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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>
<h3 id="4295"><a href="lwg-active.html#4295">4295</a>. [fund.ts.v3] <code class='backtick'>experimental::observer_ptr</code> should have more constexpr</h3>
<p><b>Section:</b> 8.2.6 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#memory.observer.ptr.special">[fund.ts.v3::memory.observer.ptr.special]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-07-14 <b>Last modified:</b> 2025-08-26</p>
<p><b>Priority: </b>4
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In the Library Fundamentals TS, the <code class='backtick'>swap</code> overload, <code class='backtick'>make_observer_ptr</code>
function, and comparisons for <code class='backtick'>observer_ptr</code> could be constexpr, but are not.
The member <code class='backtick'>swap</code> is already constexpr in the TS, and the non-member <code class='backtick'>swap</code>
is constexpr in libc++ but not the comparisons. The proposed resolution has
been implemented and tested in libstdc++.
</p>
<p>
If we ever rebase the TS on a new C++ standard (or put <code class='backtick'>observer_ptr</code> into
the standard) the comparisons should all be updated like so:
</p>

<blockquote>
<pre><code>
template &lt;class W1, class W2&gt;
<ins>constexpr</ins> bool operator==(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
<i>Returns</i>: <code class='backtick'>p1.get() == p2.get()</code>.
</blockquote>

<pre><code><del>bool operator!=(...);</del></code></pre>

<pre><code>
template &lt;class W&gt;
<ins>constexpr</ins> bool operator==(observer_ptr&lt;W&gt; p, nullptr_t) noexcept;
<del>bool operator==(...);</del>
</code></pre>
<blockquote>
<i>Returns</i>: <code class='backtick'>not p</code>.
</blockquote>

<pre><code><del>bool operator!=(...);
bool operator!=(...);</del></code></pre>

<pre><code>
template &lt;class W1, class W2&gt;
<ins>constexpr</ins> bool operator&lt;<ins>=&gt;</ins>(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
<i>Returns</i>:
    <code><del>less&lt;W3&gt;</del><ins>compare_three_way</ins>()(p1.get(), p2.get())</code><del>,
    where <code class='backtick'>W3</code> is the composite pointer type (C++20 §7) of <code class='backtick'>W1*</code> and <code class='backtick'>W2*</code></del>.
</blockquote>

<pre><code><del>bool operator&gt;(...);
bool operator&lt;=(...);
bool operator&gt;=(...);</del></code></pre>
</blockquote>


<p><i>[2025-08-21; Reflector poll]</i></p>

<p>
Set priority to 4 after reflector poll.
Seven votes for P0 but one vote for NAD, so not made Tentatively Ready.
</p>



<p id="res-4295"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4939" title=" Working Draft, C++ Extensions for Library Fundamentals, Version 3">N4939</a>.
</p>

<ol>
<li>
Modify 8.1 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#memory.syn">[fund.ts.v3::memory.syn]</a> as indicated:
<blockquote>
<pre><code>
#include &lt;memory&gt;

namespace std {
  namespace experimental::inline fundamentals_v3 {

    <i>// 8.2, Non-owning (observer) pointers</i>
    template &lt;class W&gt; class observer_ptr;

    <i>// 8.2.6, observer_ptr specialized algorithms</i>
    template &lt;class W&gt;
    <ins>constexpr</ins> void swap(observer_ptr&lt;W&gt;&amp;, observer_ptr&lt;W&gt;&amp;) noexcept;
    template &lt;class W&gt;
    <ins>constexpr</ins> observer_ptr&lt;W&gt; make_observer(W*) noexcept;
    <i>// (in)equality operators</i>
    template &lt;class W1, class W2&gt;
    <ins>constexpr</ins> bool operator==(observer_ptr&lt;W1&gt;, observer_ptr&lt;W2&gt;);

    template &lt;class W1, class W2&gt;
    <ins>constexpr</ins> bool operator!=(observer_ptr&lt;W1&gt;, observer_ptr&lt;W2&gt;);
    template &lt;class W&gt;
    <ins>constexpr</ins> bool operator==(observer_ptr&lt;W&gt;, nullptr_t) noexcept;
    template &lt;class W&gt;
    <ins>constexpr</ins> bool operator!=(observer_ptr&lt;W&gt;, nullptr_t) noexcept;
    template &lt;class W&gt;
    <ins>constexpr</ins> bool operator==(nullptr_t, observer_ptr&lt;W&gt;) noexcept;
    template &lt;class W&gt;
    <ins>constexpr</ins> bool operator!=(nullptr_t, observer_ptr&lt;W&gt;) noexcept;
    <i>// ordering operators</i>
    template &lt;class W1, class W2&gt;
    <ins>constexpr</ins> bool operator&lt;(observer_ptr&lt;W1&gt;, observer_ptr&lt;W2&gt;);
    template &lt;class W1, class W2&gt;
    <ins>constexpr</ins> bool operator&gt;(observer_ptr&lt;W1&gt;, observer_ptr&lt;W2&gt;);
    template &lt;class W1, class W2&gt;
    <ins>constexpr</ins> bool operator&lt;=(observer_ptr&lt;W1&gt;, observer_ptr&lt;W2&gt;);
    template &lt;class W1, class W2&gt;
    <ins>constexpr</ins> bool operator&gt;=(observer_ptr&lt;W1&gt;, observer_ptr&lt;W2&gt;);

