Hi Andrii, I've just tried this: class delaunay_triangulation { public: class Builder { delaunay_triangulation& dt; public: Builder(delaunay_triangulation& dt_): dt(dt_) {} void reserve(size_t num_sites) { dt.reserve(num_sites); } template <typename SEvent> void process_single_site(const SEvent& site) { dt.process_single_site(site); } template <typename SEvent> std::pair<void*,void*> insert_new_edge(const SEvent& site1, const SEvent& site2) { return dt.insert_new_edge(site1,site2); } template <typename SEvent, typename CEvent> std::pair<void*, void*> insert_new_edge(const SEvent& site1, const SEvent& site3, const CEvent& circle, void* data12, void* da$ return dt.insert_new_edge(site1, site3, circle, data12, data23); } void build() { dt.build(); } }; delaunay_triangulation(): builder_(*this) {} typedef std::pair<int,int> point_t; typedef std::pair<point_t,point_t> edge_t; typedef std::vector<edge_t> edges_t; edges_t edges; Builder* builder() { return &builder_; } // Why not a reference? void reserve(size_t /*num_sites*/) {} template <typename SEvent> void process_single_site(const SEvent& /*site*/) {} template <typename SEvent> std::pair<void*, void*> insert_new_edge(const SEvent& site1, const SEvent& site2) { edge_t e(point_t(site1.x0(),site1.y0()), point_t(site2.x0(),site2.y0())); edges.push_back(e); return std::pair<void*,void*>(NULL,NULL); } template <typename SEvent, typename CEvent> std::pair<void*, void*> insert_new_edge(const SEvent& site1, const SEvent& site3, const CEvent& /*circle*/, void* /*data12*/, void* /*data23*/) { return insert_new_edge(site1,site3); } void build() {} private: Builder builder_; }; It does seem to create a triangulation as you can see here: http://chezphil.org/tmp/delaunay.png A couple of observations: - Again you're using void* in here, which doesn't look great. Is this just to break circular references, or something? Why can't you use the actual types? - Your "builder" class, which just forwards to the other class, doesn't seem very useful to me. I can see the idea - you're breaking the "lifecycle" of the object into a "build" phase and then a "use" phase - but I think there are probably better ways to do it if it's worth doing at all. Anyway, I think you potentially have something very useful here. Thanks! Regards, Phil.