.joinable() 
              << '\n';
 
    t.join();
    std::cout << "after joining, joinable: " << t.joinable() 
              << '\n';
}

输出结果如下：

before starting, joinable: false
after starting, joinable: true
after joining, joinable: false

可知如果 jthread 对象标识活跃的执行线程则为 true ，否则为 false 。