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3017. list splice functions should use addressof

Section: 23.3.7.6 [forward.list.ops], 23.3.9.5 [list.ops] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-09-05 Last modified: 2023-02-07

Priority: 0

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Discussion:

[forwardlist.ops] p1 and 23.3.9.5 [list.ops] p3 say &x != this, but should use addressof. We really need front matter saying that when the library says &x it means std::addressof(x).

[ 2017-11-03 Moved to Tentatively Ready after 9 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4687.

1. Edit [forwardlist.ops] p1 as indicated:

   void splice_after(const_iterator position, forward_list& x);
   void splice_after(const_iterator position, forward_list&& x);
   

   -1- Requires: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()). get_allocator() == x.get_allocator(). &addressof(x) != this.

2. Edit 23.3.9.5 [list.ops] p3 as indicated:

   void splice(const_iterator position, list& x);
   void splice(const_iterator position, list&& x);
   

   -3- Requires: &addressof(x) != this.

3. Edit 23.3.9.5 [list.ops] p3 as indicated:

   template <class Compare> void merge(list& x, Compare comp);
   template <class Compare> void merge(list&& x, Compare comp);
   

   -22- Requires: comp shall define a strict weak ordering (26.8 [alg.sorting]), and both the list and the argument list shall be sorted according to this ordering.

   -23- Effects: If (&addressof(x) == this) does nothing; otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()). The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i, in the range other than the first, the condition comp(*i, *(i - 1)) will be false. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

   -24- Remarks: Stable (16.4.6.8 [algorithm.stable]). If (&addressof(x) != this) the range [x.begin(), x.end()) is empty after the merge. No elements are copied by this operation. The behavior is undefined if get_allocator() != x.get_allocator().

   -25- Complexity: At most size() + x.size() - 1 applications of comp if (&addressof(x) != this); otherwise, no applications of comp are performed. If an exception is thrown other than by a comparison there are no effects.