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| <meta charset="utf-8"> | ||
| <title>Issue 4017: Behavior of std::views::split on an empty range</title> | ||
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| <hr> | ||
| <p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p> | ||
| <h3><a name="4017" href="lwg-active.html#4017">4017</a>. Behavior of <tt>std::views::split</tt> on an empty range</h3> | ||
| <p><b>Section:</b> 26.7.17.3 <a href="https://wg21.link/range.split.iterator">[range.split.iterator]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a> | ||
| <b>Submitter:</b> Barry Revzin <b>Opened:</b> 2023-11-19 <b>Last modified:</b> 2023-11-25</p> | ||
| <p><b>Priority: </b>Not Prioritized | ||
| </p> | ||
| <p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p> | ||
| <p><b>Discussion:</b></p> | ||
| <p> | ||
| Consider the following example (which uses <tt>fmt::println</tt> instead of <tt>std::println</tt>, | ||
| but they do the same thing in C++23): | ||
| </p> | ||
| <blockquote><pre> | ||
| #include <iostream> | ||
| #include <string> | ||
| #include <ranges> | ||
| #include <fmt/ranges.h> | ||
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||
| 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(""), ' ')); | ||
| } | ||
| </pre></blockquote> | ||
| <p> | ||
| The output of this program (as specified today) is | ||
| </p> | ||
| <blockquote><pre> | ||
| [[], ['x'], []] | ||
| [[], []] | ||
| [['x']] | ||
| [] | ||
| </pre></blockquote> | ||
| <p> | ||
| The principle set out in LWG <a href="lwg-defects.html#3478">3478</a> is that splitting a sequence containing <tt>N</tt> | ||
| delimiters should lead to <tt>N+1</tt> subranges. That principle was broken if the <tt>N</tt>-th | ||
| delimiter was at the end of the sequence, which was fixed by <a href="https://wg21.link/P2210">P2210</a>. | ||
| <p/> | ||
| 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. | ||
| <p/> | ||
| Proposed Resolution: Currently, <tt>split_view::iterator</tt>'s constructor unconditionally initializes | ||
| <tt><i>trailing_empty_</i></tt> to <tt>false</tt>. Instead, change 26.7.17.3 <a href="https://wg21.link/range.split.iterator">[range.split.iterator]</a>/1 | ||
| to initialize <tt><i>trailing_empty_</i></tt> to <tt><i>cur_</i> == <i>next_</i>.begin()</tt> (i.e. | ||
| <tt><i>trailing_empty_</i></tt> is typically <tt>false</tt>, but if we're empty on initialization then we | ||
| have to have a trailing empty range). | ||
| <p/> | ||
| The following demo shows my implementation from <a href="https://wg21.link/P2210">P2210</a>, adjusted to fix this: | ||
| <a href="https://godbolt.org/z/axWb64j9f">godbolt.org/z/axWb64j9f</a> | ||
| </p> | ||
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| <p id="res-4017"><b>Proposed resolution:</b></p> | ||
| <p> | ||
| This wording is relative to <a href="https://wg21.link/N4964">N4964</a>. | ||
| </p> | ||
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| <ol> | ||
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| <li><p>Modify 26.7.17.3 <a href="https://wg21.link/range.split.iterator">[range.split.iterator]</a> as indicated:</p> | ||
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| <blockquote> | ||
| <pre> | ||
| constexpr <i>iterator</i>(split_view& parent, iterator_t<V> current, subrange<iterator_t<V>> next); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -1- <i>Effects</i>: Initializes <tt><i>parent_</i></tt> with <tt>addressof(parent)</tt>, <tt><i>cur_</i></tt> with | ||
| <tt>std::move(current)</tt>, <del>and</del> <tt><i>next_</i></tt> with <tt>std::move(next)</tt><ins>, and | ||
| <tt><i>trailing_empty_</i></tt> with <tt><i>cur_</i> == <i>next_</i>.begin()</tt></ins>. | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </body> | ||
| </html> |
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