eigen/doc/examples/QuickStart_example2_fixed.cpp

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

using Eigen::Matrix3d;
using Eigen::Vector3d;

int main() {
  Matrix3d m = Matrix3d::Random();
  m = (m + Matrix3d::Constant(1.2)) * 50;
  std::cout << "m =" << std::endl << m << std::endl;
  Vector3d v(1, 2, 3);

  std::cout << "m * v =" << std::endl << m * v << std::endl;
}