Qt 5.9 | Qt Concurrent | QtConcurrent Namespace

Qt 5.9.9 Reference Documentation

# QtConcurrent Namespace

The QtConcurrent namespace provides high-level APIs that make it possible to write multi-threaded programs without using low-level threading primitives. More...

Header: | #include <QtConcurrent>
qmake: | QT += concurrent
Since: | Qt 4.4

- Obsolete members

## Types

enum | ReduceOption { UnorderedReduce, OrderedReduce, SequentialReduce }
flags | ReduceOptions

## Functions

void | blockingFilter(Sequence &sequence, FilterFunction filterFunction)
Sequence | blockingFiltered(const Sequence &sequence, FilterFunction filterFunction)
Sequence | blockingFiltered(ConstIterator begin, ConstIterator end, FilterFunction filterFunction)
T | blockingFilteredReduced(const Sequence &sequence, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
T | blockingFilteredReduced(ConstIterator begin, ConstIterator end, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
void | blockingMap(Sequence &sequence, MapFunction function)
void | blockingMap(Iterator begin, Iterator end, MapFunction function)
T | blockingMapped(const Sequence &sequence, MapFunction function)
T | blockingMapped(ConstIterator begin, ConstIterator end, MapFunction function)
T | blockingMappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
T | blockingMappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
QFuture<void> | filter(Sequence &sequence, FilterFunction filterFunction)
QFuture<T> | filtered(const Sequence &sequence, FilterFunction filterFunction)
QFuture<T> | filtered(ConstIterator begin, ConstIterator end, FilterFunction filterFunction)
QFuture<T> | filteredReduced(const Sequence &sequence, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
QFuture<T> | filteredReduced(ConstIterator begin, ConstIterator end, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
QFuture<void> | map(Sequence &sequence, MapFunction function)
QFuture<void> | map(Iterator begin, Iterator end, MapFunction function)
QFuture<T> | mapped(const Sequence &sequence, MapFunction function)
QFuture<T> | mapped(ConstIterator begin, ConstIterator end, MapFunction function)
QFuture<T> | mappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
QFuture<T> | mappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)
QFuture<T> | run(Function function, ...)
QFuture<T> | run(QThreadPool *pool, Function function, ...)

## Detailed Description

The QtConcurrent namespace provides high-level APIs that make it possible to write multi-threaded programs without using low-level threading primitives.

See the Qt Concurrent module documentation for an overview of available functions, or see below for detailed information on each function.

## Type Documentation

### enum QtConcurrent::ReduceOption
flags QtConcurrent::ReduceOptions

This enum specifies the order of which results from the map or filter function are passed to the reduce function.

Constant | Value | Description
--- | --- | ---
`QtConcurrent::UnorderedReduce` | `0x1` | Reduction is done in an arbitrary order.
`QtConcurrent::OrderedReduce` | `0x2` | Reduction is done in the order of the original sequence.
`QtConcurrent::SequentialReduce` | `0x4` | 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.)

The ReduceOptions type is a typedef for QFlags<ReduceOption>. It stores an OR combination of ReduceOption values.

## Function Documentation

### void QtConcurrent::blockingFilter(Sequence &sequence, FilterFunction filterFunction)

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.

Note: This function will block until all items in the sequence have been processed.

See also Concurrent Filter and Filter-Reduce.

### Sequence QtConcurrent::blockingFiltered(const Sequence &sequence, FilterFunction filterFunction)

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.

Note: This function will block until all items in the sequence have been processed.

See also filtered() and Concurrent Filter and Filter-Reduce.

### Sequence QtConcurrent::blockingFiltered(ConstIterator begin, ConstIterator end, FilterFunction filterFunction)

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.

Note: This function will block until the iterator reaches the end of the sequence being processed.

See also filtered() and Concurrent Filter and Filter-Reduce.

### T QtConcurrent::blockingFilteredReduced(const Sequence &sequence, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

Note: This function will block until all items in the sequence have been processed.

See also filteredReduced() and Concurrent Filter and Filter-Reduce.

