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29 \page objecttrees.html
30 \title Object Trees & Ownership
31 \ingroup qt-basic-concepts
32 \brief Information about the parent-child pattern used to describe
33 object ownership in Qt.
37 \link QObject QObjects\endlink organize themselves in object trees.
38 When you create a QObject with another object as parent, it's added to
39 the parent's \link QObject::children() children() \endlink list, and
40 is deleted when the parent is. It turns out that this approach fits
41 the needs of GUI objects very well. For example, a \l QShortcut
42 (keyboard shortcut) is a child of the relevant window, so when the
43 user closes that window, the shorcut is deleted too.
45 \l QWidget, the base class of everything that appears on the screen,
46 extends the parent-child relationship. A child normally also becomes a
47 child widget, i.e. it is displayed in its parent's coordinate system
48 and is graphically clipped by its parent's boundaries. For example,
49 when the application deletes a message box after it has been
50 closed, the message box's buttons and label are also deleted, just as
51 we'd want, because the buttons and label are children of the message
54 You can also delete child objects yourself, and they will remove
55 themselves from their parents. For example, when the user removes a
56 toolbar it may lead to the application deleting one of its \l QToolBar
57 objects, in which case the tool bar's \l QMainWindow parent would
58 detect the change and reconfigure its screen space accordingly.
60 The debugging functions \l QObject::dumpObjectTree() and \l
61 QObject::dumpObjectInfo() are often useful when an application looks or
64 \target note on the order of construction/destruction of QObjects
65 \section1 Construction/Destruction Order of QObjects
67 When \l {QObject} {QObjects} are created on the heap (i.e., created
68 with \e new), a tree can be constructed from them in any order, and
69 later, the objects in the tree can be destroyed in any order. When any
70 QObject in the tree is deleted, if the object has a parent, the
71 destructor automatically removes the object from its parent. If the
72 object has children, the destructor automatically deletes each
73 child. No QObject is deleted twice, regardless of the order of
76 When \l {QObject} {QObjects} are created on the stack, the same
77 behavior applies. Normally, the order of destruction still doesn't
78 present a problem. Consider the following snippet:
80 \snippet doc/src/snippets/code/doc_src_objecttrees.cpp 0
82 The parent, \c window, and the child, \c quit, are both \l {QObject}
83 {QObjects} because QPushButton inherits QWidget, and QWidget inherits
84 QObject. This code is correct: the destructor of \c quit is \e not
85 called twice because the C++ language standard \e {(ISO/IEC 14882:2003)}
86 specifies that destructors of local objects are called in the reverse
87 order of their constructors. Therefore, the destructor of
88 the child, \c quit, is called first, and it removes itself from its
89 parent, \c window, before the destructor of \c window is called.
91 But now consider what happens if we swap the order of construction, as
92 shown in this second snippet:
94 \snippet doc/src/snippets/code/doc_src_objecttrees.cpp 1
96 In this case, the order of destruction causes a problem. The parent's
97 destructor is called first because it was created last. It then calls
98 the destructor of its child, \c quit, which is incorrect because \c
99 quit is a local variable. When \c quit subsequently goes out of scope,
100 its destructor is called again, this time correctly, but the damage has