Re: [boost] New Boost.XInt Library, request preliminary review

Subject: Re: [boost] New Boost.XInt Library, request preliminary review
From: Chad Nelson (chad.thecomfychair_at_[hidden])
Date: 2010-03-28 18:11:05

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On 03/28/2010 04:50 PM, Scott McMurray wrote:

> What operations result in NaNs, anyways? 

Copying from the documentation:

- ---------------------8<----------------------
To create a Not-a-Number, use the integer constructor that exists solely
for that purpose. No other library function will return a Not-a-Number
except under the following circumstances:

* If a function that returns a non-reference integer tries to throw a
blockable exception, but exceptions are blocked;

* If the string-to-integer functions are asked to convert the string
"#NaN#";

* If you call the function invmod, and there is no valid inverse;

* If you call the function random_prime with a callback, and the
callback function cancels it.
- ---------------------8<----------------------

> Are there any whose preconditions can't be trivially checked?

If you know your modular math, and never block exceptions, then no.

> The only ones that come to mind are checking for 0 before division
> and checking for negatives before taking a square root, both of which
> are just as easily checked outside the operation as inside it.

Hm... you're right, taking the square root of a negative number *should*
throw an exception. The library presently (and erroneously) returns
zero. Thanks for the accidental catch. ;-)

> So I'd say just throw instead of generating the NaN in the first
> place.  With unbounded integers there's no underflow or overflow to
> worry about, so the advantage of the special values is questionable.
> (And I could make a wrapper to add the special values -- NaN, +Inf,
> -Inf, 1/+Inf, 1/-Inf -- if I needed.)

As mentioned in my last response to Jeffrey Hellrung, the NaN value is
returned when the library needs to throw a blockable exception, but
exceptions are blocked. Throwing an exception in that case is exactly
what I *don't* want to do.
- -- 
Chad Nelson
Oak Circle Software, Inc.
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