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:
U function(const T &t);
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 mapped content.
This example shows how to apply a scale function to all the items in a sequence:
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:
U function(T &t);
Note that the return value and return type of the map function are not used.
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.
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:
V function(T &result, const U &intermediate)
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.
The reduce function will be called once for each result returned by the map function, and should merge the intermediate into the 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 neccesary. 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.
Additional API Features
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:
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.
QList<QImage> images = ...;
// each call blocks until the entire operation is finished
QList<QImage> future = QtConcurrent::blockingMapped(images, scaled);
QtConcurrent::blockingMap(images, scale);
QImage collage = QtConcurrent::blockingMappedReduced(images, scaled, addToCollage);
Note that the result types above are not QFuture objects, but real result types (in this case, QList<QImage> and QImage).
// squeeze all strings in a QStringList
QStringList strings = ...;
QFuture<void> squeezedStrings = QtConcurrent::map(strings, &QString::squeeze);
// swap the rgb values of all pixels on a list of images
QList<QImage> images = ...;
QFuture<QImage> bgrImages = QtConcurrent::mapped(images, &QImage::rgbSwapped);
// create a set of the lengths of all strings in a list
QStringList strings = ...;
QFuture<QSet<int> > wordLengths = QtConcurrent::mappedReduced(string, &QString::length, &QSet<int>::insert);
Note that when using QtConcurrent::mappedReduced(), you can mix the use of normal and member functions freely:
// can mix normal functions and member functions with QtConcurrent::mappedReduced() // compute the average length of a list of strings
extern void computeAverage(int &average, int length);
QStringList strings = ...;
QFuture<int> averageWordLength = QtConcurrent::mappedReduced(strings, &QString::length, computeAverage);
// create a set of the color distribution of all images in a list
extern int colorDistribution(const QImage &string);
QList<QImage> images = ...;
QFuture<QSet<int> > totalColorDistribution = QtConcurrent::mappedReduced(images, colorDistribution, QSet<int>::insert);
Note that Qt does not provide support for bound functions. This is provided by 3rd party libraries like Boost or 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.
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 width and the transformation mode:
Calls mapFunction once for each item in sequence. The return value of each mapFunction is passed to reduceFunction.
Note that while mapFunction is called concurrently, only one thread at a time will call reduceFunction. The order in which reduceFunction is called is determined by reduceOptions.
Note: This function will block until all items in the sequence have been processed.
This is an overloaded member function, provided for convenience.
Calls mapFunction once for each item from begin to end. The return value of each mapFunction is passed to reduceFunction.
Note that while mapFunction is called concurrently, only one thread at a time will call reduceFunction. The order in which reduceFunction is called is undefined.
Note: This function will block until the iterator reaches the end of the sequence being processed.
QFuture<void> QtConcurrent::map ( Sequence & sequence, MapFunction function )
Calls function once for each item in sequence. The function is passed a reference to the item, so that any modifications done to the item will appear in sequence.
QFuture<void> QtConcurrent::map ( Iterator begin, Iterator end, MapFunction function )
This is an overloaded member function, provided for convenience.
Calls function once for each item from begin to end. The 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.
QFuture<T> QtConcurrent::mapped ( const Sequence & sequence, MapFunction function )
Calls function once for each item in 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.
QFuture<T> QtConcurrent::mapped ( ConstIterator begin, ConstIterator end, MapFunction function )
This is an overloaded member function, provided for convenience.
Calls function once for each item from begin to 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.
Calls mapFunction once for each item in sequence. The return value of each mapFunction is passed to reduceFunction.
Note that while mapFunction is called concurrently, only one thread at a time will call reduceFunction. The order in which reduceFunction is called is determined by reduceOptions.
This is an overloaded member function, provided for convenience.
Calls mapFunction once for each item from begin to end. The return value of each mapFunction is passed to reduceFunction.
Note that while mapFunction is called concurrently, only one thread at a time will call reduceFunction. By default, the order in which reduceFunction is called is undefined.
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