Declare QPixmap as global variable cause runtime error???

[trandinhduy]

Hi,

In my .cpp file, I declare some global variables of type QPixmap. When I run the app, it crashes when the app is starting (even before the constructor of the class is called).

Here is the code:

// Own
#include "mainwindow.h"
...


QPixmap defaultImage; //This causes runtime error
QPixmap waitingImage; //This causes runtime error

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{      
    ui->setupUi(this);

    ....
}

Note that I still not use these variables elsewhere, just declare them and the app is crashed when starting.

Anyone can help???

Thanks.

[danhicksbyron]

It may be that you can't create complex Qt static objects, since QApplication had not yet run to initialize the runtime environment.

[divanov]

You must construct a QApplication before using QPaintDevice.

[trandinhduy]

You must construct a QApplication before using QPaintDevice.

Could you please make it more detail or give an example about this. I do not understand about what you mean?

My app runs normally before I add these two variables (just adding, still not using it).

Thank you.

[danhicksbyron]

Basically we're saying you can't do what you want to do. In your main() you have to code a QApplication() as your very first Qt operation. This creates the Qt environment. Static initialization occurs before your main() is executed, so there is no Qt environment when the QPixmaps attempt to initialize themselves.

I can't think of a way to reliably create a QApplication before other objects are statically inited. Even if you coded the QApplication as a static (and that actually worked, which is doubtful), you'd have no guarantee that it would occur before other static initializations.

[trandinhduy]

Oh, I got your idea. Thanks

Just clarify a little bit:

I am using QMainWindow as a main class, so these global variables are init before my main class is init. This cause the error?

Is there any way I can solve this problem????

Thanks.

[danhicksbyron]

Well, yes, but more importantly QApplication (look in your main.cpp) is inited before anything else, including your QMainWindow. But it's not inited before static objects are inited (due to the way C++ works), and that's the problem.

[trandinhduy]

Ahhhh, finally solve it.

I just declare global variables QPixmap as a pointer, then I init them in the constructor of MainWindow.

Thank you guys.

[divanov]

Just note that using global variables is a sign of a bad design.

[trandinhduy]

Just note that using global variables is a sign of a bad design.

Oh really. .

It would be great if you can help me to improve myself.

In my case: I have some images to use for many widgets at the same time. For that reason, what should I do??? Should I use singleton by placing those images in the MainWindow and call get method to get them???

Thank you.

[danhicksbyron]

Keep in mind that Qt does pixmap caching for you, so you don't have to go overboard in trying to avoid reloading QPixmaps.