<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
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  <meta name="generator" content="mpark/wg21" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
  <meta name="dcterms.date" content="2025-01-13" />
  <title>Allow static data members in local and unnamed classes</title>
  <style>
code{white-space: pre-wrap;}
span.smallcaps{font-variant: small-caps;}
span.underline{text-decoration: underline;}
div.column{display: inline-block; vertical-align: top; width: 50%;}
div.csl-block{margin-left: 1.5em;}
ul.task-list{list-style: none;}
pre > code.sourceCode { white-space: pre; position: relative; }
pre > code.sourceCode > span { display: inline-block; line-height: 1.25; }
pre > code.sourceCode > span:empty { height: 1.2em; }
.sourceCode { overflow: visible; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
{ counter-reset: source-line 0; }
pre.numberSource code > span
{ position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
{ content: counter(source-line);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
color: #aaaaaa;
}
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
div.sourceCode
{ background-color: #f6f8fa; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span { } 
code span.al { color: #ff0000; } 
code span.an { } 
code span.at { } 
code span.bn { color: #9f6807; } 
code span.bu { color: #9f6807; } 
code span.cf { color: #00607c; } 
code span.ch { color: #9f6807; } 
code span.cn { } 
code span.co { color: #008000; font-style: italic; } 
code span.cv { color: #008000; font-style: italic; } 
code span.do { color: #008000; } 
code span.dt { color: #00607c; } 
code span.dv { color: #9f6807; } 
code span.er { color: #ff0000; font-weight: bold; } 
code span.ex { } 
code span.fl { color: #9f6807; } 
code span.fu { } 
code span.im { } 
code span.in { color: #008000; } 
code span.kw { color: #00607c; } 
code span.op { color: #af1915; } 
code span.ot { } 
code span.pp { color: #6f4e37; } 
code span.re { } 
code span.sc { color: #9f6807; } 
code span.ss { color: #9f6807; } 
code span.st { color: #9f6807; } 
code span.va { } 
code span.vs { color: #9f6807; } 
code span.wa { color: #008000; font-weight: bold; } 
code.diff {color: #898887}
code.diff span.va {color: #006e28}
code.diff span.st {color: #bf0303}
</style>
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body {
margin: 5em;
font-family: serif;

hyphens: auto;
line-height: 1.35;
text-align: justify;
}
@media screen and (max-width: 30em) {
body {
margin: 1.5em;
}
}
div.wrapper {
max-width: 60em;
margin: auto;
}

ul {
list-style-type: none;
padding-left: 2.5em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
ol {
padding-left: 2.5em;
}
a {
text-decoration: none;
color: #4183C4;
}
a.hidden_link {
text-decoration: none;
color: inherit;
}
li {
margin-top: 0.6em;
margin-bottom: 0.6em;
}
h1, h2, h3, h4 {
position: relative;
line-height: 1;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
font-family: sans-serif;
font-weight: normal;
font-size: 83%;
}
a.self-link:hover { opacity: 1; }
a.self-link::before { content: "§"; }
ul > li:before {
content: "\2014";
position: absolute;
margin-left: -1.5em;
}

#TOC ul > li:before {
content: none;
}
#TOC > ul {
padding-left: 0;
}

.toc-section-number {
margin-right: 0.5em;
}
:target { background-color: #C9FBC9; }
:target .codeblock { background-color: #C9FBC9; }
:target ul { background-color: #C9FBC9; }
.abbr_ref { float: right; }
.folded_abbr_ref { float: right; }
:target .folded_abbr_ref { display: none; }
:target .unfolded_abbr_ref { float: right; display: inherit; }
.unfolded_abbr_ref { display: none; }
.secnum { display: inline-block; min-width: 35pt; }
.header-section-number { display: inline-block; min-width: 35pt; }
.annexnum { display: block; }
div.sourceLinkParent {
float: right;
}
a.sourceLink {
position: absolute;
opacity: 0;
margin-left: 10pt;
}
a.sourceLink:hover {
opacity: 1;
}
a.itemDeclLink {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
opacity: 0;
}
a.itemDeclLink:hover { opacity: 1; }
span.marginalizedparent {
position: relative;
left: -5em;
}
li span.marginalizedparent { left: -7em; }
li ul > li span.marginalizedparent { left: -9em; }
li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
div.footnoteNumberParent {
position: relative;
left: -4.7em;
}
a.marginalized {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
}
a.enumerated_item_num {
position: relative;
left: -3.5em;
display: inline-block;
margin-right: -3em;
text-align: right;
width: 3em;
}
div.para { margin-bottom: 0.6em; margin-top: 0.6em; text-align: justify; }
div.section { text-align: justify; }
div.sentence { display: inline; }
span.indexparent {
display: inline;
position: relative;
float: right;
right: -1em;
}
a.index {
position: absolute;
display: none;
}
a.index:before { content: "⟵"; }

