Re: delete [] of Derived objects through Base object which has
virtual dtor
This is actually a response to the post you're responding to,
but since I cannot see it...
Jim Langston wrote:
Jim Langston wrote:
James Kanze wrote:
On Apr 29, 4:32 am, "subramanian10...@yahoo.com, India"
<subramanian10...@yahoo.com> wrote:
Suppose
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?
Think about how you index into an array. And what pointer
arithmetic (which is how indexing is implemented) would mean if
you had to take into account the dynamic size. How would you
find p[1] without knowing the size of p[0]? And in the case of
p[2], what should be multiplied by 2, if p[0] and p[1] have
different sizes.
The problem here in C++ (which it inherits from C) is that even
at the user level, it makes the pointer arithmetic evident, and
doesn't distinguish between pointers to a single object, and
pointers to the first element in an array. Some of the things
you can do with a pointer (i.e. convert from Derived* to Base*)
only make sense for single objects, and other (anything
involving pointer arithmetic) only makes sense if the pointer is
in fact the address of an array.
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.
I found it. 5.3.5.3
Quote: In the first alternative (delete object), 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. In the second alternative (delete
array) if the dynamic type of the object to be deleted differs
from its static type, the behavior is undefined.
That covers the delete. Something like:
struct Base
{
virtual ~Base() {}
int i ; // Take up some room.
virtual void function() ;
}
struct Derived : Base
{
int j ; // Ensure that Derived is bigger than
// Base.
virtual void function() ;
} ;
Base* p = new Derived[ 20 ] ;
++ p ;
p->function() ;
is also illegal. It can't work; there's no way to implement it
so that it does work (within the usual C++ memory model). But
finding why in the standard... It's very, very indirect, but I
suppose that the definition of pointer addition (++ is defined
in terms of addition) would cover it: "When an expression that
has integral type is added to or subtracted from a pointer, the
result has the type of the pointer operand. If the pointer
operand points to an element of an array object,[...]" In this
case, the pointer points to a Base subobject, and that Base
subobject is NOT an element of an array object. (The elements
of the array object are all Derived.) I'd prefer something more
explicit, but I think that the intent is clear.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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