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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="2024-09-09" />
  <title>Designated-initializers for Base 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 { } /* Normal */
      code span.al { color: #ff0000; } /* Alert */
      code span.an { } /* Annotation */
      code span.at { } /* Attribute */
      code span.bn { color: #9f6807; } /* BaseN */
      code span.bu { color: #9f6807; } /* BuiltIn */
      code span.cf { color: #00607c; } /* ControlFlow */
      code span.ch { color: #9f6807; } /* Char */
      code span.cn { } /* Constant */
      code span.co { color: #008000; font-style: italic; } /* Comment */
      code span.cv { color: #008000; font-style: italic; } /* CommentVar */
      code span.do { color: #008000; } /* Documentation */
      code span.dt { color: #00607c; } /* DataType */
      code span.dv { color: #9f6807; } /* DecVal */
      code span.er { color: #ff0000; font-weight: bold; } /* Error */
      code span.ex { } /* Extension */
      code span.fl { color: #9f6807; } /* Float */
      code span.fu { } /* Function */
      code span.im { } /* Import */
      code span.in { color: #008000; } /* Information */
      code span.kw { color: #00607c; } /* Keyword */
      code span.op { color: #af1915; } /* Operator */
      code span.ot { } /* Other */
      code span.pp { color: #6f4e37; } /* Preprocessor */
      code span.re { } /* RegionMarker */
      code span.sc { color: #9f6807; } /* SpecialChar */
      code span.ss { color: #9f6807; } /* SpecialString */
      code span.st { color: #9f6807; } /* String */
      code span.va { } /* Variable */
      code span.vs { color: #9f6807; } /* VerbatimString */
      code span.wa { color: #008000; font-weight: bold; } /* Warning */
      code.diff {color: #898887}
      code.diff span.va {color: #006e28}
      code.diff span.st {color: #bf0303}
  </style>
  <style type="text/css">
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: 2em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
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;
}
: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>
  <style type="text/css">a {
color : #4183C4;
text-decoration: underline;
}
a.marginalized {
text-decoration: none;
}
a.self-link {
text-decoration: none;
}
h1#toctitle {
border-bottom: 1px solid #cccccc;
}
#TOC li {
margin-top: 1px;
margin-bottom: 1px;
}
#TOC ul>li:before { display: none; }
h3.subtitle { margin-top: -15px; }
h1:target { background-color: transparent; }
h2:target { background-color: transparent; }
h3:target { background-color: transparent; }
h4:target { background-color: transparent; }
h5:target { background-color: transparent; }
h6:target { background-color: transparent; }
code span.co { font-family: monospace; }
table tr {
background-color: white;
}
table tr:nth-child(2n) {
background-color: #f6f8fa;
}
#title-block-header > table tr:nth-child(2n) {
background-color: white;
}
td > div.sourceCode {
background-color: inherit;
}
table {
border-collapse: collapse;
}
table td, table th {
border: 1px solid #cccccc;
}
table th {
border-bottom: 1px solid black;
text-align: center;
}
table tr:first-child th {
border-top: 0;
}
table tr:last-child td {
border-bottom: 0;
}
table tr td:first-child,
table tr th:first-child {
border-left: 0;
}
table tr td:last-child,
table tr th:last-child {
border-right: 0;
}
table tbody tr:first-child td {
border-top: 1px solid black;
}
#title-block-header td { border: 0; }
@media all {
body {
margin: 2em;
}
}
@media screen and (min-width: 480px) {
body {
margin: 5em;
}
}
#refs code{padding-left: 0px; text-indent: 0px;}
:root {
--diff-ins: #C9FBC9;
--diff-strongins: #acf2bd;
--diff-del: #FFC8EB;
--diff-strongdel: #ff8888;
}
span.diffins {
background-color: var(--diff-strongins);
}
span.diffdel {
background-color: var(--diff-strongdel);
}
div.rm { text-decoration: line-through; }
div.rm code.sourceCode { text-decoration: line-through; }
div.addu, span.addu {
color: #006e28;
background-color: var(--diff-ins);
}

div.rm pre, div.add pre { background-color: #f6f8fa; }
div.addu pre { background-color: var(--diff-ins); }
div.add, div.add pre { background-color: var(--diff-ins); }
div.addu blockquote {
