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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 QtConcurrent \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 QtConcurrent \ingroup thread */ /*! \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 \title Concurrent Map and Map-Reduce \ingroup thread \brief The 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 code/src_concurrent_qtconcurrentmap.cpp 7 Note that the result types above are not QFuture objects, but real result types (in this case, QList 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 code/src_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 code/src_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 code/src_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 code/src_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 code/src_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 code/src_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 code/src_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 code/src_concurrent_qtconcurrentmap.cpp 7 Note that the result types above are not QFuture objects, but real result types (in this case, QList 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 code/src_concurrent_qtconcurrentmap.cpp 8 Note that when using QtConcurrent::mappedReduced(), you can mix the use of normal and member functions freely: \snippet code/src_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 code/src_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 code/src_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 code/src_concurrent_qtconcurrentmap.cpp 11 The return value from boost::bind() is a function object (functor) with the following signature: \snippet code/src_concurrent_qtconcurrentmap.cpp 12 This matches what QtConcurrent::mapped() expects, and the complete example becomes: \snippet code/src_concurrent_qtconcurrentmap.cpp 13 */ /*! \fn QFuture QtConcurrent::map(Sequence &sequence, MapFunction function) \relates 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 QtConcurrent::map(Iterator begin, Iterator end, MapFunction function) \relates 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 QtConcurrent::mapped(const Sequence &sequence, MapFunction function) \relates 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 QtConcurrent::mapped(ConstIterator begin, ConstIterator end, MapFunction function) \relates 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 QtConcurrent::mappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions) \relates 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 QtConcurrent::mappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions) \relates 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 ints = QtConcurrent::blockingMapped >(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 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 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() */