What happens under the hood if I returned the result matrix from a method, e.g.

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struct C {
  Eigen::MatrixXd data;
  Eigen::MatrixXd f();
  void compute();
  int current_row;
};

void C::compute() {
  // do something
}

Eigen::MatrixXd C::f() {
  return data.block(0, 0, current_row+1, data.cols() );
}

int main(void) {
  C c;
  c.compute();
  Eigen::MatrixXd m = C.f();
  return 0;
}


What performance hit (memory allocation, etc.) do I get? Is this the right way of returning a matrix? Thanks.

In this case the block will be copied into a temporary matrix. To avoid that, your function should return a Block<MatrixXd,Dynamic,Dynamic> object which is what data.block(...) returns.