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Updating API
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‎3394_annotations/annotations.md‎

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@@ -615,15 +615,7 @@ namespace std::meta {
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// [meta.reflection.annotation], annotation reflection
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+ consteval vector<info> annotations_of(info item);
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+ consteval vector<info> annotations_of(info item, info type);
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+
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+ template<class T>
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+ consteval optional<T> annotation_of(info item);
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+
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+ template<class T>
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+ consteval bool has_annotation(info item);
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+ template<class T>
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+ consteval bool has_annotation(info item, T const& value);
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+ consteval vector<info> annotations_of_with_type(info item, info type);
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+
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+ consteval info annotate(info item, info value, source_location loc = source_location::current());
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}

‎3394_annotations/d3394r2.html‎

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<meta charset="utf-8" />
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<meta name="generator" content="mpark/wg21" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
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<meta name="dcterms.date" content="2025-01-29" />
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<meta name="dcterms.date" content="2025-02-12" />
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<title>Annotations for Reflection</title>
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<style>
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code{white-space: pre-wrap;}
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</tr>
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<tr>
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<td>Date:</td>
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<td>2025-01-29</td>
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<td>2025-02-12</td>
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</tr>
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<tr>
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<td style="vertical-align:top">Project:</td>
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</div>
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<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span>
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Revision History<a href="#revision-history" class="self-link"></a></h1>
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<p>Since <span class="citation" data-cites="P3394R1">[<a href="https://wg21.link/p3394r1" role="doc-biblioref">P3394R1</a>]</span>:</p>
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<p>Since <span class="citation" data-cites="P3394R1"><a href="https://wg21.link/p3394r1" role="doc-biblioref">[P3394R1]</a></span>:</p>
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<ul>
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<li>loosened the P2996 scope restrictions around injected declarations:
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allow the definition of an entity to produce injected declaration of
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itself</li>
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</ul>
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<p>Since <span class="citation" data-cites="P3394R0">[<a href="https://wg21.link/p3394r0" role="doc-biblioref">P3394R0</a>]</span>:</p>
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<p>Since <span class="citation" data-cites="P3394R0"><a href="https://wg21.link/p3394r0" role="doc-biblioref">[P3394R0]</a></span>:</p>
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<ul>
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<li>added wording</li>
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</ul>
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<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span>
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Introduction<a href="#introduction" class="self-link"></a></h1>
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<p>Ever since writing <span class="title"><span class="citation" data-cites="P1240R0">[<a href="https://wg21.link/p1240r0" role="doc-biblioref">P1240R0</a>]</span></span>, but more so since <span class="title"><span class="citation" data-cites="P2996R0">[<a href="https://wg21.link/p2996r0" role="doc-biblioref">P2996R0</a>]</span></span>, we have been requested
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to add a capability to annotate declarations in a way that reflection
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can observe. For example, Jeremy Ong presented compelling arguments in a
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<p>Ever since writing <span class="title"><span class="citation" data-cites="P1240R0"><a href="https://wg21.link/p1240r0" role="doc-biblioref">[P1240R0] (Scalable Reflection in
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C++)</a></span></span>, but more so since <span class="title"><span class="citation" data-cites="P2996R0"><a href="https://wg21.link/p2996r0" role="doc-biblioref">[P2996R0]
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(Reflection for C++26)</a></span></span>, we have been requested to add
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a capability to annotate declarations in a way that reflection can
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observe. For example, Jeremy Ong presented compelling arguments in a
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post to the <a href="https://lists.isocpp.org/sg7/2023/10/0450.php">SG7
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reflector</a>. Corentin Jabot also noticed the need while P1240 was
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evolving and wrote <span class="title"><span class="citation" data-cites="P1887R0">[<a href="https://wg21.link/p1887r0" role="doc-biblioref">P1887R0</a>]</span></span>, which proposes syntax
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not entirely unlike what we present here.</p>
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evolving and wrote <span class="title"><span class="citation" data-cites="P1887R0"><a href="https://wg21.link/p1887r0" role="doc-biblioref">[P1887R0] (Typesafe Reflection on
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attributes)</a></span></span>, which proposes syntax not entirely unlike
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what we present here.</p>
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<p>In early versions of P2996 (and P1240 before that), a workaround was
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to encode properties in the template arguments of alias template
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specializations:</p>
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class is complete. Before completeness allows the class to observe the
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annotation during instantiation. After completeness allows the
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annotation callback to observe properties of the type. In some sense,
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Herb Sutter’s <span class="title"><span class="citation" data-cites="P0707R4">[<a href="https://wg21.link/p0707r4" role="doc-biblioref">P0707R4</a>]</span></span> was adding annotations
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on classes, invoked on class completeness, that allow mutation of the
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class.</p>
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Herb Sutter’s <span class="title"><span class="citation" data-cites="P0707R4"><a href="https://wg21.link/p0707r4" role="doc-biblioref">[P0707R4] (Metaclasses: Generative
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C++)</a></span></span> was adding annotations on classes, invoked on
