[boost] [futures] reference-counted futures

From: Frank Mori Hess (fmhess_at_[hidden])
Date: 2008-05-27 20:56:52

It's occurred to me that support for reference counting futures (which 
Peter Dimov has suggested a couple times) could be added simply by adding 
a method to the promise class

bool promise::has_future() const;

The has_future query would not alter the state of the promise (it wouldn't 
put it into some unusable "expired" state).  It would simply allow the 
promise-setting thread to query if there are any futures and take whatever 
action it chooses.  So it wouldn't put any burden on someone who wants to 
code a fire-and-forget function that returns a future that may be safely 
discarded.  In a typical implementation, has_future might boil down to a 
call of shared_ptr::unique().

This works because threads don't block waiting on promises, only on 
futures.  When a future's promise is destroyed without ever being 
fulfilled, action has to be taken to give the waiting futures an exception 
to prevent a thread from blocking forever in vain.  But if a promise's 
last future disappears, it doesn't need to take any action.  All it needs 
to do is provide a query that can be used by interested code to detect 
that fact.

Also, for debugging a

unsigned promise::future_count() const;

method could be provided, which might be implemented by returning the value 
of shared_ptr::use_count minus one.  

Any thoughts?  These functions could be added independently of whether or 
not an explicit future::cancel() is supported.