Message-related functionality factored out into msg_t class.

This patch addresses serveral issues:
1. It gathers message related functionality scattered over whole
   codebase into a single class.
2. It makes zmq_msg_t an opaque datatype. Internals of the class
   don't pollute zmq.h header file.
3. zmq_msg_t size decreases from 48 to 32 bytes. That saves ~33%
   of memory in scenarios with large amount of small messages.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/msg.hpp

@@ -23,28 +23,105 @@
 
 #include <stddef.h>
 
-#include "../include/zmq.h"
-
+#include "config.hpp"
 #include "atomic_counter.hpp"
 
 namespace zmq
 {
 
-    //  Shared message buffer. Message data are either allocated in one
-    //  continuous block along with this structure - thus avoiding one
-    //  malloc/free pair or they are stored in used-supplied memory.
-    //  In the latter case, ffn member stores pointer to the function to be
-    //  used to deallocate the data. If the buffer is actually shared (there
-    //  are at least 2 references to it) refcount member contains number of
-    //  references.
+    //  Note that this structure needs to be explicitly constructed
+    //  (init functions) and destructed (close function).
 
-    struct msg_content_t
+    class msg_t
     {
-        void *data;
-        size_t size;
-        zmq_free_fn *ffn;
-        void *hint;
-        zmq::atomic_counter_t refcnt;
+    public:
+
+        //  Mesage flags.
+        enum
+        {
+            more = 1,
+            shared = 128
+        };
+
+        //  Signature for free function to deallocate the message content.
+        typedef void (free_fn_t) (void *data, void *hint);
+
+        bool check ();
+        int init ();
+        int init_size (size_t size_);
+        int init_data (void *data_, size_t size_, free_fn_t *ffn_,
+            void *hint_);
+        int init_delimiter ();
+        int close ();
+        int move (msg_t &src_);
+        int copy (msg_t &src_);
+        void *data ();
+        size_t size ();
+        unsigned char flags ();
+        void set_flags (unsigned char flags_);
+        void reset_flags (unsigned char flags_);
+        bool is_delimiter ();
+
+        //  After calling this function you can copy the message in POD-style
+        //  refs_ times. No need to call copy.
+        void add_refs (int refs_);
+
+        //  Removes references previously added by add_refs.
+        void rm_refs (int refs_);
+
+    private:
+
+        //  Shared message buffer. Message data are either allocated in one
+        //  continuous block along with this structure - thus avoiding one
+        //  malloc/free pair or they are stored in used-supplied memory.
+        //  In the latter case, ffn member stores pointer to the function to be
+        //  used to deallocate the data. If the buffer is actually shared (there
+        //  are at least 2 references to it) refcount member contains number of
+        //  references.
+        struct content_t
+        {
+            void *data;
+            size_t size;
+            free_fn_t *ffn;
+            void *hint;
+            zmq::atomic_counter_t refcnt;
+        };
+
+        //  Different message types.
+        enum type_t
+        {
+            type_min = 101,
+            type_vsm = 101,
+            type_lmsg = 102,
+            type_delimiter = 103,
+            type_max = 103
+        };
+
+        //  Note that fields shared between different message types are not
+        //  moved to tha parent class (msg_t). This way we ger tighter packing
+        //  of the data. Shared fields can be accessed via 'base' member of
+        //  the union.
+        union {
+            struct {
+                unsigned char type;
+                unsigned char flags;
+            } base;
+            struct {
+                unsigned char type;
+                unsigned char flags;
+                unsigned char size;
+                unsigned char data [max_vsm_size];
+            } vsm;
+            struct {
+                unsigned char type;
+                unsigned char flags;
+                content_t *content;
+            } lmsg;
+            struct {
+                unsigned char type;
+                unsigned char flags;
+            } delimiter;
+        } u;
     };
 
 }