### T QtConcurrent::blockingFilteredReduced(ConstIterator begin, ConstIterator end, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

Note: This function will block until the iterator reaches the end of the sequence being processed.

See also filteredReduced() and Concurrent Filter and Filter-Reduce.

### void QtConcurrent::blockingMap(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.

Note: This function will block until all items in the sequence have been processed.

See also map() and Concurrent Map and Map-Reduce.

### void QtConcurrent::blockingMap(Iterator begin, Iterator end, MapFunction function)

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.

Note: This function will block until the iterator reaches the end of the sequence being processed.

See also map() and Concurrent Map and Map-Reduce.

### T QtConcurrent::blockingMapped(const Sequence &sequence, MapFunction function)

Calls function once for each item in 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.

See also mapped() and Concurrent Map and Map-Reduce.

### T QtConcurrent::blockingMapped(ConstIterator begin, ConstIterator end, MapFunction function)

Calls function once for each item from begin to end and returns a container with the results. Specify the type of container as the a template argument, like this:

      QList<int> ints = QtConcurrent::blockingMapped<QList<int> >(beginIterator, endIterator, fn);

Note: This function will block until the iterator reaches the end of the sequence being processed.

See also mapped() and Concurrent Map and Map-Reduce.

### T QtConcurrent::blockingMappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

See also mapped() and Concurrent Map and Map-Reduce.

### T QtConcurrent::blockingMappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

See also blockingMappedReduced() and Concurrent Map and Map-Reduce.

### QFuture<void> QtConcurrent::filter(Sequence &sequence, FilterFunction filterFunction)

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.

See also Concurrent Filter and Filter-Reduce.

### QFuture<T> QtConcurrent::filtered(const Sequence &sequence, FilterFunction filterFunction)

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.

See also Concurrent Filter and Filter-Reduce.

### QFuture<T> QtConcurrent::filtered(ConstIterator begin, ConstIterator end, FilterFunction filterFunction)

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.

See also Concurrent Filter and Filter-Reduce.

### QFuture<T> QtConcurrent::filteredReduced(const Sequence &sequence, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

See also Concurrent Filter and Filter-Reduce.

### QFuture<T> QtConcurrent::filteredReduced(ConstIterator begin, ConstIterator end, FilterFunction filterFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

See also Concurrent Filter and Filter-Reduce.

### 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.

See also Concurrent Map and Map-Reduce.

### QFuture<void> QtConcurrent::map(Iterator begin, Iterator end, MapFunction function)

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.

See also Concurrent Map and Map-Reduce.

### 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.

See also Concurrent Map and Map-Reduce.

### QFuture<T> QtConcurrent::mapped(ConstIterator begin, ConstIterator end, MapFunction function)

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.

See also Concurrent Map and Map-Reduce.

### QFuture<T> QtConcurrent::mappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

See also Concurrent Map and Map-Reduce.

### QFuture<T> QtConcurrent::mappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

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.

Note: QtConcurrent::OrderedReduce results in the ordered reduction.

See also Concurrent Map and Map-Reduce.

### QFuture<T> QtConcurrent::run(Function function, ...)

Equivalent to

      QtConcurrent::run(QThreadPool::globalInstance(), function, ...);

Runs function in a separate thread. The thread is taken from the global QThreadPool. Note that function may not run immediately; function will only be run once a thread becomes available.

T is the same type as the return value of function. Non-void return values can be accessed via the QFuture::result() function.

Note that the QFuture returned by QtConcurrent::run() does not support canceling, pausing, or progress reporting. The QFuture returned can only be used to query for the running/finished status and the return value of the function.

See also Concurrent Run.

### QFuture<T> QtConcurrent::run(QThreadPool *pool, Function function, ...)

Runs function in a separate thread. The thread is taken from the QThreadPool pool. Note that function may not run immediately; function will only be run once a thread becomes available.

T is the same type as the return value of function. Non-void return values can be accessed via the QFuture::result() function.

Note that the QFuture returned by QtConcurrent::run() does not support canceling, pausing, or progress reporting. The QFuture returned can only be used to query for the running/finished status and the return value of the function.

This function was introduced in Qt 5.4.

See also Concurrent Run.