[boost] Re: Lets restart the decimal type discussion

From: Stefan Slapeta (stefan_at_[hidden])
Date: 2005-01-13 10:17:19

Martin wrote:

[...]
> 
> ---
> The discussion on decimal types, including the review of a decimal library two 
> years ago, indicates that to be accepted in boost a solution should allow some 
> (or all 4) types to be used as base in something like "basic_decimal<>".
> 
> So lets discuss how a basic_decimal interface should look like. The main 
> problem I see lies in the construction:
> 
> Type 1 & 2: basic_decimal(value)
> Type 3:     basic_decimal(value, precision)
> Type 4:     basic_decimal(value, precision, rounding_mode)
> 
> Rounding is not a big problem since a sensible default can be used but how to 
> solve the precision? Creating a decimal type 3 or 4 without specifying a 
> precision is bad since there is no good value to use as default. Precision 
> also has a different meaning for a decimal type 1 & 2 (i.e. only affects 
> extractions, not operations).
> 

IMO any kind of automatic rounding and precision should not be part of a 
basic_decimal type at all! The behaviour of basic_decimal should stick 
to the decimal proposal of Mike Cowlishaw and he explains very well in 
his FAQ section why it's not a good idea to have normalized decimal 
arithmetic:

http://www2.hursley.ibm.com/decimal/decifaq.html

However, it could be possible to provide some add on, maybe controlled 
by a policy, that also handles rounding, precision and normalization.

[...]
> 
> The other members of basic_decimal isn't very complicated:
> A full set of arithmetic operations (+,-,*,/,%) and assignment must be 
> included. 
> Operations should work on integers and maybe also string literals. For decimal 
> type 1 & 2 it makes sense to also allow operations on doubles.
> Assignment should work on integers, string literals and double for all types.
> 

These are the operations of IEEE 754R; they should be at least supported 
in some way.

     * abs
     * add and subtract
     * compare
     * divide
     * divide-integer
     * max
     * min
     * minus and plus
     * multiply
     * normalize
     * quantize
     * remainder
     * remainder-near
     * round-to-integral-value
     * same-quantum
     * square-root
     * power

And don't forget relational operators....

Stefan