QtFuture Namespace

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 6.1] template <typename T> QFuture<T> QtFuture::makeExceptionalFuture(const QException &exception)

Creates and returns a QFuture which already has an exception exception.

 QException e;
 auto f = QtFuture::makeExceptionalFuture<int>(e);
 ...
 try {
     f.result(); // throws QException
 } catch (QException &) {
     // handle exception here
 }

This function was introduced in Qt 6.1.

See also QFuture, QException, and QtFuture::makeReadyFuture().

[since 6.1] template <typename T> QFuture<T> QtFuture::makeExceptionalFuture(std::exception_ptr exception)

This is an overloaded function.

Creates and returns a QFuture which already has an exception exception.

 struct TestException
 {
 };
 ...
 auto exception = std::make_exception_ptr(TestException());
 auto f = QtFuture::makeExceptionalFuture<int>(exception);
 ...
 try {
     f.result(); // throws TestException
 } catch (TestException &) {
     // handle exception here
 }

This function was introduced in Qt 6.1.

See also QFuture, QException, and QtFuture::makeReadyFuture().

[since 6.1] template <typename T, typename> QFuture<std::decay_t<T>> QtFuture::makeReadyFuture(T &&value)

This is an overloaded function.

Creates and returns a QFuture which already has a result value. The returned QFuture has a type of std::decay_t<T>, where T is not void.

 auto f = QtFuture::makeReadyFuture(std::make_unique<int>(42));
 ...
 const int result = *f.takeResult(); // result == 42

This function was introduced in Qt 6.1.

See also QFuture and QtFuture::makeExceptionalFuture().

[since 6.1] QFuture<void> QtFuture::makeReadyFuture()

This is an overloaded function.

Creates and returns a void QFuture. Such QFuture can't store any result. One can use it to query the state of the computation. The returned QFuture will always be in the finished state.

 auto f = QtFuture::makeReadyFuture();
 ...
 const bool started = f.isStarted(); // started == true
 const bool running = f.isRunning(); // running == false
 const bool finished = f.isFinished(); // finished == true

This function was introduced in Qt 6.1.

See also QFuture, QFuture::isStarted(), QFuture::isRunning(), QFuture::isFinished(), and QtFuture::makeExceptionalFuture().

[since 6.1] template <typename T> QFuture<T> QtFuture::makeReadyFuture(const QList<T> &values)

This is an overloaded function.

Creates and returns a QFuture which already has multiple results set from values.

 const QList<int> values { 1, 2, 3 };
 auto f = QtFuture::makeReadyFuture(values);
 ...
 const int count = f.resultCount(); // count == 3
 const auto results = f.results(); // results == { 1, 2, 3 }

This function was introduced in Qt 6.1.

See also QFuture and QtFuture::makeExceptionalFuture().