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:
bool function(const T &t);
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:
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.
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.
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.
The reduce function will be called once for each result kept by the filter function, and should merge the intermediate into the 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 neccesary. The QtConcurrent::ReduceOptions enum provides a way to control the order in which the reduction is done.
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.
QStringList strings = ...;
// each call blocks until the entire operation is finished
QStringList lowerCaseStrings = QtConcurrent::blockingFiltered(strings, allLowerCase);
QtConcurrent::blockingFilter(strings, allLowerCase);
QSet<QString> dictionary = QtConcurrent::blockingFilteredReduced(strings, allLowerCase, addToDictionary);
Note that the result types above are not QFuture objects, but real result types (in this case, QStringList and QSet<QString>).
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 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.
Calls filterFunction once for each item in sequence. If filterFunction returns true, the item is kept in sequence; otherwise, the item is removed from sequence.
Calls filterFunction once for each item in sequence and returns a new Sequence of kept items. If filterFunction returns true, a copy of the item is put in the new Sequence. Otherwise, the item will not appear in the new Sequence.
This is an overloaded member function, provided for convenience.
Calls filterFunction once for each item from begin to end and returns a new Sequence of kept items. If filterFunction returns true, a copy of the item is put in the new Sequence. Otherwise, the item will not appear in the new Sequence.
Calls filterFunction once for each item in sequence. If filterFunction returns true for an item, that item is then passed to reduceFunction. In other words, the return value is the result of reduceFunction for each item where filterFunction returns true.
Note that while filterFunction is called concurrently, only one thread at a time will call reduceFunction. The order in which reduceFunction is called is undefined if reduceOptions is QtConcurrent::UnorderedReduce. If reduceOptions is QtConcurrent::OrderedReduce, reduceFunction is called in the order of the original sequence.
This is an overloaded member function, provided for convenience.
Calls filterFunction once for each item from begin to end. If filterFunction returns true for an item, that item is then passed to reduceFunction. In other words, the return value is the result of reduceFunction for each item where filterFunction returns true.
Note that while filterFunction is called concurrently, only one thread at a time will call reduceFunction. The order in which reduceFunction is called is undefined if reduceOptions is QtConcurrent::UnorderedReduce. If reduceOptions is QtConcurrent::OrderedReduce, the reduceFunction is called in the order of the original sequence.
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