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The QtConcurrent::filter(), QtConcurrent::filtered() and QtConcurrent::filteredReduced() functions filter items in a sequence such as a QList or a QVector in parallel. QtConcurrent::filter() modifies a sequence in-place, QtConcurrent::filtered() returns a new sequence containing the filtered content, and QtConcurrent::filteredReduced() returns a single result. Each of the above functions have 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_qtconcurrentfilter.cpp 6 Note that the result types above are not QFuture objects, but real result types (in this case, QStringList and QSet). \section1 Concurrent Filter QtConcurrent::filtered() takes an input sequence and a filter function. This filter function is then called for each item in the sequence, and a new sequence containing the filtered values is returned. The filter function must be of the form: \snippet code/src_concurrent_qtconcurrentfilter.cpp 0 T must match the type stored in the sequence. The function returns true if the item should be kept, false if it should be discarded. This example shows how to keep strings that are all lower-case from a QStringList: \snippet code/src_concurrent_qtconcurrentfilter.cpp 1 The results of the filter 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::filter(): \snippet code/src_concurrent_qtconcurrentfilter.cpp 2 Since the sequence is modified in place, QtConcurrent::filter() does not return any results via QFuture. However, you can still use QFuture and QFutureWatcher to monitor the status of the filter. \section1 Concurrent Filter-Reduce QtConcurrent::filteredReduced() is similar to QtConcurrent::filtered(), but instead of returing a sequence with the filtered 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_qtconcurrentfilter.cpp 3 T is the type of the final result, U is the type of items being filtered. Note that the return value and return type of the reduce function are not used. Call QtConcurrent::filteredReduced() like this: \snippet code/src_concurrent_qtconcurrentfilter.cpp 4 The reduce function will be called once for each result kept by the filter function, and should merge the \e{intermediate} into the \e{result} variable. QtConcurrent::filteredReduced() 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. \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_qtconcurrentfilter.cpp 5 \section2 Using Member Functions QtConcurrent::filter(), QtConcurrent::filtered(), and QtConcurrent::filteredReduced() accept pointers to member functions. The member function class type must match the type stored in the sequence: \snippet code/src_concurrent_qtconcurrentfilter.cpp 7 Note that when using QtConcurrent::filteredReduced(), you can mix the use of normal and member functions freely: \snippet code/src_concurrent_qtconcurrentfilter.cpp 8 \section2 Using Function Objects QtConcurrent::filter(), QtConcurrent::filtered(), and QtConcurrent::filteredReduced() 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_qtconcurrentfilter.cpp 13 \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 filter function 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 use QString::contains(): \snippet code/src_concurrent_qtconcurrentfilter.cpp 9 QString::contains() takes 2 arguments (including the "this" pointer) and can't be used with QtConcurrent::filtered() directly, because QtConcurrent::filtered() expects a function that takes one argument. To use QString::contains() with QtConcurrent::filtered() we have to provide a value for the \e regexp argument: \snippet code/src_concurrent_qtconcurrentfilter.cpp 10 The return value from boost::bind() is a function object (functor) with the following signature: \snippet code/src_concurrent_qtconcurrentfilter.cpp 11 This matches what QtConcurrent::filtered() expects, and the complete example becomes: \snippet code/src_concurrent_qtconcurrentfilter.cpp 12 */ /*! \fn QFuture QtConcurrent::filter(Sequence &sequence, FilterFunction filterFunction) \relates Calls \a filterFunction once for each item in \a sequence. If \a filterFunction returns true, the item is kept in \a sequence; otherwise, the item is removed from \a sequence. */ /*! \fn QFuture QtConcurrent::filtered(const Sequence &sequence, FilterFunction filterFunction) \relates Calls \a filterFunction once for each item in \a sequence and returns a new Sequence of kept items. If \a filterFunction returns true, a copy of the item is put in the new Sequence. Otherwise, the item will \e not appear in the new Sequence. */ /*! \fn QFuture QtConcurrent::filtered(ConstIterator begin, ConstIterator end, FilterFunction filterFunction) \relates Calls \a filterFunction once for each item from \a begin to \a end and returns a new Sequence of