object.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page object.html
    \title Object Model
    \ingroup qt-basic-concepts
    \brief A description of the powerful features made possible by Qt's dynamic object model.
 
    The standard C++ object model provides very efficient runtime
    support for the object paradigm. But its static nature is
    inflexibile in certain problem domains. Graphical user interface
    programming is a domain that requires both runtime efficiency and
    a high level of flexibility. Qt provides this, by combining the
    speed of C++ with the flexibility of the Qt Object Model.
 
    Qt adds these features to C++:
 
    \list
    \li a very powerful mechanism for seamless object
       communication called \l{signals and slots}
    \li queryable and designable \l{Qt's Property System}{object
       properties}
    \li powerful \l{The Event System}{events and event filters}
    \li contextual \l{Internationalization with Qt}{string translation for
       internationalization}
    \li sophisticated interval driven \l {Timers}{timers} that make it possible
       to elegantly integrate many tasks in an event-driven GUI
    \li hierarchical and queryable \l{Object Trees & Ownership}{object
       trees} that organize object ownership in a natural way
    \li guarded pointers (QPointer) that are automatically
       set to 0 when the referenced object is destroyed, unlike normal C++
       pointers which become dangling pointers when their objects are destroyed
    \li a \l{metaobjects.html#qobjectcast}{dynamic cast} that works across
       library boundaries.
    \li support for \l{Creating Custom Qt Types}{custom type} creation.
    \endlist
 
    Many of these Qt features are implemented with standard C++
    techniques, based on inheritance from QObject. Others, like the
    object communication mechanism and the dynamic property system,
    require the \l{Meta-Object System} provided
    by Qt's own \l{moc}{Meta-Object Compiler (moc)}.
 
    The meta-object system is a C++ extension that makes the language
    better suited to true component GUI programming.
 
    \section1 Important Classes
 
    These classes form the basis of the Qt Object Model.
 
    \annotatedlist objectmodel
 
    \target Identity vs Value
    \section1 Qt Objects: Identity vs Value
 
    Some of the added features listed above for the Qt Object Model,
    require that we think of Qt Objects as identities, not values.
    Values are copied or assigned; identities are cloned. Cloning
    means to create a new identity, not an exact copy of the old
    one. For example, twins have different identities. They may look
    identical, but they have different names, different locations, and
    may have completely different social networks.
 
    Then cloning an identity is a more complex operation than copying
    or assigning a value. We can see what this means in the Qt Object
    Model.
 
    \b{A Qt Object...}
 
    \list
 
    \li might have a unique \l{QObject::objectName()}.  If we copy a Qt
    Object, what name should we give the copy?
 
    \li has a location in an \l{Object Trees & Ownership}
    {object hierarchy}. If we copy a Qt Object, where should the copy
    be located?
 
    \li can be connected to other Qt Objects to emit signals to them or
    to receive signals emitted by them. If we copy a Qt Object, how
    should we transfer these connections to the copy?
 
    \li can have \l{Qt's Property System} {new properties} added to it
    at runtime that are not declared in the C++ class. If we copy a Qt
    Object, should the copy include the properties that were added to
    the original?
 
    \endlist
 
    For these reasons, Qt Objects should be treated as identities, not
    as values. Identities are cloned, not copied or assigned, and
    cloning an identity is a more complex operation than copying or
    assigning a value. Therefore, QObject and all subclasses of
    QObject (direct or indirect) have their \l{No copy constructor}
    {copy constructor and assignment operator} disabled.
 
  */