The point is that when I want to use a new underlying type for calculations, I have a single point of customization, about 100-200 lines. The specialization of decimal_traits<T> for that type T and the typedef for decimal. I don't have to touch the 300.000+ lines of code that use it. What do you prefer?
With your solution: 1. typedef basic_decimal<decimal64> decimal 2. typedef basic_decimal<decimalBCD> decimal With my solution 1. typedef decimal64 decimal; 2. typedef decimalBCD decimal; Same amount of code to change. (We both need to implement the interface, you do it in decimal_traits, I do it in decimal64 & decimalBCD). What worries me with your solution is that your decimal_traits class implies a way to implement things that might not be the most efficient one for each implementation. Lets take an example: Assume that basic_decimal implements "<=" as "traits::less_than || traits::equal" which works fine if "less_than" and "equal" are "cheap" operations. In decimal64, both "less_than" & "equal" might need to scale one of the operands before the comparison so the scaling occurs twice in "<=". With my solution I am free to implement the interface in the most efficient way for each class.
I think it's most useful if you work on decimal64 independently of money/currency issues. Make it a cool new float-point type and basic_decimal will be able to use it :)
Do you have an example of an application where decimal floating point is useful? To me floating point means that you give up precision to be able to handle a large range of values. But if loss of precision is acceptable, you can just as well use binary floating point.
Another benefit of the indirection through basic_decimal<T>: FWIW, there is one thing that my classes don't do right now and which I always dreamed of: "Correct" epsilon handling. Like
operator== respects a given epsilon native_less(a,b) like the current operator< for the underlying type
Isn't the whole point with decimal math that values are stored exactly? Why would you need epsilon for exact values?