Question

I want to display an image in a “functional” programming style. Basically, I operate a number of processes on my image and I sometimes want to display the result. So I am trying:

int display(string file, int argc, char *argv[])
{
    QApplication a1(argc,argv);
    QImage myImage;
    myImage.load(file.c_str());
    QLabel myLabel;
    myLabel.setPixmap(QPixmap::fromImage(myImage));
    myLabel.show();
    return a1.exec();
}
int main(int argc, char *argv[])
{
    MyImageDataStructure img;
    img.erosion(5);//process 1
    img.save("lenaero.png");
    display("lenaeor.png",argc,argv); // display the first result
    img.dilation(5);//process 2
    img.save("lenaopening.png");
    display("lenaopening.png",argc,argv); // display the second result
    return 1;
}

But at the second execution of the display function, an error occurs. Do you have any ideas to solve this problem while keeping this logic?

Thanks

Note: I don’t want to include external libraries except Qt and I understand that I want to work outside the Qt logic.

Answer 1

I don’t really understand what you mean by “display an image in a functional style”. I think what you mean is that you want a function to do the display part for an image. In that case, there seems to be a misconception on your part as to what a “functional” paradigm actually is.

Nevertheless, your problem is very different. You need to move the QApplication declaration into the main function:

int main(...) {
    QApplication a(argc, argv);

    // ...

}

And move it out of the display() function.

EDIT

I re-read your code, and there are a lot of things wrong with it. You SHOULD NOT be creating a separate application every time you wish to display an image. A much better thing would be to use a QDialog. Create a dialog, with a label as an image and call show on it every time you wish to display the image.

Useful links:

http://qt-project.org/doc/qt-4.8/QDialog.html

http://sector.ynet.sk/qt4-tutorial/my-first-qt-gui-application.html (Nice overview of creating and showing dialogs)