[boost] Re: Any interest in a decimal and money class?

From: Martin (adrianm_at_[hidden])
Date: 2005-01-09 14:39:31

> Your decimal64 is both an interface *and* an implementation. I just 
> suggest to separate it. Let basic_decimal take care of the interface, 
> while decimal64 takes care of the implementation.

I can agree that the interface could be formally specified as a basic_decimal 
but still don't understand the benefits of a decimal_traits vs a class that 
implements the basic_decimal interface.
There are several string types which implements the basic_string interface 
(fixed_string, const_string, flex_string etc) and they all work without 
string_traits. Since I am familiar with the string type maybe you can explain 
what the benefits would be if all string types were implemented using 
string_traits instead.

One thing that would be nice is if basic_decimal were an ABC and all decimal 
types were derived from it. That way a function could work on any decimal type 
without templates or recompiling. Drawback is that it would introduce a 
virtual call for each operation but since a decimal type is probably slow 
anyway I don't think it matters.

> Maybe I don't understand the question, but I think it should be obvious 
> that a decimal based float point types has a lot of advantages. It's not 
> free of any rounding issues, but the problems are more intuitive. Most 
> people don't have a problem to accept rounding in general, it's just 
> examples like
> 
> double d = 0;
> for( int i=0; i<100; ++i ) d += .01;
> if( d == 1 ) { ... }

Floating point is difficult to use and you must be aware of its weaknesess. 
Decimal floating point solves the addition problem above but as soon as you do 
a division the problems are the same as with binary floating point.
Expressions often involves several operations:
x = (a + b /c - d)*e;
A test like (x == 1) will probaly fail for both a decimal and binary floating 
point type.

> If you really try to store the values exactly, consider:
> 
> decimal64 d = 1;
> d /= 3;
> d *= 3;
> if( d == 1 ) { ... }
> 
> will this work for your type? How?

It will not work since decimal64 is a fixed precision decimal type and will 
only store a fixed number of decimals. The above will only work for a rational 
type.

With exact I mean that if the decimal type got 2 decimals then these are 
stored exactly. 1/3 becomes exactly 0.33. How do you expect epsilon to work 
here? Should == return true for 0.32 0.33 0.34 or an even wider range? Sound 
confusing to me.