QFuture Class

Member Function Documentation

QFuture::QFuture()

Constructs an empty, canceled future.

QFuture::QFuture(const QFuture<T> &other)

Constructs a copy of other.

See also operator=().

QFuture::~QFuture()

Destroys the future.

Note that this neither waits nor cancels the asynchronous computation. Use waitForFinished() or QFutureSynchronizer when you need to ensure that the computation is completed before the future is destroyed.

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> QFuture<T>::const_iterator QFuture::begin() const

Returns a const STL-style iterator pointing to the first result in the future.

See also constBegin() and end().

void QFuture::cancel()

Cancels the asynchronous computation represented by this future. Note that the cancellation is asynchronous. Use waitForFinished() after calling cancel() when you need synchronous cancellation.

Results currently available may still be accessed on a canceled future, but new results will not become available after calling this function. Any QFutureWatcher object that is watching this future will not deliver progress and result ready signals on a canceled future.

Be aware that not all running asynchronous computations can be canceled. For example, the future returned by QtConcurrent::run() cannot be canceled; but the future returned by QtConcurrent::mappedReduced() can.

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> QFuture<T>::const_iterator QFuture::constBegin() const

Returns a const STL-style iterator pointing to the first result in the future.

See also begin() and constEnd().

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> QFuture<T>::const_iterator QFuture::constEnd() const

Returns a const STL-style iterator pointing to the imaginary result after the last result in the future.

See also constBegin() and end().

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> QFuture<T>::const_iterator QFuture::end() const

Returns a const STL-style iterator pointing to the imaginary result after the last result in the future.

See also begin() and constEnd().

bool QFuture::isCanceled() const

Returns true if the asynchronous computation has been canceled with the cancel() function; otherwise returns false.

Be aware that the computation may still be running even though this function returns true. See cancel() for more details.

bool QFuture::isFinished() const

Returns true if the asynchronous computation represented by this future has finished; otherwise returns false.

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> bool QFuture::isResultReadyAt(int index) const

Returns true if the result at index is immediately available; otherwise returns false.

See also resultAt(), resultCount(), and takeResult().

bool QFuture::isRunning() const

Returns true if the asynchronous computation represented by this future is currently running; otherwise returns false.

bool QFuture::isStarted() const

Returns true if the asynchronous computation represented by this future has been started; otherwise returns false.

[since 6.0] bool QFuture::isSuspended() const

Returns true if a suspension of the asynchronous computation has been requested, and it is in effect, meaning that no more results or progress changes are expected.

This function was introduced in Qt 6.0.

See also setSuspended(), toggleSuspended(), and isSuspending().

[since 6.0] bool QFuture::isSuspending() const

Returns true if the asynchronous computation has been suspended with the suspend() function, but the work is not yet suspended, and computation is still running. Returns false otherwise.

To check if suspension is actually in effect, use isSuspended() instead.

This function was introduced in Qt 6.0.

See also setSuspended(), toggleSuspended(), and isSuspended().

[since 6.0] bool QFuture::isValid() const

Returns true if a result or results can be accessed or taken from this QFuture object. Returns false after the result was taken from the future.

This function was introduced in Qt 6.0.

See also takeResult(), result(), results(), and resultAt().

[since 6.0] template <typename Function, typename = std::enable_if_t<std::is_invocable_r_v<T, Function>>> QFuture<T> QFuture::onCanceled(Function &&handler)

Attaches a cancellation handler to this future. The returned future behaves exactly as this future (has the same state and result) unless this future is cancelled. The handler is a callable which takes no arguments and returns a value of the type packaged by this future. After cancellation, the returned future packages the value returned by handler.

If attached before the cancellation, handler will be invoked in the same thread that reports the future as finished after the cancellation. If the handler is attached after this future has already been canceled, it will be invoked immediately in the thread that executes onCanceled(). Therefore, the handler cannot always make assumptions about which thread it will be run on. Use the overload that takes a context object if you want to control which thread the handler is invoked on.

