QtFuture Namespace | Qt Core 6.4.1

- Qt 6.4
- Qt Core
- QtFuture Namespace
- Qt 6.4.1 Reference Documentation

# QtFuture Namespace

Contains miscellaneous identifiers used by the QFuture class. More...

Header: | #include <QFuture>
CMake: | find_package(Qt6 REQUIRED COMPONENTS Core) target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: | QT += core

## Classes

struct | WhenAnyResult

## Types

enum class | Launch { Sync, Async, Inherit }

## Functions

QFuture<ArgsType<Signal>> | connect(Sender *sender, Signal signal)
QFuture<OutputSequence> | whenAll(InputIt first, InputIt last)
QFuture<OutputSequence> | whenAll(Futures &&... futures)
QFuture<QtFuture::WhenAnyResult<T>> | whenAny(InputIt first, InputIt last)

## Detailed Description

## Classes

### class WhenAnyResult

QtFuture::WhenAnyResult is used to represent the result of QtFuture::whenAny(). More...

## Type Documentation

### `[since 6.0]` enum class QtFuture::Launch

Represents execution policies for running a QFuture continuation.

Constant | Value | Description
--- | --- | ---
`QtFuture::Launch::Sync` | `0` | The continuation will be launched in the same thread that fulfills the promise associated with the future to which the continuation was attached, or if it has already finished, the continuation will be invoked immediately, in the thread that executes `then()`.
`QtFuture::Launch::Async` | `1` | The continuation will be launched in a separate thread taken from the global QThreadPool.
`QtFuture::Launch::Inherit` | `2` | The continuation will inherit the launch policy or thread pool of the future to which it is attached.

`Sync` is used as a default launch policy.

This enum was introduced or modified in Qt 6.0.

See also QFuture::then() and QThreadPool::globalInstance().

## Function Documentation

### template <typename Sender, typename Signal, typename> QFuture<ArgsType<Signal>> QtFuture::connect(Sender *sender, Signal signal)

Creates and returns a QFuture which will become available when the sender emits the signal. If the signal takes no arguments, a QFuture<void> is returned. If the signal takes a single argument, the resulted QFuture will be filled with the signal's argument value. If the signal takes multiple arguments, the resulted QFuture is filled with std::tuple storing the values of signal's arguments. If the sender is destroyed before the signal is emitted, the resulted QFuture will be canceled.

For example, let's say we have the following object:

     class Object : public QObject
     {
         Q_OBJECT
         ...
     signals:
         void noArgSignal();
         void singleArgSignal(int value);
         void multipleArgs(int value1, double value2, const QString &value3);
     };

We can connect its signals to QFuture objects in the following way:

     Object object;
     QFuture<void> voidFuture = QtFuture::connect(&object, &Object::noArgSignal);
     QFuture<int> intFuture = QtFuture::connect(&object, &Object::singleArgSignal);

     using Args = std::tuple<int, double, QString>;
     QFuture<Args> tupleFuture = QtFuture::connect(&object, &Object::multipleArgs)

We can also chain continuations to be run when a signal is emitted:

     QtFuture::connect(&object, &Object::singleArgSignal).then([](int value) {
         // do something with the value
     });

You can also start the continuation in a new thread or a custom thread pool using QtFuture::Launch policies. For example:

     QtFuture::connect(&object, &Object::singleArgSignal).then(QtFuture::Launch::Async, [](int value) {
         // this will run in a new thread
     });

Throwing an exception from a slot invoked by Qt's signal-slot connection is considered to be an undefined behavior, if it is not handled within the slot. But with QFuture::connect(), you can throw and handle exceptions from the continuations:

     QtFuture::connect(&object, &Object::singleArgSignal).then([](int value) {
         ...
         throw std::exception();
         ...
     }).onFailed([](const std::exception &e) {
         // handle the exception
     }).onFailed([] {
         // handle other exceptions
     });

Note: The connected future will be fulfilled only once, when the signal is emitted for the first time.

See also QFuture and QFuture::then().

### `[since 6.3]` template <typename OutputSequence, typename InputIt> QFuture<OutputSequence> QtFuture::whenAll(InputIt first, InputIt last)

Returns a new QFuture that succeeds when all futures from first to last complete. first and last are iterators to a sequence of futures packaging type `T`. `OutputSequence` is a sequence containing all completed futures from first to last, appearing in the same order as in the input. If the type of `OutputSequence` is not specified, the resulting futures will be returned in a `QList` of `QFuture<T>`. For example:

     QList<QFuture<int>> inputFutures {...};

     // whenAll has type QFuture<QList<QFuture<int>>>
     auto whenAll = QtFuture::whenAll(inputFutures.begin(), inputFutures.end());

     // whenAllVector has type QFuture<std::vector<QFuture<int>>>
     auto whenAllVector =
             QtFuture::whenAll<std::vector<QFuture<int>>>(inputFutures.begin(), inputFutures.end());

Note: The output sequence must support random access and the `resize()` operation.

