virtual destructors for classes only with virtual functions?

[heted7]

Hi,

Most of the books on C++ say something like this: "A virtual destructor
should be defined if the class contains at least one virtual member
function."

My question is: why is it only for the case when the class contains at
least one virtual member function? Shouldn't the destructor always be
virtual, whenever there's a possibility that an inherited object will
be destructed through a base class pointer? (This does not require,
that the base class have any other virtual function!)

Thanks!

ps: can someone please explain me briefly, why a concrete base class
cannot have an abstract derived class?

[Julián Albo]

heted7 wrote:

My question is: why is it only for the case when the class contains at
least one virtual member function? Shouldn't the destructor always be
virtual, whenever there's a possibility that an inherited object will
be destructed through a base class pointer? (This does not require,
that the base class have any other virtual function!)

Because it will be uncommon that you wants to destroy polimorphically
objetcts that are not used polimorphically , and the majority of the books
talks about more common cases. And to add it to class in this case has some
overhead and many people does not want to pay this price.

But if you have reasons to add a virtual destructor, do it. The books says
that in that case you must add it, not that you must not do it in any other
case.

--
Salu2

[Pete Becker]

Julián Albo wrote:

But if you have reasons to add a virtual destructor, do it. The books says
that in that case you must add it, not that you must not do it in any other
case.

And, just as not saying there are other cases where you need it is an
oversimplificat ion, saying that you always need it when you have virtual
functions is an oversimplificat ion.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)

[heted7]

Thanks for both of you!

[Abecedarian]

Pete Becker wrote:

saying that you always need it when you have virtual
functions is an oversimplificat ion.

An example, please!

::A::

[E. Robert Tisdale]

heted7 wrote:

Most of the books on C++ say something like this,
"A virtual destructor should be defined
if the class contains at least one virtual member function."
This is generally bad advice. Ignore it.
My question is, "Why is it only for the case
when the class contains at least one virtual member function?"
Shouldn't the destructor always be virtual whenever there's a possibility
that an inherited object will be destructed through a base class pointer?
(This does not require, that the base class have any other virtual function!)

Don't define a virtual destructor unless you have a specific use for it.
Generally, C++ functions shouldn't be destroying objects passed to them
by reference (or by reference through a pointer).

[Abecedarian]

heted7 wrote:

Most of the books on C++ say something like this: "A virtual destructor should be defined if the class contains at least one virtual member
function."

This is generally a good rule of thumb. Don't ignore it.
See also: http://www.artima.com/cppsource/bigtwo.html

::A::

[Peter Koch Larsen]

"E. Robert Tisdale" <E.************ **@jpl.nasa.gov > skrev i en meddelelse
news:d5******** **@nntp1.jpl.na sa.gov...

heted7 wrote:

Most of the books on C++ say something like this,
"A virtual destructor should be defined
if the class contains at least one virtual member function."
This is generally bad advice. Ignore it.

My question is, "Why is it only for the case
when the class contains at least one virtual member function?"
Shouldn't the destructor always be virtual whenever there's a
possibility
that an inherited object will be destructed through a base class pointer?
(This does not require, that the base class have any other virtual
function!)

That's simply not true. The destructor SHOULD be virtual unless you have a
compelling reason for it not being so. The only compelling reason i can
think of would be performance-related. So unless you profile your code and
decide that the overhead of the virtual function is excessive (and with a
good compiler, there should be no overhead if the type is statically known),
do keep that destructor virtual.
Don't define a virtual destructor unless you have a specific use for it.
Generally, C++ functions shouldn't be destroying objects passed to them
by reference (or by reference through a pointer).

Also I recommend that if you have a class with virtual functions and no
virtual destructor, you sohuld remove the new-operator for that class. If
the class is meant to be used by others, this is nearly a must.

/Peter

[E. Robert Tisdale]

Peter Koch Larsen wrote:

The destructor SHOULD be virtual
unless you have a compelling reason for it not being so.

Why?

[Pete Becker]

E. Robert Tisdale wrote:

Peter Koch Larsen wrote:

The destructor SHOULD be virtual
unless you have a compelling reason for it not being so.

Why?

To increase code bloat, of course.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)