source: trunk/doc/src/porting/qt4-threads.qdoc@ 603

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/*!
    \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.
*/