Efficient storage for messages

I am creating an open source multimedia controller in ruby. The idea  
is to use ruby to glue together extensions written in C/C++.  
Actually, the server will be used for a project in which we use  
serial communication, artificial intelligence signal recognition and  
midi. This is the current processing line:

serial in  ---> buffer ---> (window on the buffer) pattern  
recognition ---> midi out

I am now starting to write a proper message class to pass the data  
between the different objects. Data can be large (if we process  
video) and needs to be processed fast (eventually using BLAS or Blitz+ 
+).

struct RKMessage {
   long len;     // can be used as a direct value (integer)
   double value; // direct value (floating point)
   int  ctype;   // type of data RK_DIRECT_LONG, RK_DIRECT_DOUBLE,  
RK_LIST_DOUBLE, ...
   void * ptr;  // storage for lists, matrices
};

Is it a good idea to use a single class (RKMessage) to pass messages  
or should I create specialized classes for each kind of data  
providing common accessors ?

Could I create subclasses of RKMessage, sharing the same struct as  
their internal storage ?

Can a specialized class have additional data, or is it "one class",  
"one struct" ?

I understand these questions are very general and might reflect my  
lack of ruby internals knowledge but I would greatly appreciate any  
advice or comment.

Thanks for your help,

Gaspard



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