Convert Eigen Matrix to C array

Question

The Eigen library can map existing memory into Eigen matrices.

float array[3];
Map(array, 3).fill(10);
int data[4] = 1, 2, 3, 4;
Matrix2i mat         


        

Answer 1

The solution with Map above segfaults when I try it (see comment above).

Instead, here's a solution that works for me, copying the data into a std::vector from an Eigen::Matrix. I pre-allocate space in the vector to store the result of the Map/copy.

Eigen::MatrixXf m(2, 2);
m(0, 0) = 3;
m(1, 0) = 2.5;
m(0, 1) = -1;
m(1, 1) = 0;

cout << m << "\n";

// Output:
//    3  -1
// 2.5   0

// Segfaults with this code: 
//
// float* p = nullptr;
// Eigen::Map(p, m.rows(), m.cols()) = m;

// Better code, which also copies into a std::vector:

// Note that I initialize vec with the matrix size to begin with:
std::vector vec(m.size());
Eigen::Map(vec.data(), m.rows(), m.cols()) = m;

for (const auto& x : vec)
  cout << x << ", ";
cout << "\n";

// Output: 3, 2.5, -1, 0