Re: [eigen] Issues regarding Quaternion-alignment and const Maps

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

On Wed, Jul 7, 2010 at 10:53 AM, Hauke Heibel
<hauke.heibel@xxxxxxxxxxxxxx> wrote:
> On Wed, Jul 7, 2010 at 10:26 AM, Gael Guennebaud
> <gael.guennebaud@xxxxxxxxx> wrote:
>> On Wed, Jul 7, 2010 at 9:56 AM, Christoph Hertzberg
>> <chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>> The second issue is a bit more complicated:
>>> Eigen::Map severely violates const-correctness. I.e., it is possible to
>>> create a non-const Map from a (const double*).
>>> If you still accept API-changes, I would suggest having a Map and a ConstMap
>>> (similar to iterators in STL).
>>
>> yes, that's not very nice. The only advantage of our current approach
>> is that keep the number of type instantiation to a minimum. The user
>> can still manually enforce constness by declaring the Map object
>> const:
>>
>> const double* data= ...;
>> const Map<MatrixXf&> map(data,....);
>>
>> But, sure, this is far to be satisfactory, for many reasons.
>>
>> This issue also exists with named Block objects:
>>
>> foo(const MatrixXf& m) {
>>  Block<MatrixXf> bl(mat,0,0,,4,4);
>>  bl.setZero();
>> }
>>
>> API-wise, what about const qualifying the matrix type to enforce constness:
>>
>> // here data can be a double* or a const double*
>> Map<const MatrixXf> mat(data,...);
>>
>> // here data must be a double*
>> Map<MatrixXf> mat(data,...);
>>
>> and we would do the same for Block. (Block and Map are based on the
>> same implementation).
>>
>> Again the drawback is that we are artificially increasing the number
>> of types, and consequently compilation time and binary sizes....
>
> This issue was also bugging me. I am voting for this option but I
> think it requires some new macros because our traits are only
> specialized for clean types.
>
> But then it seems to be working out of the box though I did not check
> everything - at least this example works (and only with const
> double*):
>
> #include <iostream>
>
> #include <Eigen/Eigen>
>
> using namespace Eigen;
>
> void main()
> {
>  MatrixXd m = MatrixXd::Random(50,50);
>  Map<const MatrixXd> mymap(m.data(), m.rows(), m.cols());
>  const double* data_ptr = mymap.data();
> }
>
> I cannot push anything because I used tr1 traits like
> std::remove_const but that one is easily implemented.

we already have it:

ei_unconst<type>::type

> A general cleanup of our traits/template helpers would be nice so they
> are conforming with the standard - just an idea, I could even do it.

would be nice.

gael

> Regarding Gael's comment about the increased types and build times, I
> think const correctness should be preferred over shorter build times.
>
> - Hauke
>
>
>