I have a eigen matrix of type "MatrixXd" that contains a single columns (this is known only at run time )and i want to convert it to a "const eigen::Vector<double>" without copying the data (In case that matrix happen to have more than one colums at run time i could either raise an error or use only the first row).
I tried the following code :

Eigen::MatrixXd m = Eigen::MatrixXd::Random(10000,1);
const Eigen::VectorXd m_vec(m.col(0));

it compile and runs, but the second line produces a copy of the data. In theory that seems possible avoid a copy of the data : the data is continuous and well aligned and because i use the "const" there will not be any resize or modification of elements of the resulting vector . However i'm not able to do that and i had to resort
to use a map (of type "const Map<VectorXr,true>") instead. This is not very convenient for me because it implies changing all my function interfaces to use a map instead of a vector. Is there anyway i could obtain an object of type "const Eigen::VectorXd" from my single column matrix ? if not, what makes it impossible?

MatrixXd::ColXpr m_vec(m.col(0));

or:

MatrixXd::ConstColXpr m_vec(m.col(0));


should do the job.

ggael wrote:MatrixXd::ColXpr m_vec(m.col(0));

or:

MatrixXd::ConstColXpr m_vec(m.col(0));


should do the job.

I tried it but it still does copies before or within the function call. here is my code that i used to test it. In order to make my intial question more explicit : i do not want to change the interface of my_function, because the function that i actually want to use is from a library of which i can't change the interface.

Code: Select all

#include <Eigen/Core>
#include <iostream>

const double* my_function(const Eigen::VectorXd &m)
{return m.data();}

void main(void){

  Eigen::MatrixXd m = Eigen::MatrixXd::Random(10000,1);

  // const Eigen::VectorXd m_vec(m.col(0));        // causes a copy
  const Eigen::MatrixXd::ColXpr m_vec(m.col(0));   // will copy at the function call
  // Eigen::MatrixXd::ConstColXpr m_vec(m.col(0)); // does not compile

  const double *ptr=my_function( m_vec);
  if (m.data()==ptr)
  {std::cout<<"great, no copy occurred";}
  else
  {std::cout<<"too bad, a copy occurred";}

}


well, then you're screwed. you should check if there is really no way for you to get an overload for this function. A very ugly and unsafe solution:

struct UglyWrapper {
double* ptr;
DenseIndex size;
};

UglyWrapper vec;
vec.ptr = m.col(j).data();
vec.size = m.rows();

my_function(reinterpret_cast<VectorXd&>(vec));