Re: [eigen] Is it really impossible to modify a column of a matrix with in function?

[Helmut Jarausch <jarausch@xxxxxxxxxxxxxxxxxxx>]

On 08/30/2011 12:35:15 PM, Christoph Hertzberg wrote:
> On 30.08.2011 11:58, Helmut Jarausch wrote:
> > void set_Col(VectorXi&  V) {
> >    V[0]=1;
> > }
> >
> > int main() {
> >    MatrixXi A(2,3);
> >    A<<  1,2,3,
> >         4,5,6;
> >    set_Col(A.col(1));
> >    std::cout<<  "A=\n"<<  A<<  std::endl;
> >
> > }
> >
> >
> > It looks as if a column cannot be passed by a non-const reference.
> > Why doesn't  the col-method return a non-const reference for a non-
> > const object (the col-method could be overloaded with/without the
> > const attribute)?
> 
> That's because A.col(1) does not return a VectorXi but a view on the 
> corresponding column. Unfortunately passing this to another function
> is 
> a bit hackish. To stay generic you need to pass a 
> MatrixBase<Derived>&
> 
> and because A.col() returns a temporary it needs to be a const-ref, 
> which you need to const-cast within your method.
> Look here for details:
> http://eigen.tuxfamily.org/dox/
> TopicFunctionTakingEigenTypes.html#TopicPlainFunctionsFailing

Many thanks!

It would be nice if there were a proxy class doing this for me.
E.g. within my own (dated) matrix/vector class I had proxy classes
Vector_L and Matrix_L which were used as return values and which can be 
passed as const reference. All write-access operators, like operator[], 
operator>> and operator== do const casting internally.

Helmut.