Re: [boost] physical-quantities (was: runtime unit-modifiable...)

From: Matthias Schabel (boost_at_[hidden])
Date: 2004-01-10 14:32:20

>>      // test ideal gas law
>>      SI<double>::temperature    T = (273.+37.)*_kelvin;
>>      SI<double>::pressure       P = 1.01325e5*_pascals;
>>      SI<double>::length         r = 0.5e-6*_meters;
>>      SI<double>::volume         V = (4.0/3.0)*3.141592*power<3>(r);
>>      SI<double>::quantity       n(idealGasLaw(P,V,T));
>
> But isnt this a bit  runtime-slow Matt?

Not unless the compiler can't inline away the trivial multiplication;  
the various
constants defining dimensioned units (_kelvin, _pascals, etc...) are  
instances of
an empty class - no data members - so the multiplication is just a way  
of getting
the type information to propagate in the runtime system to make this  
sort of syntax
possible.  In other words, looking at the first line,

SI<double>::temperature

is a dimensioned_quantity (value plus dimensioned  unit)

_kelvin

is a dimensioned_unit, a class with no data members, and the  
multiplication operator
just decorates the value passed to it (273.+37.) with the appropriate  
unit information.

> BTW I looked into your ct problems. VC 7.1 has some neat  
> specialisations.
> (Doesnt work on gcc)
> have you tried masks?
> Basically 7 dimensions --> 127 (I think) mask possibilities
> If interested Will try to look out the code.

Not sure how that will help with user-defined quantities outside of SI.  
  I suppose if one
was willing to enumerate every allowable tag class, it would be  
possible to have two
mpl::vectors, one with the integer indices of the tags and another with  
the exponents.
I think at this point I'm going to stick to my efforts to improve  
compile time through use
of mpl::set and see where that gets me...

------------------------------------------------------------------------ 
---------------------------
Matthias Schabel, Ph.D.                           	
Utah Center for Advanced Imaging Research
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mschabel at ucair med utah edu