diff --git a/geom/tri.hpp b/geom/tri.hpp
index 533edff3..6e1fc845 100644
--- a/geom/tri.hpp
+++ b/geom/tri.hpp
@@ -43,7 +43,8 @@ namespace util::geom {
 
     namespace surface {
         template <size_t S, typename T>
-        struct sampler<tri<S,T>> {
+        class sampler<tri<S,T>> {
+        public:
             sampler (tri<S,T> _target):
                 base (_target.a),
                 v0 (_target.b - _target.a),
@@ -58,6 +59,7 @@ namespace util::geom {
                 return base + dist (gen) * v0 + dist (gen) * v1;
             }
 
+        private:
             util::point<S,T> base;
             util::vector<S,T> v0, v1;
         };