a.index:target {
display: inline;
}
.indexitems {
margin-left: 2em;
text-indent: -2em;
}
div.itemdescr {
margin-left: 3em;
}
.bnf {
font-family: serif;
margin-left: 40pt;
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.ncbnf {
font-family: serif;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
}
.ncsimplebnf {
font-family: serif;
font-style: italic;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
background: inherit; 
}
span.textnormal {
font-style: normal;
font-family: serif;
white-space: normal;
display: inline-block;
}
span.rlap {
display: inline-block;
width: 0px;
}
span.descr { font-style: normal; font-family: serif; }
span.grammarterm { font-style: italic; }
span.term { font-style: italic; }
span.terminal { font-family: monospace; font-style: normal; }
span.nonterminal { font-style: italic; }
span.tcode { font-family: monospace; font-style: normal; }
span.textbf { font-weight: bold; }
span.textsc { font-variant: small-caps; }
a.nontermdef { font-style: italic; font-family: serif; }
span.emph { font-style: italic; }
span.techterm { font-style: italic; }
span.mathit { font-style: italic; }
span.mathsf { font-family: sans-serif; }
span.mathrm { font-family: serif; font-style: normal; }
span.textrm { font-family: serif; }
span.textsl { font-style: italic; }
span.mathtt { font-family: monospace; font-style: normal; }
span.mbox { font-family: serif; font-style: normal; }
span.ungap { display: inline-block; width: 2pt; }
span.textit { font-style: italic; }
span.texttt { font-family: monospace; }
span.tcode_in_codeblock { font-family: monospace; font-style: normal; }
span.phantom { color: white; }

span.math { font-style: normal; }
span.mathblock {
display: block;
margin-left: auto;
margin-right: auto;
margin-top: 1.2em;
margin-bottom: 1.2em;
text-align: center;
}
span.mathalpha {
font-style: italic;
}
span.synopsis {
font-weight: bold;
margin-top: 0.5em;
display: block;
}
span.definition {
font-weight: bold;
display: block;
}
.codeblock {
margin-left: 1.2em;
line-height: 127%;
}
.outputblock {
margin-left: 1.2em;
line-height: 127%;
}
div.itemdecl {
margin-top: 2ex;
}
code.itemdeclcode {
white-space: pre;
display: block;
}
span.textsuperscript {
vertical-align: super;
font-size: smaller;
line-height: 0;
}
.footnotenum { vertical-align: super; font-size: smaller; line-height: 0; }
.footnote {
font-size: small;
margin-left: 2em;
margin-right: 2em;
margin-top: 0.6em;
margin-bottom: 0.6em;
}
div.minipage {
display: inline-block;
margin-right: 3em;
}
div.numberedTable {
text-align: center;
margin: 2em;
}
div.figure {
text-align: center;
margin: 2em;
}
table {
border: 1px solid black;
border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none; 
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
}
td.empty {
padding: 0px;
padding-left: 1px;
}
td.left {
text-align: left;
}
td.right {
text-align: right;
}
td.center {
text-align: center;
}
td.justify {
text-align: justify;
}
td.border {
border-left: 1px solid black;
}
tr.rowsep, td.cline {
border-top: 1px solid black;
}
tr.even, tr.odd {
border-bottom: 1px solid black;
}
tr.capsep {
border-top: 3px solid black;
border-top-style: double;
}
tr.header {
border-bottom: 3px solid black;
border-bottom-style: double;
}
th {
border-bottom: 1px solid black;
}
span.centry {
font-weight: bold;
}
div.table {
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
}
span.indented {
display: block;
margin-left: 2em;
margin-bottom: 1em;
margin-top: 1em;
}
ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }

code.sourceCode > span { display: inline; }
</style>
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<h1 class="title" style="text-align:center">Allow static data members in
local and unnamed classes</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3588R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2025-01-13</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>
      Brian Bi<br>&lt;<a href="mailto:bbi10@bloomberg.net" class="email">bbi10@bloomberg.net</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#abstract" id="toc-abstract"><span class="toc-section-number">1</span> Abstract</a></li>
<li><a href="#background" id="toc-background"><span class="toc-section-number">2</span> Background</a></li>
<li><a href="#why-this-is-useful" id="toc-why-this-is-useful"><span class="toc-section-number">3</span> Why this is useful</a></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">4</span> Proposal</a></li>
<li><a href="#implementation-experience" id="toc-implementation-experience"><span class="toc-section-number">5</span> Implementation experience</a></li>
<li><a href="#proposed-wording" id="toc-proposed-wording"><span class="toc-section-number">6</span> Proposed wording</a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">7</span> References</a></li>
</ul>
</div>
<h1 data-number="1" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>Local and unnamed classes (as well as classes nested within) are not
permitted to declare static data members. This restriction dates back to
C++98, when there was no way to provide a definition for such a member.
In modern C++, static data members can have inline definitions, so this
rationale is obsolete. Static data members can be useful in local
classes for the same reasons why they are useful in non-local classes,
so this paper proposes to allow them in C++26.</p>
<h1 data-number="2" id="background"><span class="header-section-number">2</span> Background<a href="#background" class="self-link"></a></h1>
<p>C++03 §[class.static.data] stated as follows:</p>
<blockquote>
<p>[…] Unnamed classes and classes contained directly or indirectly
within unnamed classes shall not contain
<code class="sourceCode cpp"><span class="kw">static</span></code> data
members. [<em>Note</em>: this is because there is no mechanism to
provide the definitions for such
<code class="sourceCode cpp"><span class="kw">static</span></code> data
members. ]</p>
<p>[…] A local class shall not have
<code class="sourceCode cpp"><span class="kw">static</span></code> data
members.</p>
</blockquote>
<p>No rationale is explicitly provided for the latter restriction.
However, we can see that “there is no mechanism to provide the
definitions” applies to static data members of local classes as well.
There was one additional possible issue, which is the need to generate
an external symbol based on the identity of the local class, but it
seems that this applies equally well to functions (<em>i.e.</em>, it
seems that a member function of a local class in an inline function
would need to have a weak external symbol since its address could be
taken), and in any case, thanks to <span class="citation" data-cites="N2657">[<a href="https://wg21.link/n2657" role="doc-biblioref">N2657</a>]</span>, unnamed and local classes became
valid template arguments from C++11 and it is certain that all
implementations can generate appropriate mangled names for local classes
and members of local and unnamed classes. There do not appear to be any
remaining technical obstacles to allowing such classes to have static
data members.</p>
<p><span class="citation" data-cites="CWG728">[<a href="https://wg21.link/cwg728" role="doc-biblioref">CWG728</a>]</span>
proposed to relax some restrictions on local classes; more specifically,
to allow them to be templates, declare member templates, declare
friends, and declare static data members. It was determined that this
was a feature request, and a paper was submitted, <span class="citation" data-cites="P2044R2">[<a href="https://wg21.link/p2044r2" role="doc-biblioref">P2044R2</a>]</span>. However, that paper addressed
only the issue of member templates. <em>This</em> paper deals only with
the simpler feature of static data members.</p>
<h1 data-number="3" id="why-this-is-useful"><span class="header-section-number">3</span> Why this is useful<a href="#why-this-is-useful" class="self-link"></a></h1>
<p>C++ entities should generally be declared in the narrowest scope in
which they are needed, which may be a single function. For example, in
Google Test, the <code class="sourceCode cpp">TEST</code> macro is used
to define a single function which then verifies some expected
properties. If a class is required only for that single test, it is
convenient to declare it as a local class. It can be desirable for such
a local class to have a static data member, such as a static data member
that tracks the number of live objects of the class.</p>
<p>Here is another example: suppose that a C++ library defines a custom
version of the tuple protocol in which a tuple-like class is expected to
declare a static data member named
<code class="sourceCode cpp">tuple_size</code>, initialized to a
compile-time constant. If a unit test needs to declare such a class and
pass it as a template argument to the component under test (which is
expecting it to be a tuple), it would be convenient to be able to scope
that class to the smallest enclosing block.</p>
<p>In the former case, the static data member would be declared <code class="sourceCode cpp"><span class="kw">static</span> <span class="kw">inline</span></code>.
In the latter case, it would be declared <code class="sourceCode cpp"><span class="kw">static</span> <span class="kw">constexpr</span></code>,
which makes it implicitly inline. A non-inline static data member in a
local class cannot be defined, but I see no reason not to also allow it.
It could be used just for its type (<em>i.e.</em>, in unevaluated
contexts only). Allowing such a variable to inhabit class scope would
