[boost] Re: Dimensional Analysis

From: Andy Little (andy_at_[hidden])
Date: 2005-01-30 16:15:06

"Phil Richards" <news_at_[hidden]> wrote in message 
news:20050130142206.305CF5C14_at_derisoft.derived-software.demon.co.uk...
> On 2005-01-30, Andy Little <andy_at_[hidden]> wrote:
>>  "Phil Richards" <news_at_[hidden]> wrote in message
>>  news:20050129141037.DE6D0E305_at_derisoft.derived-software.demon.co.uk...
> [wrt dimensional analysis being independent from units]
>> > Indeed it does.  I use one every day... we don't care about unit
>> > conversions because we don't do any.
>>  I would hope that a general purpose physical quantities lib would deal 
>> with
>>  units, even if you dont use anything but the base units. The Mars lander
>>  crash and all that. IOW you need to be explicit about what units you are
>>  using.
>
> Yes, but I think what is being raised is that a "physical quantities
> library" and a "dimensional analysis library" don't have to be the same
> thing.  The latter is a much simpler proposition, and, for many people
> is all that they want.

Strictly speaking dimensional-analysis is only used for checking (what I 
have called) 'abstract quantities'. As soon as a value is involved you need 
units in order to decipher what the value means.  Therefore IMO a 
dimensional analysis type should not have a Runtime 'value' member and 
should be limited to compile time only. Then the dimensional analysis part 
can certainly be separated out as a self contained module. The two 
requirements on it  would be some form of compile time math operations and 
also ideally some useful error message scheme on invalid operations.  One 
could then roll ones own physical-quantity with that as a building block as 
reqd I guess.

> It's all we need because *all* our interfaces to the outside world are
> defined entirely in terms of SI units.  We make unit conversion an SEP[*].

IOW you are a user node on a generic physical quantities hub ... Let pqs 
take the strain ... :-)

[*] That somebody else is me I guess.. no problem. However I hear what you 
are saying re boost::dimensional_analysis.

regards
Andy Little