The example below demonstrates how to attach a cancellation handler:

QFuture<int> testFuture = ...;
auto resultFuture = testFuture.then([](int res) {
    // Block 1
    ...
    return 1;
}).then([](int res) {
    // Block 2
    ...
    return 2;
}).onCanceled([] {
    // Block 3
    ...
    return -1;
});

If testFuture is canceled, Block 3 will be called and the resultFuture will have -1 as its result. Unlike testFuture, it won't be in a Canceled state. This means that you can get its result, attach countinuations to it, and so on.

Also note that you can cancel the chain of continuations while they are executing via the future that started the chain. Let's say testFuture.cancel() was called while Block 1 is already executing. The next continuation will detect that cancellation was requested, so Block 2 will be skipped, and the cancellation handler (Block 3) will be called.

This function was introduced in Qt 6.0.

See also then() and onFailed().

[since 6.1] template <typename Function, typename = std::enable_if_t<std::is_invocable_r_v<T, Function>>> QFuture<T> QFuture::onCanceled(QObject *context, Function &&handler)

This is an overloaded function.

Attaches a cancellation handler to this future, to be called when the future is canceled. The handler is a callable which doesn't take any arguments. It will be invoked in the thread of the context object. This can be useful if the cancellation needs to be handled in a specific thread.

If the context is destroyed before the chain has finished, the future is canceled. See then() for details.

See the documentation of the other overload for more details about handler.

This function was introduced in Qt 6.1.

See also then() and onFailed().

[since 6.0] template <typename Function, typename = std::enable_if_t<!QtPrivate::ArgResolver<Function>::HasExtraArgs>> QFuture<T> QFuture::onFailed(Function &&handler)

Attaches a failure handler to this future, to handle any exceptions. The returned future behaves exactly as this future (has the same state and result) unless this future fails with an exception.

The handler is a callable which takes either no argument or one argument, to filter by specific error types, similar to the catch statement. It returns a value of the type packaged by this future. After the failure, the returned future packages the value returned by handler.

The handler will only be invoked if an exception is raised. If the exception is raised after this handler is attached, the handler is executed in the thread that reports the future as finished as a result of the exception. If the handler is attached after this future has already failed, it will be invoked immediately, in the thread that executes onFailed(). Therefore, the handler cannot always make assumptions about which thread it will be run on. Use the overload that takes a context object if you want to control which thread the handler is invoked on.

The example below demonstrates how to attach a failure handler:

QFuture<int> future = ...;
auto resultFuture = future.then([](int res) {
    ...
    throw Error();
    ...
}).onFailed([](const Error &e) {
    // Handle exceptions of type Error
    ...
    return -1;
}).onFailed([] {
    // Handle all other types of errors
    ...
    return -1;
});

auto result = resultFuture.result(); // result is -1

If there are multiple handlers attached, the first handler that matches with the thrown exception type will be invoked. For example:

QFuture<int> future = ...;
future.then([](int res) {
    ...
    throw std::runtime_error("message");
    ...
}).onFailed([](const std::exception &e) {
    // This handler will be invoked
}).onFailed([](const std::runtime_error &e) {
    // This handler won't be invoked, because of the handler above.
});

If none of the handlers matches with the thrown exception type, the exception will be propagated to the resulted future:

QFuture<int> future = ...;
auto resultFuture = future.then([](int res) {
    ...
    throw Error("message");
    ...
}).onFailed([](const std::exception &e) {
    // Won't be invoked
}).onFailed([](const QException &e) {
    // Won't be invoked
});

try {
    auto result = resultFuture.result();
} catch(...) {
    // Handle the exception
}

This function was introduced in Qt 6.0.

See also then() and onCanceled().

[since 6.1] template <typename Function, typename = std::enable_if_t<!QtPrivate::ArgResolver<Function>::HasExtraArgs>> QFuture<T> QFuture::onFailed(QObject *context, Function &&handler)

This is an overloaded function.

