diff --git a/BaseLib/ConfigTree-impl.h b/BaseLib/ConfigTree-impl.h
index ed4cc075f75175de0e86feea8fdaa5f153bb89d7..0d233614521ad0884073a44ce1f4394b60256b60 100644
--- a/BaseLib/ConfigTree-impl.h
+++ b/BaseLib/ConfigTree-impl.h
@@ -26,6 +26,8 @@ public:
 
     Iterator begin() const { return _begin; }
     Iterator end()   const { return _end; }
+    std::size_t size() const { return std::distance(_begin, _end); }
+
 private:
     Iterator _begin;
     Iterator _end;