Wait until all threads are finished in main thread using Qt/C++

Question

I want to make a condition in code to wait until all threads are finished in the main thread final slot, following is the test code ..

testClass::testClass()
{
    m_count = 0;
    m_flag = false;
    for( int i = 0; i < 3; i++)
    {
        QThread *thread = new QThread();
        WorkerThread *worker = new WorkerThread();

        connect(thread, SIGNAL(started()), worker, SLOT(startThread()));
        connect(worker, SIGNAL(workerFinished()), this, SLOT(threadFinished()));
        connect(worker, SIGNAL(workerFinished()), thread, SLOT(quit()));
        connect(thread, SIGNAL(finished()), worker, SLOT(deleteLater()));
        connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
        thread->start();
        m_workerList.append(worker);
     }
}

void testClass::threadFinished()
{
    // wait untill all thread are finished, how to achive this ?
    if(!m_flag)
    {
        // print << m_flag;
        m_flag = true;
    }
}

void WorkerThread::startThread()
{
    emit workerFinished();
}

here testClass is in main thread and I want the application to wait in threadFinished slot until all threads I started in testClass constructor to finish, could anyone suggest best way to do this ?

I am using Qt 5.4.0 in Windows 7.

Answer 1

If using boost in addition to Qt is an option, you can use a thread group and call join_all, which waits until all threads in the group are done.

Also, QThreadPool provides the waitForDone() function ("Waits for each thread to exit and removes all threads from the thread pool."), however you might have to restructure your worker objects slightly to comply to the QRunnable interface.

A more basic solution would be to set a QVector or QMap member variable with one entry for each thread and set them to true when the respective thread finishes. Continue program execution in your slot only when all threads have reached "finished = true" state in your member variable.