[Insight-users] itkMatrix error: discards qualifiers??

[Luis Ibanez]

Hi Michiel,


It is excellent that you are diving into the code
looking for deeper understanding !


The message that you get indicates that you are
attempting to change the value of a variable that
has been declared "const" previously.

The "const" declaration is a promise made to the
compiler, stating that you will not change the value
of that variable.

When the compiler catch you breaking that promise,
it get angry at you and gives you the error message:

"error: passing ‘const   X to  Z discards qualifiers"


Where the "qualifier" refers to the "const" property
of that variable.


The error message indicates that you are calling
the operator=() method on a const matrix (as left
argument of the = ).


You did so, with an expression such as:


  const MatrixType  myConstMatrixThatWillNeverChange;

  myConstMatrixThatWillNeverChange = anotherMatrix;


This may also happens implicitly if the matrix
happens to be a member variable of a class,
and you attempt to do such assignment inside
a "const" method of the class.


If you share with use the exact declaration of your
"direction" matrix we could tell you more about how
to get around the problem.


       Regards,


             Luis



--------------------------------------------------------------------------
On Mon, Mar 29, 2010 at 12:01 PM, michiel mentink
<michael.mentink at st-hughs.ox.ac.uk> wrote:
>
> In an autodidactic effort, I'm trying to rotate an image by 'hand', by
> multiplying the 'direction' matrix of an image by a rotation matrix.
>
> To do that, I'm copying bits out of itkEuler3DTransform.txx
> In that file, a RotationX matrix is defined:
>
> Matrix<TScalarType,3,3> RotationX
>
> However, when I try doing that, I'm getting the following error:
>
> error: passing ‘const itk::Matrix<double, 3u, 3u>’ as ‘this’ argument of
> ‘const itk::Matrix<T, NRows, NColumns>& itk::Matrix<T, NRows,
> NColumns>::operator=(const itk::Matrix<T, NRows, NColumns>&) [with T =
> double, unsigned int NRows = 3u, unsigned int NColumns = 3u]’ discards
> qualifiers
>
> 1) What does this mean?
> 2) How do I get my beloved Matrix?
>
> #include <itkMatrix.h>
> #include <vnl/vnl_matrix.h>
>
>   typedef itk::Matrix<double,3,3> Matrix;
>
>   Matrix RotationX;
>  // ImageType::DirectionType RotationX;
>
> //  vnl_matrix_fixed<double,3,3> RotationX;
>
>   RotationX[0][0] = 1; RotationX[0][1] =   0; RotationX[0][2] = 0;
>   RotationX[1][0] = 0; RotationX[1][1] =  cx; RotationX[1][2] = sx;
>   RotationX[2][0] = 0; RotationX[2][1] = -sx; RotationX[2][2] = cx;
>
>   direction = direction*RotationX;
>
> cheers,
>
> Michael
>
>
>
>
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