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 (i.e. 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, i.e. 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();
}