Question

Short summary:
My Qt class crashes at class construction part when calling the parent QWidget constructor. Error message shows: “First-chance exception at 0x0048fca9 in MyApplication.exe: 0xC0000005: Access violation reading location 0x00000000.”

Details:
I’m working on a project to add an overlay user interface to the one client, much like what steam has when you press “tab+shift” while game play. The original application is written in visual studio environment without any UI stuff, and I decide to use Qt to do the job.

Since the project is pretty complex to transform to a Qt project with the .pro file, I want to basically use Qt as another library (including the lib and include folder and use it in the project).

It works fine when I write it in C language. The menu show up but it’s simply not clickable. It seems like in order to make the Qt event loop to work, I have to declare my custom widget as a class rather than group of C functions.

I added my Qt class into the project, modify the include and lib folder, generate the Moc file, including it in the project. Everything compiles but when I run the program, it shows the error message. Tracking into the code shows that the constructor fails when it tried to construct QWidget.

I have searched online but most tutorial tells you how to work in Qt Creator or with Visual Studio Addin from scratch.

Solved thanks to Smi:

The problem was that I initialize QApplication later than QWidget gets initialized. Adding QApplication before calling constructor fixed the problem.

Answer 1

One possible reason for QWidget constructor failing is that you have forgot to initialize the QApplication object. The Qt documentation says:

For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any given time.

And also:

Since the QApplication object does so much initialization, it must be created before any other objects related to the user interface are created.