Re: [boost] Re: new compile-time dimensional analysis and unit library

From: Matthias Schabel (boost_at_[hidden])
Date: 2003-11-18 18:29:59

> Hi Matt I have had a look at your code and compiled on VC7.1 MSos with  
> a
> couple of mods.

Cool - that's good news.  Metrowerks pukes on it; a little too much  
MPL, I guess.
I'm still mystified by the asymmetry between the normal and assignment  
forms of the
constructor (commented out in test_units.cpp.  Could you try  
uncommenting those
statements in VC7.1 to see what pops out?  I don't have access to that  
compiler...

> template<class T>
> void write_typeid()
> {
>   //  std::system((std::string("c++filt ")+
>                 // std::string(typeid(T).name())).c_str());
>     std::cout << typeid(T).name()<< std::endl;
> }

Yeah, sorry about that - gcc's typeid gives the mangled class name  
which is about useless
for debugging complex MPL sequences.

> As far as the rest goes, the MPL stuff looks very impressive
> and I like the removal of  dimensions with power 0.
> On that subject it would be nice once having arrived at a  
> dimensionless pq
> to convert to the value_type direct.
> ie ideally a dimensionless pq is never constructed

Glad you like it; some parts were a bear to get right.  At the moment,  
I allow dimensionless
quantities to be implicitly converted to/from the underlying value  
type. I like preserving
the dimensionlessness (d15s?) so that you can distinguish between raw  
values and
types which result from evaluation of expressions containing  
dimensioned quantities...

>  If you wrote up documentation on that I would be interested and
> comparing my stuff with yours I have to admit that most on Boost will
> probably prefer yours... because its nice and technical :-)
> but to convey what youre doing  it obviously needs documentation.

At this point I feel like the dimensional analysis part is pretty much  
stable,
but I'm still working on schemes for the dimensioned quantities which  
will
maximize flexibility.  In particular, I'm trying to concoct a good way  
to take
two disparate models and merge them - say you have one model with
length_tag, time_tag, and mass_tag and another with angle_tag, it would
be nice to be able to easily create a meta-model encompassing both...  
I'm
also interested to hear what others are looking for in such a library.

> If you do set up a working group on units/d.a./pqs whatever I would be
> interested.

Will do.

> What I am trying to achieve is best summed up in this previous post.
>
> http://lists.boost.org/MailArchives/boost/msg55344.php
>
> Would be interesting to see how it would look with your scheme ?

I'm personally on the explicit conversion only side of the fence - the  
idea of
setting up a system for intelligently determining the minimum required  
precision
for the result of an arbitrary unit-containing expression sounds like  
it would be
an interesting application for expression templates, though I suspect  
it will be
very difficult to accomplish for general (rather than POD) value  
types...

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