call to non static member function without an object argument

@MalitheBIG , @Chaki
In order to use Qt you do need to know C++. This forum is for helping people with Qt questions. We do try, but we are not a C++ tutorial forum. That's just how it is.

There's nothing wrong with learning to program and not yet knowing C++ well enough. Nobody blames you for that. It's just that here is not the best place to ask about C++ questions, there are better, more suitable C++ learning forums out there.

Having said that. The Qt connect() statement requires 4 arguments, 2 pairs of 2, as follows:

connect(signallingObject, signallingMethod,
        slotObject, slotMethod);

signallingObject is some object on which something happens --- like a click.
signallingMethod is a method (belonging to the signallingObject class, declared as a signals method in the .h file).
slotObject is some object which wants to know when the event happens --- like, show a message when signallingObject gets clicked.
slotMethod is a method (belonging to the slotObject class, declared as a slots method in the .h file). slotMethod should accept the parameters which signallingMethod sends.

So a simple example example looks like

// In, say, your `MainWindow` class
connect(ui->someButton, &QPushButton::clicked,
        this, &MainWindow::onButtonClicked);

/*slot*/ MainWindow::onButtonClicked()
{
    qDebug() << "Button was clicked";
}

The important think is that the connect() will usually look like:

connect(signallingObject, &SignallingObjectClass::method,
        slotObject, &SlotObjectClass::method);

You would benefit hugely from taking some time to study some of the tutorial signals & slots examples out there on the web. They are better than asking people here to come up with examples. If you Google for qt5 signal slot tutorial you get several suggestions, not only in the Qt docs, which is where I would start from:
https://doc.qt.io/qt-5/signalsandslots.html
https://www.bogotobogo.com/Qt/Qt5_SignalsSlotsGui.php
https://prognotes.net/2019/12/qt-5-signals-and-slots-tutorial/
https://www.vikingsoftware.com/getting-the-most-of-signal-slot-connections/
https://wiki.qt.io/New_Signal_Slot_Syntax

P.S.
I will strongly recommend one thing. In @Chaki's code, and in a lot of older examples on the web, the old style SIGNAL() & SLOT() macros are used for the methods. Do not do this. It might look simpler than the new style &Class::method, but it allows you to write nonsense, which gets through the compiler but doesn't work at runtime. Using the new style will only allow you to write something sensible at compile time, and if it rejects what you try it means it was not correct. The new style is covered in the https://wiki.qt.io/New_Signal_Slot_Syntax link I gave above.

KroMignon

@MalitheBIG said in call to non static member function without an object argument:

@jsulm Naja wenn man sowas schreibt finde ich das nicht ganz so nett! Man könnte ja etwas Nettigkeit and den Tag legen!

Why are you fell hurt when he asked you to learn first C++ basics before trying to develop with Qt, which is a C++ framework!
Would you also be hurt if someone tells you to first learn Python before trying to use Django?

When you try to build a house, do you start with the roof before the foundations?

jsulm

I suggest to stop the off-topic discussion here. If somebody wants to continue please create a new thread and link to this one.

Pl45m4

@MalitheBIG @Chaki

lol, you guys feel offended, when getting told to look at the docs for about 5 mins again...
I think we had the same discussion with some other users before.
If you looked at the docs, and still don't know what to do, you should really consider to improve your C++ knowledge.... you don't need to be an expert in C++ to use this basic Qt stuff.

This is also a volunteer's forum. Most people here can help you with your Qt and / or C++ issue, but nobody will teach you basic C++.

Btw: If you would have posted the same question elsewhere (SO, cough), you would get roasted very badly ;-)