Glas List: [glas] evaluation and dispatch of functions

From: Karl Meerbergen (Karl.Meerbergen_at_[hidden])
Date: 2005-09-21 10:17:20

Hello,

One of the issues I am thinking about now is how to dispatch a function 
to the right piece of code.
For example, norm_2(v), is going to be (slightly) different for dense 
and sparse collections and depends on whether we use the blas nrm2() 
functions.

At this stage, we already have a collection_type in the dense_vector and 
sparse_vector collections, but this might not be enough. I would like to 
be able to add a number of attributes in an attribute list in order to 
be able to dispatch a function, depending on the attribute.

I think that boost::mpl::list or boost::mpl::vector would be good 
candidates to contain a sequence of attributes: these can be 
dense_collection_type (or sparse_collection_type), blas_type, 
fixed_size_type etc. The function norm_2() interrogates the attributes 
in order to know which implementation should be chosen.

If, in addition, we allow the creation of new collection types by adding 
atributes to old collection types, and implement the corresponding 
implementations for norm_2(), the glas dispatch system should be able to 
select a user implementation for this function.

template <class V>
typename norm_2_traits<V, attribute<V>::type>::result_type norm_2( V 
const& v ) {
  return norm_2_traits<V, attribute<V>::type>( v ) ;
}

where the norm_2_traits are specialized for each attribute type.

It would also be possible to work out a system with nesting:
the norm_2_traits first looks at the first attribute and passes the 
remaining attributes to another instantiation of norm_2_traits, e.g. a 
vector having dense_collection_type and blas_type as attributes would 
first be dispatched to the dense specialization where it then will be 
sent to the dense blas specialization.

For example, a glas user could add an attribute my_attribute to 
dense_vector and then use its own implementation for norm_2().

I am working out an example for the 2-norm to make this idea more concrete.

In the meantime I would like to know your opinion about all this.

Best regards,

Karl

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