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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

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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.