Attaches a failure handler to this future, to handle any exceptions that the future raises, or that it has already raised. Returns a QFuture of the same type as this future. The handler will be invoked only in case of an exception, in the thread of the context object. This can be useful if the failure needs to be handled in a specific thread. For example:

// somewhere in the main thread
auto future = QtConcurrent::run([] {
    // This will run in a separate thread
    ...
    throw std::exception();
}).onFailed(this, [] {
   // Update UI elements
});

The failure handler attached into QtConcurrent::run updates the UI elements and cannot be invoked from a non-gui thread. So this is provided as a context to .onFailed(), to make sure that it will be invoked in the main thread.

If the context is destroyed before the chain has finished, the future is canceled. See then() for details.

See the documentation of the other overload for more details about handler.

This function was introduced in Qt 6.1.

See also then() and onCanceled().

int QFuture::progressMaximum() const

Returns the maximum progressValue().

See also progressValue() and progressMinimum().

int QFuture::progressMinimum() const

Returns the minimum progressValue().

See also progressValue() and progressMaximum().

QString QFuture::progressText() const

Returns the (optional) textual representation of the progress as reported by the asynchronous computation.

Be aware that not all computations provide a textual representation of the progress, and as such, this function may return an empty string.

int QFuture::progressValue() const

Returns the current progress value, which is between the progressMinimum() and progressMaximum().

See also progressMinimum() and progressMaximum().

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> T QFuture::result() const

Returns the first result in the future. If the result is not immediately available, this function will block and wait for the result to become available. This is a convenience method for calling resultAt(0). Note that result() returns a copy of the internally stored result. If T is a move-only type, or you don't want to copy the result, use takeResult() instead.

See also resultAt(), results(), and takeResult().

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> T QFuture::resultAt(int index) const

Returns the result at index in the future. If the result is not immediately available, this function will block and wait for the result to become available.

See also result(), results(), takeResult(), and resultCount().

int QFuture::resultCount() const

Returns the number of continuous results available in this future. The real number of results stored might be different from this value, due to gaps in the result set. It is always safe to iterate through the results from 0 to resultCount().

See also result(), resultAt(), results(), and takeResult().

template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> QList<T> QFuture::results() const

Returns all results from the future. If the results are not immediately available, this function will block and wait for them to become available. Note that results() returns a copy of the internally stored results. Getting all results of a move-only type T is not supported at the moment. However you can still iterate through the list of move-only results by using STL-style iterators or read-only Java-style iterators.

See also result(), resultAt(), takeResult(), resultCount(), and isValid().

void QFuture::resume()

Resumes the asynchronous computation represented by the future(). This is a convenience method that simply calls setSuspended(false).

See also suspend().

[since 6.0] void QFuture::setSuspended(bool suspend)

If suspend is true, this function suspends the asynchronous computation represented by the future(). If the computation is already suspended, this function does nothing. QFutureWatcher will not immediately stop delivering progress and result ready signals when the future is suspended. At the moment of suspending there may still be computations that are in progress and cannot be stopped. Signals for such computations will still be delivered.

If suspend is false, this function resumes the asynchronous computation. If the computation was not previously suspended, this function does nothing.

Be aware that not all computations can be suspended. For example, the QFuture returned by QtConcurrent::run() cannot be suspended; but the QFuture returned by QtConcurrent::mappedReduced() can.

This function was introduced in Qt 6.0.

See also isSuspended(), suspend(), resume(), and toggleSuspended().

[since 6.0] void QFuture::suspend()

Suspends the asynchronous computation represented by this future. This is a convenience method that simply calls setSuspended(true).

This function was introduced in Qt 6.0.

See also resume().

[since 6.0] template <typename U = T, typename = QtPrivate::EnableForNonVoid<U>> T QFuture::takeResult()

Call this function only if isValid() returns true, otherwise the behavior is undefined. This function takes (moves) the first result from the QFuture object, when only one result is expected. If there are any other results, they are discarded after taking the first one. If the result is not immediately available, this function will block and wait for the result to become available. The QFuture will try to use move semantics if possible, and will fall back to copy construction if the type is not movable. After the result was taken, isValid() will evaluate as false.

This function was introduced in Qt 6.0.

