How to extend Eigen's Tensor class?

Question

I have created a function to display the shape of a Tensor on eigen.

template<typename Scalar_, int rank>
void shape(const Eigen::Tensor<Scalar_, rank>& x)
{
  cout << "( ";  
  for (int i(0); i<x.NumDimensions; i++){
      cout << x.dimensions()[i];
      cout << ",";

  }
  cout << ")";  
}

To use it I have to do :

Tensor <double,2> t (2,2)
shape(t)

I would like to integrate it directly into the tensor class so that we can call it like this:

Tensor <double,2> t (2,2)
t.shape

How to extend Eigen's Tensor class to add functions?