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3032. ValueSwappable requirement missing for push_heap and make_heap

Section: 27.8.8 [alg.heap.operations] Status: WP Submitter: Robert Douglas Opened: 2017-11-08 Last modified: 2023-02-13

Priority: 3

View all other issues in [alg.heap.operations].

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Discussion:

In discussion of D0202R3 in Albuquerque, it was observed that pop_heap and sort_heap had constexpr removed for their requirement of ValueSwappable. It was then observed that push_heap and make_heap were not similarly marked as having the ValueSwappable requirement. The room believed this was likely a specification error, and asked to open an issue to track it.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 3; Marshall to investigate

Previous resolution [SUPERSEDED]:

This wording is relative to N4700.

1. Change 27.8.8.2 [push.heap] as indicated:

   template<class RandomAccessIterator>
     void push_heap(RandomAccessIterator first, RandomAccessIterator last);
   template<class RandomAccessIterator, class Compare>
     void push_heap(RandomAccessIterator first, RandomAccessIterator last,
                    Compare comp);
   

   -1- Requires: The range [first, last - 1) shall be a valid heap. RandomAccessIterator shall satisfy the requirements of ValueSwappable (16.4.4.3 [swappable.requirements]). The type of *first shall satisfy the MoveConstructible requirements (Table 23) and the MoveAssignable requirements (Table 25).

2. Change 27.8.8.4 [make.heap] as indicated:

   template<class RandomAccessIterator>
     void make_heap(RandomAccessIterator first, RandomAccessIterator last);
   template<class RandomAccessIterator, class Compare>
     void make_heap(RandomAccessIterator first, RandomAccessIterator last,
                    Compare comp);
   

   -1- Requires: RandomAccessIterator shall satisfy the requirements of ValueSwappable (16.4.4.3 [swappable.requirements]). The type of *first shall satisfy the MoveConstructible requirements (Table 23) and the MoveAssignable requirements (Table 25).

[2022-11-06; Daniel comments and syncs wording with recent working draft]

For reference, the finally accepted paper was P0202R3 and the constexpr-ification of swap-related algorithms had been realized later by P0879R0 after resolution of CWG 1581 and more importantly CWG 1330.

[Kona 2022-11-09; Move to Ready]

[2023-02-13 Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Change 27.8.8.2 [push.heap] as indicated:

   template<class RandomAccessIterator>
     constexpr void push_heap(RandomAccessIterator first, RandomAccessIterator last);
   
   template<class RandomAccessIterator, class Compare>
     constexpr void push_heap(RandomAccessIterator first, RandomAccessIterator last,
                              Compare comp);
                    
   template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
            class Proj = identity>
     requires sortable<I, Comp, Proj>
     constexpr I
       ranges::push_heap(I first, S last, Comp comp = {}, Proj proj = {});
   template<random_access_range R, class Comp = ranges::less, class Proj = identity>
     requires sortable<iterator_t<R>, Comp, Proj>
     constexpr borrowed_iterator_t<R>
       ranges::push_heap(R&& r, Comp comp = {}, Proj proj = {});
   

   -1- Let comp be less{} and proj be identity{} for the overloads with no parameters by those names.

   -2- Preconditions: The range [first, last - 1) is a valid heap with respect to comp and proj. For the overloads in namespace std, RandomAccessIterator meets the Cpp17ValueSwappable requirements (16.4.4.3 [swappable.requirements]) and the type of *first meets the Cpp17MoveConstructible requirements (Table 32) and the Cpp17MoveAssignable requirements (Table 34).

   -3- Effects: Places the value in the location last - 1 into the resulting heap [first, last).

   -4- Returns: last for the overloads in namespace ranges.

   -5- Complexity: At most log(last - first) comparisons and twice as many projections.

2. Change 27.8.8.4 [make.heap] as indicated:

   template<class RandomAccessIterator>
     constexpr void make_heap(RandomAccessIterator first, RandomAccessIterator last);
   
   template<class RandomAccessIterator, class Compare>
     constexpr void make_heap(RandomAccessIterator first, RandomAccessIterator last,
                              Compare comp);
                              
   template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
            class Proj = identity>
     requires sortable<I, Comp, Proj>
     constexpr I
       ranges::make_heap(I first, S last, Comp comp = {}, Proj proj = {});
   template<random_access_range R, class Comp = ranges::less, class Proj = identity>
     requires sortable<iterator_t<R>, Comp, Proj>
     constexpr borrowed_iterator_t<R>
       ranges::make_heap(R&& r, Comp comp = {}, Proj proj = {});
   

   -1- Let comp be less{} and proj be identity{} for the overloads with no parameters by those names.

   -2- Preconditions: For the overloads in namespace std, RandomAccessIterator meets the Cpp17ValueSwappable requirements (16.4.4.3 [swappable.requirements]) and the type of *first meets the Cpp17MoveConstructible (Table 32) and Cpp17MoveAssignable (Table 34) requirements.

   -3- Effects: Constructs a heap with respect to comp and proj out of the range [first, last).

   -4- Returns: last for the overloads in namespace ranges.

   -5- Complexity: At most 3(last - first) comparisons and twice as many projections.