border-left: 4px solid #00a000;
padding: 0 15px;
color: #006e28;
text-decoration: none;
}
div.addu blockquote code.sourceCode { text-decoration: none; }
div.addu blockquote pre { text-decoration: none; }
div.addu blockquote pre code { text-decoration: none; }
div.quote {
border-left: 7px solid #ccc;
background: #f9f9f9;
margin: 1.5em 10px;
padding-left: 20px;
}
code.diff span.va { color: #000000; background-color: var(--diff-ins); }
code.diff span.st { color: #000000; background-color: var(--diff-del); }
div.std blockquote { color: #000000; background-color: #F1F1F1;
border: 1px solid #D1D1D1;
padding-left: 0.5em; padding-right: 0.5em; }
div.std.ins blockquote {
color: #000000; background-color: #C8FFC8;
border: 1px solid #B3EBB3;
}
div.ins > div.example {
color: #000000; background-color: #C8FFC8;
border: 1px solid #B3EBB3;
}
div.std div.sourceCode { background-color: inherit; margin-left: 1em; }
div.std blockquote del { text-decoration: line-through;
color: #000000; background-color: var(--diff-del);
border: none; }
code del { border: 1px solid #ECB3C7; }
span.orange {
background-color: #ffa500;
}
span.yellow {
background-color: #ffff00;
}</style>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Designated-initializers for
Base Classes</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2287R3 <a href="https://wg21.link/P2287">[Latest]</a> <a href="https://wg21.link/P2287/status">[Status]</a></td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-09-09</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>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history" id="toc-revision-history"><span class="toc-section-number">1</span> Revision
History<span></span></a></li>
<li><a href="#introduction" id="toc-introduction"><span class="toc-section-number">2</span> Introduction<span></span></a>
<ul>
<li><a href="#design-space" id="toc-design-space"><span class="toc-section-number">2.1</span> Design Space<span></span></a></li>
</ul></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">3</span> Proposal<span></span></a>
<ul>
<li><a href="#naming-the-base-classes" id="toc-naming-the-base-classes"><span class="toc-section-number">3.1</span> Naming the base
classes<span></span></a></li>
<li><a href="#naming-all-the-subobjects" id="toc-naming-all-the-subobjects"><span class="toc-section-number">3.2</span> Naming all the
subobjects<span></span></a></li>
<li><a href="#impact-on-existing-code" id="toc-impact-on-existing-code"><span class="toc-section-number">3.3</span> Impact on Existing
Code<span></span></a></li>
<li><a href="#implementation-experience" id="toc-implementation-experience"><span class="toc-section-number">3.4</span> Implementation
Experience<span></span></a></li>
</ul></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">4</span> Wording<span></span></a>
<ul>
<li><a href="#strategy" id="toc-strategy"><span class="toc-section-number">4.1</span> Strategy<span></span></a></li>
<li><a href="#actual-wording" id="toc-actual-wording"><span class="toc-section-number">4.2</span> Actual
Wording<span></span></a></li>
<li><a href="#feature-test-macro" id="toc-feature-test-macro"><span class="toc-section-number">4.3</span> Feature-test
Macro<span></span></a></li>
</ul></li>
<li><a href="#acknowledgements" id="toc-acknowledgements"><span class="toc-section-number">5</span>
Acknowledgements<span></span></a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">6</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span>
Revision History<a href="#revision-history" class="self-link"></a></h1>
<p><span class="citation" data-cites="P2287R2">[<a href="https://wg21.link/p2287r2" role="doc-biblioref">P2287R2</a>]</span> proposed only allowing
indirectly initializing members using the designated syntax. R3
additionally allows directly initializing base class subobjects
<em>without</em> a designator. Also adds an implementation.</p>
<p><span class="citation" data-cites="P2287R1">[<a href="https://wg21.link/p2287r1" role="doc-biblioref">P2287R1</a>]</span> proposed a novel way of naming
base classes, as well as a way for naming indirect non-static data
members. This revision <em>only</em> supports naming direct or indirect
non-static data members, with no mechanism to name base classes.
