C++ :: Forward Declaring Typedefs

I'm using some rather large external libraries, and I want to load them in my .cpp file only. so, my header looks like this:

namespace {
// hidden declarations
namespace geometry {
class Point;
class Polygon;
class Box;
//etc
}
}

In the declaration of the main class in that header, I merely use these as pointers or references. The .cpp file looks as follows:

// Boost
#include <boost/geometry/geometry.hpp>
#include <boost/geometry/geometries/polygon.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/multi/geometries/multi_polygon.hpp>

[Code] .....

This doesn't work however:

error C2371: '`anonymous-namespace'::geometry::Point' : redefinition; different basic types
1> e:..........NavigationMesh.h(10) : see declaration of '`anonymous-namespace'::geometry::Point'

What can I do to forward declare external classes like this?