I don’t think the issue is with how you instantiate the QWidget. The way Qt works when one object is deleted all its children get deleted. The difference here is just when the QWidget will get deleted – when you use the “new” the object will only get deleted when the application exits and the OS reclaims the memory.

My guess it that you will get the same error using the pointer if you add after line 20 a

Most likely the issue is with Qt trying to delete the “textedit” or “layout” objects when the QWidget gets deleted.

Please note that is not good practice to leave for the OS to reclaim memory even if it seems to work.

The following code should work:

replacing with (starting on line 6):

should also work.

I think you are right fcrochik ,and I still have a question about your answer that’s why just

but not

Like I said Qt will re-parent layout and textedit on setLayout and addWidget. When you delete window they will be deleted automatically.

This is a paragraph from the “C++ GUI Programming with Qt 4 – 1st edition by Jasmin Blanchette and Mark Summerfield (page 28):

Qt’s parent–child mechanism is implemented in QObject. When we create an object (a widget, validator, or any other kind) with a parent, the parent adds the object to the list of its children. When the parent is deleted, it walks through its list of children and deletes each child. The children themselves then delete all of their children, and so on recursively until none remain.

This is a very good book and the first edition is available as a pdf for free. Look into the “Qt books” wiki.

Thanks for your response. If I just follow the Qt doc “Getting Started Programming with Qt”,I think I would get mad. :-)

jandin wrote:

Thanks for your response. If I just follow the Qt doc “Getting Started Programming with Qt”,I think I would get mad. :-)

Have a look at the book I mentioned before. I usually can’t stand programing books but I found this one very well written and easy to follow. Too bad I didn’t know about it when I started with Qt :(

jandin wrote:

1. #include <QtGui>
2. #include "mainwindow.h"
3.  
4. int main(int argv, char **args)
5. {
6.     QApplication app(argv, args);
7.     QTextEdit textEdit;
8.     QVBoxLayout layout;
9.     layout.addWidget(&textEdit);
10.  
11. //    1.If I use the object,I will get an error whe I quit the application.
12. //    Debug it,I get a signal SIGSEGV which means Segmentation fault
13.     QWidget window;
14.     window.setLayout(&layout);
15.     window.show();
16. //     2.But if I use the poniter ,it is ok.
17. //     QWidget *window=new QWidget;
18. //     window->setLayout(&layout);
19. //     window->show();
20.     return app.exec();
21. }
22. </blockquote>
23.  
24. You really need to learn C++ and figure out why the two ways of doing things are different.
25.  
26. First of all, as people already said, you're implicitly using QObject parent/children mechanism, which means that destroying an object will automatically destroy all of its children.
27.  
28. Second, what's going on here is mandated by the C++ standard: automatic object destruction happens in the reverse order than automatic object construction, that is, the last object created is the one that is destroyed first.
29.  
30. What's the last object that was built? The "QWidget window". And its dtor wipes out (with _operator delete_) all its children, namely the layout and the textedit, which were allocated on the stack. Uh oh... operator delete on an object on the stack. That's *WRONG!*
31.  
32. What happens then? The dtors of the layout and of the textedit objects are run (*WRONG!* and *WRONG!* again, or, *WRONG²!*). Well, it's too late. Your program's memory layout is already FUBAR, and you've probably already got a weird crash by now.
33.  
34. If you use the @QWidget *window = new QWidget

approach instead your application will leak memory (WRONG!) and tools like valgrind will complain and report: you never free the memory allocated there.

Of course, adding a

1. delete window

(GOOD!) line after app.exec() will crash your application for the same reason stated above (WRONG!).

So, what you can do about that? Either

• allocate everything on the heap with operator new, then delete ONLY the widget with operator delete;
• allocate everything on the stack, but ensure that automatic object deletion doesn’t interfere with QObject parent/children management, f.i.
  
  1. QWidget w;
  2. QLayout l;
  3. QTextEdit te;
  4. w.setLayout(&l);
  5. l.addWidget(&te);
  6. w.show();
  
  will do the right thing (but think about why it will).

peppe wrote:

jandin wrote:

1. #include <QtGui>
2. #include "mainwindow.h"
3.  
4. int main(int argv, char **args)
5. {
6.     QApplication app(argv, args);
7.     QTextEdit textEdit;
8.     QVBoxLayout layout;
9.     layout.addWidget(&textEdit);
10.  
11. //    1.If I use the object,I will get an error whe I quit the application.
12. //    Debug it,I get a signal SIGSEGV which means Segmentation fault
13.     QWidget window;
14.     window.setLayout(&layout);
15.     window.show();
16. //     2.But if I use the poniter ,it is ok.
17. //     QWidget *window=new QWidget;
18. //     window->setLayout(&layout);
19. //     window->show();
20.     return app.exec();
21. }
22. </blockquote>
23.  
24. You really need to learn C++ and figure out why the two ways of doing things are different.
25.  
26. First of all, as people already said, you're implicitly using QObject parent/children mechanism, which means that destroying an object will automatically destroy all of its children.
27.  
28. Second, what's going on here is mandated by the C++ standard: automatic object destruction happens in the reverse order than automatic object construction, that is, the last object created is the one that is destroyed first.
29.  
30. What's the last object that was built? The "QWidget window". And its dtor wipes out (with _operator delete_) all its children, namely the layout and the textedit, which were allocated on the stack. Uh oh... operator delete on an object on the stack. That's *WRONG!*
31.  
32. What happens then? The dtors of the layout and of the textedit objects are run (*WRONG!* and *WRONG!* again, or, *WRONG²!*). Well, it's too late. Your program's memory layout is already FUBAR, and you've probably already got a weird crash by now.
33.  
34. If you use the @QWidget *window = new QWidget

approach instead your application will leak memory (WRONG!) and tools like valgrind will complain and report: you never free the memory allocated there.

Of course, adding a

1. delete window

(GOOD!) line after app.exec() will crash your application for the same reason stated above (WRONG!).

So, what you can do about that? Either

• allocate everything on the heap with operator new, then delete ONLY the widget with operator delete;
• allocate everything on the stack, but ensure that automatic object deletion doesn’t interfere with QObject parent/children management, f.i.
  
  1. QWidget w;
  2. QLayout l;
  3. QTextEdit te;
  4. w.setLayout(&l);
  5. l.addWidget(&te);
  6. w.show();
  
  will do the right thing (but think about why it will).

thanks peppe,I can understand it totally now.

1. allocate everything on the heap with operator new, then delete ONLY the widget with operator delete;
2. allocate everything on the stack, but ensure that automatic object deletion doesn’t interfere with QObject parent/children management

So the example in the Getting Started Programming with Qt [doc.qt.nokia.com] is wrong. It should be corrected.

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:

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.

Gerolf wrote:

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

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

jandin wrote:

Thanks for your response. If I just follow the Qt doc “Getting Started Programming with Qt”,I think I would get mad. :-)

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 [developer.qt.nokia.com], it has a working version of the sample.

Gerolf wrote:

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:

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

And here we go with another “the thread has changed while posting” … :-)

Volker wrote:

Gerolf wrote:

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

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

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.

Gerolf wrote:

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.

Yes it does. And does the right thing.