Transitive Const in OO programming

[eao197]

	Hi!

I'm sorry, I haven't ability to follow D2.0 discussions last time, so may  
be my question had been solved anywhere. But I wonder how to express in  
D2.0 C++ classes like that:

// Collection of client's specific data.
class client_info_t
   {
   public :
     client_info_t()
       : m_socket( 0 )
       , m_queue( 0 )
       {}

     //
     // Getters/Setters.
     //

     ACE_SOCK_Stream *
     socket() const { return m_socket; }

     void
     set_socket( ACE_SOCK_Stream * s ) { m_socket = s; }

     ACE_Message_Queue *
     queue() const { return m_queue; }

     void
     set_queue( ACE_Message_Queue * q ) { m_queue = q; }

   private :
     ACE_SOCK_Stream * m_socket;
     ACE_Message_Queue * m_queue;
   };

// Clients map.
class client_map_t
   {
   public :
     ...
     // Puts new client info into map.
     void
     insert( const std::string & client_id, const client_info_t & info );

     // Removes client info from map.
     void
     remove( const std::string & client_id );

     // Fetches client info from map.
     // Throws invalid_agrument if client_id is unknown.
     const client_info_t &
     fetch( const std::string & client_id ) const;
     ...
   private :
     std::map< std::string, client_info_t > m_map;
   };

client_map_t::fetch is const method, so it doesn't allow anyone to change  
map content. If someone wants to send some data to a client, it can use  
const client_map_t::fetch() and retrive const client_info_t object. Then  
he can use const client_info_t::socket() to send data via non-const  
ACE_SOCK_Stream pointer. But he can't change content of client_info_t  
because he has only const reference to client_info_t.


I could suggest that in D2.0 fetch() sould return object of different  
type. For example:

class ConstrainedClientInfo
   {
   public :
      this( SocketStream s, MessageQueue q ) { m_socket = s; m_queue = q; }
      SocketStream socket() { return m_socket; }
      MessageQueue queue() { return m_queue; }
   private :
      SocketStream m_socket;
      MessageQueue m_queue;
   }

and

class ClientMap
   {
   public :
     ...
     ConstrainedClientInfo fetch( string client_id );
     ...
   private :
      ClientInfo[string] m_map;
   };

But in such case ClientMap.fetch cannot be const (may be I'm wrong?)  
because if ClientMap.fetch is const then m_map is also const and  
ClientInfo reference from m_map is also const, and  
SocketStream/MessageQueue references from ClientInfo from m_map is also  
const.

-- 
Regards,
Yauheni Akhotnikau