QThread: Destroyed while thread is still running?

Question

I would like to start my QThread when I push on button Run . But the compiler outputs following error:

QThread: Destroyed while thread is still running
ASSERT failure in QThread::setTerminationEnabled(): "Current thread was not started with QThread.", file thread\qthread_win.cp.

I don't know what is wrong with my code.

Any help would be appreciated.

Here is my code:

SamplingThread::SamplingThread( QObject *parent):
   QwtSamplingThread( parent ),
   d_frequency( 5.0 )
{
   init();
}

MainWindow::MainWindow( QWidget *parent ):
QMainWindow( parent )
{.......
  .....
   run= new QPushButton ("Run",this);
   stop= new QPushButton("Stop",this);
   connect(run, SIGNAL(clicked()),this, SLOT (start()));
}

MainWindow::start
{
   SamplingThread samplingThread;
   samplingThread.setFrequency( frequency() );
   samplingThread.start();
}

int main( int argc, char **argv )
{
   QApplication app( argc, argv );
   MainWindow window;
   window.resize( 700, 400 );
   window.show();
   bool ok = app.exec();
   return ok;
}

Answer 1

As the error message states: QThread: Destroyed while thread is still running . You are creating your SamplingThread object inside the MainWindow::start method but it goes out of scope (ie is destroyed) when that method terminates. There are two easy ways that I see:

1. You make your SamplingThread a member of your MainWindow so its lifetime is the same as for the MainWindow instance

2. You use a pointer, ie you create the SamplingThread using

   SamplingThread *samplingThread = new SamplingThread;

Does this help?

Edit: to illustrate the two cases, a very crude example to show the two cases

#include <iostream>
#include <QApplication>
#include <QThread>

class Dummy
{
public:
  Dummy();
  void start();
private:
  QThread a;
};

Dummy::Dummy() :
  a()
{
}


void Dummy::start()
{
  a.start();
  QThread *b = new QThread;
  b->start();

  if( a.isRunning() ) {
    std::cout << "Thread a is running" << std::endl;
  }
  if( b->isRunning() ) {
    std::cout << "Thread b is running" << std::endl;
  }
}

int main(int argc, char** argv)
{
  QApplication app(argc,argv);
  Dummy d;
  d.start();
  return app.exec();
}

Answer 2

This is basics of C++! You are creating local object of QThread on stack not on heap, so it gets destroys immediately when you leave method MainWindow::start .

It should be done like that:

MainWindow::MainWindow( QWidget *parent ):
QMainWindow( parent )
{
   ...

   samplingThread = SamplingThread(this);
   samplingThread->setFrequency( frequency() );

   run= new QPushButton ("Run",this);
   stop= new QPushButton("Stop",this);
   connect(run, SIGNAL(clicked()), samplingThread, SLOT(start()));
}

MainWindow::~MainWindow() {
   samplingThread->waitFor(5000);
}

Answer 3

There are two different "threads" involved: One is the actual thread, the other is the C++ object representing it (and to be correct, there is another thread from which this code is started in the first place).

The error just says that the thread is still running at the point where the C++ object representing it is destroyed. In your code, the reason is that the QThread instance is local to start() . Maybe you want to store the QThread in a member.