"Matt Calabrese" wrote
Expression reorginization can optimize an overall expression quite a bit, which is why I decided to make it a part of the library. As a simple example, take the following expression after analysis:
meters1 + feet1 + meters2 + feet2
Without expression rearrangement, a conversion could potentially take place at every operations. For instance, without rearrangement, the following logic could occur (obviously it would all be implicit, I am using just made up functions for clarity here):
convert_to_feet( convert_to_meters( ( convert_to_feet( meters1 ) + feet1 ) ) + meters2 ) + feet2
However, with expression rearrangement, we can group like terms:
feet1 + feet2 + convert_to_feet( meters1 + meters2 )
The example works for addition but what about multiplication and division? Is the example realistic? FWIW pqs assumes/("prods") that all calculations would actually be done in SI units. (The whole idea behind SI is to use the so-called "coherent" units and phase out the others). In this scenario the conversion to-from "incoherent" units would only occur for user input/output. OTOH meters result = millimeters1 + meters2; In the above pqs would convert the intermediate result to millimeters under the rule that millimeters is the most fine grained unit. I guess that improvements could be made with expression templates. OTOH will the rules regarding the result of an operation be simple to understand? Maybe that doesnt matter. BTW Are there not subtleties with temporaries using ET that users need to know about? [cut]
Implementation-wise, I am using MPL maps to represent classifications and unit types, unlike Andy Little's if I remember correctly, providing more compile-time efficient type comparisons.
I used a home brewed version. It would be interesting to see if mpl::map is faster, but I think it will be slower because mine wasnt actually very generic ;-) . However the use of compile time double will have a major detrimental impact on compile times unfortunately. I dont know what one can do about that.
I also provide a variety of simple ways of creating new unit types and classification types using compile-time metafunctions and variadic template argument lists.
One problem in pqs was explaining how to add new types and units. If this can be simplified that would be great. regards Andy Little