If `first` equals `last`, this function returns a ready QFuture that contains an empty `OutputSequence`.

The returned future always completes successfully after all the specified futures complete. It doesn't matter if any of these futures completes with error or is canceled. You can use `.then()` to process the completed futures after the future returned by `whenAll()` succeeds:

     QList<QFuture<int>> inputFutures {...};

     QtFuture::whenAll(inputFutures.begin(), inputFutures.end())
             .then([](const QList<QFuture<int>> &results) {
                 for (auto future : results) {
                     if (future.isCanceled())
                         // handle the cancellation (possibly due to an exception)
                     else
                         // do something with the result
                 }
             });

Note: If the input futures complete on different threads, the future returned by this method will complete in the thread that the last future completes in. Therefore, the continuations attached to the future returned by `whenAll()` cannot always make assumptions about which thread they will be run on. Use the overload of `.then()` that takes a context object if you want to control which thread the continuations are invoked on.

This function was introduced in Qt 6.3.

### `[since 6.3]` template <typename OutputSequence, typename Futures> QFuture<OutputSequence> QtFuture::whenAll(Futures &&... futures)

Returns a new QFuture that succeeds when all futures packaging arbitrary types complete. `OutputSequence` is a sequence of completed futures. The type of its entries is `std::variant<Futures...>`. For each `QFuture<T>` passed to `whenAll()`, the entry at the corresponding position in `OutputSequence` will be a `std::variant` holding that `QFuture<T>`, in its completed state. If the type of `OutputSequence` is not specified, the resulting futures will be returned in a QList of `std::variant<Futures...>`. For example:

     QFuture<int> intFuture = ...;
     QFuture<QString> stringFuture = ...;
     QFuture<void> voidFuture = ...;

     using FuturesVariant = std::variant<QFuture<int>, QFuture<QString>, QFuture<void>>;

     // whenAll has type QFuture<QList<FuturesVariant>>
     auto whenAll = QtFuture::whenAll(intFuture, stringFuture, voidFuture);

     // whenAllVector has type QFuture<std::vector<FuturesVariant>>
     auto whenAllVector =
             QtFuture::whenAll<std::vector<FuturesVariant>>(intFuture, stringFuture, voidFuture);

Note: The output sequence should support random access and the `resize()` operation.

The returned future always completes successfully after all the specified futures complete. It doesn't matter if any of these futures completes with error or is canceled. You can use `.then()` to process the completed futures after the future returned by `whenAll()` succeeds:

     QFuture<int> intFuture = ...;
     QFuture<QString> stringFuture = ...;
     QFuture<void> voidFuture = ...;

     using FuturesVariant = std::variant<QFuture<int>, QFuture<QString>, QFuture<void>>;

     QtFuture::whenAll(intFuture, stringFuture, voidFuture)
             .then([](const QList<FuturesVariant> &results) {
                 ...
                 for (auto result : results)
                 {
                     // assuming handleResult() is overloaded based on the QFuture type
                     std::visit([](auto &&future) { handleResult(future); }, result);
                 }
                 ...
             });

Note: If the input futures complete on different threads, the future returned by this method will complete in the thread that the last future completes in. Therefore, the continuations attached to the future returned by `whenAll()` cannot always make assumptions about which thread they will be run on. Use the overload of `.then()` that takes a context object if you want to control which thread the continuations are invoked on.

This function was introduced in Qt 6.3.

### `[since 6.3]` template <typename T, typename InputIt> QFuture<QtFuture::WhenAnyResult<T>> QtFuture::whenAny(InputIt first, InputIt last)

Returns a new QFuture that succeeds when any of the futures from first to last completes. first and last are iterators to a sequence of futures packaging type `T`. The returned future packages a value of type `QtFuture::WhenAnyResult<T>` which in turn packages the index of the first completed `QFuture` and the `QFuture` itself. If first equals last, this function returns a ready `QFuture` that has `-1` for the `index` field in the QtFuture::WhenAnyResult struct and a default-constructed `QFuture<T>` for the `future` field. Note that a default-constructed QFuture is a completed future in a cancelled state.

The returned future always completes successfully after the first future from the specified futures completes. It doesn't matter if the first future completes with error or is canceled. You can use `.then()` to process the result after the future returned by `whenAny()` succeeds:

     QList<QFuture<int>> inputFutures = ...;

     QtFuture::whenAny(inputFutures.begin(), inputFutures.end())
             .then([](const QtFuture::WhenAnyResult<int> &result) {
                 qsizetype index = result.index;
                 QFuture<int> future = result.future;
                 // ...
             });

Note: If the input futures complete on different threads, the future returned by this method will complete in the thread that the first future completes in. Therefore, the continuations attached to the future returned by `whenAny()` cannot always make assumptions about which thread they will be run on. Use the overload of `.then()` that takes a context object if you want to control which thread the continuations are invoked on.

This function was introduced in Qt 6.3.

See also QtFuture::WhenAnyResult.