use signal&slot between two tabs

I have a QTabWidget, and two classes, display1 and display2, are two tab members. I want to transfer some data, say a bool flag, between these two tabs:

Code:

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tabWidget = new QTabWidget;
display1 = new Display1 ();
display2= new Display2()
tabWidget->addTab(display1 , tr("Online"));
tabWidget->addTab(display2, tr("Offline"));
 
connect(Display1, SIGNAL(sendout(bool)), Display2, SLOT(receive(bool)));

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In class Display1.h:

Code:

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signals:
      void sendout(bool flag);

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In class Display1.cpp, one of its methods does this:

Code:

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bool flag = true;
emit sendout(flag);

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In class Display2.h:

Code:

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public slots:
	void receive(bool flag);

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In class Display2.cpp:

Code:

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void Display2::receive(bool flag)
{
	printf("I got one message. \n");
}

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I got LINK ERROR:
Error 6 error LNK2001: unresolved external symbol "protected: void __thiscall Display1::sendout(bool)" (?sendout@Display1@@IAEX_N@Z) Display1.obj

I have no idea where is the problem....Can anyone help me? Thanks

You didn't moc the header file. Are you using qmake?

Quote:

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Originally Posted by wysota

You didn't moc the header file. Are you using qmake?

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I have the moc already. This is strange. When I delete the SIGNAL in Display1, everything is running. The problem seems in Display1. But I can't findany error there...

What do you mean you have the moc? Are you compiling and linking the file that moc generates for your class? Did you remember about the Q_OBJECT macro?

Quote:

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Originally Posted by wysota

What do you mean you have the moc? Are you compiling and linking the file that moc generates for your class? Did you remember about the Q_OBJECT macro?

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Oh, I find out that in the .h files I don't have the configurations any more. Strange, I've set them in the past.... Maybe erased during copy and paste.

Thanks very much!!!!!!! I think my problem is solved!!!!

I know why... because I changed to release model from debug model
(I am using visual studio)....

Hi, I have a another question.
If I want to emit something not simple as bool but my own class instance, this also can be done via SIGNAL & SLOTS?

Quote:

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Originally Posted by stella1016

Hi, I have a another question.
If I want to emit something not simple as bool but my own class instance, this also can be done via SIGNAL & SLOTS?

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Yes, but just read about Signal and Slots in Docu in case you want e.g. Queued Connection.

I doubt that whether SIGNAL & SLOT can be synchronized...
In my old project, I need to update a QGLWidget with the new camera output data. I used SIGNAL & SLOT. But sometimes the two SIGNALS emitted, but only one SLOT responds.
If anyone interested, I can post the code.

Quote:

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Originally Posted by stella1016

I doubt that whether SIGNAL & SLOT can be synchronized...

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Signals & slots can be synchronized.

Quote:

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In my old project, I need to update a QGLWidget with the new camera output data. I used SIGNAL & SLOT. But sometimes the two SIGNALS emitted, but only one SLOT responds.

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This is hardly possible. You must have done something wrong.

I hope I did something wrong. Maybe you can help me solve my old problem. I post my code here. My app is to display the camera output images in QGLWidget.

MyThread: in its run method, it does the while-loop

Code:

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run()
{
   while(flag)
   {
	IplImage *lp_color = t_camera.get_image();
	myQLWidget->setCurrentImageIpl(lp_color);
 	cvReleaseImage(&lp_color);
    }
}

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MyQLWidget: in charge of updating the texture when it receives any image.
I am using SIGNAL & SLOT in MyQLWidget, because in method setCurrentImageIpl(), I cannot directly call updateGL(). This is because OpenGl & Thread issue... So in setCurrentImageIpl() I emit a SIGNAL to a SLOT, the SLOT will do updateGL() for me.

Code:

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MyQLWidget::setCurrentImageIpl()
{
 
    ...(set image)
    imageData = ...;
    update = true;
    emit newData();
 
}

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newData is connect to SLOT updateImage()

Code:

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MyQLWidget::updateImage()
{
  this->updateGL();
}
 
MyQLWidget::paintGL()
{
  if(update)
       glTexSubImage2D = (.....imageData);
}

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During displaying the first few frames that grasped by camera, my program crashes. Not every time. I don't know where is the problem. When it crashes, the reason is that the imageData is not set jet, so when update texture it crashes.

I set breakpoint, and found that in the first few steps, ...emit... executes twice, but updateImage() only once. Maybe this causes the crash, I am not sure.

Hard to explain what I did in my code here. :(

why I posted twice..

Quote:

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Originally Posted by stella1016

I hope I did something wrong. Maybe you can help me solve my old problem. I post my code here. My app is to display the camera output images in QGLWidget.

MyThread: in its run method, it does the while-loop

Code:

---

run()
{
   while(flag)
   {
	IplImage *lp_color = t_camera.get_image();
	myQLWidget->setCurrentImageIpl(lp_color);
 	cvReleaseImage(&lp_color);
    }
}

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This is bad. You can't access widgets from other threads and I can't stress that more. You should have emitted a signal from the thread (from the thread and not the thread object!) that would be caught by a slot in the widget.

Quote:

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So in setCurrentImageIpl() I emit a SIGNAL to a SLOT, the SLOT will do updateGL() for me.

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This is wrong. You are emitting a signal on behalf of the widget from a worker thread. Qt checks which thread owns the sending and receiving objects, sees that it is the GUI thread, so makes a direct function call to updateGL() from the worker thread which is wrong and will cause segmentation faults sooner or later.

Quote:

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During displaying the first few frames that grasped by camera, my program crashes.

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That's exactly what I mean :)

Quote:

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Originally Posted by wysota

This is bad. You can't access widgets from other threads and I can't stress that more. You should have emitted a signal from the thread (from the thread and not the thread object!) that would be caught by a slot in the widget.

Quote:

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So in setCurrentImageIpl() I emit a SIGNAL to a SLOT, the SLOT will do updateGL() for me.

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This is wrong. You are emitting a signal on behalf of the widget from a worker thread. Qt checks which thread owns the sending and receiving objects, sees that it is the GUI thread, so makes a direct function call to updateGL() from the worker thread which is wrong and will cause segmentation faults sooner or later.



That's exactly what I mean :)

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Thanks for your replay.
I don't fully understand what you mean...So how can I make things right?

Read about signals and multithreading, that's for a start. Then you need to make sure that:
1. you don't access any widget method from non-GUI thread directly
2. queued connections are used for signal-slot connections between threads (and I don't mean passing the optional argument to connect() but to make sure Qt will choose queued connections by itself).