C++ Eigen initialise dynamic matrix with raw data

Question

Suppose I have raw data, whose size I don't know at compile time, and that's why I need to store it in a dynamically sized matrix. I know I can initialise a static-sized matrix as follows:

  std::vector<double> v {1.1, 2.2, 3.3, 4.4};  // "Raw data".
  Eigen::Matrix<double, 2, 2> m(v.data());
  std::cout << m << std::endl;

But is there a way of similarly initialising, or (even better) setting the data of a dynamic matrix? Something like the following (which doesn't compile)?

  std::vector<double> v {1.1, 2.2, 3.3, 4.4};  // "Raw data".
  Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic> m(v.data(), 2, 2);
  std::cout << m << std::endl;

I know from a comment in this post that I can just use Eigen::Map, but as far as I understand, Eigen::Map doesn't own the memory, so I can't, for example, return it from a function. I know that I can set the matrix element-wise but that feels so dumb LOL.

If just using the `Eigen::Map` is not working in your case (because the raw memory gets destructed or overwritten), you can copy the `Map` to an `Eigen::MatrixXd` object. — chtz, Jul 14 '22 at 15:02

score 3 · Accepted Answer · edited Jul 14 '22 at 18:21

If you want to copy the raw data, assign the Map to a normal matrix.

std::vector<double> v {1.1, 2.2, 3.3, 4.4};
Eigen::MatrixXd m = Eigen::MatrixXd::Map(v.data(), 2, 2);

BTW: You don't need to deal with the template parameters such as Matrix<double, 2, 2> or Matrix<double, Dynamic, Dynamic>: There are type definitions for common cases. Just use Matrix2d and MatrixXd in your example. The type definitions are listed here for matrix and here for array

C++ Eigen initialise dynamic matrix with raw data

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