[eigen] Assertion in DenseStorageBase.h:487: Nonnegativity of rows and cols sufficient?

[Martin Senst <martin.senst@xxxxxx>]

Hi everyone,

sorry to bother you again with the topic of zero-sized matrices, but as I've 
written in the thread about sum() of empty vectors, I need them to avoid ugly 
special-case-treatments.


I'm wondering wheter it's okay to change the assertion in DenseStorageBase.h, 
lines 486 and 487, from

ei_assert(rows > 0 && (RowsAtCompileTime == Dynamic || RowsAtCompileTime == 
rows)
&& cols > 0 && (ColsAtCompileTime == Dynamic || ColsAtCompileTime == cols));

to

ei_assert(rows >= 0 && (RowsAtCompileTime == Dynamic || RowsAtCompileTime == 
rows)
&& cols >= 0 && (ColsAtCompileTime == Dynamic || ColsAtCompileTime == cols));



The issue is that currently, code like

MatrixXd A(0, 0);
MatrixXd B(0, N);
MatrixXd C(N, 0);

fails due to this assertion. If compiled with NDEBUG, this code works as 
expected, so I guess that the current assertion is overly strict.

As a workaround, something like
MatrixXd C = MatrixXd(N, 1).leftCols(0);
works, but of course that's pretty ugly.

Cheers
Martin