kept items. If \a filterFunction returns true, a copy of the item is put in the new Sequence. Otherwise, the item will \e not appear in the new Sequence. */ /*! \fn QFuture QtConcurrent::filteredReduced(const Sequence &sequence, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions) \relates Calls \a filterFunction once for each item in \a sequence. If \a filterFunction returns true for an item, that item is then passed to \a reduceFunction. In other words, the return value is the result of \a reduceFunction for each item where \a filterFunction returns true. Note that while \a filterFunction is called concurrently, only one thread at a time will call \a reduceFunction. The order in which \a reduceFunction is called is undefined if \a reduceOptions is QtConcurrent::UnorderedReduce. If \a reduceOptions is QtConcurrent::OrderedReduce, \a reduceFunction is called in the order of the original sequence. */ /*! \fn QFuture QtConcurrent::filteredReduced(ConstIterator begin, ConstIterator end, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions) \relates Calls \a filterFunction once for each item from \a begin to \a end. If \a filterFunction returns true for an item, that item is then passed to \a reduceFunction. In other words, the return value is the result of \a reduceFunction for each item where \a filterFunction returns true. Note that while \a filterFunction is called concurrently, only one thread at a time will call \a reduceFunction. The order in which \a reduceFunction is called is undefined if \a reduceOptions is QtConcurrent::UnorderedReduce. If \a reduceOptions is QtConcurrent::OrderedReduce, the \a reduceFunction is called in the order of the original sequence. */ /*! \fn void QtConcurrent::blockingFilter(Sequence &sequence, FilterFunction filterFunction) Calls \a filterFunction once for each item in \a sequence. If \a filterFunction returns true, the item is kept in \a sequence; otherwise, the item is removed from \a sequence. \note This function will block until all items in the sequence have been processed. */ /*! \fn Sequence QtConcurrent::blockingFiltered(const Sequence &sequence, FilterFunction filterFunction) Calls \a filterFunction once for each item in \a sequence and returns a new Sequence of kept items. If \a filterFunction returns true, a copy of the item is put in the new Sequence. Otherwise, the item will \e not appear in the new Sequence. \note This function will block until all items in the sequence have been processed. \sa filtered() */ /*! \fn Sequence QtConcurrent::blockingFiltered(ConstIterator begin, ConstIterator end, FilterFunction filterFunction) Calls \a filterFunction once for each item from \a begin to \a end and returns a new Sequence of kept items. If \a filterFunction returns true, a copy of the item is put in the new Sequence. Otherwise, the item will \e not appear in the new Sequence. \note This function will block until the iterator reaches the end of the sequence being processed. \sa filtered() */ /*! \fn T QtConcurrent::blockingFilteredReduced(const Sequence &sequence, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions) Calls \a filterFunction once for each item in \a sequence. If \a filterFunction returns true for an item, that item is then passed to \a reduceFunction. In other words, the return value is the result of \a reduceFunction for each item where \a filterFunction returns true. Note that while \a filterFunction is called concurrently, only one thread at a time will call \a reduceFunction. The order in which \a reduceFunction is called is undefined if \a reduceOptions is QtConcurrent::UnorderedReduce. If \a reduceOptions is QtConcurrent::OrderedReduce, \a reduceFunction is called in the order of the original sequence. \note This function will block until all items in the sequence have been processed. \sa filteredReduced() */ /*! \fn T QtConcurrent::blockingFilteredReduced(ConstIterator begin, ConstIterator end, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions) Calls \a filterFunction once for each item from \a begin to \a end. If \a filterFunction returns true for an item, that item is then passed to \a reduceFunction. In other words, the return value is the result of \a reduceFunction for each item where \a filterFunction returns true. Note that while \a filterFunction is called concurrently, only one thread at a time will call \a reduceFunction. The order in which \a reduceFunction is called is undefined if \a reduceOptions is QtConcurrent::UnorderedReduce. If \a reduceOptions is QtConcurrent::OrderedReduce, the \a reduceFunction is called in the order of the original sequence. \note This function will block until the iterator reaches the end of the sequence being processed. \sa filteredReduced() */