Template Factory Pattern Problem

John <jschwartzman@comcast.net>
Sat, 25 Sep 2010 00:50:18 CST
{ Please fit your text within 70 columns or so. Gratuitous linebreaks,
  especially with the //-style comments, break the code. -mod }

I have a series of message classes all derived from a common base
class, RcvHdrMsg. Different message classes may contain 0 or more
arguments. I've been using a factory pattern to create messages from
serialization. (based on Object Factory from Chapter 8 of Modern C++
Design). The factory works fine when I define each message as its own
class, but, of course, adding new messages means defining an
implementation for each message class, which is essentially a copy and
paste operation.

I've been trying to simplify by creating a template class for messages
with no arguments and replacing some message classes with typedefs.
I'm having trouble moving the anonymous namespace create function into
a header file. I can't figure out how to declare createMsg() function
so that the NoArgMsg::reg() method can find it.

The compilation errors are shown below the file. I'm using gcc 4.5 on
Linux x86-64. Thanks in advance for your help.

// file NoArgMsg.h

#ifndef __NOARGMSG_H__
#define __NOARGMSG_H_

#include "RcvHdrMsg.h" // includes RcvMsg.h and RcvMsgFactory.h


template<byte MSGTYPE>
class NoArgMsg : public RcvHdrMsg
    NoArgMsg() : RcvHdrMsg(MSGTYPE) {}
    NoArgMsg(uint nMsgId = 0) : RcvHdrMsg(MSGTYPE, nMsgId) {}

    static bool reg(RcvMsgFactory& cf)
        { return cf.registerMsg(MSGTYPE, createMsg<MSGTYPE>); } // <== line 41
    static bool unreg(RcvMsgFactory& cf)
                            { return cf.unregisterMsg(MSGTYPE); }

typedef NoArgMsg<RcvMsg::eCMD_RESET> CmdResetMsg;
typedef NoArgMsg<RcvMsg::eCMD_DISABLE> CmdDisableMsg;
typedef NoArgMsg<RcvMsg::eCMD_PROGRAM> CmdProgramMsg;
typedef NoArgMsg<RcvMsg::eCMD_SERVER_STOP> CmdServerStopMsg;
typedef NoArgMsg<RcvMsg::eDEL_ALL_SAVED_SMS> DeleteAllSavedSmsMsg;


 * RcvMsgFactory ctor method hidden in anonymous namespace
namespace // ********************* anonymous namespace *********************
    template<byte MSGTYPE>
    transport::RcvMsg* createMsg()
        return new transport::NoArgMsg<MSGTYPE>;
} // ********************** end of anonymous namespace *********************

#endif // ************************ ! __NOARGMSG_H__ ************************

ERRORS follow:

NoArgMsg.h: In static member function ?static bool
NoArgMsg.h:41:36: error: ?createMsg? was not declared in this scope
NoArgMsg.h:41:55: error: expected primary-expression before ?)? token
NoArgMsg.h: In static member function ?static bool
      [with unsigned char MSGTYPE = 5u]?:
NoArgMsg.h:41:62: warning: control reaches end of non-void function
NoArgMsg.h: In static member function ?static bool
 transport::NoArgMsg<MSGTYPE>::reg(transport::RcvMsgFactory&) [with
unsigned char MSGTYPE
                    = 6u]?:
NoArgMsg.h:41:62: warning: control reaches end of non-void function

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