[eigen] Commenting issue #110, disabling operator[] for matrices

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Hi,

https://bitbucket.org/eigen/eigen/issue/110/disabling-operator-for-a-matrix

here we are discussing disabling operator[] on matrices.

We can perfectly disable it on matrices while keeping it on vectors
(e.g. with a static assertion).

The motivation for disabling it on matrices is to guard against this
dreadful c++ code:

   matrix[i,j]

which in c++ compiles as

   matrix[i]

which addresses the i-th entry in the storage order of the matrix.

If you want that functionality, there still is the synonym

  matrix(i)

And the method coeff(int) if you want to avoid the assertion.

People have already proposed adding an expression to view a matrix as
a vector, following its storage order. But you can already do that
with Map. If you really want, we can add a conveniene method
asVector() to return such a Map object. But given the possibility of
doing matrix(index), i don't see too much motivation for that...

Benoit