"Robert Ramey" <ramey@rrsd.com> writes:
David Abrahams wrote:
There are important numerical applications where large collections of individual floating values (e.g. vectors and matrices) have to be computed in parallel, and one or more NaNs in the result do not render the whole calculation useless.
I suppose that's possible. I'm not familiar with such applications.
Exactly.
It sounds to me that they would be implemented in special hardware
No.
and sort of out of the C++ mainstream.
Wow, that's exactly the kind of thing politicians say about each other when they want to paint their views as radical and worthy of dismissal.
Besides all that, eliminating NaNs from the serialization problem space is really impossible. Even if we struck them from the C++ language, someone would effectively recreate them using optional<float> or the equivalent.
Actually that would be an improvement in that the "union" would be explicit, visible and modifiable.
It's currently visible, since it is possible to test for NaN. And as far as I know, it's modifiable. What, exactly, do you mean by "modifiable?" I can explicitly request a NaN from numeric_limits.
I would argue that such code is already broken anyway - it just looks like its working.
Really, you still think so?
My view on this is in another post.
If so, I'd like to know how you measure "brokenness."
A program which produces an undefined result is "broke"
I think what you mean to say is that it's nonportable.
BTW - as far as the serialization system is concerned I never had a problem with the idea of recovering the exact kind of "undefined" data. Its just that there's no way to do it with archives which might be ported from one machine to another as there is no guarentee that the reading machine has the same set of of undefined results as the source machine.
That doesn't mean there's no way to do it. I can think of any number of ways, including, for example, having a replaceable handler for translating unrepresentable values.
Its even worse, there is way to write portable code which will generate any specific one of the "undefined" types.
numeric_limits<float>::quiet_NaN() Is "reasonably portable."
One might hack something together that would recover some undefined type but it wouldn't be guarenteed to the the same original type of undefined type.
Type of undefined type? -- Dave Abrahams Boost Consulting www.boost-consulting.com