TITLE: deletion and constness

(Newsgroups: comp.lang.c++, 11 Aug 98)


NARAN: Siemel Naran

> "char const*" means that you promise not to change *x, the stuff that x
> points to.  Deleting x means changing changing x.  So it's illegal.  But
> const_cast removes constness, changing "char const*" to "char*", and now
> the delete is legal.


BECKER: Pete Becker <petebecker@acm.org>


There was a time when this was the rule. It was such a pain that it was
changed:

	[Note: a pointer to a const type can be the operand of a
	delete­expression; it is not necessary to cast away the
	constness (5.2.11) of the pointer xpression before it is
	used as the operand of the delete­expression. ]

NARAN:

> If the memory for x was allocated in the constructor with a call to new[],
> then I think the const_cast is well defined and delete[] properly delete
> the memory.  If the memory for x was allocated by another object -- that
> is, this object does not own x but just received a pointer to x from
> somewhere else -- then the const_cast should result in undefined behaviour.
> This is what makes sense to me, because constness can only be casted
> away by the object that owns the constness (or at least, this is how it
> should be).  But can someone please clarify this?

BECKER:

Be careful to distinguish between a const object and a const access
path. For example:

const char text[] = "Hello, world!";
const char *ptr = text.
*const_cast<char*>(ptr) = 'a';	// trouble: text[0] not modifiable

Here, text contains const char elements. They are not permitted to be
modified. Casting away const to turn ptr into a pointer to non-const
char results in undefined behavior.

char text[] = "Hello, world!";
const char *ptr = text;
*const_cast<char*>(ptr) = 'a';	// OK

Here, text contains modifiable char elements. You can cast away the
const in ptr and modify elements of text, because those elements are in
fact modifiable.