diff --git a/include/boost/detail/shared_count.hpp b/include/boost/detail/shared_count.hpp
index 120fd47..2e0dc47 100644
--- a/include/boost/detail/shared_count.hpp
+++ b/include/boost/detail/shared_count.hpp
@@ -174,6 +174,7 @@ public:
     }
 };
 
+class weak_count;
 
 class shared_count
 {