QtConcurrent Namespace | Qt Concurrent 6.4.1

- Qt 6.4
- Qt Concurrent
- QtConcurrent Namespace
- Qt 6.4.1 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>
CMake: | find_package(Qt6 REQUIRED COMPONENTS Concurrent) target_link_libraries(mytarget PRIVATE Qt6::Concurrent)
qmake: | QT += concurrent

## Classes

class | QTaskBuilder

## Types

enum class | FutureResult { Ignore }
| InvokeResultType
enum | ReduceOption { UnorderedReduce, OrderedReduce, SequentialReduce }
flags | ReduceOptions

## Functions

QFuture<void> | filter(QThreadPool *pool, Sequence &sequence, KeepFunctor &&filterFunction)
QFuture<void> | filter(Sequence &sequence, KeepFunctor &&filterFunction)
QFuture<ResultType> | filteredReduced(QThreadPool *pool, Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | filteredReduced(Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | filteredReduced(QThreadPool *pool, Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | filteredReduced(Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | filteredReduced(QThreadPool *pool, Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | filteredReduced(Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | filteredReduced(QThreadPool *pool, Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | filteredReduced(Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<void> | map(QThreadPool *pool, Sequence &&sequence, MapFunctor &&function)
QFuture<void> | map(Sequence &&sequence, MapFunctor &&function)
QFuture<void> | map(QThreadPool *pool, Iterator begin, Iterator end, MapFunctor &&function)
QFuture<void> | map(Iterator begin, Iterator end, MapFunctor &&function)
QFuture<ResultType> | mappedReduced(QThreadPool *pool, Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | mappedReduced(Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | mappedReduced(QThreadPool *pool, Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<ResultType> | mappedReduced(Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))
QFuture<T> | run(Function function, ...)
QFuture<T> | run(QThreadPool *pool, Function function, ...)
QTaskBuilder<Task> | task(Task &&task)

## Detailed Description

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

## Classes

### class QTaskBuilder

The QTaskBuilder class is used for adjusting task parameters. More...

## Type Documentation

### enum class QtConcurrent::FutureResult

This enum type is used to invoke a special overload of QtConcurrent::QTaskBuilder::spawn(QtConcurrent::FutureResult) that doesn't return a future object.

Constant | Value | Description
--- | --- | ---
`QtConcurrent::FutureResult::Ignore` | `0` | An auxiliary tag which introduced to improve code readability.

### `[alias]` InvokeResultType

The simplified definition of this type looks like this:

     template <class Task, class ...Args>
     using InvokeResultType = std::invoke_result_t<std::decay_t<Task>, std::decay_t<Args>...>;

The real implementation also contains a compile-time check for whether the task can be invoked with the specified arguments or not.

### 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

### template <typename Sequence, typename KeepFunctor> QFuture<void> QtConcurrent::filter(QThreadPool *pool, Sequence &sequence, KeepFunctor &&filterFunction)

Calls filterFunction once for each item in sequence. All calls to filterFunction are invoked from the threads taken from the QThreadPool pool. If filterFunction returns `true`, the item is kept in sequence; otherwise, the item is removed from sequence.

Note that this method doesn't have an overload working with iterators, because it invalidates the iterators of the sequence it operates on.

See also Concurrent Filter and Filter-Reduce.

### template <typename Sequence, typename KeepFunctor> QFuture<void> QtConcurrent::filter(Sequence &sequence, KeepFunctor &&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 that this method doesn't have an overload working with iterators, because it invalidates the iterators of the sequence it operates on.

See also Concurrent Filter and Filter-Reduce.

### template <typename ResultType, typename Sequence, typename KeepFunctor, typename ReduceFunctor> QFuture<ResultType> QtConcurrent::filteredReduced(QThreadPool *pool, Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))

Calls filterFunction once for each item in sequence. All calls to filterFunction are invoked from the threads taken from the QThreadPool pool. 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.

### template <typename ResultType, typename Sequence, typename KeepFunctor, typename ReduceFunctor> QFuture<ResultType> QtConcurrent::filteredReduced(Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions 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.

### template <typename ResultType, typename Sequence, typename KeepFunctor, typename ReduceFunctor, typename InitialValueType> QFuture<ResultType> QtConcurrent::filteredReduced(QThreadPool *pool, Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))

Calls filterFunction once for each item in sequence. All calls to filterFunction are invoked from the threads taken from the QThreadPool pool. 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`. The result value is initialized to initialValue when the function is called, and the first call to reduceFunction will operate on this value.

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.

### template <typename ResultType, typename Sequence, typename KeepFunctor, typename ReduceFunctor, typename InitialValueType> QFuture<ResultType> QtConcurrent::filteredReduced(Sequence &&sequence, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions 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`. The result value is initialized to initialValue when the function is called, and the first call to reduceFunction will operate on this value.

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.

### template <typename ResultType, typename Iterator, typename KeepFunctor, typename ReduceFunctor> QFuture<ResultType> QtConcurrent::filteredReduced(QThreadPool *pool, Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))

Calls filterFunction once for each item from begin to end. All calls to filterFunction are invoked from the threads taken from the QThreadPool pool. 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.

### template <typename ResultType, typename Iterator, typename KeepFunctor, typename ReduceFunctor> QFuture<ResultType> QtConcurrent::filteredReduced(Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions 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.

