QT4 Memory Management

Question

I come from a fairly strong C background, and have a rather solid foundation in C++. More recently I've been working with C# and other higher level languages. A project I'm looking at working on could really benefit from using QT4, but I have some questions on memory management I can't seem to understand. I've read the QT4 documentation and it hasn't helped me much. So that is why I'm here.

1) Okay so to start with, I understand that the easiest way to use QT4 objects is to declare them locally:

void MyFunc()
{
     QString foo;
     // do stuff to foo

}

This is simple enough, I can take that object, and pass it around and know that when it goes out of scope it will be destroyed. But here's my question.

1)If I create a QList and add objects to it, and then the QList goes out of scope, will it try to deallocate the child objects?

2)If a QT4 routine returns a pointer to an object, am I then responsible for the de-allocation of that object?

3)If I create a subclass of a QWidget, and add it to a QWindow, how do I insure that when the QWindow is destroyed, that it takes my widget with it?

Thanks for the help.

Answer 1

The answer is the Parent/Child object relationship that exist in Qt. When an parent object goes out of scope, or is destroyed by any other means, Qt ensures that all it's child objects are destroyed to.

More about this behavior can be found here -> http://doc.trolltech.com/4.4/objecttrees.html

Answer 2

If I create a QList and add objects to it, and then the QList goes out of scope, will it try to deallocate the child objects?

QList is just like std::list. It will destroy the contained objects when it is destroyed.

If a Qt4 routine returns a pointer to an object, am I then responsible for the de-allocation of that object?

Usually no, the docs should specify what happens. An exception are the take* functions (e.g: QTableWidget::takeItem).

If I create a subclass of a QWidget, and add it to a QWindow, how do I insure that when the QWindow is destroyed, that it takes my widget with it?

It depends on how you create the subclass object.

• You could add it as a member of the window widget (there is no QWindow, by the way) and it will be destroyed just like any member variable.
• You could allocate it with new and pass it the window as parent and it will be deleted thanks to the Qt object tree implementation (as cake mentioned)
• You could do the memory management yourself.

When a QWidget (or any QObject) is destroyed, it will remove itself from its parent's to-delete list so you can delete it yourself and not worry about double deletes.

Answer 3

You might want to start here: Object Trees as this explains the parent/child relationship Qt uses (and all the other links given so far are out of date or 3rd party).

1)If I create a QList and add objects to it, and then the QList goes out of scope, will it try to deallocate the child objects?

Yes, it behaves just like std::list. Just like std::list, it will only deallocate not delete. This is where some of the Qt pointer classes come in handy

2)If a QT4 routine returns a pointer to an object, am I then responsible for the de-allocation of that object?

As rpg said, usually not, but if you are the documentation will say so.

3)If I create a subclass of a QWidget, and add it to a QWindow, how do I insure that when the QWindow is destroyed, that it takes my widget with it?

When you create your subclass, be sure to use the parent parameter in the ctor. This way the object will be deleted by Qt. If needed, you can set the parent after with QObject::setParent ( QObject * parent ).