diff --git a/libuavcan/include/uavcan/marshal/scalar_codec.hpp b/libuavcan/include/uavcan/marshal/scalar_codec.hpp
index 990cf5fa0d..8df53da8b3 100644
--- a/libuavcan/include/uavcan/marshal/scalar_codec.hpp
+++ b/libuavcan/include/uavcan/marshal/scalar_codec.hpp
@@ -6,7 +6,6 @@
 
 #include <cassert>
 #include <limits>
-#include <string>
 #include <uavcan/stdint.hpp>
 #include <uavcan/impl_constants.hpp>
 #include <uavcan/util/compile_time.hpp>
diff --git a/libuavcan/include/uavcan/transport/outgoing_transfer_registry.hpp b/libuavcan/include/uavcan/transport/outgoing_transfer_registry.hpp
index 1bee6db283..cd4360ce47 100644
--- a/libuavcan/include/uavcan/transport/outgoing_transfer_registry.hpp
+++ b/libuavcan/include/uavcan/transport/outgoing_transfer_registry.hpp
@@ -5,7 +5,7 @@
 #pragma once
 
 #include <cassert>
-#include <sstream>
+#include <string>
 #include <uavcan/stdint.hpp>
 #include <uavcan/impl_constants.hpp>
 #include <uavcan/map.hpp>
@@ -34,8 +34,8 @@ public:
         , destination_node_id_(destination_node_id)
     {
         assert((transfer_type == TransferTypeMessageBroadcast) == destination_node_id.isBroadcast());
-
-        /* Service response transfers must use the same Transfer ID as matching service request transfer,
+        /*
+         * Service response transfers must use the same Transfer ID as matching service request transfer,
          * so this registry is not applicable for service response transfers at all.
          */
         assert(transfer_type != TransferTypeServiceResponse);
@@ -52,14 +52,7 @@ public:
             (destination_node_id_ == rhs.destination_node_id_);
     }
 
-    std::string toString() const
-    {
-        std::ostringstream os;
-        os << "dtid=" << int(data_type_id_.get())
-           << " tt=" << int(transfer_type_)
-           << " dnid=" << int(destination_node_id_.get());
-        return os.str();
-    }
+    std::string toString() const;
 };
 UAVCAN_PACKED_END
 
diff --git a/libuavcan/src/transport/uc_outgoing_transfer_registry.cpp b/libuavcan/src/transport/uc_outgoing_transfer_registry.cpp
new file mode 100644
index 0000000000..5940579c51
--- /dev/null
+++ b/libuavcan/src/transport/uc_outgoing_transfer_registry.cpp
@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) 2014 Pavel Kirienko <pavel.kirienko@gmail.com>
+ */
+
+#include <cstdio>
+#include <uavcan/transport/outgoing_transfer_registry.hpp>
+
+namespace uavcan
+{
+/*
+ * OutgoingTransferRegistryKey
+ */
+std::string OutgoingTransferRegistryKey::toString() const
+{
+    using namespace std;
+    char buf[40];
+    (void)snprintf(buf, sizeof(buf), "dtid=%u tt=%u dnid=%u",
+                   int(data_type_id_.get()), int(transfer_type_), int(destination_node_id_.get()));
+    return std::string(buf);
+}
+
+}