Object Trees & Ownership

Construction/Destruction Order of QObjects

When QObjects are created on the heap (i.e., created with new), a tree can be constructed from them in any order, and later, the objects in the tree can be destroyed in any order. When any QObject in the tree is deleted, if the object has a parent, the destructor automatically removes the object from its parent. If the object has children, the destructor automatically deletes each child. No QObject is deleted twice, regardless of the order of destruction.

When QObjects are created on the stack, the same behavior applies. Normally, the order of destruction still doesn't present a problem. Consider the following snippet:

 int main()
 {
     QWidget window;
     QPushButton quit("Quit", &window);
     ...
 }

The parent, window, and the child, quit, are both QObjects because QPushButton inherits QWidget, and QWidget inherits QObject. This code is correct: the destructor of quit is not called twice because the C++ language standard (ISO/IEC 14882:2003) specifies that destructors of local objects are called in the reverse order of their constructors. Therefore, the destructor of the child, quit, is called first, and it removes itself from its parent, window, before the destructor of window is called.

But now consider what happens if we swap the order of construction, as shown in this second snippet:

 int main()
 {
     QPushButton quit("Quit");
     QWidget window;

     quit.setParent(&window);
     ...
 }

In this case, the order of destruction causes a problem. The parent's destructor is called first because it was created last. It then calls the destructor of its child, quit, which is incorrect because quit is a local variable. When quit subsequently goes out of scope, its destructor is called again, this time correctly, but the damage has already been done.