Predeclaring a template's member typedef

I use a library (which I cannot modify) and that declares a PointCloud template that includes a Ptr typedef

namespace pcl
{
    template <typename PointT>
    class PCL_EXPORTS PointCloud
    {
        ...
        typedef boost::shared_ptr<PointCloud<PointT> > Ptr;
    }
}

Now, I need to predeclare Ptr and I have no idea how to. I have done

namespace pcl
{
    class PointXYZ;
    template<class pointT> class PointCloud;
}

but I am stuck here and whatever I do, I do not seem to be able to predeclare the Ptr typedef.

Any idea?