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Mail Archives: cygwin-developers/1998/03/27/10:29:55

From: rdparker AT butlermfg DOT org (Parker, Ron)
Subject: RE: Mark ~fhandler_base virtual?
27 Mar 1998 10:29:55 -0800 :
Message-ID: <c=US%a=_%p=Butler_Manufactu%l=BUILDINGS/BLDGSHUB/00089B3B.cygnus.cygwin32.developers@nttwr2.tower.bldgs.butlermfg.org>
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To: Larry Hall <lhall AT rfk DOT com>,
"'cygwin32-developers'"
<cygwin32-developers AT cygnus DOT com>

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The only size effect in declaring a destructor as virtual is that it 
lengthens an objects vtable by sizeof(void*), admittedly 4-bytes is 
not an issue. However, in a class that has no virtual functions it 
also adds a vtable to the class and a vtable pointer to all of the 
objects of that class. This increases the size of the objects by 
sizeof(void*).  This is only becomes a memory issue for small classes 
that have a large number of instances.

By all means if a class has subclasses that contain explicitly 
declared destructors, the superclass should have a virtual destructor. 
 See my previous message for more information on the need of virtual 
destructors.

-----Original Message-----
From: Larry Hall [mailto:lhall AT rfk DOT com]
Sent: Wednesday, March 25, 1998 8:04 AM
To: Geoffrey Noer; cygwin32-developers AT cygnus DOT com
Cc: noer AT cygnus DOT com
Subject: Re: Mark ~fhandler_base virtual?


At 06:39 PM 3/24/98 -0800, Geoffrey Noer wrote:
>I've been cleaning up warnings generated by compiling cygwin32 with
>-Wall.  One of them is that fhandler.h declares virtual functions 
but
>has a non-virtual destructor.
>
>Should I make ~fhandler_base virtual?  I *think* it's not necessary
>but wouldn't hurt either.  The function set_name (which is the only
>thing that calls new and thus needs the destructor) is only called 
by
>fhandler_base functions.
>
>--
>Geoffrey Noer
>noer AT cygnus DOT com
>

I haven't looked at fhandler_base but assuming its name is indicative 
of
its function and the fact that there are virtual functions on this 
class,
a virtual destructor is recommended.  In general, if a class is a base 
class, its destructor should be virtual.  Also, there is no harm in 
making
a destructor virtual even if the class is not a base class (or isn't 
"yet").


Larry Hall                              lhall AT rfk DOT com
RFK Partners, Inc.                      (781) 239-1053
8 Grove Street                          (781) 239-1655 - FAX
Wellesley, MA  02181                    http://www.rfk.com


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