----- Original Message ----- From: "Anthony Williams" <anthony_w.geo@yahoo.com> To: <boost@lists.boost.org> Sent: Sunday, May 11, 2008 11:53 AM Subject: Re: [boost] Review Request: future library (N2561/Williams version)
* Finally, I've added a set_wait_callback() function to both promise and packaged_task. This allows for lazy-futures which don't actually run the operation to generate the value until the value is needed: no threading required. It also allows for a thread pool to do task stealing if a pool thread waits for a task that's not started yet. The callbacks must be thread-safe as they are potentially called from many waiting threads simultaneously. At the moment, I've specified the callbacks as taking a non-const reference to the promise or packaged_task for which they are set, but I'm open to just making them be any callable function, and leaving it up to the user to call bind() to do that.
Hi, Why you don't allow multiple callbacks? I suposse that this is related to the implementation of do_callback void do_callback(boost::unique_lock<boost::mutex>& lock) { if(callback && !done) { boost::function<void()> local_callback=callback; relocker relock(lock); local_callback(); } } You need to call all the callbacks with the mutex unlock, and you need to protect from other concurrent set_wait_callback. So you will need to copy the list of callbacks before unlock. Is this correct? Anyway, IMO, having a single call back could motivate a user wrapper which offer "bugy" multiple callbacks. Vicente