[ublas] specializations of row() and column() for vector_of_vector

From: Gunter Winkler (guwi17_at_[hidden])
Date: 2006-12-19 08:45:56

Hallo,

the attached patch provides specializations of row(M, i) and column(M, i) for 
generalized_vector_of_vector. This type stores a list of rows/columns which 
can be used directly as row/column instead of the proxy. The only difference 
is that matrix_row has an index() member function (which is undocumented and 
not part of the vector expression concept), and the sub vector type of gvov 
usually has no index() member function.

The performance gain is visible but small. However, the patch should help to 
avoid unnecessary proxy objects.

mfg
Gunter

PS: similar optimizations are possible for compressed matrix and coordinate 
matrix. In general a sub vector of a matrix could be seen as a simple pair of 
iterators (it_begin, it_end) contained in a vector expression "facade".