Fix -Wmismatched-tags

src/i_decoder.hpp

@@ -30,8 +30,9 @@ namespace zmq
 
     //  Interface to be implemented by message decoder.
 
-    struct i_decoder
+    class i_decoder
     {
+    public:
         virtual ~i_decoder () {}
 
         virtual void set_msg_sink (i_msg_sink *msg_sink_) = 0;

src/i_msg_sink.hpp

@@ -29,8 +29,9 @@ namespace zmq
 
     //  Interface to be implemented by message sink.
 
-    struct i_msg_sink
+    class i_msg_sink
     {
+    public:
         virtual ~i_msg_sink () {}
 
         //  Delivers a message. Returns 0 if successful; -1 otherwise.

src/i_msg_source.hpp

@@ -29,8 +29,9 @@ namespace zmq
 
     //  Interface to be implemented by message source.
 
-    struct i_msg_source
+    class i_msg_source
     {
+    public:
         virtual ~i_msg_source () {}
 
         //  Fetch a message. Returns 0 if successful; -1 otherwise.