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‎issue3778.html‎

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@@ -67,6 +67,7 @@ <h3 id="3778"><a href="lwg-defects.html#3778">3778</a>. <code>vector&lt;bool&gt;
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<b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2022-09-13 <b>Last modified:</b> 2023-11-22</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all other</b> <a href="lwg-index.html#vector.bool.pspc">issues</a> in [vector.bool.pspc].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>

‎issue4228.html‎

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<!DOCTYPE html>
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<html lang="en">
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<title>Issue 4228: Does vector&lt;bool, Allocator&gt; mandate that Allocator::value_type is bool?</title>
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<meta property="og:title" content="Issue 4228: Does vector&lt;bool, Allocator&gt; mandate that Allocator::value_type is bool?">
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<meta property="og:description" content="C++ library issue. Status: New">
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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="4228"><a href="lwg-active.html#4228">4228</a>. Does <code>vector&lt;bool, Allocator&gt;</code> mandate that <code class='backtick'>Allocator::value_type</code> is <code class='backtick'>bool</code>?</h3>
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<p><b>Section:</b> 23.3.12.1 <a href="https://wg21.link/vector.bool.pspc">[vector.bool.pspc]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2025-03-18 <b>Last modified:</b> 2025-03-22</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all other</b> <a href="lwg-index.html#vector.bool.pspc">issues</a> in [vector.bool.pspc].</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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<a href="https://wg21.link/N5008">N5008</a> 23.3.12.1 <a href="https://wg21.link/vector.bool.pspc">[vector.bool.pspc]</a>/2 says:
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</p>
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<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
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<p>
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Unless described below, all operations have the same requirements and semantics as the primary <code class='backtick'>vector</code> template,
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except that operations dealing with the <code class='backtick'>bool</code> value type map to bit values in the container storage and
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<code class='backtick'>allocator_traits::construct</code> (20.2.9.3 <a href="https://wg21.link/allocator.traits.members">[allocator.traits.members]</a>) is not used to construct these values.
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</p>
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</blockquote>
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<p>
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23.2.2.5 <a href="https://wg21.link/container.alloc.reqmts">[container.alloc.reqmts]</a>/5 says:
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</p>
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<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
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<p>
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<i>Mandates</i>: <code class='backtick'>allocator_type::value_type</code> is the same as <code class='backtick'>X::value_type</code>.
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</p>
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</blockquote>
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<p>
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Is <code>vector&lt;bool, allocator&lt;int&gt;&gt;</code> forbidden? There's implementation divergence:
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MSVC's STL enforces the mandate, while libc++ and libstdc++ accept this code, discovered while
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running libc++'s tests with MSVC's STL.
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<p/>
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Preferred resolution: I would be satisfied with resolving this as NAD, with a record that LWG believes
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that "all operations have the same requirements" means that the <i>Mandate</i> applies. I suppose that
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an editorial note could also be added to this effect, although I don't believe it's necessary.
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<p/>
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Alternate resolution: If LWG believes that the <i>Mandate</i> does not apply, and that <code>vector&lt;bool&gt;</code>
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should be unique among containers in accepting <code>allocator&lt;Anything&gt;</code>, then I believe that
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a normative sentence should be added to 23.3.12.1 <a href="https://wg21.link/vector.bool.pspc">[vector.bool.pspc]</a>/2, specifically creating an exemption
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to 23.2.2.5 <a href="https://wg21.link/container.alloc.reqmts">[container.alloc.reqmts]</a>/5.
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</p>
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<p id="res-4228"><b>Proposed resolution:</b></p>
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</body>
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</html>

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