Re: [ublas] help writing a function that works for matrix<> and matrix_range<>

From: Gunter Winkler (guwi17_at_[hidden])
Date: 2006-11-20 05:37:01

On Sunday 19 November 2006 00:13, Manoj Rajagopalan wrote:

> template<typename E>
> void initialize(matrix_expression<E> A,
>                  typename E::value_type const& value)
> {
> ...
> } 

Dealing with matrix expression directly is tricky, because of the CRTP usage.

1) You always have to use references to a matrix_expression as function 
parameter, because the (default) copy constructor of matrix_expression does 
not copy anything. (matrix_expression has no member variables and thus its 
size is zero!)

template<typename E>
void initialize(matrix_expression<E>& A,
                typename E::value_type const& value)
{
  E& A_ref = A(); // this is the "official" way (just hiding the cast)
  A_ref = scalar_matrix<typename E::value_type>(A_ref.size1(), A_ref.size2(), 
value);
}

2) Sometimes you have to explicitly create instances (because you need 
something to refer to and you can't have a direct instance of a 
matrix_expression<X>)

     matrix_range< matrix<int> > sub_A(A,range(1,3),range(1,3));
     initialize(sub_A, 8);

In short:
  Never instantiate matrix_expression because it contains no data.
Longer:
  matrix_expression exists only because you need some common type
  for all expressions. The price you pay is to use references everywhere.

mfg
Gunter