Multithreading in C++: The correct way to join threads

Question

I am new to multithreading so any suggestions would be appreciated. The following program takes in a vector of integers(eg. 1 2 3 4 5 6 7) and processes each of them as threads. I was wondering if the method with which I join the threads is correct, and if I can make any improvements.

I hope my explanation is clear! Here is my code snippet. It's not the complete code, I'm just ensuring I'm going the right way:

    //vector's name is 'inputs' and it contains integers

    for (int unsigned i = 0; i < inputs.size(); i++) {
        thread thread_obj(thread_function, inputs.at(i));  
        thread_obj.detach();
        
    }
    
    for (int unsigned i = 0; i < inputs.size(); i++) {
      thread_obj.join();
    }
   
    

Answer 1

Your code has undefined behavior so no, it is not correct. The issue here is that you can only call join() if joinable() is true and because you called detach() , joinable() will return false .

The good news is it is really quite a simple fix. You just need to remove the call to detach . To complete out the code just populate a vector of threads and then join them all like

std::vector<std::thread> threads;
threads.reserve(inputs.size());
for (int unsigned i = 0; i < inputs.size(); i++) {
    threads.push_back(std::thread{thread_function, inputs.at(i)};  
}
// now all threads are running, or waiting to run
for (int unsigned i = 0; i < inputs.size(); i++) {
    threads[i].join();
}
// now all threads have been joined