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From: mark@lznv.ATT.COM (MARK SMITH)
Newsgroups: comp.lang.c++
Subject: Virtual Functions in IPC environment
Message-ID: <1120@lznv.ATT.COM>
Date: Mon, 31-Aug-87 16:24:22 EDT
Article-I.D.: lznv.1120
Posted: Mon Aug 31 16:24:22 1987
Date-Received: Fri, 4-Sep-87 07:18:25 EDT
Organization: AT&T ISL Middletown NJ USA
Lines: 33


I am building a message-based system consisting of client/server processes
which I would like to implement using C++.  As such, for a given message
I would like to define the base class

class MsgHdr { IPC private data items; public: IPC access functions(); };

and a set of derived classes

class DerivedN: public MsgHdr {
	private data for DerivedN;
public:
	virtual void Operate();		// server: do DerivedN-specific fct
};

However, I encounter two difficulties:

(1) Although the intention is clearly to allow Operate() only to be
usable by the server, I can't find a way to disallow a client process from
linking with Operate.  Since I am memory-conscious (I'm using swapping UNIX), 
this is a critical fault.

(2) Similarly, the client can't send a DerivedN and expect Operate to work, 
since the server gets a DerivedN Operate linked in the address space
of the client.  Memory fault core dumped.

Is there some better abstraction?  I came up with some ugly (more C-like)
workarounds involving run-time virtual function linking (for (1)) 
and readdressing (for (2)).  Shared libraries are out, too.
I'm beginning to believe there's no advantage in using C++ objects if
they are to be passed across process boundaries.

Mark Smith --- { all that jazz } ! lznv ! mark