eigen/doc/examples/QuickStart_example.cpp

#include <iostream>
#include <Eigen/Dense>

using Eigen::MatrixXd;

int main() {
  MatrixXd m(2, 2);
  m(0, 0) = 3;
  m(1, 0) = 2.5;
  m(0, 1) = -1;
  m(1, 1) = m(1, 0) + m(0, 1);
  std::cout << m << std::endl;
}