QObject::connect: Cannot connect (null)::clicked() to ScanFrame::OnBtnStartScanClicked()

[Kira]

Hello All,
I am using qt 5.14 with msvc 2019 compiler.
I created a form file class in the visual studio then add a button to the form and when I am trying to call slot I am getting the following message

For reference:
connect(_ui->btnStart, SIGNAL(clicked()), this, SLOT(OnBtnStartScanClicked()));

[jsulm]

@Kira As the error message says the sender (_ui->btnStart) is null.
Is your UI initialised when you're trying to connect?
Also, connect() does not call the slot, it only connects a signal to a slot.

[Kira]

@jsulm : Yes, Ui is connected

Given below ins the lines from the constructor function
_ui = new Ui::Frame();
connect(_ui->btnStart, SIGNAL(clicked()), this, SLOT(OnBtnStartScanClicked()));

[J.Hilk]

@Kira
what exactly is the error/warning message and is there a difference, when you change it to Qt5-Syntax?

[jsulm]

@Kira said in QObject::connect: Cannot connect (null)::clicked() to ScanFrame::OnBtnStartScanClicked():

btnStart

Is this really initialised?

[Kira]

@jsulm : Its part of qtdesigner.

[Kira]

@J-Hilk : Ok thanks will try it

[SGaist]

Hi,

Don't forget to call setupUi from the ui object otherwise you are not going to get what you expect.

[Kira]

@SGaist : Its being called _ui->setupUi(this);

[Kira]

@jsulm: Also I am trying to display image on QLabel its not being displayed.
I checked the slot is being called but the image is not being displayed.
Please refer to the below slot code.

QPixmap pix;
pix.load("cat.jpg");
pix.scaled(_ui->lblROICamera->size(), Qt::KeepAspectRatio);

_ui->lblCamera->setPixmap(QPixmap::fromImage(img));

[SGaist]

Because the path to that picture is relative so unless you have that file in the same folder as the executable, it won't be loaded.

[Kira]

@SGaist :its present. I verified, actually, I haved worked with images multiple times, don't know y its showing strange behavior;

[J.Hilk]

@Kira said in QObject::connect: Cannot connect (null)::clicked() to ScanFrame::OnBtnStartScanClicked():

its present

executable, not project file!

[Kira]

@J-Hilk :ohh.
Executable is being generated but its not in the debug folder. I have provided a separated folder for the exe file

[jsulm]

@Kira Is the image in the SAME directory as the executable?

[Kira]

@jsulm : One more thing I am using a promoted widget which is QFrame consisting of QLabel and QPushbutton.
When I put label in the base class the image gets displayed but when I write the same code in the promoted widget it is not being displayed.

Base Widget label:

After pasting the similar code in promoted widget:

[jsulm]

@Kira Without code I have no idea...

[Kira]

@jsulm : Every connect statement I get a warning : avoid unnamed object with custom constructor and destructor.
Also, I also figured the problem is in the slot of the promoted widget. When I try to write the code to display the image in constructor it works fine but when I add the same code in the slot it failed to display the image.
I have also checked that the slot is invoked every time by placing a breakpoint in the respective slot.

[jsulm]

@Kira Sorry, without code and exact error messages others can only guess...

[Kira]

@jsulm : Thanks for highlighting the issue.
As mentioned earlier, I had created a base widget and promoted widget.
In the designer, I promoted the widget, and in the class file of the base widget, I created an object of the promoted widget to pass some initialization parameter. There I called the signal to slot connection.

I finally figured that the connections start working after I added them after
_ui->setupUi(this); or called them in showEvent().
Since _ui->setupUi(this); is declared in the constructor of the promoted widget. I never thought this could be the issue.
I finally got the example working by adding them in the showEvent()