Re: virtual destructors in base classes
"Charles Wang[MSFT]" <changliw@online.microsoft.com> wrote in message
news:u4IO%23imYIHA.2164@TK2MSFTNGHUB02.phx.gbl
The reason of "Make sure base classes have virtual destructors" in
"Effective C++" was to avoid undefined behaviors caused by compilers.
By C++ standard, when you try to delete a derived class object via
its base class pointer and when the base class has a nonvirtual
destructor, the result is undefined.
Of course if you never delete a derived class object via its base class
pointer, you don't care about this.
Actually Microsoft VC++ ATL
compiler
There ain't no such thing as Microsoft VC++ ATL compiler. There's VC++
compiler, and ATL library that can be compiled by it. The compiler
doesn't do anything special for ATL library that it won't do for any
other library.
implement its mechanism to ensure that you can get the
expected result
Implement what mechanism? What result would you expect to get? If you
are saying that VC++ compiler can magically find and call the derived
class' destructor when presented with a base class pointer where that
base class has a non-virtual destructor, then you are wrong. This would
be a physical impossibility.
COM objects based on ATL are not normally destroyed with 'delete' by the
client. Instead, the client calls IUnknown::Release() through one of the
object's interfaces to decrement the object's reference count, and once
said count reaches zero, the object destroys itself, usually by calling
'delete this;'. ATL arranges for IUnknown::Release to always be
implemented by the most-derived class in the class hierarchy. So the
issue of deleting through the base pointer fails to arise.
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925