objecttrees.qdoc

Go to the documentation of this file.

// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page objecttrees.html
    \title Object Trees & Ownership
    \ingroup qt-basic-concepts
    \brief Information about the parent-child pattern used to describe
    object ownership in Qt.
 
    \section1 Overview
 
    \l{QObject}{QObjects} organize themselves in object trees.
    When you create a QObject with another object as parent, it's added to
    the parent's \l{QObject::children()}{children()} list, and
    is deleted when the parent is. It turns out that this approach fits
    the needs of GUI objects very well. For example, a \l QShortcut
    (keyboard shortcut) is a child of the relevant window, so when the
    user closes that window, the shortcut is deleted too.
 
    \l QQuickItem, the basic visual element of the Qt Quick module, inherits
    from QObject, but has a concept of the \e {visual parent} which
    differs from that of the \e {QObject parent}. An item's visual parent
    may not necessarily be the same as its object parent. See
    \l {Concepts - Visual Parent in Qt Quick} for more details.
 
    \l QWidget, the fundamental class of the Qt Widgets module,
    extends the parent-child relationship. A child normally also becomes a
    child widget, i.e. it is displayed in its parent's coordinate system
    and is graphically clipped by its parent's boundaries. For example,
    when the application deletes a message box after it has been
    closed, the message box's buttons and label are also deleted, just as
    we'd want, because the buttons and label are children of the message
    box.
 
    You can also delete child objects yourself, and they will remove
    themselves from their parents. For example, when the user removes a
    toolbar it may lead to the application deleting one of its \l QToolBar
    objects, in which case the tool bar's \l QMainWindow parent would
    detect the change and reconfigure its screen space accordingly.
 
    The debugging functions \l QObject::dumpObjectTree() and \l
    QObject::dumpObjectInfo() are often useful when an application looks or
    acts strangely.
 
    \target note on the order of construction/destruction of QObjects
    \section1 Construction/Destruction Order of QObjects
 
    When \l {QObject} {QObjects} are created on the heap (i.e., created
    with \e 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 \l {QObject} {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:
 
    \snippet code/doc_src_objecttrees.cpp open
    \snippet code/doc_src_objecttrees.cpp example1
    \snippet code/doc_src_objecttrees.cpp close
 
    The parent, \c window, and the child, \c quit, are both \l {QObject}
    {QObjects} because QPushButton inherits QWidget, and QWidget inherits
    QObject. This code is correct: the destructor of \c quit is \e not
    called twice because the C++ language standard \e {(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, \c quit, is called first, and it removes itself from its
    parent, \c window, before the destructor of \c window is called.
 
    But now consider what happens if we swap the order of construction, as
    shown in this second snippet:
 
    \snippet code/doc_src_objecttrees.cpp open
    \snippet code/doc_src_objecttrees.cpp example2
    \snippet code/doc_src_objecttrees.cpp close
 
    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, \c quit, which is incorrect because \c
    quit is a local variable. When \c quit subsequently goes out of scope,
    its destructor is called again, this time correctly, but the damage has
    already been done.
*/