### template <typename ResultType, typename Iterator, typename KeepFunctor, typename ReduceFunctor, typename InitialValueType> QFuture<ResultType> QtConcurrent::filteredReduced(QThreadPool *pool, Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))

Calls filterFunction once for each item from begin to end. All calls to filterFunction are invoked from the threads taken from the QThreadPool pool. 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`. The result value is initialized to initialValue when the function is called, and the first call to reduceFunction will operate on this value.

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.

### template <typename ResultType, typename Iterator, typename KeepFunctor, typename ReduceFunctor, typename InitialValueType> QFuture<ResultType> QtConcurrent::filteredReduced(Iterator begin, Iterator end, KeepFunctor &&filterFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions 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`. The result value is initialized to initialValue when the function is called, and the first call to reduceFunction will operate on this value.

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.

### template <typename Sequence, typename MapFunctor> QFuture<void> QtConcurrent::map(QThreadPool *pool, Sequence &&sequence, MapFunctor &&function)

Calls function once for each item in sequence. All calls to function are invoked from the threads taken from the QThreadPool pool. The function takes a reference to the item, so that any modifications done to the item will appear in sequence.

See also Concurrent Map and Map-Reduce.

### template <typename Sequence, typename MapFunctor> QFuture<void> QtConcurrent::map(Sequence &&sequence, MapFunctor &&function)

Calls function once for each item in sequence. The function takes a reference to the item, so that any modifications done to the item will appear in sequence.

See also Concurrent Map and Map-Reduce.

### template <typename Iterator, typename MapFunctor> QFuture<void> QtConcurrent::map(QThreadPool *pool, Iterator begin, Iterator end, MapFunctor &&function)

Calls function once for each item from begin to end. All calls to function are invoked from the threads taken from the QThreadPool pool. The function takes 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.

### template <typename Iterator, typename MapFunctor> QFuture<void> QtConcurrent::map(Iterator begin, Iterator end, MapFunctor &&function)

Calls function once for each item from begin to end. The function takes 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.

### template <typename ResultType, typename Sequence, typename MapFunctor, typename ReduceFunctor> QFuture<ResultType> QtConcurrent::mappedReduced(QThreadPool *pool, Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))

Calls mapFunction once for each item in sequence. All calls to mapFunction are invoked from the threads taken from the QThreadPool pool. 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.

### template <typename ResultType, typename Sequence, typename MapFunctor, typename ReduceFunctor> QFuture<ResultType> QtConcurrent::mappedReduced(Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, QtConcurrent::ReduceOptions 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.

### template <typename ResultType, typename Sequence, typename MapFunctor, typename ReduceFunctor, typename InitialValueType> QFuture<ResultType> QtConcurrent::mappedReduced(QThreadPool *pool, Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))

Calls mapFunction once for each item in sequence. All calls to mapFunction are invoked from the threads taken from the QThreadPool pool. The return value of each mapFunction is passed to reduceFunction. The result value is initialized to initialValue when the function is called, and the first call to reduceFunction will operate on this value.

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.

### template <typename ResultType, typename Sequence, typename MapFunctor, typename ReduceFunctor, typename InitialValueType> QFuture<ResultType> QtConcurrent::mappedReduced(Sequence &&sequence, MapFunctor &&mapFunction, ReduceFunctor &&reduceFunction, InitialValueType &&initialValue, QtConcurrent::ReduceOptions reduceOptions = ReduceOptions(UnorderedReduce | SequentialReduce))

Calls mapFunction once for each item in sequence. The return value of each mapFunction is passed to reduceFunction. The result value is initialized to initialValue when the function is called, and the first call to reduceFunction will operate on this value.

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.

### template <typename T> 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.

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

In basic mode the QFuture returned can only be used to query for the running/finished status and the return value of the function. In particular, canceling or pausing can be issued only if the computations behind the future has not been started.

In run with promise mode, the function is expected to return void and must take an additional argument of `QPromise<T> &` type, placed as a first argument in function's argument list. T is the result type and it is the same for the returned `QFuture<T>`.

In run with promise mode, similar to basic mode, the QFuture returned can be used to query for the running/finished status and the value reported by the function. In addition, it may be used for suspending or canceling the running task, fetching multiple results from the called function or monitoring progress reported by the function.

See also Concurrent Run (basic mode) and Concurrent Run With Promise.

### `[since 5.4]` template <typename T> 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.

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

In basic mode the QFuture returned can only be used to query for the running/finished status and the return value of the function. In particular, canceling or pausing can be issued only if the computations behind the future has not been started.

In run with promise mode, the function is expected to return void and must take an additional argument of `QPromise<T> &` type, placed as a first argument in function's argument list. T is the result type and it is the same for the returned `QFuture<T>`.

In run with promise mode, similar to basic mode, the QFuture returned can be used to query for the running/finished status and the value reported by the function. In addition, it may be used for suspending or canceling the running task, fetching multiple results from the called function or monitoring progress reported by the function.

This function was introduced in Qt 5.4.

See also Concurrent Run (basic mode) and Concurrent Run With Promise.

### `[since 6.0]` template <typename Task> QTaskBuilder<Task> QtConcurrent::task(Task &&task)

Creates an instance of QtConcurrent::QTaskBuilder. This object can be used to adjust some parameters and run task in a separate thread.

This function was introduced in Qt 6.0.

See also Concurrent Task and QtConcurrent::QTaskBuilder.