See also result(), results(), resultAt(), and isValid().

[since 6.0] template <typename Function> QFuture<QFuture<T>::ResultType<Function>> QFuture::then(Function &&function)

This is an overloaded function.

Attaches a continuation to this future, allowing to chain multiple asynchronous computations if desired, using the Sync policy. function is a callable that takes an argument of the type packaged by this future if this has a result (is not a QFuture<void>). Otherwise it takes no arguments. This method returns a new QFuture that packages a value of the type returned by function. The returned future will be in an uninitialized state until the attached continuation is invoked, or until this future fails or is canceled.

You can chain multiple operations like this:

QFuture<int> future = ...;
future.then([](int res1){ ... }).then([](int res2){ ... })...

Or:

QFuture<void> future = ...;
future.then([](){ ... }).then([](){ ... })...

The continuation can also take a QFuture argument (instead of its value), representing the previous future. This can be useful if, for example, QFuture has multiple results, and the user wants to access them inside the continuation. Or the user needs to handle the exception of the previous future inside the continuation, to not interrupt the chain of multiple continuations. For example:

QFuture<int> future = ...;
    future.then([](QFuture<int> f) {
        try {
            ...
            auto result = f.result();
            ...
        } catch (QException &e) {
            // handle the exception
        }
    }).then(...);

If the previous future throws an exception and it is not handled inside the continuation, the exception will be propagated to the continuation future, to allow the caller to handle it:

QFuture<int> future = ...;
auto continuation = future.then([](int res1){ ... }).then([](int res2){ ... })...
...
// future throws an exception
try {
    auto result = continuation.result();
} catch (QException &e) {
    // handle the exception
}

In this case the whole chain of continuations will be interrupted.

This function was introduced in Qt 6.0.

See also onFailed() and onCanceled().

[since 6.0] template <typename Function> QFuture<QFuture<T>::ResultType<Function>> QFuture::then(QtFuture::Launch policy, Function &&function)

This is an overloaded function.

Attaches a continuation to this future, allowing to chain multiple asynchronous computations. When the asynchronous computation represented by this future finishes, function will be invoked according to the given launch policy. A new QFuture representing the result of the continuation is returned.

Depending on the policy, continuation will be invoked in the same thread as this future, in a new thread, or will inherit the launch policy and thread pool of this future. If no launch policy is specified (see the overload taking only a callable), the Sync policy will be used.

In the following example both continuations will be invoked in a new thread (but in the same one).

QFuture<int> future = ...;
future.then(QtFuture::Launch::Async, [](int res){ ... }).then([](int res2){ ... });

In the following example both continuations will be invoked in new threads using the same thread pool.

QFuture<int> future = ...;
future.then(QtFuture::Launch::Async, [](int res){ ... })
      .then(QtFuture::Launch::Inherit, [](int res2){ ... });

See the documentation of the other overload for more details about function.

This function was introduced in Qt 6.0.

See also onFailed() and onCanceled().

[since 6.0] template <typename Function> QFuture<QFuture<T>::ResultType<Function>> QFuture::then(QThreadPool *pool, Function &&function)

This is an overloaded function.

Attaches a continuation to this future, allowing to chain multiple asynchronous computations if desired. When the asynchronous computation represented by this future finishes, function will be scheduled on pool.

This function was introduced in Qt 6.0.

See also onFailed() and onCanceled().

[since 6.1] template <typename Function> QFuture<QFuture<T>::ResultType<Function>> QFuture::then(QObject *context, Function &&function)

This is an overloaded function.

Attaches a continuation to this future, allowing to chain multiple asynchronous computations if desired. When the asynchronous computation represented by this future finishes, function will be invoked in the thread of the context object. This can be useful if the continuation needs to be invoked in a specific thread. For example:

// somewhere in the main thread
auto future = QtConcurrent::run([] {
    // This will run in a separate thread
    ...
}).then(this, [] {
   // Update UI elements
});

The continuation attached into QtConcurrent::run updates the UI elements and cannot be invoked from a non-gui thread. So this is provided as a context to .then(), to make sure that it will be invoked in the main thread.

