ZMQII-38: Compiler complains about missing virtual constructors

src/i_endpoint.hpp

@@ -25,6 +25,8 @@ namespace zmq
 
     struct i_endpoint
     {
+        virtual ~i_endpoint () {}
+
         virtual void attach_pipes (class reader_t *inpipe_,
             class writer_t *outpipe_) = 0;
         virtual void detach_inpipe (class reader_t *pipe_) = 0;

src/i_inout.hpp

@@ -29,6 +29,8 @@ namespace zmq
 
     struct i_inout
     {
+        virtual ~i_inout () {}
+
         //  Engine asks to get a message to send to the network.
         virtual bool read (::zmq_msg_t *msg_) = 0;