vector_of_vector

Hi. I'd like to have a matrix<T> stored in such a way that I can quickly
use each row (or else column) as a vector<T>. Seeing the vector_of_vector
matrix type, at least the name suggests that this is what I need, but I
suspect this is not so because I can't get it to do that. Does anyone have
a hint on how to extract an entire row of a matrix into a vector as
efficiently as possible?

Thanks