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From: jimad@microsoft.com (Jim ADCOCK)
Newsgroups: comp.std.c++
Subject: Re: virtual functions and virtual destructors
Message-ID: <1991Sep03.194231.28991@microsoft.com>
Date: 3 Sep 91 19:42:31 GMT
References: <1991Aug28.011034.6690@microsoft.com> <643@mole-end.UUCP>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 43

In article <643@mole-end.UUCP> mat@mole-end.UUCP (Mark A Terribile) writes:
|In article <1991Aug28.011034.6690@microsoft.com>, jimad@microsoft.com (Jim ADCOCK) writes:
|> A well know C++ programming error is to have a class without a 
|> virtual destructor and then to delete an instance via a pointer to 
|> a base class rather than via a pointer to the actual class. ...
|
|> Unfortunately, C++ compilers cannot in general diagnose this error. ...
|
|> Therefore, I'd like to suggest for consideration:
|>   If a class has at least one virtual function, and has a destructor,
|>   that destructor be automatically be considered declared virtual.  
|>   Everything else to remain the same.
|
|> The downside ... is that if a class has virtual function[s] but no
|> destructor, it would still be illegal ... to delete that object via a base
|> pointer.  ...
|
|It's hard to find the right place to issue the warning.  You can issue it
|when you see the base class, when you see the derived class, when you see
|the `new' on the derived class, when you see the derived class pointer
|converted to the base class pointer, etc.   The most restrictive case that
|will still work seems to me to be

Agreed that its hard to find the right place to issue an *error* message,
thus a compiler cannot generally be expected to diagnose this error.
However, a compiler can issue warnings about what it feels are bad
programming practices wherever it wants.  In particular, I'd want a 
compiler to issue a warning at the closing brace of a class with any
virtual functions and without a virtual destructor.

I don't propose that all classes with virtual functions must have 
a destructor [which can then be forced to be virtual.]  I mearly propose
that if a class has virtual functions and a destructor, then that destructor
automatically be considered virtual.  This 'fixes' most of the problem,
but can't fix the problem where a programmer insists on having a class
with virtual functions, insists on no destructor, and then deletes such an 
object through a pointer type different than the object's actual
exact type.  To fix the problem "everywhere" would require that any
class with a virtual function must have a virtual destructor, and I
think some people might complain about forcing the [compiler generated]
creation of a destructor just so it could be considered virtual.  
Anybody have any comments on how these issues tie into how compilers
handle exceptions?