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‎issue4000.html‎

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@@ -67,6 +67,8 @@ <h3 id="4000"><a href="lwg-active.html#4000">4000</a>. <code>flat_map::insert_ra
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-10-23 <b>Last modified:</b> 2024-02-22</p>
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<p><b>Priority: </b>3
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#flat.map.modifiers">active issues</a> in [flat.map.modifiers].</p>
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<p><b>View all other</b> <a href="lwg-index.html#flat.map.modifiers">issues</a> in [flat.map.modifiers].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>

‎issue4239.html‎

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Issue 4239: flat_map's transparent comparator no longer works for string literals</title>
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<meta property="og:title" content="Issue 4239: flat_map's transparent comparator no longer works for string literals">
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<meta property="og:description" content="C++ library issue. Status: New">
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<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4239.html">
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<meta property="og:type" content="website">
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<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>
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<h3 id="4239"><a href="lwg-active.html#4239">4239</a>. <code class='backtick'>flat_map</code>'s transparent comparator no longer works for string literals</h3>
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<p><b>Section:</b> 23.6.8.7 <a href="https://wg21.link/flat.map.modifiers">[flat.map.modifiers]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Hui Xie <b>Opened:</b> 2025-03-29 <b>Last modified:</b> 2025-03-29</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#flat.map.modifiers">active issues</a> in [flat.map.modifiers].</p>
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<p><b>View all other</b> <a href="lwg-index.html#flat.map.modifiers">issues</a> in [flat.map.modifiers].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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According to the spec, the following code should hard error
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</p>
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<blockquote><pre>
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std::flat_map&lt;std::string, int, std::less&lt;&gt;&gt; m;
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m.try_emplace("abc", 5); // <span style="color:red;font-weight:bolder">hard error</span>
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</pre></blockquote>
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<p>
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The reason is that we specify in 23.6.8.7 <a href="https://wg21.link/flat.map.modifiers">[flat.map.modifiers]</a> p21 the effect to be
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as if <code class='backtick'>ranges::upper_bound</code> is called.
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</p>
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<p>
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<code class='backtick'>ranges::upper_bound</code> requires <code class='backtick'>indirect_strict_weak_order</code>, which requires the comparator to be
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invocable for all combinations. In this case, it requires
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</p>
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<blockquote><pre>
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const char (&amp;)[4] &lt; const char (&amp;)[4]
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</pre></blockquote>
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<p>
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to be well-formed, which is no longer the case in C++26 after <a href="https://wg21.link/P2865R6">P2865R6</a>.
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<p/>
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We should just use <code class='backtick'>std::upper_bound</code> instead. libstdc++ already uses <code class='backtick'>std::upper_bound</code>.
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libc++ uses <code class='backtick'>ranges::upper_bound</code> but clang has not yet implemented <a href="https://wg21.link/P2865" title=" Remove Deprecated Array Comparisons from C++26">P2865</a>
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properly.
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</p>
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<p id="res-4239"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 23.6.8.7 <a href="https://wg21.link/flat.map.modifiers">[flat.map.modifiers]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class K, class... Args&gt;
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constexpr pair&lt;iterator, bool&gt; try_emplace(K&amp;&amp; k, Args&amp;&amp;... args);
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template&lt;class K, class... Args&gt;
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constexpr iterator try_emplace(const_iterator hint, K&amp;&amp; k, Args&amp;&amp;... args);
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</pre>
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<blockquote>
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<p>
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-19- <i>Constraints</i>: [&hellip;]
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<p/>
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-20- <i>Preconditions</i>: [&hellip;]
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<p/>
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-21- <i>Effects</i>: If the map already contains an element whose key is equivalent to <code class='backtick'>k</code>,
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<code class='backtick'>*this</code> and <code class='backtick'>args...</code> are unchanged. Otherwise equivalent to:
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</p>
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<blockquote><pre>
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auto key_it = <ins>upper_bound(<i>c</i>.keys.begin(), <i>c</i>.keys.end(), k, <i>compare</i>)</ins><del>ranges::upper_bound(<i>c</i>.keys, k, <i>compare</i>)</del>;
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auto value_it = <i>c</i>.values.begin() + distance(<i>c</i>.keys.begin(), key_it);
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<i>c</i>.keys.emplace(key_it, std::forward&lt;K&gt;(k));
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<i>c</i>.values.emplace(value_it, std::forward&lt;Args&gt;(args)...);
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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</body>
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</html>

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