std :: thread :: join无限期地阻塞了main

Question

std::thread::join does not return, even if the thread routine is exited.

Guess, I have a class.

class A
{
public:
    A()
    {
        this->thr = std::thread(&A::foo, this);
    }
    ~A()
    {
        this->join();
    }
    void join()
    {
        this->cond.notify_all();
        if (this->thr.joinable())
        {
            this->thr.join();
        }
    }

private:
    void foo()
    {
        std::mutex mtx;
        std::unique_lock<std::mutex> lck(mtx);
        this->cond.wait(lck);
        MessageBox(L"I'm done!");
    }

private:
    std::thread thr;
    std::condition_variable cond;
};

My application contains the only instance of A . It is a global variable. If A::join is called from the destructor, std::thread::join blocks forever. If I call A::join manually (eg before exiting main ), everything is alright.

My main looks like this:

A a;
int main()
{
    auto timeout = std::chrono::seconds(3);
    std::this_thread::sleep_for(timeout);

    // a.join();
}

By the way, MessageBox is always executed.

Is that the same problem as here ?

Answer 1

Yes it is the same bug as in the referenced link since your example also hangs on _Thrd_join . You could be interrested in this question which contains a far more detailed analysis.

Answer 2

from your comment

"It doesn't seem to be a great solution. I works, but what if the main is not so trivial? There're a lot of different ways of exiting from my application. Thus, I've got to join my threads manually every time I need it?"

how about making A a std::unique_ptr within your main. that way, no matter how your main exits, it'll always destroy A before exiting main() and you won't have this problem.