implicit-sharing.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
 
/* TODO: Move some of the documentation from QSharedDataPointer into this
   document. */
 
/*!
    \group shared
    \brief How to maximize resource usage by implicit data sharing.
    \title Implicitly Shared Classes
 
    These \l{Qt Core} classes provides a safe and efficient way of sharing and
    manipulating data by \l{Implicit Sharing}{implicitly sharing} data.
 
*/
 
/*!
    \page implicit-sharing.html
    \title Implicit Sharing
    \ingroup qt-basic-concepts
 
    \brief Reference counting for fast copying.
 
    \keyword implicit data sharing
    \keyword implicit sharing
    \keyword implicitly shared
    \keyword reference counting
    \keyword shared implicitly
    \keyword shared classes
 
    Many C++ classes in Qt use implicit data sharing to maximize
    resource usage and minimize copying. Implicitly shared classes are
    both safe and efficient when passed as arguments, because only a
    pointer to the data is passed around, and the data is copied only
    if and when a function writes to it, i.e., \e {copy-on-write}.
 
    \tableofcontents
 
    \section1 Overview
 
    A shared class consists of a pointer to a shared data block that
    contains a reference count and the data.
 
    When a shared object is created, it sets the reference count to 1. The
    reference count is incremented whenever a new object references the
    shared data, and decremented when the object dereferences the shared
    data. The shared data is deleted when the reference count becomes
    zero.
 
    \target deep copy
    \target shallow copy
 
    When dealing with shared objects, there are two ways of copying an
    object. We usually speak about \e deep and \e shallow copies. A deep
    copy implies duplicating an object. A shallow copy is a reference
    copy, i.e. just a pointer to a shared data block. Making a deep copy
    can be expensive in terms of memory and CPU. Making a shallow copy is
    very fast, because it only involves setting a pointer and incrementing
    the reference count.
 
    Object assignment (with operator=()) for implicitly shared objects is
    implemented using shallow copies.
 
    The benefit of sharing is that a program does not need to duplicate
    data unnecessarily, which results in lower memory use and less copying
    of data. Objects can easily be assigned, sent as function arguments,
    and returned from functions.
 
    Implicit sharing mostly takes place behind the scenes;
    the programmer rarely needs to worry about it. However, Qt's
    container iterators have different behavior than those from
    the STL. Read \l{Implicit sharing iterator problem}.
 
    In multithreaded applications, implicit sharing takes place, as explained in
    \l{Thread-Support in Qt Modules#Threads and Implicitly Shared Classes}
    {Threads and Implicitly Shared Classes}.
 
    When implementing your own implicitly shared classes, use the
    QSharedData and QSharedDataPointer classes.
 
    \section1 Implicit Sharing in Detail
 
    Implicit sharing automatically detaches the object from a shared
    block if the object is about to change and the reference count is
    greater than one. (This is often called \e {copy-on-write} or
    \e {value semantics}.)
 
    An implicitly shared class has control of its internal data. In
    any member functions that modify its data, it automatically detaches
    before modifying the data. Notice, however, the special case with
    container iterators; see \l{Implicit sharing iterator problem}.
 
    The QPen class, which uses implicit sharing, detaches from the shared
    data in all member functions that change the internal data.
 
    Code fragment:
    \snippet code/doc_src_groups.cpp 0
 
 
    \section1 List of Classes
 
    The classes listed below automatically detach from common data if
    an object is about to be changed. The programmer will not even
    notice that the objects are shared. Thus you should treat
    separate instances of them as separate objects. They will always
    behave as separate objects but with the added benefit of sharing
    data whenever possible. For this reason, you can pass instances
    of these classes as arguments to functions by value without
    concern for the copying overhead.
 
    Example:
    \snippet code/doc_src_groups.cpp 1
 
    In this example, \c p1 and \c p2 share data until QPainter::begin()
    is called for \c p2, because painting a pixmap will modify it.
 
    \warning Be careful with copying an implicitly shared container
    (QMap, QList, etc.) while you use
    \l{STL-style iterators}{STL-style iterator}. See \l{Implicit sharing iterator problem}.
 
    \target implicitly shared classes
    \annotatedlist shared
*/