diff --git a/poly2tri/sweep/cdt.cc b/poly2tri/sweep/cdt.cc
index 4dfe6a6..293efdc 100644
--- a/poly2tri/sweep/cdt.cc
+++ b/poly2tri/sweep/cdt.cc
@@ -52,12 +52,12 @@ void CDT::Triangulate()
   sweep_->Triangulate(*sweep_context_);
 }
 
-std::vector<p2t::Triangle*> CDT::GetTriangles()
+const std::vector<p2t::Triangle*>& CDT::GetTriangles() const
 {
   return sweep_context_->GetTriangles();
 }
 
-std::list<p2t::Triangle*> CDT::GetMap()
+const std::list<p2t::Triangle*>& CDT::GetMap() const
 {
   return sweep_context_->GetMap();
 }
diff --git a/poly2tri/sweep/cdt.h b/poly2tri/sweep/cdt.h
index 7c54ea9..ed6f113 100644
--- a/poly2tri/sweep/cdt.h
+++ b/poly2tri/sweep/cdt.h
@@ -83,12 +83,12 @@ public:
   /**
    * Get CDT triangles
    */
-  std::vector<Triangle*> GetTriangles();
+  const std::vector<Triangle*>& GetTriangles() const;
 
   /**
    * Get triangle map
    */
-  std::list<Triangle*> GetMap();
+  const std::list<Triangle*>& GetMap() const;
 
   private:
 
diff --git a/poly2tri/sweep/sweep_context.cc b/poly2tri/sweep/sweep_context.cc
index 7b9432f..c5a2af9 100644
--- a/poly2tri/sweep/sweep_context.cc
+++ b/poly2tri/sweep/sweep_context.cc
@@ -57,12 +57,24 @@ void SweepContext::AddPoint(Point* point) {
   points_.push_back(point);
 }
 
-std::vector<Triangle*> &SweepContext::GetTriangles()
+std::vector<Triangle*>& SweepContext::GetTriangles()
+{
+  return const_cast<std::vector<Triangle*>&>(
+      static_cast<const SweepContext&>(*this).GetTriangles());
+}
+
+const std::vector<Triangle*>& SweepContext::GetTriangles() const
 {
   return triangles_;
 }
 
-std::list<Triangle*> &SweepContext::GetMap()
+std::list<Triangle*>& SweepContext::GetMap()
+{
+  return const_cast<std::list<Triangle*>&>(
+    static_cast<const SweepContext&>(*this).GetMap());
+}
+
+const std::list<Triangle*>& SweepContext::GetMap() const
 {
   return map_;
 }
diff --git a/poly2tri/sweep/sweep_context.h b/poly2tri/sweep/sweep_context.h
index 11d5739..64029d6 100644
--- a/poly2tri/sweep/sweep_context.h
+++ b/poly2tri/sweep/sweep_context.h
@@ -91,7 +91,9 @@ AdvancingFront* front() const;
 void MeshClean(Triangle& triangle);
 
 std::vector<Triangle*> &GetTriangles();
+const std::vector<Triangle*> &GetTriangles() const;
 std::list<Triangle*> &GetMap();
+const std::list<Triangle*> &GetMap() const;
 
 std::vector<Edge*> edge_list;