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261 lines
9.2 KiB
261 lines
9.2 KiB
/****************************************************************************
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**
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** Qt Shared Classes Documentation
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**
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** Copyright (C) 1992-2008 Trolltech ASA. All rights reserved.
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**
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** This file is part of the Qt GUI Toolkit.
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**
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** This file may be used under the terms of the GNU General
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** Public License versions 2.0 or 3.0 as published by the Free
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** Software Foundation and appearing in the files LICENSE.GPL2
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** and LICENSE.GPL3 included in the packaging of this file.
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** Alternatively you may (at your option) use any later version
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** of the GNU General Public License if such license has been
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** publicly approved by Trolltech ASA (or its successors, if any)
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** and the KDE Free Qt Foundation.
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**
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** Please review the following information to ensure GNU General
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** Public Licensing retquirements will be met:
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** http://trolltech.com/products/qt/licenses/licensing/opensource/.
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** If you are unsure which license is appropriate for your use, please
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** review the following information:
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** http://trolltech.com/products/qt/licenses/licensing/licensingoverview
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** or contact the sales department at sales@trolltech.com.
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**
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** This file may be used under the terms of the Q Public License as
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** defined by Trolltech ASA and appearing in the file LICENSE.QPL
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** included in the packaging of this file. Licensees holding valid Qt
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** Commercial licenses may use this file in accordance with the Qt
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** Commercial License Agreement provided with the Software.
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**
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** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
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** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
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** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
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** herein.
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**
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**********************************************************************/
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/*!
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\page shclass.html
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\title Shared Classes
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\keyword reference counting
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\keyword implicit sharing
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\keyword explicit sharing
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\keyword implicitly shared
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\keyword explicitly shared
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\keyword explicit sharing
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\keyword shared implicitly
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\keyword shared explicitly
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Many C++ classes in Qt use \e explicit and \e implicit data sharing
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to maximize resource usage and minimize copying of data.
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\tableofcontents
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\section1 Overview
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A shared class consists of a pointer to a shared data block that
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contains a reference count and the data.
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When a shared object is created, it sets the reference count to 1. The
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reference count is incremented whenever a new object references the
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shared data, and decremented when the object dereferences the shared
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data. The shared data is deleted when the reference count becomes
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zero.
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\keyword deep copy
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\keyword shallow copy
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When dealing with shared objects, there are two ways of copying an
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object. We usually speak about \e deep and \e shallow copies. A deep
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copy implies duplicating an object. A shallow copy is a reference
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copy, i.e. just a pointer to a shared data block. Making a deep copy
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can be expensive in terms of memory and CPU. Making a shallow copy is
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very fast, because it only involves setting a pointer and incrementing
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the reference count.
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Object assignment (with operator=()) for implicitly and explicitly
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shared objects is implemented using shallow copies. A deep copy can be
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made by calling a copy() function or by using \l QDeepCopy.
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The benefit of sharing is that a program does not need to duplicate
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data unnecessarily, which results in lower memory use and less copying
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of data. Objects can easily be assigned, sent as function arguments,
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and returned from functions.
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Now comes the distinction between \e explicit and \e implicit sharing.
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Explicit sharing means that the programmer must be aware of the fact
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that objects share common data. Implicit sharing means that the
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sharing mechanism takes place behind the scenes and the programmer
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does not need to worry about it.
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\section1 A QByteArray Example
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QByteArray is an example of a shared class that uses explicit sharing.
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Example:
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\code
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//Line a= b= c=
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QByteArray a(3),b(2) // 1: {?,?,?} {?,?}
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b[0] = 12; b[1] = 34; // 2: {?,?,?} {12,34}
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a = b; // 3: {12,34} {12,34}
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a[1] = 56; // 4: {12,56} {12,56}
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QByteArray c = a; // 5: {12,56} {12,56} {12,56}
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a.detach(); // 6: {12,56} {12,56} {12,56}
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a[1] = 78; // 7: {12,78} {12,56} {12,56}
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b = a.copy(); // 8: {12,78} {12,78} {12,56}
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a[1] = 90; // 9: {12,90} {12,78} {12,56}
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\endcode
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The assignment \c {a = b} on line 3 throws away \c a's original shared
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block (the reference count becomes zero), sets \c a's shared block to
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point to \c b's shared block and increments the reference count.
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On line 4, the contents of \c a is modified. \c b is also modified,
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because \c a and \c b refer to the same data block. This is the
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difference between explicit and implicit sharing (explained below).
