Glas List: Re: [glas] MTL3 design ? [was MTL project site is up]

From: Patrick Kowalzick (patrick.kowalzick_at_[hidden])
Date: 2005-02-03 03:24:54

From: "David Abrahams" <dave_at_[hidden]>
> Guntram Berti <berti_at_[hidden]> writes:

[snip]

>> I think it will not be uncommon to wish to forbid even assignment
>> between vectors of equal size, for example if one represents
>> cell-based and the other one vertex-based quantities (speaking
>> FEM). So one could introduce mechanisms to forbid assigment at
>> compile time (i.e. making those vectors different types).
>
> Or const?

IMO for a "user" there is an easy way needed to create a bunch of vectors 
with different _semantic_ meanings. One example is Guntrams FEM example. 
Another one might be the representations of geometrical entities, e.g the 
semantic difference between points and vectors. This truly depends on the 
mathematical context.

Having different types makes it much easier declaring functions for some 
action. As an example take two vectors[3], where one represents a point and 
the other a line in a 2D homogeneous space. The affine transformations for 
both types must not be equal, so the functions should be overloaded:
    point affine_transformation( const matrix & a, const point & x );
    line affine_transformation( const matrix & a, const line & x );

Both points and lines _are_ vectors, but behave completly different. E.g. 
addition of both is forbidden.

In uBlas I inherit from the bounded_vector (but I am not completly sure 
about side-effects).

Regards,
Patrick