Re: [eigen] is MAP oneway ? (urgent)

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

2009/6/24 Helmut Jarausch <jarausch@xxxxxxxxxxxxxxxxxxx>:
> On 24 Jun, Gael Guennebaud wrote:
>> On Wed, Jun 24, 2009 at 2:13 PM, Helmut
>> Jarausch<jarausch@xxxxxxxxxxxxxxxxxxx> wrote:
>
>>> Is there something like "overlaying" an Eigen-Vector onto some raw
>>> C-array?
>>
>> this is exactly what Map do, however you have to deal with Map object
>> directly, e.g.
>>
>> double* c_like_data = whatever;
>>
>> Eigen::Map<VectorXd> v(c_like_data, size);
>>
>> then you can use "v" just like any other Eigen matrix/vector:
>>
>> v *= 10;
>>
>> is equivalent to:
>> for (int i=0; i<size; ++i)
>>  c_like_data[i] *= 10;
>>
>
> But now, I have the ugly problem with reference types
> #include <iostream>
> using std::cerr;  using std::endl;
> #include <Eigen/Core>
> // import most common Eigen types
> USING_PART_OF_NAMESPACE_EIGEN
>
> void process(VectorXd& DATA);
>
> int main() {
>  double data[4] = {1, 2, 3, 4};
>
>  Eigen::Map<VectorXd> D(data,4);
>  process(D); // ERROR! invalid initialization of reference
>              // of type 'Eigen::VectorXd& ....

Of course: process takes a VectorXd reference, but D is not a VectorXd.

The biggest common denominator between VectorXd and Map<VectorXd> is
MatrixBase, so you want process to take a MatrixBase reference:

template<typename Derived>
void process(const MatrixBase<Derived>& DATA);

Benoit