Re: [boost] formal review request : mcs::units

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-02-07 12:42:40

ISWT

>>> I would suggest that you have a
>>> pod_quantity for defining the various
>>> constants so that they can be safely
>>> used during the dynamic initilization
>>> phase.
>
> template<class Unit, class T = double>
> struct pod_quantity {
>     //like quantity but without constructors/assignment operators
>     T val_;
> };
>
> constants.hpp
>
> pod_quantity<si::velocity> c = { 299792458 };

The problem with this is that you lose the type-safety of forcing  
explicit value+unit
construction of quantities. That is, if someone decided to express  
the constant in
cgs units instead and just did a global search and replace for si:: - 
 > cgs:: the code
would suddenly have a bug because there would be no way for the  
compiler to know
that the specified constant value was meant to be in SI units. By  
requiring constructs
like

quantity<si::velocity> c(299792458*si::meters/si::second);

you would get an error here:

quantity<cgs::velocity> c(299792458*cgs::meters/cgs::second);

because there is no meter defined in cgs. There has to be a better  
solution to the
dynamic initialization problem...anyone?

Matthias

----------------------------------------------------------------
Matthias Schabel, Ph.D.
Assistant Professor, Department of Radiology
Utah Center for Advanced Imaging Research
729 Arapeen Drive
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801-587-9413 (work)
801-585-3592 (fax)
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matthias dot schabel at hsc dot utah dot edu
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