Path: utzoo!attcan!uunet!rocket!newshost!dove
From: dove@batman.uucp (Webster &)
Newsgroups: comp.lang.c++
Subject: Will C++ 2.1 fix the virtual function linkage problem?
Message-ID: <DOVE.90Jun2102921@batman.uucp>
Date: 2 Jun 90 14:29:21 GMT
Sender: news@rocket.UUCP
Distribution: comp
Organization: Lockheed Sanders Inc.
Lines: 60

The problem:

Suppose a user of a library (e.g. InterViews) has set up many virtual
member functions for derived classes that inherit class declarations
from the library.  Assume that these member functions are being
invoked somewhere inside the library that the user is aware of in
general, but not in detail.  Now assume that a new release of the
library comes out and the details of the calling syntax for some of
these virtual functions is superficially changed, and the user has not
read all the new documentation carefully or the documentation fails to
mention the change.

This causes the derived class virtual functions to "fail to link" with
the base class declarations leading to program failure.  There is no
compile time warning since the derived virtual declarations are now
treated as new function members starting at the derived class level.

I have had this happen several times on programs I controlled.  I see
this as a severe risk.  Is this being fixed?

I have previously (5/12/87) discussed this issue and I gather it has
come up as a recurring theme.  I think I have a fix.  Since I have not
been reading this list for a while, forgive me if it has been
discussed.

Suppose you allow the derived class to inherit the declaration of the
virtual member.  

class base_class {
public: 
	virtual base_virtual();
};

int base_class::base_virtual() { ... default behavior ... }

class derived_class : public base_class {
}

int derived_class::base_virtual() { ... derived behavior ... }


This would be back compatible with existing code, not require any new
keywords, and allow me to fix my problem with a library line
InterViews without requiring any changes to the library.  This allows
me to explicitly state (by ommision) that I am expecting there to be a
matching inherited virtual and would yield a compile time error if
that linkage failed.

Comments?

Web
--
Dr. Webster Dove				Dr. Webster Dove
Experimental Computing Systems ('X')		Lockheed Sanders Inc.  
Signal Processing Center of Technology		144 Daniel Webster Hwy.
Lockheed Sanders Inc.				Merrimack, NH.  03054  
						
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