如何安全地终止线程？ （使用指针） c++

Question

I am currently learning multithreading in c++11 and I am confused with the way to terminate a thread safely.

In c++, I know the way to create threads and use thread.join() to safely ensure main() to wait for all threads to finish before quitting itself.

However, I found that some multithread codes implemented via pointers are able to run even without using thread.join().

class Greating
{
public:

    Greating(const int& _i):i_(_i){}
    ~Greating(){}
    int i_;
    void say()
    {
        std::cout << "Hello World" << i_ << std::endl;
    }

};

int main(){
    Greating greating1(1);
    Greating greating2(2);

    std::thread t1(&Greating::say, greating1);
    std::thread t2(&Greating::say, greating2);
    return 0;
}

The code shown above will absolutely report the error "terminate called without an active exception Aborted (core dumped)", because I did not use t1.join() and t2.join().

However, I found in some codes when they use the pointer to manage the thread, this does not become a problem, as shown below.

class Greating
{
public:

    Greating(const int& _i):i_(_i){}
    ~Greating(){}
    int i_;
    void say()
    {
        std::cout << "Hello World" << i_ << std::endl;
    }

};

int main(){
    Greating greating1(1);
    Greating greating2(2);

    std::thread* tt1 = new std::thread(&Greating::say, greating1);
    std::thread* tt2 = new std::thread(&Greating::say, greating2);
    return 0;
}

The output is:

Hello WorldHello World12
Hello World12

There is no error reported. This made me very confused.

So my question is:

1. Why when we use pointer to manage the thread, we could not use the function thread.join()?
2. How to correctly terminate a thread? (probably wait for the callable function to finish?)

Thanks very much!

Answer 1

When creating objects with dynamic allocation, you have to deallocate the memory with operator delete so it calls appropriate destructor.

In the first example, two std::thread objects are created. At the end of main function, the destructor std::thread::~thread is called. Since the threads are not joined, the destructor reports an error.

On the other hand, in the second example, you called operator new so you create objects with dynamic allocation. But, you didn't call operator delete , so the destructor is not called. That is, the program didn't check whether the threads are joined.

Therefore, the only way to correctly terminate a thread is to call std::thread::join . If you want to use pointers, you have to do as following:

std::thread *th = new std::thread(foo);
...
th->join();
delete th;