Basically only what Matthias suggested.</p>
<p><span class="citation" data-cites="P2287R0">[<a href="https://wg21.link/p2287r0" role="doc-biblioref">P2287R0</a>]</span> proposed a single syntax for a
<em>designated-initializer</em> that identifies a base class. Based on a
reflector suggestion from Matthias Stearn, this revision extends the
syntax to allow the brace-elision version of
<em>designated-initializer</em>: allow naming indirect non-static data
members as well. Also actually correctly targeting EWG this time.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span>
Introduction<a href="#introduction" class="self-link"></a></h1>
<p><span class="citation" data-cites="P0017R1">[<a href="https://wg21.link/p0017r1" role="doc-biblioref">P0017R1</a>]</span> extended aggregates to allow an
aggregate to have a base class. <span class="citation" data-cites="P0329R4">[<a href="https://wg21.link/p0329r4" role="doc-biblioref">P0329R4</a>]</span> gave us designated
initializers, which allow for much more expressive and functional
initialization of aggregates. However, the two do not mix: a designated
initializer can currently only refer to a direct non-static data
members. This means that if I have a type like:</p>
<div class="std">
<blockquote>
<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">struct</span> A <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>    <span class="dt">int</span> a;</span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> B <span class="op">:</span> A <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>    <span class="dt">int</span> b;</span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
</div>
<p>While I can initialize an <code class="sourceCode cpp">A</code> like
<code class="sourceCode cpp">A<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span></code>,
I cannot designated-initialize <code class="sourceCode cpp">B</code>. An
attempt like <code class="sourceCode cpp">B<span class="op">{{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span></code>
runs afoul of the rule that the initializers must either be all
designated or none designated. But there is currently no way to
designate the base class here.</p>
<p>Which means that my only options for initializing a
<code class="sourceCode cpp">B</code> are to fall-back to regular
aggregate initialization and write either <code class="sourceCode cpp">B<span class="op">{{</span><span class="dv">1</span><span class="op">}</span>, <span class="dv">2</span><span class="op">}</span></code>
or <code class="sourceCode cpp">B<span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span><span class="op">}</span></code>.
Neither are especially satisfactory.</p>
<h2 data-number="2.1" id="design-space"><span class="header-section-number">2.1</span> Design Space<a href="#design-space" class="self-link"></a></h2>
<p>There are basically three potential approaches for being able to
designated-initialize <code class="sourceCode cpp">B</code>:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="co">// provide a way to name the base class</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> b1 <span class="op">=</span> B<span class="op">{.</span>A<span class="op">={.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span>;</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a><span class="co">// allow mixing designated and non-designated</span></span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> b2 <span class="op">=</span> B<span class="op">{{.</span>a<span class="op">=</span><span class="dv">3</span><span class="op">}</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">4</span><span class="op">}</span>;</span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a><span class="co">// allow directly initializing indirect members</span></span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> b3 <span class="op">=</span> B<span class="op">{.</span>a<span class="op">=</span><span class="dv">5</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">6</span><span class="op">}</span>;</span></code></pre></div>
</blockquote>
</div>
<p>A previous revision of this paper proposed allowing only
<code class="sourceCode cpp">b1</code> — coming up with a way to name
the base class. This is much more complicated than naming an aggregate
member because base classes aren’t just
<code class="sourceCode cpp"><em>identifier</em></code>s, they can
include template parameters. This revision eschews that approach
entirely.</p>
<p>This leaves the other two options. When this paper was discussed in
<a href="https://wiki.edg.com/bin/view/Wg21varna/NotesEWGP2287">Varna</a>,
there was some desire expressed for having braces around the base class
subobject (as in <code class="sourceCode cpp">b2</code> above). While
allowing for optional braces is straightforward enough (and is simply a
matter of refining the restriction on mixing designated and
non-designated initializers), I would be strongly opposed to mandating
braces.</p>
<p>Designated-initialization mirrors assignment. The goal is to do this
substitution:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Assignment</strong>