  } <i>// namespace experimental::inline fundamentals_v3</i>

  <i>// 8.2.7, observer_ptr hash support</i>
  template &lt;class T&gt; struct hash;
  template &lt;class T&gt; struct hash&lt;experimental::observer_ptr&lt;T&gt;&gt;;

} <i>// namespace std</i>
</code></pre>
</blockquote>
</li>

<li>
Modify 8.2.6 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#memory.observer.ptr.special">[fund.ts.v3::memory.observer.ptr.special]</a> as indicated:
<blockquote>


<pre><code>
template &lt;class W&gt;
  <ins>constexpr</ins> void swap(observer_ptr&lt;W&gt;&amp; p1, observer_ptr&lt;W&gt;&amp; p2) noexcept;
</code></pre>
<blockquote>
-2- <i>Effects</i>: <code class='backtick'>p1.swap(p2)</code>.
</blockquote>

<pre><code>
template &lt;class W&gt; <ins>constexpr</ins> observer_ptr&lt;W&gt; make_observer(W* p) noexcept;
</code></pre>
<blockquote>
-4- <i>Returns</i>: <code>observer_ptr&lt;W&gt;{p}</code>.
</blockquote>


<pre><code>
template &lt;class W1, class W2&gt;
  <ins>constexpr</ins> bool operator==(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
-6- <i>Returns</i>: <code class='backtick'>p1.get() == p2.get()</code>.
</blockquote>

<pre><code>
template &lt;class W1, class W2&gt;
  <ins>constexpr</ins> bool operator!=(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
-8- <i>Returns</i>: <code class='backtick'>not (p1 == p2)</code>.
</blockquote>

<pre><code>
template &lt;class W&gt;
  <ins>constexpr</ins> bool operator==(observer_ptr&lt;W&gt; p, nullptr_t) noexcept;
template &lt;class W&gt;
  <ins>constexpr</ins> bool operator==(nullptr_t, observer_ptr&lt;W&gt; p) noexcept;
</code></pre>
<blockquote>
-10- <i>Returns</i>: <code class='backtick'>not p</code>.
</blockquote>

<pre><code>
template &lt;class W&gt;
  <ins>constexpr</ins> bool operator!=(observer_ptr&lt;W&gt; p, nullptr_t) noexcept;
template &lt;class W&gt;
  <ins>constexpr</ins> bool operator!=(nullptr_t, observer_ptr&lt;W&gt; p) noexcept;
</code></pre>
<blockquote>
-12- <i>Returns</i>: <code class='backtick'>(bool)p</code>.
</blockquote>

<pre><code>
template &lt;class W1, class W2&gt;
  <ins>constexpr</ins> bool operator&lt;(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
-14- <i>Returns</i>:
    <code>less&lt;W3&gt;()(p1.get(), p2.get())</code>,
    where <code class='backtick'>W3</code> is the composite pointer type (C++20 §7) of <code class='backtick'>W1*</code> and <code class='backtick'>W2*</code>.
</blockquote>

<pre><code>
template &lt;class W1, class W2&gt;
  <ins>constexpr</ins> bool operator&gt;(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
-16- <i>Returns</i>: <code>p2 &lt; p1</code>.
</blockquote>

<pre><code>
template &lt;class W1, class W2&gt;
  <ins>constexpr</ins> bool operator&lt;=(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
-16- <i>Returns</i>: <code>not (p2 &lt; p1)</code>.
</blockquote>

<pre><code>
template &lt;class W1, class W2&gt;
  <ins>constexpr</ins> bool operator&gt;=(observer_ptr&lt;W1&gt; p1, observer_ptr&lt;W2&gt; p2);
</code></pre>
<blockquote>
-16- <i>Returns</i>: <code>not (p1 &lt; p2)</code>.
</blockquote>


</blockquote>
</li>
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