Re: Copying singular iterators

From:

"MathGis" <mhilferink@objectvision.nl>

Newsgroups:

comp.std.c++

Date:

Wed, 7 Mar 2007 08:52:27 CST

Message-ID:

<1173264063.203082.104120@s48g2000cws.googlegroups.com>

On Mar 7, 5:11 am, a...@acm.org ("Andrew Koenig") wrote:

"Sylvain Pion" <Sylvain.P...@sophia.inria.fr> wrote in message

news:esje06$r76$1@news-sop.inria.fr...

I am wondering why there is such a strict requirement on singular values
of iterators. Iterators are meant to be small pointer-like objects,
and I can't see any useful reason to disallow copies/assignments of
singular values (which would of course propagate the "singular-ness").

One reason is that copying an uninitialized pointer is undefined:

int* p;
int* q = p; // undefined behavior

So the requirement on singular iterator values is necessary in order to
allow pointers to be iterators.

However, I think the following code is legal:

  typedef int* int_ptr;
  int_ptr p = int_ptr(); // default ctor on 'built-in-type' assignes
NULL
  assert(p == NULL); // p has the 'singular' value for int_ptr
  int_ptr q = p; // the 'singular' value is copied. (q is copy
constructed)
  q = p; // now the value of p is also assigned to q (redundant, for
demonstration purposes only)
  for (; p != q; ++p) // NULL can be compared, but should not be
incremented
     do_something(*p); // this line should only be called with non-
singluar int_ptr values

so, why shouldn't the following be legal?

  list<int>::iterator p = list<int>::iterator(); // use default ctor
to assign the singular value
  list<int>::iterator q = p; // the 'singular' value is copied. (q is
copy constructed)
  q = p; // now the value of p is also assigned to q (redundant, for
demonstration purposes only)
  for (; p != q; ++p) // compare two singular values
     do_something(*p); // this line should only be called with non-
singluar iterator values

in my code I have been indicating empty ranges of T with default
constructed vector<T>::iterators and I was disappointed when moving to
VC8 (iterator debug mode) to find that the standard indeed prohibits
this. I still wonder why.

Maarten Hilferink
Object Vision
Netherlands

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