Creating a completed Deferred is very heavy

[vpicaver]

Chaining asyncfuture is useful to chain QFuture's together. Sometimes at the begin of the chain, it's useful to have completed QFuture that come from a fixed value. To achieve this with asyncfuture, a programmer needs to use deferred() and then complete it with deferred::complete(). Although this works, deferred creates a Deferred object and a that holds a DeferredFuture. A DeferredFuture is a fairly heavy weight object since inherits QObject and hold a mutex. A DeferrerFuture is usually stored as a shared object and require reference counting.

To reduce this overhead for the specific job having a QFuture that's automatically complete, I propose having a QFuture<void> finished() and QFuture<T> finished<T>() functions.

They would be implement as the following:

inline QFuture<void> finished() {
   QFutureInterface<void> fi;
   fi.reportFinished();
   return QFuture<void>(&fi);
}

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

[remster]

Ack! This is needed.