Hi Noah,
Would something like this work for creating a quantity that's unit can change during runtime and allow support for multiple units all converting to a particular "system" (as you specify the meaning)?
I am drawing a "line in the sand" at implementation of runtime unit systems; this is not because it can't or shouldn't be done, but because it is basically a completely different problem domain with different objectives and performance criteria. It would certainly be possible to integrate a runtime unit and quantity library, if someone else wanted to implement it, with my proposed library by simply specializing the unit and quantity classes something like this: struct runtime_system { }; struct runtime_dim { }; template<> class unit<runtime_system,runtime_dim> { ... }; typedef unit<runtime_system,runtime_dim> runtime_unit; template<class Y> class quantity<runtime_unit,Y = double> { ... }; That being said, it is possible to have compile-time units with runtime varying unit conversions (as I alluded to in the previous post on currency conversion). You basically need to specialize the conversion_helper class (in conversion.hpp) to do what you want it to...
For instance, I work on fluid flow analysis programs and have started working on a unit library based on ch. 3 in the MPL book. One thing I need is for user entry of arbitrary (and sometimes user defined) units. So an entry of pressure might be in psi or inches of mercury; it also might be in gage (with some user defined reference atmospheric pressure) or absolute. These values need to be stored in those units (otherwise you run afoul of float migration) and are the ultimate entry data parameters for all the system's equations. How does your library meet this need?
My library does not deal with issues of runtime units, therefore, does not provide any facility for doing these sorts of things. I do believe that this is an interesting area, and potentially worthy of implementation, but I don't have the time, inclination, or expertise to do it. I would like to keep the focus in the present discussion on zero runtime overhead dimensional analysis systems and reserve consideration of runtime units for another time/ library...
Another issue we have is that equations are more often than not referenced from some source written in arbitrary units. It would be nice to be able to reflect this in code so that if some source says "* 9.32 psi a" this could be done in the code itself with minimal to 0 impact on runtime performance. Does your library meet this need?
I envision something akin to:
typedef quantity<SI::pressure> length_qty;
X f() { static length_qty const p9 = 9.32 * psi; }
There are various ways to accomplish this with the library; explicit conversion of units between unit systems would allow something like you wrote : static quantity<SI::pressure> const p9(9.32*psi); assuming you either 1) defined a pounds-inches unit system with appropriate conversions to the SI system or 2) simply defined psi as a constant in SI units Either option is probably acceptable; the former is best if many of the program's computations will be done in a pounds-inches system, the latter if you are just occasionally using non-SI units in a predominantly SI code. Cheers, Matthias