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  <title>Introspection of Closure Types</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Introspection of Closure
Types</h1>
<table style="border:none;float:right">
  <tbody><tr>
    <td>Document #:</td>
    <td>P3273R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-05-20</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      EWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
      Andrei Alexandrescu<br>&lt;<a href="mailto:andrei@nvidia.com" class="email">andrei@nvidia.com</a>&gt;<br>
      David Olsen<br>&lt;<a href="mailto:dolsen@nvidia.com" class="email">dolsen@nvidia.com</a>&gt;<br>
      Daveed Vandevoorde<br>&lt;<a href="mailto:daveed@edg.com" class="email">daveed@edg.com</a>&gt;<br>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
      Michael Garland<br>&lt;<a href="mailto:mgarland@nvidia.com" class="email">mgarland@nvidia.com</a>&gt;<br>
    </td>
  </tr>
</tbody></table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#introduction" id="toc-introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#motivation" id="toc-motivation"><span class="toc-section-number">2</span> Motivation<span></span></a></li>
<li><a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#proposed-changes-specify-storage-of-reference-captures" id="toc-proposed-changes-specify-storage-of-reference-captures"><span class="toc-section-number">3</span> Proposed Changes: Specify Storage of
Reference Captures<span></span></a></li>
<li><a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#discussion" id="toc-discussion"><span class="toc-section-number">4</span> Discussion<span></span></a></li>
<li><a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#bibliography" id="toc-bibliography"><span class="toc-section-number">5</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">1</span>
Introduction<a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#introduction" class="self-link"></a></h1>
<p>Recent proposals of reflection facilities for C++ (such as <span class="citation" data-cites="P2996R2">[<a href="https://wg21.link/p2996r2" role="doc-biblioref">P2996R2</a>]</span> and <span class="citation" data-cites="P3157R0">[<a href="https://wg21.link/p3157r0" role="doc-biblioref">P3157R0</a>]</span>) raise important questions
regarding the applicability of reflection to the layout of closure
types. The ability to introspect closure types has important
applications to applications that need to carry computation and
data—packaged together in closures—across address spaces: inter-process
communication, networking, serialization, and GPU execution. We propose
to strenghten the layout guarantees of closure types in ways that allow
introspection to work appropriately. To the best of our knowledge, the
guarantees we propose are already observed by current implementations,
so we estimate no or low impact on existing compiler infrastructure.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="motivation"><span class="header-section-number">2</span>
Motivation<a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#motivation" class="self-link"></a></h1>
<p>Closure objects are a simple, syntactically compact, and convenient
means to package computation and data together. Captures allow closures
to carry arbitrary amounts of data within. Because of these advantages,
closures are used extensively in C++ code either as a means to customize
algorithms, or in various applications of the <a href="https://en.wikipedia.org/wiki/Command_pattern">Command</a> design
pattern.</p>
<p>The CUDA C++ dialect aimed at running algorithms on GPU devices with
dedicated computational and memory hardware pays special attention to
closure types, making them transparently available for execution on GPU
hardware. Libraries such as <a href="https://developer.nvidia.com/thrust">Thrust</a> and <a href="https://docs.nvidia.com/cuda/cub/index.html">Cub</a> make good use
of device lambda functions as a central feature.</p>
<p>Introspection of closure types promises new opportunities for such
codes as well as any code that needs to marshal data and computation
across memory address spaces. To realize such opportunities, closure
types must make their state available for introspection so that custom
code can carry marshaling operations, or disallow certain data
statically or dynamically.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="proposed-changes-specify-storage-of-reference-captures"><span class="header-section-number">3</span> Proposed Changes: Specify Storage
of Reference Captures<a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#proposed-changes-specify-storage-of-reference-captures" class="self-link"></a></h1>
<p>Given that the introspection primitives proposed in P2996 can query
class types and that closure types are class types (per <span>7.5.5.2 <a href="https://wg21.link/expr.prim.lambda.closure">[expr.prim.lambda.closure]</a></span>),
it follows that introspection should apply by definition to closure
types. However, there is one specific issue with reference captures: per
<span>7.5.5.3 <a href="https://wg21.link/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a></span>,
the language definition leaves it to the implementation whether member
variables are declared in the closure type to accommodate those
captures. This makes it impossible for code that uses introspection for
closure objects to portably take account of all captured data.</p>
<p>We propose to change wording in <span>7.5.5.3 <a href="https://wg21.link/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a></span>/12:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#pnum_1" id="pnum_1">12</a></span>
An entity is <em>captured by reference</em> if it is implicitly or
explicitly captured but not captured by copy. <span class="rm" style="color: #bf0303"><del>It is unspecified whether additional unnamed
non-static data members are declared in the closure type for entities
captured by reference. If declared, such non-static data members shall
be of literal type.</del></span> <span class="addu">For each entity
captured by reference, an unnamed non-static data member is declared in
the closure type. The declaration order of these members is unspecified.
The type of such a data member is an lvalue reference to:</span></p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#pnum_2" id="pnum_2">(12.1)</a></span>
<span class="addu">the referenced type if the entity is a reference to
an object,</span></li>
<li><span class="marginalizedparent"><a class="marginalized" href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#pnum_3" id="pnum_3">(12.2)</a></span>
<span class="addu">the referenced function type if the entity is a
reference to a function, or</span></li>
<li><span class="marginalizedparent"><a class="marginalized" href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#pnum_4" id="pnum_4">(12.3)</a></span>
<span class="addu">the type of the corresponding captured entity
otherwise.</span></li>
</ul>
</blockquote>
</div>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="discussion"><span class="header-section-number">4</span>
Discussion<a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#discussion" class="self-link"></a></h1>
<p>As mentioned, to our knowledge, all implementations already implement
by-reference lambda captures by means of reference or pointer members,
so we anticipate no or small impact on compiler implementations.</p>
<p>To the extent the intent of the existing wording was meant to allow
optimizations (e.g., use direct access for reference captures when the
lambda is executed directly upon introduction), we note that all
optimizations are still possible if reflection facilities are not used
for any given lambda.</p>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">5</span>
References<a href="https://brevzin.github.io/cpp_proposals/3273_introspect-closure/p3273r0.html#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" data-entry-spacing="1" role="doc-bibliography">
<div id="ref-P2996R2" class="csl-entry" role="doc-biblioentry">
[P2996R2] Barry Revzin, Wyatt Childers, Peter Dimov, Andrew Sutton,
Faisal Vali, Daveed Vandevoorde, Dan Katz. 2024-02-15. Reflection for
C++26. <a href="https://wg21.link/p2996r2"><div class="csl-block">https://wg21.link/p2996r2</div></a>
</div>
<div id="ref-P3157R0" class="csl-entry" role="doc-biblioentry">
[P3157R0] Andrei Alexandrescu, Bryce Lelbach, Michael Garland.
2024-02-15. Generative Extensions for Reflection. <a href="https://wg21.link/p3157r0"><div class="csl-block">https://wg21.link/p3157r0</div></a>
</div>
</div>
</div>
</div>


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