Why is the recommended QDialog instantiation the way it is? - c++

Question

I have a Qt Widgets application, created and edited in Qt-Creator.
The main window (MainWindow class) has a menubar, with a button to open a small dialog (with text or widgets for settings).
To create a new "window" I open the "create new file" dialog in Qt-Creator and select Qt Designer Form Class, which creates the needed header, source, and ui files (dialogabout.h, dialogabout.cpp, dialogabout.ui).
If I follow along with the docs, I then open the QDialog like so:
QDialog * widget = new QDialog;
Ui::DialogAbout about_ui;
about_ui.setupUi(widget);
widget->exec();
This works, but if I modify the new dialog's instantiator to connect a pushbutton to the close signal, the connect statement (along with any other code there) is never reached.
DialogAbout::DialogAbout(QWidget *parent) :
QDialog(parent),
ui(new Ui::DialogAbout)
{
ui->setupUi(this);
qDebug() << "I'm alive!"; // No output happens
connect(ui->pushButton_close, SIGNAL(clicked(bool)), this, SIGNAL(please_close())); // No signal created on pushbutton click.
}
I suspect that this is because I haven't explicitly done widget = new DialogAbout(this). If I instead instantiate the new dialog this different way:
DialogAbout * newwindow;
newwindow = new DialogAbout(this);
newwindow->exec();
Then the connect statement and qDebug work.
My question is: what are the pitfalls of deviating from the documentation's recommended way to create dialogs? Is there a way to get this functionality with the prior instantiation method?

Answer 1

Note that DialogAbout is not the same as Ui::DialogAbout. Ui::DialogAbout is a class of build placed in the UI namespace, created automatically by uic. In your project, the name of this file should be "ui_dialogabout h".
class Ui_DialogAbout
{
public:
QPushButton *pushButton_close;
void setupUi(QDialog *DialogAbout)
{
...
} // setupUi
void retranslateUi(QDialog *DialogAbout)
{
...
} // retranslateUi
};
namespace Ui {
class DialogAbout: public Ui_DialogAbout {};
} // namespace Ui
Here you use a class QDialog and uses the Ui::DialogAbout to build a layout in it. Note that Ui::DialogAbout has the function to create the components in QDialog.
QDialog * widget = new QDialog;
Ui::DialogAbout about_ui;
about_ui.setupUi(widget);
widget->exec();
If you specialize QDialog for DialogAbout your code should look like this:
DialogAbout * widget = new DialogAbout();
Ui::DialogAbout about_ui;
about_ui.setupUi(widget);
widget->exec();
But as setupUi() is already within DialogAbout, you cannot call again, resulting in:
DialogAbout * widget = new DialogAbout();
widget->exec();