Fixed-Point Decimal Type

From: Bill Seymour (bsey_at_[hidden])
Date: 2001-05-09 12:01:39

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.

- 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.)

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

- 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.)

- Construction from const char* for faking decimal constants.

- Explicit conversion to/from double.

Are there more requirements that I haven't thought of?
All comments and flames are welcome.

--Bill Seymour