How to assign / copy a Boost::multi_array

I want to assign a copy of a boost::multi_array. How can I do this. The object where I want to assign it to has been initialized with the default constructors.

This code does not work, because the dimensions and size are not the same

class Field {
  boost::multi_array<char, 2> m_f;

  void set_f(boost::multi_array<short, 2> &f) {
    m_f = f;
  }
}

What to use instead of m_f = f ?