Qt: Can child objects be composed in their parent object?

Question

In Qt, can I embed child widgets in their parent via composition, or do I have to create them with new?

class MyWindow : public QMainWindow
{
    ...
private:
    QPushButton myButton;
}

MyWindow::MyWindow ()
 : mybutton("Do Something", this)
{
   ...
}

The documentation says that any object derived from QObject will automatically destroyed when its parent is destroyed; this implies a call to delete, whcih in the above example would crash.

Do I have to use the following?

QPushButton* myButton;

myButton = new QPushButton("Do Something", this);

EDIT

The answers are quite diverse, and basically boil down to three possibilities:

• Yes, composition is ok. Qt can figure out how the object was allocated and only delete heap-allocated objects (How does this work?)
• Yes, composition is ok, but don't specify a parent, since the parent would otherwise call delete on the object (But won't a parent-less widget turn into a top-level window?)
• No, widgets always have to be heap-allocated.

Which one is correct?

Answer 1

The non-static, non-heap member variables are deleted when that particular object's delete sequence starts. Only when all members are deleted, will it go to the destructor of the base class. Hence QPushButton myButton member will be deleted before ~QMainWindow() is called. And from QObject documentation: "If we delete a child object before its parent, Qt will automatically remove that object from the parent's list of children". Hence no crash will occur.

Answer 2

Object trees & ownership answers your question. Basically when the child object is created on the heap it will be deleted by its parent.

On the other hand when the child object is created on the stack the order of destruction is important. The child will be destroyed before its parent and will remove itself from its parent's list so that its destructor is not called twice.

There is also an example in that link that shows problematic order of destruction.