eigen/doc/examples/function_dynBlock.cpp

#include <Eigen/Core.h>
USING_EIGEN_DATA_TYPES
using namespace std;
int main(int, char**)
{
    Matrix4d m = Matrix4d::identity();
    m.dynBlock(2,0,2,2) = m.dynBlock(0,0,2,2);
    cout << m << endl;
    return 0;
}