Re: [ZooLib-dev] virtual functions but non-virtual destructor

[Andrew G. <ag...@em...>]

On 02 Jan 2003 02:20:22 -0500, Michael D. Crawford wrote:
> In building ZooLib under project builder (with g++) I get a lot of
warning 
> messages about classes that have some virtual functions but don't have
virtual 
> destructors.  ZMutexBase is one, I think there are others.
> 
> That is probably not what we want.
> 
> If you have a non-virtual destructor and you delete an object whose
real type 
> is a derived type, by deleting a pointer of the base type, you will
call the 
> base class' destructor but not the derived class' destructor.
> 
> That would be OK if you never had any derived classes.  It is common
for STL 
> templates to not have virtual destructors.  They are not meant to be
used as 
> base classes.
> 
> If you have some virtual member functions in a class, that suggests
that this 
> is meant to be a base class.  If you have a pointer of base type that
really 
> points to an object of derived type, and call one of the virtual
functions 
> through the pointer, you will get a call to the derived class' version
of the 
> function, if there is one.
> 
> Generally you either want no virtual functions at all (in which case
you 
> shouldn't inherit from the class) or if you do intend this to be a
base class, 
> the destructor should be virtual.

In general this is correct. However in this case ZMutexBase is intended
not as a base class for implementation-inheritance, but as an interface
definition. The indication that this is the case is that the destructor
and constructor are protected, and copy-constructor and assignment are
private. You cannot use any aspect of a ZMutexBase *other* than it's
virtual methods -- you can't create, assign, copy from or delete with a
ZMutexBase reference or pointer.

A+
-- 
Andrew Green                           mailto:ag...@em...
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