</div></th>
<th><div style="text-align:center">
<strong>Initialization</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div>

<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="op">[&amp;]{</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>    B b;</span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>    b<span class="op">.</span>a <span class="op">=</span> <span class="dv">5</span>;</span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>    b<span class="op">.</span>b <span class="op">=</span> <span class="dv">6</span>;</span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> b;</span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a><span class="op">}()</span>;</span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>B<span class="op">{.</span>a<span class="op">=</span><span class="dv">5</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">6</span><span class="op">}</span></span></code></pre></div>

</div></td>
</tr>
</tbody>
</table>
<p>We can directly assign into <code class="sourceCode cpp">a</code>, so
we should be able to directly initialize into it. Of course, in C the
two forms are identical (assuming C had lambdas) while in C++ they
aren’t necessarily due to the existence of constructors. This makes
designated initialization in C a much simpler problem. Plus C doesn’t
have base classes. But the essence should be the same — mandating braces
breaks that.</p>
<p>Additionally, in many cases, the point of inheritance of aggregates
is not being we actually need an “is-a” relationship but rather to
compose aggregate members. I don’t need
<code class="sourceCode cpp">B</code> to be an
<code class="sourceCode cpp">A</code>, I need
<code class="sourceCode cpp">B</code> to have the same members as
<code class="sourceCode cpp">A</code> with the same convenient access
syntax — as opposed to having a member of type
<code class="sourceCode cpp">A</code> and needing an extra accessor in
the chain. Forcing the user to initialize
<code class="sourceCode cpp">B</code> with braces forces the user to be
aware of what is basically an implementation detail. It’s frustrating
enough to deal with the requirement that initializers be in-order,
additionally requiring braces is too much.</p>
<p>Lastly, aggregate initialize <em>already</em> allows for brace
elision. <code class="sourceCode cpp">B<span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span><span class="op">}</span></code>
is well-formed today, despite the
<code class="sourceCode cpp"><span class="dv">1</span></code>
initializing a base class subobject and
<code class="sourceCode cpp"><span class="dv">2</span></code>
initializing a direct member. So it strikes me as especially contrary to
the design to suddenly mandate braces here.</p>
<p>But <em>allowing</em> (not mandating) braces? That seems totally
fine. Also, notably, gcc in <code class="sourceCode cpp"><span class="op">-</span>std<span class="op">=</span>c<span class="op">++</span><span class="dv">17</span></code>
mode — still to this day on trunk — supports <code class="sourceCode cpp">B<span class="op">{{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span></code>.
We actually had some code break while upgrading to C++20 that
initialized aggregates in this way. While I think <code class="sourceCode cpp">B<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span></code>
is the clearest way to initialize this type, <code class="sourceCode cpp">B<span class="op">{{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span></code>
is still great — and both are substantial improvements over the best you
can do in valid C++20 today, which would be either <code class="sourceCode cpp">B<span class="op">{{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>, <span class="dv">2</span><span class="op">}</span></code>
or simply <code class="sourceCode cpp">B<span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span><span class="op">}</span></code>.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>This paper proposes extending designated initialization syntax
to:</p>
<ol type="1">
<li>allow directly initializing base class aggregate elements, and</li>
<li>allow mixing designated and non-designated initializers, if all of
the non-designated initializers are at the beginning of the
<em>initializer-list</em> and are used to initialize base class
subobjects.</li>
</ol>
<p>In short, based on the above declarations of
<code class="sourceCode cpp">A</code> and
<code class="sourceCode cpp">B</code>, this proposal allows all of the
following declarations:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>B<span class="op">{{</span><span class="dv">1</span><span class="op">}</span>, <span class="dv">2</span><span class="op">}</span>         <span class="co">// already valid in C++17</span></span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>B<span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span><span class="op">}</span>           <span class="co">// already valid in C++17</span></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>B<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span>     <span class="co">// proposed</span></span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>B<span class="op">{{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span>   <span class="co">// proposed</span></span>
