Re: [boost] Stream input and output of NaN and infinity

From: David Abrahams (dave_at_[hidden])
Date: 2006-05-15 15:29:36

"Robert Ramey" <ramey_at_[hidden]> writes:

> Peter Dimov wrote:
>
>> It depends. Where do you draw the line? Is inf a number? Is -0.0 a
>> number? You have to have NaN if you want to be able to represent x/y
>> as a float.
>
> That's the problem.  x/y is not a valid operation if y is equal to 0. So
> it can't be represented as a number.
>
> The fact that C++ permits such an operation makes C++ different
> than arithmetic.  The fact that C++ uses operators like "/" and defines them
> similar to - but not identical to - the way they are defined by
> standard arithmetic is the source of all these problems.  

Well, you'd better come up with a new number representation, then.

      assert(2/3*3 == 2);         // boom
      assert(2.0/3.0*3.0 == 2.0); // boom

> I say that C++ should be changed to so that the floats and operators
> which apply to them should implement what people expect from
> arithmetic operators.

That would be great, wouldn't it, LOL? (there, I get to use that one
this time)

There are two kinds of programmers in the world: those who expect
floating values to behave like idealized Real numbers, and those whose
programs using floating values actually work.

In fact, among those whose programs work, C++ floats and operators
that apply to them *do* implement what people expect from arithmetic
operators.  The only way they could get more predictable for these
people would be to mandate that they be ieee standard floats (which do
include NaN, FWIW).  C++ floating types fit solidly into a long
tradition of floating point numbers as supported in many other
languages.

I just want to close by saying that the domain of floating point isn't
really amenable to glib pronouncements about what is "broken."  If you
need more evidence of that, read
http://docs.sun.com/source/806-3568/ncg_goldberg.html

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com