Improve documentation.

git-svn-id: https://svn.dealii.org/trunk@12880 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/lac/include/lac/sparse_matrix.h b/deal.II/lac/include/lac/sparse_matrix.h
index 1508743b22b4055fa2e0efa978d6b1aec932c039..6c271f29df7d3592fe30c803098aeeedf8f0496d 100644 (file)
--- a/deal.II/lac/include/lac/sparse_matrix.h
+++ b/deal.II/lac/include/lac/sparse_matrix.h
@@ -589,8 +589,23 @@ class SparseMatrix : public virtual Subscriptor
     virtual ~SparseMatrix ();
 
                                     /** 
-                                     * Pseudo operator only copying
-                                     * empty objects.
+                                     * Copy operator. Since copying
+                                     * entire sparse matrices is a
+                                     * very expensive operation, we
+                                     * disallow doing so except for
+                                     * the special case of empty
+                                     * matrices of size zero. This
+                                     * doesn't seem particularly
+                                     * useful, but is exactly what
+                                     * one needs if one wanted to
+                                     * have a
+                                     * <code>std::vector@<SparseMatrix@<double@>
+                                     * @></code>: in that case, one
+                                     * can create a vector (which
+                                     * needs the ability to copy
+                                     * objects) of empty matrices
+                                     * that are then later filled
+                                     * with something useful.
                                      */
     SparseMatrix<number>& operator = (const SparseMatrix<number> &);