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/****************************************************************************
**
** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the QtCore module 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.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\namespace QtConcurrent
\inmodule QtCore
\since 4.4
\brief The QtConcurrent namespace provides high-level APIs that make it
possible to write multi-threaded programs without using low-level
threading primitives.
See the \l {Concurrent Programming}{Qt Concurrent} chapter in
the \l{threads.html}{threading} documentation.
\inheaderfile QtCore
\ingroup thread
*/
/*!
\namespace QtConcurrent::internal
\internal
\brief The QtConcurrent::internal namespace contains QtConcurrent
implementation details.
*/
/*!
\enum QtConcurrent::ReduceOption
This enum specifies the order of which results from the map or filter
function are passed to the reduce function.
\value UnorderedReduce Reduction is done in an arbitrary order.
\value OrderedReduce Reduction is done in the order of the
original sequence.
\value SequentialReduce Reduction is done sequentially: only one
thread will enter the reduce function at a time. (Parallel reduction
might be supported in a future version of Qt Concurrent.)
*/
/*!
\headerfile <QtConcurrentMap>
\title Concurrent Map and Map-Reduce
\ingroup thread
\brief The <QtConcurrentMap> header provides concurrent Map and MapReduce.
These functions are a part of the \l {Concurrent Programming}{Qt Concurrent} framework.
The QtConcurrent::map(), QtConcurrent::mapped() and
QtConcurrent::mappedReduced() functions run computations in parallel on
the items in a sequence such as a QList or a QVector. QtConcurrent::map()
modifies a sequence in-place, QtConcurrent::mapped() returns a new
sequence containing the modified content, and QtConcurrent::mappedReduced()
returns a single result.
Each of the above functions has a blocking variant that returns
the final result instead of a QFuture. You use them in the same
way as the asynchronous variants.
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 7
Note that the result types above are not QFuture objects, but real result
types (in this case, QList<QImage> and QImage).
\section1 Concurrent Map
QtConcurrent::mapped() takes an input sequence and a map function. This map
function is then called for each item in the sequence, and a new sequence
containing the return values from the map function is returned.
The map function must be of the form:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 0
T and U can be any type (and they can even be the same type), but T must
match the type stored in the sequence. The function returns the modified
or \e mapped content.
This example shows how to apply a scale function to all the items
in a sequence:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 1
The results of the map are made available through QFuture. See the
QFuture and QFutureWatcher documentation for more information on how to
use QFuture in your applications.
If you want to modify a sequence in-place, use QtConcurrent::map(). The
map function must then be of the form:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 2
Note that the return value and return type of the map function are not
used.
Using QtConcurrent::map() is similar to using QtConcurrent::mapped():
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 3
Since the sequence is modified in place, QtConcurrent::map() does not
return any results via QFuture. However, you can still use QFuture and
QFutureWatcher to monitor the status of the map.
\section1 Concurrent Map-Reduce
QtConcurrent::mappedReduced() is similar to QtConcurrent::mapped(), but
instead of returning a sequence with the new results, the results are
combined into a single value using a reduce function.
The reduce function must be of the form:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 4
T is the type of the final result, U is the return type of the map
function. Note that the return value and return type of the reduce
function are not used.
Call QtConcurrent::mappedReduced() like this:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 5
The reduce function will be called once for each result returned by the map
function, and should merge the \e{intermediate} into the \e{result}
variable. QtConcurrent::mappedReduced() guarantees that only one thread
will call reduce at a time, so using a mutex to lock the result variable
is not necessary. The QtConcurrent::ReduceOptions enum provides a way to
control the order in which the reduction is done. If
QtConcurrent::UnorderedReduce is used (the default), the order is
undefined, while QtConcurrent::OrderedReduce ensures that the reduction
is done in the order of the original sequence.
\section1 Additional API Features
\section2 Using Iterators instead of Sequence
Each of the above functions has a variant that takes an iterator range
instead of a sequence. You use them in the same way as the sequence
variants:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 6
\section2 Blocking Variants
Each of the above functions has a blocking variant that returns
the final result instead of a QFuture. You use them in the same
way as the asynchronous variants.
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 7
Note that the result types above are not QFuture objects, but real result
types (in this case, QList<QImage> and QImage).
\section2 Using Member Functions
QtConcurrent::map(), QtConcurrent::mapped(), and
QtConcurrent::mappedReduced() accept pointers to member functions.
The member function class type must match the type stored in the sequence:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 8
Note that when using QtConcurrent::mappedReduced(), you can mix the use of
normal and member functions freely:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 9
\section2 Using Function Objects
QtConcurrent::map(), QtConcurrent::mapped(), and
QtConcurrent::mappedReduced() accept function objects, which can be used to
add state to a function call. The result_type typedef must define the
result type of the function call operator:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 14
\section2 Using Bound Function Arguments
Note that Qt does not provide support for bound functions. This is
provided by 3rd party libraries like
\l{http://www.boost.org/libs/bind/bind.html}{Boost} or
\l{http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf}{C++
TR1 Library Extensions}.
If you want to use a map function that takes more than one argument you can
use boost::bind() or std::tr1::bind() to transform it onto a function that
takes one argument.
