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### 3017. `list splice` functions should use `addressof`

Section: 99 [forwardlist.ops], 24.3.10.5 [list.ops] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-09-05 Last modified: 2021-02-25

Priority: 0

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Discussion:

99 [forwardlist.ops] p1 and 24.3.10.5 [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 99 [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 24.3.10.5 [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 24.3.10.5 [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 (27.8 [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.