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4017. Behavior of std::views::split on an empty range

Section: 26.7.17.3 [range.split.iterator], 26.7.16.3 [range.lazy.split.outer] Status: New Submitter: David Stone Opened: 2023-11-19 Last modified: 2024-06-24

Priority: 3

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Discussion:

Consider the following example (which uses fmt::println instead of std::println, but they do the same thing in C++23):

#include <iostream>
#include <string>
#include <ranges>
#include <fmt/ranges.h>

int main()
{
  fmt::println("{}", std::views::split(std::string(" x "), ' '));
  fmt::println("{}", std::views::split(std::string(" "), ' '));
  fmt::println("{}", std::views::split(std::string("x"), ' '));
  fmt::println("{}", std::views::split(std::string(""), ' '));
}

The output of this program (as specified today) is

[[], ['x'], []]
[[], []]
[['x']]
[]

The principle set out in LWG 3478 is that splitting a sequence containing N delimiters should lead to N+1 subranges. That principle was broken if the N-th delimiter was at the end of the sequence, which was fixed by P2210.

However, the principle is still broken if the sequence contains zero delimiters. A non-empty sequence will split into one range, but an empty sequence will split into zero ranges. That last line is incorrect — splitting an empty range on a delimiter should yield a range of an empty range — not simply an empty range.

Proposed Resolution: Currently, split_view::iterator's constructor unconditionally initializes trailing_empty_ to false. Instead, change 26.7.17.3 [range.split.iterator]/1 to initialize trailing_empty_ to cur_ == next_.begin() (i.e. trailing_empty_ is typically false, but if we're empty on initialization then we have to have a trailing empty range).

The following demo shows Barry Revzin's implementation from P2210, adjusted to fix this: godbolt.org/z/axWb64j9f

[2024-03-11; Reflector poll]

Set priority to 3 after reflector poll.

[2024-03; Reflector comments]

"For split, we need to adjust the definition of end() for the common_range case (which may require introducing a new constructor to the iterator); right now it would compare ranges::end(base_) against a value-initialized iterator, which is not in the domain of ==. For lazy_split, we need to also change the non-forward overload."

"What should splitting an empty range on an empty pattern produce? Right now the behavior is that splitting a range of N > 0 elements with an empty pattern produces a range of N single-element ranges. I suppose you can argue that an empty pattern matches between adjacent elements but not at the start or end, so that an empty range, like a single-element range, contains 0 delimiters so should produce a range of one empty range. But it's also at least arguable that this should produce an empty range instead, so that we maintain the N element <-> N subrange and 1 element per subrange invariant.

Proposed resolution:

This wording is relative to N4964.

1. Modify 26.7.17.3 [range.split.iterator] as indicated:

   constexpr iterator(split_view& parent, iterator_t<V> current, subrange<iterator_t<V>> next);
   

   -1- Effects: Initializes parent_ with addressof(parent), cur_ with std::move(current), and next_ with std::move(next), and trailing_empty_ with cur_ == next_.begin().

2. Modify 26.7.16.3 [range.lazy.split.outer] as indicated:

   constexpr outer-iterator(Parent& parent, iterator_t<Base> current)
     requires forward_range<Base>;
   

   -3- Effects: Initializes parent_ with addressof(parent), and current_ with std::move(current), and trailing_empty_ with current_ == ranges::end(parent.base_).