As an example, we'll use QImage::scaledToWidth():
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 10
scaledToWidth takes three arguments (including the "this" pointer) and
can't be used with QtConcurrent::mapped() directly, because
QtConcurrent::mapped() expects a function that takes one argument. To use
QImage::scaledToWidth() with QtConcurrent::mapped() we have to provide a
value for the \e{width} and the \e{transformation mode}:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 11
The return value from boost::bind() is a function object (functor) with
the following signature:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 12
This matches what QtConcurrent::mapped() expects, and the complete example
becomes:
\snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 13
*/
/*!
\fn QFuture<void> QtConcurrent::map(Sequence &sequence, MapFunction function)
\relates <QtConcurrentMap>
Calls \a function once for each item in \a sequence. The \a function is
passed a reference to the item, so that any modifications done to the item
will appear in \a sequence.
*/
/*!
\fn QFuture<void> QtConcurrent::map(Iterator begin, Iterator end, MapFunction function)
\relates <QtConcurrentMap>
Calls \a function once for each item from \a begin to \a end. The
\a function is passed a reference to the item, so that any modifications
done to the item will appear in the sequence which the iterators belong to.
*/
/*!
\fn QFuture<T> QtConcurrent::mapped(const Sequence &sequence, MapFunction function)
\relates <QtConcurrentMap>
Calls \a function once for each item in \a sequence and returns a future
with each mapped item as a result. You can use QFuture::const_iterator or
QFutureIterator to iterate through the results.
*/
/*!
\fn QFuture<T> QtConcurrent::mapped(ConstIterator begin, ConstIterator end, MapFunction function)
\relates <QtConcurrentMap>
Calls \a function once for each item from \a begin to \a end and returns a
future with each mapped item as a result. You can use
QFuture::const_iterator or QFutureIterator to iterate through the results.
*/
/*!
\fn QFuture<T> QtConcurrent::mappedReduced(const Sequence &sequence,
MapFunction mapFunction, ReduceFunction reduceFunction,
QtConcurrent::ReduceOptions reduceOptions)
\relates <QtConcurrentMap>
Calls \a mapFunction once for each item in \a sequence. The return value of
each \a mapFunction is passed to \a reduceFunction.
Note that while \a mapFunction is called concurrently, only one thread at a
time will call \a reduceFunction. The order in which \a reduceFunction is
called is determined by \a reduceOptions.
*/
/*!
\fn QFuture<T> QtConcurrent::mappedReduced(ConstIterator begin,
ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction,
QtConcurrent::ReduceOptions reduceOptions)
\relates <QtConcurrentMap>
Calls \a mapFunction once for each item from \a begin to \a end. The return
value of each \a mapFunction is passed to \a reduceFunction.
Note that while \a mapFunction is called concurrently, only one thread at a
time will call \a reduceFunction. By default, the order in which
\a reduceFunction is called is undefined.
\note QtConcurrent::OrderedReduce results in the ordered reduction.
*/
/*!
\fn void QtConcurrent::blockingMap(Sequence &sequence, MapFunction function)
Calls \a function once for each item in \a sequence. The \a function is
passed a reference to the item, so that any modifications done to the item
will appear in \a sequence.
\note This function will block until all items in the sequence have been processed.
\sa map()
*/
/*!
\fn void QtConcurrent::blockingMap(Iterator begin, Iterator end, MapFunction function)
Calls \a function once for each item from \a begin to \a end. The
\a function is passed a reference to the item, so that any modifications
done to the item will appear in the sequence which the iterators belong to.
\note This function will block until the iterator reaches the end of the
sequence being processed.
\sa map()
*/
/*!
\fn T QtConcurrent::blockingMapped(const Sequence &sequence, MapFunction function)
Calls \a function once for each item in \a sequence and returns a Sequence containing
the results. The type of the results will match the type returned my the MapFunction.
\note This function will block until all items in the sequence have been processed.
\sa mapped()
*/
/*!
\fn T QtConcurrent::blockingMapped(ConstIterator begin, ConstIterator end, MapFunction function)
Calls \a function once for each item from \a begin to \a end and returns a
container with the results. Specify the type of container as the a template
argument, like this:
\code
QList<int> ints = QtConcurrent::blockingMapped<QList<int> >(beginIterator, endIterator, fn);
\endcode
\note This function will block until the iterator reaches the end of the
sequence being processed.
\sa mapped()
*/
/*!
\fn T QtConcurrent::blockingMappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions)
\relates <QtConcurrentMap>
Calls \a mapFunction once for each item in \a sequence. The return value of
each \a mapFunction is passed to \a reduceFunction.
Note that while \a mapFunction is called concurrently, only one thread at a
time will call \a reduceFunction. The order in which \a reduceFunction is
called is determined by \a reduceOptions.
\note This function will block until all items in the sequence have been processed.
\sa mapped()
*/
/*!
\fn T QtConcurrent::blockingMappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions)
\relates <QtConcurrentMap>
Calls \a mapFunction once for each item from \a begin to \a end. The return
value of each \a mapFunction is passed to \a reduceFunction.
Note that while \a mapFunction is called concurrently, only one thread at a
time will call \a reduceFunction. The order in which \a reduceFunction is
called is undefined.
\note This function will block until the iterator reaches the end of the
sequence being processed.
\sa blockingMappedReduced()
*/
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