如何从Eigen :: Matrix获取内存所有权？

Question

A dynamically sized Eigen::Matrix holds its values in a continuous memory block. I need these values as a memory block I own. I currently copy the values over using std::memcpy .

#include <cstdlib>
#include <cstring>
#include <eigen3/Eigen/Core>
using RowMajorMatrixXf = Eigen::Matrix<float, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor>;
int main()
{
    RowMajorMatrixXf mat(1024, 2048);
    // ...
    const std::size_t num_bytes = mat.rows() * mat.cols() * sizeof(float);
    float* ptr = (float*)std::malloc(num_bytes); // raw ptr for simplicity
    std::memcpy(ptr, mat.data(), num_bytes);
    // ...
    std::free(ptr);
}

However the copying is unnecessary, since the Eigen::Matrix is no longer needed at this point. How can I acquire the ownership of the memory of the Eigen Matrix, essentially preventing the Matrix object from freeing the memory in its destructor?

Answer 1

You can better allocate your own buffer, and interpret as an Eigen's matrix using Map :

float* ptr = new float[r*c];
Map<RowMajorMatrixXf> mat(ptr,r,c);

and then use mat just like RowMajorMatrixXf, except its true type is not a RowMajorMatrixXf, so you cannot pass it by reference to a function taking a RowMajorMatrixXf& , for that use Ref<RowMajorMatrixXf> or Ref<const RowMajorMatrixXf> .