why boost::make_function_output_iterator accepts a structure halfedge2edge(mesh, border) as its parameter?

[citystrawman]

Hi, I am reading this code, and for this line of code:

PMP::border_halfedges(faces(mesh), mesh, boost::make_function_output_iterator(halfedge2edge(mesh, border)));

I checked that boost::make_function_output_iterator should accept a function as it is defined as:

template <class UnaryFunction>
inline function_output_iterator<UnaryFunction>
make_function_output_iterator(const UnaryFunction& f = UnaryFunction()) {
  return function_output_iterator<UnaryFunction>(f);
}

However, in the above code it accepts halfedge2edge(mesh, border) which is a initialization of halfedge2edge.

Can anyone explain the logic here? Thank you so much.

[citystrawman]

I see. Actually void operator()(const halfedge_descriptor& h) const makes the structure a functor.