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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.

*/