Details
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Suggestion
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Resolution: Unresolved
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P3: Somewhat important
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None
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6.2.0
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None
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All
Description
This issue was hit many times already when trying to migrate from Utils::runAsync() using QFutureInterface<T> into QtConcurrent::run() using QPromise<T> inside QtCreator.
It should be always possibile to access the QPromise<void> API from QPromise<T> object.
The responsibility of the QPromise<void> is mainly to check for cancel of for progress reporting, while QPromise<T> adds a responsibility of delivering the results.
The example use case:
// The responsibility of the promise passed here is to check for cancel and report progress. // Moreover, this helper may live in some other library and may not have access to MyType at all. void helper(QPromise<void> &promise) { ... while (...) { if (promise.isCanceled()) return; promise.setProgressValue(i); .... } .... } // The responsibility of the promise passed here is to check for cancel, to report progress and to deliver result. void asyncTask1(QPromise<MyType> &promise) { ... helper(promise); // Can't pass QPromise<void> here. ... } // Same here, we reuse helper(), but work on different type of QPromise void asyncTask2(QPromise<MyOtherType> &promise) { ... helper(promise); // Can't pass QPromise<void> here. ... promise.addResult(...); } // Main thread void runAsync() { .... auto future1 = QtConcurrent::run(asyncTask1); auto future2 = QtConcurrent::run(asyncTask2); .... }
As a quick workaround on the user side I wondered about:
template <typename T>
QPromise<void> &toVoidPromise(QPromise<T> &promise)
{
return reinterpret_cast<QPromise<void> &>(promise);
}
But I'm really not sure if that's guaranteed to work in 100% correctly.