diff --git a/src/data.hpp b/src/data.hpp
index 70d1a72..1480103 100644
--- a/src/data.hpp
+++ b/src/data.hpp
@@ -321,6 +321,8 @@
   friend class nfd::LocalControlHeader;
 };
 
+std::ostream&
+operator<<(std::ostream& os, const Data& data);
 
 inline bool
 Data::hasWire() const