Boost/Voronoi - Can't create voronoi_diagram in class, can in function

Question

I'm trying to put this:

boost::polygon::voronoi_diagram<> diagram;

... in my class, but I get this error:

error C2248: 'boost::polygon::voronoi_diagram<T>::operator =' : cannot access private member declared in class 'boost::polygon::voronoi_diagram<T>'

If I declare it in a function body instead, I do not get this error.