Re: [eigen] Assignment to MatrixBase fails assertion or SEGVs

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

ah wait, i hit "send" too fast.

There's actually a very, very good reason why your code can't work.

When you do
  MatrixBase<Matrix2f> noinit;

you're constructing an object of MatrixBase which is an empty class.
The static array float[4] never gets reserved by the compiler. Hence
that object noinit is definitively unusable; even casting it as a
Matrix2f will just result in segfaults since one'll be addressing 4
bytes at a location where typically 1 byte (size of an empty class)
was reserved.

The dynamic-size variant is broken too: the array that would normally
be allocated in the Matrix constructor, never gets allocated because
the Matrix constructor never gets called.

Conclusion: never create explicitly a MatrixBase object, only create
Matrix objects and then cast them to MatrixBase as needed.

Benoit

2009/10/30 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> Thanks for the report and test-case, that was interesting.
>
> First of all, how to make it work with current Eigen: just replace the line
>   noinit = mat;
> by
>   noinit.derived() = mat;
>
> Now the reason why your code fails is the following. As you notice,
>
>> In general, eigen sets the lhs to the size of the rhs.
>
> However, in Eigen, only a Matrix can be resized, the class MatrixBase
> doesnt have a resize() method (in the devel branch it has one but it
> still doesn't actually allow to resize).
>
> In order to fix that, we would need to make it so that
> MatrixBase<Derived>::resize() calls Matrix::resize() when the Derived
> type is Matrix.
>
> We can do that with a template helper. I'm asking for opinions here.
> Should we do that, in order to support Robert's code? If not, then we
> still must make sure that at least we fail on an assert, not a
> segfault!
>
> Benoit
>
>>
>> This assignment using MatrixBase compiles nicely but fails at run time:
>> Assertion failed: (rows() == other.rows() && cols() == other.cols()),
>> function lazyAssign, file
>> /u/lsst/products/DarwinX86/eigen/2.0.9/include/Eigen/src/Core/Assign.h, line
>> 406.
>>
>> If you use static-sized arrays, you get a SEGV (g++ 4.3.4; os/x 10.6)
>>
>> (I care because of the way that SWIG generates code (see yesterday's
>> problems);  I have almost everything working --- at least if you only want
>> one type of Eigen arrays (e.g. float, RowMajor) playing with numpy at a
>> time)
>>
>>                                R
>>
>>
>> #include "Eigen/Core.h"
>>
>> using namespace Eigen;
>>
>> int main() {
>> #if 0
>>    MatrixBase<Matrix2f> noinit;
>>    Matrix2f mat;
>> #else
>>    MatrixBase<MatrixXf> noinit;
>>    MatrixXf mat(2, 2);
>> #endif
>>
>>    noinit = mat;
>> }
>>
>>
>>
>>
>