The difference between Qwidget object and pointer

giesbert

Hi,

it goes a but farer. You should give the textedit a parent, as the text edit is not automatically reparented. So it should look like this:

@
QApplication app(argv, args);

QWidget window;
QVBoxLayout *layout = new QVBoxLayout();
window.setLayout(layout);
QTextEdit *textEdit = new QTextEdit(&window);
layout->addWidget(textEdit);

window.show();

return app.exec();

@

The top level widget can be created on the stack, so it is then deleted automatically, or you can create it on the heap and use a smart pointer (scoped ptr) to automatically delete it. But child widgets MUST be created on the heap.

? Offline

[quote author="Gerolf" date="1294128379"]it goes a but farer. You should give the textedit a parent, as the text edit is not automatically reparented.[/quote]

It actually is reparented. That's why the sample program of the getting started guide crashes.

? Offline

[quote author="jandin" date="1294109520"]Thanks for your response. If I just follow the Qt doc "Getting Started Programming with Qt",I think I would get mad. :-)[/quote]

Unfortunately the sample code in the Getting Started Guide is wrong and causes the application to crash. See the thread "Getting started example is buggy":http://developer.qt.nokia.com/forums/viewthread/2253/, it has a working version of the sample.

dangelog

[quote author="Gerolf" date="1294128379"]Hi,

it goes a but farer. You should give the textedit a parent, as the text edit is not automatically reparented. So it should look like this:[/quote]

Actually not; putting it into the layout will automatically reparent it.

dangelog

And here we go with another "the thread has changed while posting" ... :-)

giesbert

[quote author="Volker" date="1294147102"][quote author="Gerolf" date="1294128379"]it goes a but farer. You should give the textedit a parent, as the text edit is not automatically reparented.[/quote]

It actually is reparented. That's why the sample program of the getting started guide crashes.[/quote]

But it's only reparented, if the layout already is set to a widget, so creating the layout, adding widgets and the setting the layout to it's parent does not work.

dangelog

[quote author="Gerolf" date="1294155697"]
But it's only reparented, if the layout already is set to a widget, so creating the layout, adding widgets and the setting the layout to it's parent does not work.[/quote]

Yes it does. And does the right thing.

@
#include <QtGui>

int main(int argc, char **argv)
{
QApplication app(argc, argv);
QHBoxLayout *layout = new QHBoxLayout;
layout->addWidget(new QTextEdit);
QWidget w;
w.setLayout(layout);
w.show();
return app.exec();
}
@

? Offline

[quote author="peppe" date="1294156553"]Yes it does. And does the right thing.[/quote]

Correct. It is a two step process:

layout takes ownership of the text edit on addWidget()
widget w takes ownership of the layout on setLayout() and thus on all of the layout's widgets

dangelog

[quote]layout takes ownership of the text edit on addWidget()[/quote]

No :-)

A QLayout cannot own a text edit. Remember: widgets can have only another widget as parent. Not arbitrary QObjects.

What happens is that the QLayout simply "remembers" about the text edit; the reparenting happens when setLayout() is called. After that, the hierarchy will be something like

QWidget
** QLayout
** QTextEdit

(notice that the layout and the text edit are siblings and both children of the QWidget).

giesbert

Ok, you are right, I checked in the Qt source code. But I think it was different in earlier versions... Perhaps it was 3.X and I just never tried it again that way....

I had some of thos funny things with top level combo boxes some time....

? Offline

Peppe, your right. I mixed up things from what I looked up in the sources some times ago. The layout only keeps book of the widgets it manages, but does not take ownership.

But what's really important for the users: After the UI is set up, all parent-child relationships are correct. I really do like Qt :-)