eigen/doc/examples/Tutorial_BlockOperations_print_block.cpp

#include <Eigen/Dense>
#include <iostream>
using namespace Eigen;

int main()
{
  MatrixXf m(3,3);
  
  m << 1,2,3,
       4,5,6,
       7,8,9;
     
  std::cout << m.block(0,0,2,2) << std::endl;
}