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The \c a object detaches from the common data on line 6. Detaching
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means that the shared data is copied to make sure that an object has
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its own private data. Therefore, modifying \c a on line 7 does not
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affect \c b or \c c.
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Finally, on line 8 we make a deep copy of \c a and assign it to \c b,
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so that when \c a is modified on line 9, \c b remains unchanged.
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\section1 Explicit vs. Implicit Sharing
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Implicit sharing automatically detaches the object from a shared block
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if the object is about to change and the reference count is greater
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than one. (This is often called "copy-on-write".) Explicit sharing
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leaves this job to the programmer. If an explicitly shared object is
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not detached, changing an object will change all other objects that
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refer to the same data.
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Implicit sharing optimizes memory use and copying of data without
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this side effect. So why didn't we implement implicit sharing for all
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shared classes? The answer is that a class that allows direct access
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to its internal data (for efficiency reasons), like QByteArray, cannot
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be implicitly shared, because it can be changed without letting
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QByteArray know.
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An implicitly shared class has total control of its internal data. In
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any member functions that modify its data, it automatically detaches
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before modifying the data.
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The QPen class, which uses implicit sharing, detaches from the shared
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data in all member functions that change the internal data.
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Code fragment:
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\code
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void QPen::setStyle( PenStyle s )
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{
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detach(); // detach from common data
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data->style = s; // set the style member
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}
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void QPen::detach()
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{
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if ( data->count != 1 ) // only if >1 reference
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*this = copy();
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}
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\endcode
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This is clearly not possible for QByteArray, because the programmer
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can do the following:
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\code
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QByteArray array( 10 );
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array.fill( 'a' );
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array[0] = 'f'; // will modify array
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array.data()[1] = 'i'; // will modify array
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\endcode
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If we monitor changes in a QByteArray, the QByteArray class would
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become unacceptably slow.
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\section1 Explicitly Shared Classes
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All classes that are instances of the QMemArray template class are
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explicitly shared:
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\list
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\i \l QBitArray
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\i \l QPointArray
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\i \l QByteArray
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\i Any other instantiation of \link QMemArray QMemArray\<type\>\endlink
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\endlist
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These classes have a detach() function that can be called if you want
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your object to get a private copy of the shared data. They also have a
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copy() function that returns a deep copy with a reference count of 1.
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The same is true for \l QImage, which does not inherit QMemArray. \l
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QMovie is also explicitly shared, but it does not support detach() or
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copy().
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\section1 Implicitly Shared Classes
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The Qt classes that are implicitly shared are:
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\list
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\i \l QBitmap
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\i \l QBrush
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\i \l QCursor
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\i \l QFont
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\i \l QFontInfo
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\i \l QFontMetrics
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\i \l QIconSet
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\i \l QMap
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\i \l QPalette
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\i \l QPen
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\i \l QPicture
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\i \l QPixmap
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\i \l QRegion
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\i \l QRegExp
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\i \l QString
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\i \l QStringList
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\i \l QValueList
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\i \l QValueStack
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\endlist
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These classes automatically detach from common data if an object is
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about to be changed. The programmer will not even notice that the
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objects are shared. Thus you should treat separate instances of them
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as separate objects. They will always behave as separate objects but
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with the added benefit of sharing data whenever possible. For this
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reason, you can pass instances of these classes as arguments to
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functions by value without concern for the copying overhead.
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Example:
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\code
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QPixmap p1, p2;
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p1.load( "image.bmp" );
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p2 = p1; // p1 and p2 share data
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QPainter paint;
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paint.begin( &p2 ); // cuts p2 loose from p1
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paint.drawText( 0,50, "Hi" );
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paint.end();
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\endcode
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In this example, \c p1 and \c p2 share data until QPainter::begin() is
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called for \c p2, because painting a pixmap will modify it. The same
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also happens if anything is \link ::bitBlt() bitBlt()\endlink'ed into
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\c p2.
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\warning Do not copy an implicitly shared container (QMap,
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QValueVector, etc.) while you are iterating over it.
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\section1 QCString: implicit or explicit?
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\l QCString uses a mixture of implicit and explicit sharing. Functions
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inherited from QByteArray, such as data(), employ explicit sharing, while
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those only in QCString detach automatically. Thus, QCString is rather an
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"experts only" class, provided mainly to ease porting from Qt 1.x to Qt 2.0.
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We recommend that you use \l QString, a purely implicitly shared class.
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*/
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