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class completeness, that allow mutation of the class.</p>
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<p>One concrete, simpler example. We can, with this proposal as-is,
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create a <code class="sourceCode cpp">Debug</code> annotation that a
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user can add to their type and a specialization of <code class="sourceCode cpp">std<span class="op">::</span>formatter</code> for
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<code class="sourceCode cpp">Point</code>, inject an explicit
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specialization of <code class="sourceCode cpp">std<span class="op">::</span>formatter</code>.
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This would rely both on the ability for the annotation to be called back
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and language support for such injection (see <span class="title"><span class="citation" data-cites="P3294R2">[<a href="https://wg21.link/p3294r2" role="doc-biblioref">P3294R2</a>]</span></span>).</p>
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and language support for such injection (see <span class="title"><span class="citation" data-cites="P3294R2"><a href="https://wg21.link/p3294r2" role="doc-biblioref">[P3294R2] (Code
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Injection with Token Sequences)</a></span></span>).</p>
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<p>There are still open questions as to how to handle such callbacks.
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Does an annotation that gets called back merit different syntax from an
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annotation that doesn’t? Can it mutate the entity that it is attached
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be invoked when an entity is first declared with an annotation of type
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<code class="sourceCode cpp">X</code>).</p>
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<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">5</span> Wording<a href="#wording" class="self-link"></a></h1>
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<p>The wording is relative to <span class="citation" data-cites="P2996R9">[<a href="https://wg21.link/p2996r9" role="doc-biblioref">P2996R9</a>]</span>.</p>
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<p>The wording is relative to <span class="citation" data-cites="P2996R9"><a href="https://wg21.link/p2996r9" role="doc-biblioref">[P2996R9]</a></span>.</p>
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<h2 data-number="5.1" id="language"><span class="header-section-number">5.1</span> Language<a href="#language" class="self-link"></a></h2>
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<p>Change <span>6.8.2 <a href="https://wg21.link/basic.fundamental">[basic.fundamental]</a></span>
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to add “annotation” to the list of reflection kinds:</p>
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<span id="cb26-7"><a href="#cb26-7" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb26-8"><a href="#cb26-8" aria-hidden="true" tabindex="-1"></a> // [meta.reflection.annotation], annotation reflection</span>
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<span id="cb26-9"><a href="#cb26-9" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval vector&lt;info&gt; annotations_of(info item);</span></span>
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<span id="cb26-10"><a href="#cb26-10" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval vector&lt;info&gt; annotations_of(info item, info type);</span></span>
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<span id="cb26-10"><a href="#cb26-10" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval vector&lt;info&gt; annotations_of_with_type(info item, info type);</span></span>
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<span id="cb26-11"><a href="#cb26-11" aria-hidden="true" tabindex="-1"></a><span class="va">+</span></span>
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<span id="cb26-12"><a href="#cb26-12" aria-hidden="true" tabindex="-1"></a><span class="va">+ template&lt;class T&gt;</span></span>
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<span id="cb26-13"><a href="#cb26-13" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval optional&lt;T&gt; annotation_of(info item);</span></span>
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<span id="cb26-14"><a href="#cb26-14" aria-hidden="true" tabindex="-1"></a><span class="va">+</span></span>
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<span id="cb26-15"><a href="#cb26-15" aria-hidden="true" tabindex="-1"></a><span class="va">+ template&lt;class T&gt;</span></span>
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<span id="cb26-16"><a href="#cb26-16" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval bool has_annotation(info item);</span></span>
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<span id="cb26-17"><a href="#cb26-17" aria-hidden="true" tabindex="-1"></a><span class="va">+ template&lt;class T&gt;</span></span>
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<span id="cb26-18"><a href="#cb26-18" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval bool has_annotation(info item, T const&amp; value);</span></span>
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<span id="cb26-19"><a href="#cb26-19" aria-hidden="true" tabindex="-1"></a><span class="va">+</span></span>
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<span id="cb26-20"><a href="#cb26-20" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval info annotate(info item, info value, source_location loc = source_location::current());</span></span>
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<span id="cb26-21"><a href="#cb26-21" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
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<span id="cb26-12"><a href="#cb26-12" aria-hidden="true" tabindex="-1"></a><span class="va">+ consteval info annotate(info item, info value, source_location loc = source_location::current());</span></span>
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<span id="cb26-13"><a href="#cb26-13" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
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</div>
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</blockquote>
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</div>
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2024-10-15. Code Injection with Token Sequences. <a href="https://wg21.link/p3294r2"><div class="csl-block">https://wg21.link/p3294r2</div></a>
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</div>
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<div id="ref-P3394R0" class="csl-entry" role="doc-biblioentry">
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[P3394R0] Daveed Vandevoorde, Wyatt Childers, Barry Revzin, Dan Katz.
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2024-10-14. Annotations for Reflection. <a href="https://wg21.link/p3394r0"><div class="csl-block">https://wg21.link/p3394r0</div></a>
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[P3394R0] Daveed Vandevoorde, Wyatt Childers, Dan Katz,. 2024-10-14.
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Annotations for Reflection. <a href="https://wg21.link/p3394r0"><div class="csl-block">https://wg21.link/p3394r0</div></a>
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</div>
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<div id="ref-P3394R1" class="csl-entry" role="doc-biblioentry">
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[P3394R1] Daveed Vandevoorde, Wyatt Childers, Barry Revzin, Dan Katz.
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2025-01-12. Annotations for Reflection. <a href="https://wg21.link/p3394r1"><div class="csl-block">https://wg21.link/p3394r1</div></a>
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[P3394R1] Daveed Vandevoorde, Wyatt Childers, Dan Katz, Barry Revzin.
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2025-01-13. Annotations for Reflection. <a href="https://wg21.link/p3394r1"><div class="csl-block">https://wg21.link/p3394r1</div></a>
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</div>
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</div>
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</div>

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