On Mon, 10 Oct 2005 16:12:53 -0400, Matt Calabrese wrote
...snip...
For instance, I have three different main templates for working with units (instantiated with unit type, value type, an operations policy for the provided value type i.e. vector and point operations, all which have appropriate defaults):
quantity (which is a scalar quantity) quantity_vector quantity_point
A quick example of why explicitly separating these types is important is easily examplified by the common case of temperature. Firstly, consider the temperature 2 degrees celsius. In order to convert this temperature to fahrenheit, we would multiply by 9/5 and then add 32. Applying this operation, we get the result 35.6 degrees celsius, as expected. On the other hand, consider a temperature difference. Let's say at the beginning of the day, the temperature was 8 degrees celsius. At the end of the day, the temperature was 10 degrees celsius. The change in temperature from the beginning of the day to the end of the day is 2 degrees celsius. Now, to convert that change in temperature to fahrenheit, we multiply by 9/5. Note that unlike the previous example, we do not at 32. This result is 3.6degrees fahrenheit, which is obviously very different from 35.6 degrees fahrenheit, despite that both come from a conversion of 2 degrees celsius.
Wow, that's amazing. Replace quantity_vector with time_duration and quantity_point with time_point and you have the core concepts behind date-time. And the 32 versus zero -- seems an awful lot like an epoch_time adjustment. So I suppose I'm saying I believe there is something fundamental here :-)
...snip...
Aside from everything I mentioned, there are also a few other interesting things I'd like to talk about later, though I really wanted to get a working implementation uploaded before getting into a big discussion about it. I'll try to get expression rearrangement done by the end of the week and upload a version for people to play with.
Expression rearrangement is really cool stuff, but it doesn't really impact the interface does it? If you're serious about putting it in boost it's really the interface and docs that will ultimately matter the most. Personally I'd like to see a units lib proposal sooner rather than later... Jeff