[ublas] fixed size vector in boost::numeric::ublas?

Subject: [ublas] fixed size vector in boost::numeric::ublas?
From: Markus Grabner (grabner_at_[hidden])
Date: 2010-07-22 18:16:48

        Hi all!

    In boost::numeric::ublas, there exist different vector variants 
(unbounded and bounded). All of them can be resized dynamically (up to a 
maximum size defined at compile time in the case of the bounded vector). 
Therefore the vector class (or the underlying storage array) have a size 
member which indicates the current size of the vector.

However, it would be extremely useful to have a vector of fixed size, i.e., 
one which can't be resized dynamically, but has its size statically defined 
at compile time. Such a vector does not require a size member and only 
consumes N*sizeof(T) bytes of storage, where N is the (static) vector size 
and T the value type. Many libraries dealing with geometric data (OpenGL and 
OpenInventor being prominent examples) store their data in compact arrays in 
their native data types, e.g., the values (x0,y0,z0,x1,y1,z1,x2,y2,z2) 
stored in consecutive memory locations define three points in 3D space. By 
means of a fixed size vector, the entire boost::numeric::ublas functionality 
could be applied to such arrays by a simple reinterpret_cast on each 
element. This is not possible with the bounded or unbounded vectors since 
the vector must be constructed before with its size member properly 
initialized, and the actual vector data must therefore be copied explicitly 
before use.

I tried to create a fixed_array class to implement this proposal (see 
attachment), it can be used, e.g., to create the equivalent of the 
OpenInventor "SbVec3f" data type as follows:

typedef boost::numeric::ublas::vector<float,fixed_array<float,3> > vec3d_t;

It is probably not the cleanest solution, and I skipped the serialization 
stuff altogether, but it demonstrates an approach how this can be done.

Is such a feature already available in ublas and did I just miss it? If not, 
what do you think about adding it to improve boost's interoperability with 
other libraries?

        Thanks & kind regards,
                Markus

-- 
Markus Grabner
Institute for Computer Graphics and Vision
Graz University of Technology, Inffeldgasse 16a/II, 8010 Graz, Austria
WWW: http://www.icg.tugraz.at/Members/grabner