Re: Why not reject the dynamic instantiation of a class with non-virtual destructor?

From:

Alberto Ganesh Barbati <AlbertoBarbati@libero.it>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 30 Jun 2008 09:02:37 CST

Message-ID:

<NS0ak.107522$FR.362495@twister1.libero.it>

Angel Tsankov ha scritto:

Hi,

Why not treat as erroneous a new expression that creates an instance of a
class which has a (direct or indirect) base class with a non-virtual
destructor and require implementations issue a diagnostic message? This
would minimize the chances for resource leaks and for undefined behaviour
in
C++ programs.

While it may help protect a programmer from himself, that's not a good
enough reason IMHO. Tools that do static analysis can detect this
kind of problem, perhaps including your compiler when enabling certain
warnings (and I'll admit it may very well be useful if enabled from
time to time.)

That said, there several reasons to not force this rule change,
including:

1) It would break inordinate amounts of existing, completely correct
   code.

2) It would be preventing valid use-cases where objects are NOT used
   or deleted polymorphically. (The pointer may be stored in pointers
   of the dynamic type of the object, so deleting it is fine, or even
   if they are converted to base pointers, that doesn't mean they'll
   be deleted through those pointers.)

Could you provide a citation from the Standard that says it is perfectly
fine to delete an object with a non-virtual destructor through a pointer to
its dynamic type?

5.3.5/3 gives a condition for UB when the static type and dynamic type
are different:

"[...] if the static type of the operand is different from its dynamic
type, the static type shall be a base class of the operand?s dynamic
type and the static type shall have a virtual destructor or the behavior
is undefined."

If the static type and the dynamic type are the same, with the only
exception of incomplete types (5.3.5/5), 5.3.5/6 describes the effect:

"If the value of the operand of the delete-expression is not a null
pointer value, the delete-expression will invoke the destructor (if any)
for the object or the elements of the array being deleted. [...]"

HTH,

Ganesh

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