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From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2008-05-14 06:15:44

"vicente.botet" <vicente.botet_at_[hidden]> writes:

> ----- Original Message -----
> From: "Anthony Williams" <anthony_w.geo_at_[hidden]>
> To: <boost_at_[hidden]>
> Sent: Wednesday, May 14, 2008 10:55 AM
> Subject: Re: [boost] Review Request: future library (N2561/Williams version)
>> "vicente.botet" <vicente.botet_at_[hidden]> writes:
>>> 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?
>> That is correct with respect to the implementation,
> BTW, Braddock implementation do a move of the list of callbacks before doing
> the callbacks. What do you thing about this approach?

That's an interesting idea: rather than calling the callback every time some
thread waits on a future, it's only called on the first call. You could make
the callback clear itself when it was invoked if you want that behaviour, as
the promise or packaged_task is passed in to the callback.

>> but I don't actually see
>> the need for multiple callbacks. The callbacks are set as part of the
>> promise
>> or packaged_task. I can't imagine why that would require multiple
>> callbacks. In any case, the user can provide that facility on their own if
>> required.
> What about the guarded schedule of Braddock?
> template future<T>
> schedule(boost::function<T (void)> const& fn, future<void> guard =
> future<void>()) {
> promise<T> prom; // create promise
> future_wrapper<T> wrap(fn,prom);
> guard.add_callback(boost::bind(&JobQueue3::queueWrapped<T>, this,
> wrap, prom));
> return future<T>(prom); // return a future created from the promise
> }
> Several task can be scheduled guarded by the same future.

That's a completion callback, not a wait callback. My proposal doesn't offer
completion callbacks.


Anthony Williams            | Just Software Solutions Ltd
Custom Software Development |
Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL

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