eigen/doc/examples/tut_matrix_resize_fixed_size.cpp

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

int main()
{
  Eigen::Matrix4d m;
  m.resize(4,4); // no operation
  std::cout << "The matrix m is of size "
            << m.rows() << "x" << m.cols() << std::endl;
}