QPromise Class▲
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Header: QPromise
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Since: Qt 6.0
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CMake:
find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
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qmake: QT += core
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Group: QPromise is part of thread
Detailed Description▲
QPromise provides a simple way to communicate progress and results of the user-defined computation to QFuture in an asynchronous fashion. For the communication to work, QFuture must be constructed by QPromise.
You can use QPromise based workloads as an alternative to Qt Concurrent framework when fine-grained control is needed or high-level communication primitive to accompany QFuture is sufficient.
The simplest case of promise and future collaboration would be a single result communication:
QPromise<int> promise;
QFuture<int> future = promise.future();
QScopedPointer<QThread> thread(QThread::create([] (QPromise<int> promise) {
promise.start(); // notifies QFuture that the computation is started
promise.addResult(42);
promise.finish(); // notifies QFuture that the computation is finished
}, std::move(promise)));
thread->start();
future.waitForFinished(); // blocks until QPromise::finish is called
future.result(); // returns 42By design, QPromise is a move-only object. This behavior helps to ensure that whenever the promise is destroyed, the associated future object is notified and will not wait forever for the results to become available. However, this is inconvenient if one wants to use the same promise to report results from different threads. There is no specific way to do that at the moment, but known mechanisms exist, such as the use of smart pointers or raw pointers/references. QSharedPointer is a good default choice if you want to copy your promise and use it in multiple places simultaneously. Raw pointers or references are, in a sense, easier, and probably perform better (since there is no need to do a resource management) but may lead to dangling.
Here is an example of how a promise can be used in multiple threads:
QSharedPointer<QPromise<int>> sharedPromise(new QPromise<int>());
QFuture<int> future = sharedPromise->future();
// ...
sharedPromise->start();
// here, QPromise is shared between threads via a smart pointer
QScopedPointer<QThread> threads[] = {
QScopedPointer<QThread>(QThread::create([] (auto sharedPromise) {
sharedPromise->addResult(0, 0); // adds value 0 by index 0
}, sharedPromise)),
QScopedPointer<QThread>(QThread::create([] (auto sharedPromise) {
sharedPromise->addResult(-1, 1); // adds value -1 by index 1
}, sharedPromise)),
QScopedPointer<QThread>(QThread::create([] (auto sharedPromise) {
sharedPromise->addResult(-2, 2); // adds value -2 by index 2
}, sharedPromise)),
// ...
};
// start all threads
for (auto& t : threads)
t->start();
// ...
future.resultAt(0); // waits until result at index 0 becomes available. returns value 0
future.resultAt(1); // waits until result at index 1 becomes available. returns value -1
future.resultAt(2); // waits until result at index 2 becomes available. returns value -2
sharedPromise->finish();See Also▲
See also QFuture
Member Function Documentation▲
bool QPromise::addResult(T &&result, int index = -1)▲
bool QPromise::addResult(const T &result, int index = -1)
Adds result to the internal result collection at index position. If index is unspecified, result is added to the end of the collection.
Returns true when result is added to the collection.
Returns false when this promise is in canceled or finished state or when result is rejected. addResult() rejects result if there's already another result in the collection stored at the same index.
You can get a result at a specific index by calling QFuture::resultAt().
It is possible to specify an arbitrary index and request result at that index. However, some QFuture methods operate with continuous results. For instance, iterative approaches that use QFuture::resultCount() or QFuture::const_iterator. In order to get all available results without thinking if there are index gaps or not, use QFuture::results().
QPromise::QPromise()▲
Constructs a QPromise with a default state.
QPromise::QPromise(QPromise<T> &&other)▲
QPromise::~QPromise()▲
Destroys the promise.
The promise implicitly transitions to a canceled state on destruction unless finish() is called beforehand by the user.
void QPromise::finish()▲
Reports that the computation is finished. Once finished, no new results will be added when calling addResult(). This method accompanies start().
