[C++知识库]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 <iostream>
#include <thread>
#include <chrono>
 
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 <iostream>
#include <thread>
#include <chrono>
 
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 。