make it more narrowly scoped than if it were a local
<code class="sourceCode cpp"><span class="kw">extern</span></code>
variable inhabiting the enclosing function.</p>
<h1 data-number="4" id="proposal"><span class="header-section-number">4</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Allow static data members to be declared by local and unnamed classes
without restriction, except unnamed classes that have a typedef name for
linkage purposes. That is, the following would remain ill formed:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">typedef</span> <span class="kw">struct</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>    <span class="kw">inline</span> <span class="kw">static</span> <span class="dt">int</span> x <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span> S;</span></code></pre></div>
<p>§<span>9.2.4
<a href="https://wg21.link/N5001#dcl.typedef">[dcl.typedef]</a><a href="#fn1" class="footnote-ref" id="fnref1" role="doc-noteref"><sup>1</sup></a></span>p5 places various restrictions
on such classes in the vein of requiring them to be “C-like”: for
example, they can’t have member functions nor member typedefs. Since
static data members don’t exist in C, I don’t propose to drop this
restriction.</p>
<p>Static data members of local classes follow the same initialization
order rules as if their classes were non-local: for example, if
<code class="sourceCode cpp">x</code>,
<code class="sourceCode cpp">y</code>, and
<code class="sourceCode cpp">z</code> are three inline variables and in
every translation unit they appear in that order, and
<code class="sourceCode cpp">x</code> and
<code class="sourceCode cpp">z</code> belong to non-local classes while
<code class="sourceCode cpp">y</code> belongs to a local class, then
<code class="sourceCode cpp">y</code> is initialized after
<code class="sourceCode cpp">x</code> and before
<code class="sourceCode cpp">z</code>, unless
<code class="sourceCode cpp">y</code>’s enclosing function is templated,
in which case <code class="sourceCode cpp">y</code> could be initialized
in any order relative to <code class="sourceCode cpp">x</code> and
<code class="sourceCode cpp">z</code>. I believe that this rule is
intuitive and not difficult to implement.</p>
<h1 data-number="5" id="implementation-experience"><span class="header-section-number">5</span> Implementation experience<a href="#implementation-experience" class="self-link"></a></h1>
<p>GCC supports static data members in unnamed and local classes as an
extension when
<code class="sourceCode cpp"><span class="op">-</span>fpermissive</code>
is used. GCC does not allow such members to be explicitly declared
<code class="sourceCode cpp"><span class="kw">inline</span></code>, but
this seems to be an oversight: the error message says “‘inline’
specifier invalid for variable ‘x’ declared at block scope”. GCC does
allow the member to be
<code class="sourceCode cpp"><span class="kw">constexpr</span></code>,
and generates an appropriate definition in that case (<em>i.e.</em>, the
address can be taken).</p>
<p>I implemented this feature, including dynamic initialization of
inline static data members, as a patch to Clang 19.1.4. The only changes
required were as follows.</p>
<ul>
<li>Remove the restriction.</li>
<li>Add a case for instantiation of templated variables: instantiate
them eagerly when they’re static data members of local classes. (The
Standard specifies that member of local classes are always instantiated
eagerly.)</li>
<li>When generating code for a function, iterate through its local
classes as well, so that their static data members can be emitted.</li>
</ul>
<p>The patch passes the Clang unit tests, but the feature has not been
tested in a large code base.</p>
<h1 data-number="6" id="proposed-wording"><span class="header-section-number">6</span> Proposed wording<a href="#proposed-wording" class="self-link"></a></h1>
<p>Wording is relative to <span class="citation" data-cites="N5001">[<a href="https://wg21.link/n5001" role="doc-biblioref">N5001</a>]</span>.</p>
<p>Modify §<span>11.4.9.3
<a href="https://wg21.link/N5001#class.static.data">[class.static.data]</a></span>p2
as follows.</p>
<blockquote>
<p>A static data member shall not be
<code class="sourceCode cpp"><span class="kw">mutable</span></code>
([dcl.stc]). <span class="rm" style="color: #bf0303"><del>A static data
member shall not be a direct member ([class.mem]) of an unnamed
([class.pre]) or local ([class.local]) class or of a (possibly
indirectly) nested class ([class.nest]) thereof.</del></span></p>
</blockquote>
<h1 data-number="7" id="bibliography"><span class="header-section-number">7</span> References<a href="#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-CWG728" class="csl-entry" role="doc-biblioentry">
[CWG728] Faisal Vali. 2008-10-05. Restrictions on local classes. <a href="https://wg21.link/cwg728"><div class="csl-block">https://wg21.link/cwg728</div></a>
</div>
<div id="ref-N2657" class="csl-entry" role="doc-biblioentry">
[N2657] John Spicer. 2008-06-10. Local and Unnamed Types as Template
Arguments. <a href="https://wg21.link/n2657"><div class="csl-block">https://wg21.link/n2657</div></a>
</div>
<div id="ref-N5001" class="csl-entry" role="doc-biblioentry">
[N5001] Thomas Köppe. 2024-12-17. Working Draft, Programming Languages —
C++. <a href="https://wg21.link/n5001"><div class="csl-block">https://wg21.link/n5001</div></a>
</div>
<div id="ref-P2044R2" class="csl-entry" role="doc-biblioentry">
[P2044R2] Robert Leahy. 2020-04-14. Member Templates for Local Classes.
<a href="https://wg21.link/p2044r2"><div class="csl-block">https://wg21.link/p2044r2</div></a>
</div>
</div>
<section id="footnotes" class="footnotes footnotes-end-of-document" role="doc-endnotes">
<hr />
<ol>
<li id="fn1"><p>All citations to the Standard are to working draft N5001
unless otherwise specified.<a href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</section>
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