diff --git a/include/ndn-cpp/data.hpp b/include/ndn-cpp/data.hpp
index 3f0d00b..d25253f 100644
--- a/include/ndn-cpp/data.hpp
+++ b/include/ndn-cpp/data.hpp
@@ -273,6 +273,9 @@
   wire_ = Block();
 }
 
+std::ostream&
+operator << (std::ostream &os, const Data &data);
+
 } // namespace ndn
 
 #endif