[Interest] Question about QFutureWatcher<void> monitoring a QFuture<T>

Ivan Solovev ivan.solovev at qt.io
Thu Sep 29 12:35:01 CEST 2022

Hi Sean,

I'm not a QtConcurrent expert, but it looks like the reason for the error message is that an implicit conversion between QFuture<void> and QFuture<T> was removed in Qt 6. The recommended approach is to explicitly use a corresponding QFuture<void> constructor.
See the details in the documentation: https://doc.qt.io/qt-6/qtcore-changes-qt6.html#implicit-conversions-between-qfuture-and-other-types

Best regards,

From: Interest <interest-bounces at qt-project.org> on behalf of Sean Murphy via Interest <interest at qt-project.org>
Sent: Wednesday, September 28, 2022 4:15 PM
To: interest at qt-project.org <interest at qt-project.org>
Subject: [Interest] Question about QFutureWatcher<void> monitoring a QFuture<T>

In the Qt Docs, at the bottom of QFutureWatcher's Detailed
Description section there's this statement:
  Any QFuture<T> can be watched by a QFutureWatcher<void> as
  well. This is useful if only status or progress information
  is needed; not the actual result data.

That statement fits what I'm trying to do - I only need the
status/progress information. So I attempt the lines:
  QFutureWatcher<void> watcher;
  watcher.setFuture(QtConcurrent::mappedReduced(list, mapped, reduce));

This fails to compile with the error message:
  No viable conversion from 'QFuture<int>' to 'const QFuture<void>'
Where my mapped() function has the form
  U function(const T &t);
And my reduce() function has the form
  void function(int &result, const U &intermediate)

If I explicitly cast the mappedReduced() return value to a QFuture<void>:
             QtConcurrent::mappedReduced(list, mapped, reduce)));

It compiles and seems to run fine, but I just wanted to double
check this was intended behavior, or if there was a better way
to do it? From the line in the docs, I assumed that the original
assignment was fine and I wouldn't need to explicitly cast.


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