See Also▲
See also QFuture::isFinished(), QFuture::waitForFinished(), start()
QFuture<T> QPromise::future() const▲
Returns a future associated with this promise.
bool QPromise::isCanceled() const▲
Returns whether the computation has been canceled with the QFuture::cancel() function. The returned value true indicates that the computation should be finished and finish() called.
After cancellation, results currently available may still be accessed by a future, but new results will not be added when calling addResult().
void QPromise::setException(const QException &e)▲
Sets exception e to be the result of the computation.
You can set at most one exception throughout the computation execution.
This method must not be used after QFuture::cancel() or finish().
See Also▲
See also isCanceled()
void QPromise::setException(std::exception_ptr e)▲
This is an overloaded function.
void QPromise::setProgressRange(int minimum, int maximum)▲
Sets the progress range of the computation to be between minimum and maximum.
If maximum is smaller than minimum, minimum becomes the only legal value.
The progress value is reset to be minimum.
The progress range usage can be disabled by using setProgressRange(0, 0). In this case progress value is also reset to 0.
See Also▲
See also QFuture::progressMinimum(), QFuture::progressMaximum(), QFuture::progressValue()
void QPromise::setProgressValue(int progressValue)▲
Sets the progress value of the computation to progressValue. It is possible to only increment the progress value. This is a convenience method for calling setProgressValueAndText(progressValue, QString()).
In case of the progressValue falling out of the progress range, this method has no effect.
See Also▲
See also QFuture::progressValue(), setProgressRange()
void QPromise::setProgressValueAndText(int progressValue, const QString &progressText)▲
Sets the progress value and the progress text of the computation to progressValue and progressText respectively. It is possible to only increment the progress value.
This function has no effect if the promise is in canceled or finished state.
See Also▲
See also QFuture::progressValue(), QFuture::progressText(), QFuture::cancel(), finish()
void QPromise::start()▲
Reports that the computation is started. Calling this method is important to state the beginning of the computation as QFuture methods rely on this information.
Extra attention is required when start() is called from a newly created thread. In such case, the call might naturally be delayed due to the implementation details of the thread scheduling.
See Also▲
See also QFuture::isStarted(), QFuture::waitForFinished(), finish()
void QPromise::suspendIfRequested()▲
Conditionally suspends current thread of execution and waits until resumed or canceled by the corresponding methods of QFuture. This method does not block unless the computation is requested to be suspended by QFuture::suspend() or another related method. If you want to check that the execution has been suspended, use QFuture::isSuspended().
When using the same promise in multiple threads, QFuture::isSuspended() becomes true as soon as at least one thread with the promise suspends.
The following code snippets show the usage of suspension mechanism:
// Create promise and future
QPromise<int> promise;
QFuture<int> future = promise.future();
promise.start();
// Start a computation thread that supports suspension and cancellation
QScopedPointer<QThread> thread(QThread::create([] (QPromise<int> promise) {
for (int i = 0; i < 100; ++i) {
promise.addResult(i);
promise.suspendIfRequested(); // support suspension
if (promise.isCanceled()) // support cancellation
break;
}
promise.finish();
}, std::move(promise)));
thread->start();QFuture::suspend() requests the associated promise to suspend:
future.suspend();After QFuture::isSuspended() becomes true, you can get intermediate results:
future.resultCount(); // returns some number between 0 and 100
for (int i = 0; i < future.resultCount(); ++i) {
// process results available before suspension
}When suspended, you can resume or cancel the awaiting computation:
future.resume(); // resumes computation, this call will unblock the promise
// alternatively, call future.cancel() to stop the computation
future.waitForFinished();
future.results(); // returns all computation results - array of values from 0 to 99See Also▲
See also QFuture::resume(), QFuture::cancel(), QFuture::setSuspended(), QFuture::toggleSuspended()
void QPromise::swap(QPromise<T> &other)▲
Swaps promise other with this promise. This operation is very fast and never fails.
QPromise<T> &QPromise::operator=(QPromise<T> &&other)▲
Move assigns other to this promise and returns a reference to this promise.