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3880. Clarify operator+= complexity for {chunk,stride}_view::iterator

Section: 25.7.29.7 [range.chunk.fwd.iter], 25.7.32.3 [range.stride.iterator] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

The intent was that the precondition allows the call to ranges::advance, which otherwise would have time linear in the argument of operator+=, to actually be implemented using operator+= or equivalent for all but the last step. This is at best very non-obvious and should be clarified.

[Issaquah 2023-02-10; LWG issue processing]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.7.29.7 [range.chunk.fwd.iter] p13 as indicated:

   constexpr iterator& operator+=(difference_type x)
     requires random_access_range<Base>;
   

   -12- Preconditions: If x is positive, ranges::distance(current_, end_) > n_ * (x - 1) is true.

   [Note 1: If x is negative, the Effects paragraph implies a precondition. — end note]

   -13- Effects: Equivalent to:

   if (x > 0) {
     ranges::advance(current_, n_ * (x - 1));
     missing_ = ranges::advance(current_, n_ * x, end_);
   } else if (x < 0) {
     ranges::advance(current_, n_ * x + missing_);
     missing_ = 0;
   }
   return *this;
   

2. Modify 25.7.32.3 [range.stride.iterator] p14 as indicated:

   constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>;
   

   -13- Preconditions: If n is positive, ranges::distance(current_, end_) > stride_ * (n - 1) is true.

   [Note 1: If n is negative, the Effects paragraph implies a precondition. — end note]

   -14- Effects: Equivalent to:

   if (n > 0) {
     ranges::advance(current_, stride_ * (n - 1));
     missing_ = ranges::advance(current_, stride_ * n, end_);
   } else if (n < 0) {
     ranges::advance(current_, stride_ * n + missing_);
     missing_ = 0;
   }
   return *this;