How to create a QFuture with an immediately available value?

Question

I have a function which returns QFuture object as a result of a QtConcurrent::run computation. However, there could be some conditions before running the concurrent method where I need to manually return a value-holding future from my function.

QFuture<bool> foo(const QString &bar)
{
    if (bar.isEmpty()) {
        return QFuture<bool>(false); // This does not work.
        // Here I need to return from the function, but I don't know how to do it.
    }
    return QtConcurrent::run([=]() -> bool {
        // Asynchronous computations...
    });
}

How to manually create the QFuture object?
Or (more globally) how to properly return from such method?

Answer 1

When there's no data to return, things are easy - this should be the first thing to try anyway in modern C++:

return {};

Or, if you're targeting some obsolete platform (<Qt 5.6):

return QFuture<bool>();

That way you get an invalid future. There's no way to directly create a future that carries preset data, you'd have to use QFutureInterface for that:

// https://github.com/KubaO/stackoverflown/tree/master/questions/qfuture-immediate-50772976
#include <QtConcurrent>

template <typename T> QFuture<T> finishedFuture(const T &val) {
   QFutureInterface<T> fi;
   fi.reportFinished(&val);
   return QFuture<T>(&fi);
}

QFuture<bool> foo(bool val, bool valid) {
   if (!valid)
      return {};
   return finishedFuture(val);
}

int main() {
   Q_ASSERT(foo(true, true));
   Q_ASSERT(!foo(false, true));
   Q_ASSERT(foo(false, false).isCanceled());
   Q_ASSERT(foo(true, false).isCanceled());
}