diff --git a/types.hpp b/types.hpp
index 1af648d4..55e28795 100644
--- a/types.hpp
+++ b/types.hpp
@@ -25,10 +25,17 @@
 //-----------------------------------------------------------------------------
 /// Returns the number of elements of a statically allocated array
 template <typename T, size_t N> 
-constexpr size_t elems(const T (&)[N])
+constexpr size_t
+elems (const T (&)[N])
     { return N; }
 
 
+template <typename T>
+constexpr typename T::size_type
+elems (const T &t)
+    { return t.size (); }
+
+
 //-----------------------------------------------------------------------------
 template <class T>
 T