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/****************************************************************************
**
** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page qt4-threads.html
\title Thread Support in Qt 4
\contentspage {What's New in Qt 4}{Home}
\previouspage The Qt 4 Style API
Qt 4 makes it easier than ever to write multithreaded
applications. More classes have been made usable from non-GUI
threads, and the signals and slots mechanism can now be used to
communicate between threads.
\section1 General Overview
QThread now inherits QObject. It emits signals to indicate that
the thread started or finished executing, and provides a few
slots as well.
Each thread can now have its own event loop. The initial thread
starts its event loops using QCoreApplication::exec(); other
threads can start an event loop using QThread::exec(). Like
QCoreApplication, QThread also provides an
\l{QThread::exit()}{exit(int)} function and a
\l{QThread::quit()}{quit()} slot.
An event loop in a thread makes it possible for the thread to use
certain non-GUI Qt classes that require the presence of an event
loop (such as QTimer, QTcpSocket, and QProcess). It also makes it
possible to connect signals from any threads to slots of a
specific thread. When a signal is emitted, the slot isn't called
immediately; instead, it is invoked when control returns to the
event loop of the thread to which the object belongs. The slot is
executed in the thread where the receiver object lives. See
QObject::connect() for details.
Qt 4 also introduces a new synchronization class: QReadWriteLock.
It is similar to QMutex, except that it distinguishes between
"read" and "write" access to shared data and allows multiple
readers to access the data simultaneously. Using QReadWriteLock
instead of QMutex when it is possible can make multithreaded
programs more concurrent.
Since Qt 4, \l{implicitly shared} classes can safely be copied
across threads, like any other value classes. They are fully
reentrant. This is implemented using atomic reference counting
operations, which are implemented in assembly language for the
different platforms supported by Qt. Atomic reference counting is
very fast, much faster than using a mutex.
See \l{Thread Support in Qt} for more information.
\section1 Comparison with Qt 3
Earlier versions of Qt offered an option to build the library
without thread support. In Qt 4, threads are always enabled.
Qt 3 had a class called \c QDeepCopy that you could use to take a
deep copy of an implicitly shared object. In Qt 4, the atomic
reference counting makes this class superfluous.
*/
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