Re: delete [] of Derived objects through Base object which has virtual dtor

From:

"Jim Langston" <tazmaster@rocketmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 29 Apr 2008 02:48:22 -0700

Message-ID:

<2aCRj.110$go5.17@newsfe02.lga>

James Kanze wrote:

On Apr 29, 4:32 am, "subramanian10...@yahoo.com, India"
<subramanian10...@yahoo.com> wrote:

Suppose

class Base
{
public:
virtual ~Test() { ... }
// ...
};

class Derived : public Base
{
public:
virtual ~Derived() { ... }
// ...
};

int main()
{
      Base* base_ptr = new Derived[10]();
      delete [] base_ptr;
      return EXIT_SUCCESS;
}

If the Base class dtor is not not virtual, 'delete []
base_ptr' has undefined behaviour.

Will 'delete [] base_ptr' call each Derived class dtor because
the Base::~Base() is virtual ? Is the deletion in the above
code valid ?. Or does this also invoke undefined behaviour ?

As Kai-Uwe has pointed out, it is undefined behavior. More
generally, although the compiler will tranquilly convert
Derived* to Base* even if Derived* points to an array (because
it cannot know this), the resulting pointer can only be used as
a pointer to the first individual object. In you case, for
example, no only is the delete[] undefined behavior, but any use
of base_ptr to access the allocated array (e.g. base_ptr[1])
would be as well.

In general, don't use array new; prefer std::vector. And don't
try to make array elements polymorphic; it doesn't work. (Array
elements are values, and polymorphism only works through
pointers or references. If you need an array of polymorphic
types, you must use std::vector< Base* >, allocating and
deallocating each one manually.)

I understand what you are saying, but I don't understand why. Why should a
pointer from a std::vector<Base*> be treated any different than a pointer
from Base*[] ? You state that array elements are values, but aren't the
members of containers values also? And don't most implementations of
std::vector hold their data in arrays?

It seems totally... non-intuitive and wrong to me. Can you perhaps point to
where in the standard this is stated? This is something I'm going to have
to get my head around, and right now it's just not doing it.

Thanks.

--
Jim Langston
tazmaster@rocketmail.com

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