The following continuations will be also invoked from the same context, unless a different context or launch policy is specified:

auto future = QtConcurrent::run([] {
    ...
}).then(this, [] {
   // Update UI elements
}).then([] {
    // This will also run in the main thread
});

This is because by default .then() is invoked from the same thread as the previous one.

But note that if the continuation is attached after this future has already finished, it will be invoked immediately, in the thread that executes then():

QObject *context = ...;
auto future = cachedResultsReady ? QtFuture::makeReadyValueFuture(result)
                                 : QtConcurrent::run([] { /* compute result */});
auto continuation = future.then(context, [] (Result result) {
    // Runs in the context's thread
}).then([] {
    // May or may not run in the context's thread
});

In the above example if cachedResultsReady is true, and a ready future is returned, it is possible that the first .then() finishes before the second one is attached. In this case it will be resolved in the current thread. Therefore, when in doubt, pass the context explicitly.

If the context is destroyed before the chain has finished, the future is canceled. This implies that a cancellation handler might be invoked when the context is not valid anymore. To guard against this, capture the context as a QPointer:

QObject *context = ...;
auto future = ...;
auto continuation = future.then(context, [context](Result result) {
                               // ...
                           }).onCanceled([context = QPointer(context)] {
                               if (!context)
                                   return;  // context was destroyed already
                               // handle cancellation
                           });

When the context object is destroyed, cancellation happens immediately. Previous futures in the chain are not cancelled and keep running until they are finished.

This function was introduced in Qt 6.1.

See also onFailed() and onCanceled().

[since 6.0] void QFuture::toggleSuspended()

Toggles the suspended state of the asynchronous computation. In other words, if the computation is currently suspending or suspended, calling this function resumes it; if the computation is running, it is suspended. This is a convenience method for calling setSuspended(!(isSuspending() || isSuspended())).

This function was introduced in Qt 6.0.

See also setSuspended(), suspend(), and resume().

[since 6.4] template <typename U> QFuture<U> QFuture::unwrap()

Unwraps the inner future from this QFuture<T>, where T is a future of type QFuture<U>, i.e. this future has type of QFuture<QFuture<U>>. For example:

QFuture<QFuture<int>> outerFuture = ...;
QFuture<int> unwrappedFuture = outerFuture.unwrap();

unwrappedFuture will be fulfilled as soon as the inner future nested inside the outerFuture is fulfilled, with the same result or exception and in the same thread that reports the inner future as finished. If the inner future is canceled, unwrappedFuture will also be canceled.

This is especially useful when chaining multiple computations, and one of them returns a QFuture as its result type. For example, let's say we want to download multiple images from an URL, scale the images, and reduce them to a single image using QtConcurrent::mappedReduced(). We could write something like:

auto downloadImages = [] (const QUrl &url) {
    QList<QImage> images;
    ...
    return images;
};

auto processImages = [](const QList<QImage> &images) {
   return QtConcurrent::mappedReduced(images, scale, reduceImages);
}

auto show = [](const QImage &image) { ... };

auto future = QtConcurrent::run(downloadImages, url)
               .then(processImages)
               .unwrap()
               .then(show);

Here QtConcurrent::mappedReduced() returns a QFuture<QImage>, so .then(processImages) returns a QFuture<QFuture<QImage>>. Since show() takes a QImage as argument, the result of .then(processImages) can't be passed to it directly. We need to call .unwrap(), that will get the result of the inner future when it's ready and pass it to the next continuation.

In case of multiple nesting, .unwrap() goes down to the innermost level:

QFuture<QFuture<QFuture<int>>>> outerFuture;
QFuture<int> unwrappedFuture = outerFuture.unwrap();

This function was introduced in Qt 6.4.

void QFuture::waitForFinished()

Waits for the asynchronous computation to finish (including cancel()ed computations), i.e. until isFinished() returns true.

QFuture<T> &QFuture::operator=(const QFuture<T> &other)

Assigns other to this future and returns a reference to this future.