Re: [boost] Boost Units library preview

From: Andy Little (andy_at_[hidden])
Date: 2006-08-20 04:42:00

"Janek Kozicki" <janek_listy_at_[hidden]> wrote in message
news:20060819212430.739b0920_at_absurd...
> Andy Little said:     (by the date of Sat, 19 Aug 2006 13:03:42 +0100)
>
>> In fact there is a 2D and 3d vector in quan so you can do:
>>
>> quan::some_vect< quan::length:: m > position_a, position_b;
>>
>> quan::length:: m  distance_a_to_b  = magnitude(position_b - position_a);
>>
>> Because of that useage It might get confusing.
>
> I still hold my position that linear algerbra library should not be a
> part of _two_ separate libraries: Dimensions and Units.

FWIW I brought this up only in relation to magnitude. That said, I have found
that being able to write code like the above is a lot of fun, much more so than
if I was using  doubles to represent lengths. However suddenly you are
restricted in what you can do. For example:

double var1= 10;    // var1  meters
double var2 = 10 ;    // var2 represents meters.

var1 *= var2 ;        // var1 now represents an area in square meters.

Of course you cant do this  with a fixed_quantity:

quan::length::m var1(10);
quan::length::m var2(10);
var1 *= var2 ;                         // Error

I would guess that is an unacceptable restriction for many authors of algebra
libraries.

As far as I am concerned though I am more interested to see what happens if I do
play by the rules that are imposed by having strongly typed quantities, but it
is kind of difficult to ask everyone to play by these rules, so I am happy to
create vector types myself, that work with Quan.

Maybe, if we can get Quan into Boost then we will be in a stronger position and
there may then be interest in creating a linear generic algebra library for
physical quantities, but I suspect that due to the above kind of issues, there
will always be a great divide between a 'raw' linear algebra library and one
that is designed to work with physical quantities.

> Although Dave did not answer about details of his linear algebra library
> I agree with him that correctly written units+dimensions libraries
> should work with any other library - eg. small vectors (liear algebra),
> quaternions, octonions, matrices, etc...

It is difficult to define 'correctly written'.... ;-)

> Specifically it should work with any other part of boost.
>
> If it wouldn't then the user of units library would be only limited to
> this linear algebra which is bundled with it.

>From the work I have done so far it isnt that hard to make something that would
work for other quantities libraries, and in fact the main problem is agreeing on
a common result_type deduction scheme. Ideally one could use Boost.Typeof raw,
but unfortunately gcc chokes on this type of declaration:

template <typename Lhs, typename Rhs>
BOOST_TYPEOF_TPL(Lhs() * Rhs())  operator * ( Lhs lhs, Rhs rhs);

Meanwhile, in practise it is not too difficult to write some glue code for
interfacing with the other result_type deduction schemes such as Boost.Lambda
and so on.

regards
Andy Little