Re: [boost] Interest in Barton-Nackman's fallible<T>?

From: Nicola Musatti (Nicola.Musatti_at_[hidden])
Date: 2006-07-31 03:36:32

Sorry for this late answer, I've just got back from vacation.

Paul A Bristow wrote:
[...]
> If you have an array of 'fallible' data (measurements say), not all of which
> are valid,
> you can use doubles for the data, but you need an extra byte for each value
> to
> hold the valid/not valid bit - a bool in a byte, or more perhaps.
> 
> If you manage to persuade the compiler to pack the doubles and bytes
> together,
> the double won't be on speed-friendly boundaries,
> 
> or if you don't/can't pack each byte, the bool may end up taking as much
> space as the double.
> 
> So for a few items, fallible is fine, but for hundreds and up, has a
> downside.
> 
> A NaN by contrast is neatly stored in the double, and costs nothing extra,
> and MAY be used to indicate 'fallibility'.
> 
> But don't let this make you too tired to continue - there are other cases
> when fallible may be very useful.

One solution to this problem is to supplement fallible<T> with a trait 
indicating whether an in-range invalid value is available and its value 
(an even more obvious example than NaN's is fallible<T *>).

Cheers,
Nicola Musatti