Hmm, the error isn't quite what I said: 1>V:\Trunk\stable\src\Include\Tools/async.h(268): error C2664: 'boost::unique_future<R>::unique_future(boost::unique_future<R> &)' : cannot convert parameter 1 from 'boost::unique_future<R>' to 'boost::unique_future<R> &' 1> with 1> [ 1> R=boost::optional & 1> ] 1> and 1> [ 1> R=boost::optional 1> ] 1> and 1> [ 1> R=boost::optional & 1> ] But my other test cases of using unique_future (with simple types) work OK. —John