This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++11 status.

2042. Comparing `forward_list::before_begin()` to `forward_list::end()`

Section: 23.3.7.3 [forward.list.iter] Status: C++11 Submitter: Joe Gottman Opened: 2011-03-13 Last modified: 2023-02-07

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

For an object c of type forward_list<X, Alloc>, the iterators c.before_begin() and c.end() are part of the same underlying sequence, so the expression c.before_begin() == c.end() must be well-defined. But the standard says nothing about what the result of this expression should be. The forward iterator requirements says no dereferenceable iterator is equal to a non-dereferenceable iterator and that two dereferenceable iterators are equal if and only if they point to the same element. But since before_begin() and end() are both non-dereferenceable, neither of these rules applies.

Many forward_list methods, such as insert_after(), have a precondition that the iterator passed to them must not be equal to end(). Thus, user code might look like the following:

void foo(forward_list<int>& c, forward_list<int>::iterator it)
{
  assert(it != c.end());
  c.insert_after(it, 42);
}

Conversely, before_begin() was specifically designed to be used with methods like insert_after(), so if c.before_begin() is passed to this function the assertion must not fail.

[2011-03-14: Daniel comments and updates the suggested wording]

The suggested wording changes are necessary but not sufficient. Since there does not exist an equivalent semantic definition of cbefore_begin() as we have for cbegin(), this still leaves the question open whether the normative remark applies to cbefore_begin() as well. A simple fix is to define the operational semantics of cbefore_begin() in terms of before_begin().

[2011-03-24 Madrid meeting]

General agreement that this is a serious bug.

Pablo: Any objections to moving 2042 to Immediate?

No objections.

Proposed resolution:

Add to the definition of forward_list::before_begin() [forwardlist.iter] the following:

iterator before_begin();
const_iterator before_begin() const;
const_iterator cbefore_begin() const;
-1- Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by begin().
-?- Effects: cbefore_begin() is equivalent to const_cast<forward_list const&>(*this).before_begin().
-?- Remarks: before_begin() == end() shall equal false.

pFad - Phonifier reborn

Note: This service is not intended for secure transactions such as banking, social media, email, or purchasing. Use at your own risk. We assume no liability whatsoever for broken pages.

Alternative Proxies:

2042. Comparing forward_list::before_begin() to forward_list::end()

Pfad - The Proxy pFad of © 2024 Garber Painting. All rights reserved.

2042. Comparing `forward_list::before_begin()` to `forward_list::end()`