Why this program crashed?

[John Van]

Hi
I encountered a problem that confused me. When I debug the program,it crashed.

int main(int argc, char* argv[])
{
    QCoreApplication a(argc, argv);
    QImage dfs("1.png");
    QPainter painter;
    bool ret = painter.begin(&dfs);
    auto ddd = dfs.paintEngine();
    if (!dfs.isNull() && painter.isActive())
        painter.drawText(100, 100, QString("1"));
    return a.exec();
}

the qt version is 5.14.2.

Regrads!

[jsulm]

@John-Van said in Why this program crashed?:

bool ret = painter.begin(&dfs);

You do not check whether the image is null or not before you call begin(). I guess it is null because you use a relative path to the image so it is not found...

[ChrisW67]

@jsulm The QImage object exists (it's on the stack) but it may represent a "null" image for the reason you gave. The attempt to paint is guarded by a check for a "null" image, so this should not even be executed in that circumstance.

@John-Van Crashes here for me with an existing image (Qt 5 or 6).
The program is crashing because it needs the QFontDatabase to render text. The requires at least a QGuiApplication object. Under Qt 6 you get the useful warning QFontDatabase: Must construct a QGuiApplication before accessing QFontDatabase before the crash.

[jsulm]

@ChrisW67 said in Why this program crashed?:

but it may represent a "null" image for the reason you gave

That's what I was talking about

[John Van]

@ChrisW67 Thank you.The problem was resolved after I used QApplication instead of QCoreApplication.