Re: [boost] Fixed-Point Decimal Type

From: Rainer Deyke (root_at_[hidden])
Date: 2001-05-09 13:50:44

----- Original Message -----
From: "Bill Seymour" <bsey_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Wednesday, May 09, 2001 11:01 AM
Subject: [boost] Fixed-Point Decimal Type

> When J16 and WG21 begin working on specific proposals for the
> evolution of C++, I intend to suggest a fixed-point decimal type.
> The main reason is that many novices want to use floating-point
> types to store monetary values; and they're suprised when the
> pennies (or other fractional values) don't add up.  In the
> traditional "business data processing" environment in which
> I work, this is often what Bjarne calls an "embarrassment."
>
> If there's interest, I'd like to do a reference implementation
> under the auspices of Boost.  The requirements for the type
> would be:
>
> - A template class with the template parameter being the number
>   of digits to the right of the decimal point.

I think there's also a need for non-decimal fixed-point numbers (i.e.
hexadecimal digits to the right of the hexadecimal point or such).  For this
reason I would prefer it if there was instead a template argument N such
that multiplies of 1/N are reprsented exactly.

> - Represent at least 18 decimal digits exactly.  (The 18 comes
>   from prior art on IBM mainframes, and also turns out to be
>   convenient if the internal representation is a 64-bit int.)

I would make the internal representation type another template argument.

> - Support all operations that can be done on ints except implicit
>   conversions and non-decimal I/O.

This statement is somewhat ambiguous.  How are expressions that combine
integers and fixed-point values defined?

> - All operations are exact if there is no overflow and if no
>   rounding is required.  Overflow is undefined behavior.
>   Rounding is done the way accountants do it:  round to nearest
>   with 1/2-LSB always rounded "up."  (I think "up" means "away
>   from zero."  I've asked an accountant friend via e-mail but
>   haven't yet gotten an answer.)

If at all possible, I would make the rounding mode user selectable.

--
Rainer Deyke (root_at_[hidden])
Shareware computer games           -           http://rainerdeyke.com
"In ihren Reihen zu stehen heisst unter Feinden zu kaempfen" - Abigor