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<title>Issue 3000: monotonic_memory_resource::do_is_equal uses dynamic_cast unnecessarily</title>
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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#C++20">C++20</a> status.</em></p>
<h3 id="3000"><a href="lwg-defects.html#3000">3000</a>. <code>monotonic_memory_resource::do_is_equal</code> uses <code>dynamic_cast</code> unnecessarily</h3>
<p><b>Section:</b> 20.5.6.3 <a href="https://wg21.link/mem.res.monotonic.buffer.mem">[mem.res.monotonic.buffer.mem]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Pablo Halpern <b>Opened:</b> 2017-07-14 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#mem.res.monotonic.buffer.mem">issues</a> in [mem.res.monotonic.buffer.mem].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Section [mem.res.monotonic.buffer.mem], paragraph 11 says
</p>
<blockquote>
<pre>
bool do_is_equal(const memory_resource&amp; other) const noexcept override;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>this == dynamic_cast&lt;const monotonic_buffer_resource*&gt;(&amp;other)</code>.
</p>
</blockquote>
</blockquote>
<p>
The <code>dynamic_cast</code> adds nothing of value. It is an incorrect cut-and-paste from an example <code>do_is_equal</code>
for a more complex resource.
</p>

<p><i>[2017-07-16, Tim Song comments]</i></p>

<p>
The pool resource classes appear to also have this issue.
</p>

<p><i>[2017-09-18, Casey Carter expands PR to cover the pool resources.]</i></p>


<strong>Previous resolution: [SUPERSEDED]</strong>

<blockquote class="note">
<ol>
<li><p>Edit 20.5.6.3 <a href="https://wg21.link/mem.res.monotonic.buffer.mem">[mem.res.monotonic.buffer.mem]</a> as indicated:</p>

<blockquote><pre>
bool do_is_equal(const memory_resource&amp; other) const noexcept override;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>this == <del>dynamic_cast&lt;const monotonic_buffer_resource*&gt;(</del>&amp;other<del>)</del></code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[
2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib.
]</i></p>

<p><i>[2018-3-17 Adopted in Jacksonville]</i></p>



<p id="res-3000"><b>Proposed resolution:</b></p>
<p>This resolution is relative to <a href="https://wg21.link/n4687">N4687</a>.</p>

<ol>
<li><p>Edit 20.5.5.4 <a href="https://wg21.link/mem.res.pool.mem">[mem.res.pool.mem]</a> as indicated:</p>

<blockquote><pre>
bool <del>synchronized_pool_resource::</del>do_is_equal(<ins>const memory_resource&amp; other) const noexcept override;</ins>
    <del>const memory_resource&amp; other) const noexcept override;</del>
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>this == <del>dynamic_cast&lt;const synchronized_pool_resource*&gt;(</del>&amp;other<del>)</del></code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Strike 20.5.5.4 <a href="https://wg21.link/mem.res.pool.mem">[mem.res.pool.mem]</a> paragraph 10, and the immediately preceding declaration of <code>unsynchronized_pool_resource::do_is_equal</code>.</p>
</li>

<li><p>Edit 20.5.6.3 <a href="https://wg21.link/mem.res.monotonic.buffer.mem">[mem.res.monotonic.buffer.mem]</a> as indicated:</p>

<blockquote><pre>
bool do_is_equal(const memory_resource&amp; other) const noexcept override;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>this == <del>dynamic_cast&lt;const monotonic_buffer_resource*&gt;(</del>&amp;other<del>)</del></code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>




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