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a>B<span class="op">{.</span>a<span class="op">{</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>b<span class="op">{</span><span class="dv">2</span><span class="op">}}</span>   <span class="co">// proposed</span></span>
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a>B<span class="op">{.</span>b<span class="op">=</span><span class="dv">2</span>, <span class="op">.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>     <span class="co">// still ill-formed</span></span></code></pre></div>
</blockquote>
</div>
<h2 data-number="3.1" id="naming-the-base-classes"><span class="header-section-number">3.1</span> Naming the base classes<a href="#naming-the-base-classes" class="self-link"></a></h2>
<p>The original revisions of this paper dealt with how to name the
<code class="sourceCode cpp">A</code> base class of
<code class="sourceCode cpp">B</code>, and what this means for more
complicated base classes (such at those with template parameters). This
revision eschews that approach entirely: it’s simpler to just stick with
naming members, direct and indirect. After all, that’s how these
aggregates will be interacted with.</p>
<p>What this means is that while this paper proposes that this
works:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> A <span class="op">{</span> <span class="dt">int</span> a; <span class="op">}</span>;</span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> B <span class="op">:</span> A <span class="op">{</span> <span class="dt">int</span> b; <span class="op">}</span>;</span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> b <span class="op">=</span> B<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span>;</span></code></pre></div>
</blockquote>
</div>
<p>And in a type that has a base class that is not an aggregate, you can
use the mix-and-match form:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> C <span class="op">:</span> std<span class="op">::</span>string <span class="op">{</span> <span class="dt">int</span> c; <span class="op">}</span>;</span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> c1 <span class="op">=</span> C<span class="op">{</span><span class="st">&quot;hello&quot;</span>, <span class="op">.</span>c<span class="op">=</span><span class="dv">3</span><span class="op">}</span>;</span>
<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> c2 <span class="op">=</span> C<span class="op">{{</span><span class="st">&quot;hello&quot;</span><span class="op">}</span>, <span class="op">.</span>c<span class="op">=</span><span class="dv">3</span><span class="op">}</span>;</span></code></pre></div>
</blockquote>
</div>
<p>If you have a hierarchy with repeated members, you’ll likewise have
to use the mix-and-match form:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> D <span class="op">{</span> <span class="dt">int</span> x; <span class="op">}</span>;</span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> E <span class="op">:</span> D <span class="op">{</span> <span class="dt">int</span> x; <span class="op">}</span>;</span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> e1 <span class="op">=</span> E<span class="op">{.</span>x<span class="op">=</span><span class="dv">1</span><span class="op">}</span>;         <span class="co">// initializes E::x, not D::x</span></span>
<span id="cb8-5"><a href="#cb8-5" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> e2 <span class="op">=</span> E<span class="op">{{.</span>x<span class="op">=</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>x<span class="op">=</span><span class="dv">2</span><span class="op">}</span>; <span class="co">// initializes D::x to 1 and E::x to 2</span></span>
<span id="cb8-6"><a href="#cb8-6" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> e3 <span class="op">=</span> E<span class="op">{</span>D<span class="op">{</span><span class="dv">1</span><span class="op">}</span>, <span class="op">.</span>x<span class="op">=</span><span class="dv">2</span><span class="op">}</span>;   <span class="co">// likewise</span></span></code></pre></div>
</blockquote>
</div>
<p>Coming up with a way to <em>name</em> the base class subobject of a
class seems useful, but that’s largely orthogonal. It can be done
later.</p>
<h2 data-number="3.2" id="naming-all-the-subobjects"><span class="header-section-number">3.2</span> Naming all the subobjects<a href="#naming-all-the-subobjects" class="self-link"></a></h2>
<p>The current wording we have says that, from <span>9.4.2 <a href="https://wg21.link/dcl.init.aggr">[dcl.init.aggr]</a></span>/3.1:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">(3.1)</a></span>
If the initializer list is a brace-enclosed
<em>designated-initializer-list</em>, the aggregate shall be of class
type, the <em>identifier</em> in each designator shall name a direct
non-static data member of the class […]</p>
</blockquote>
</div>
<p>And, from <span>9.4.5 <a href="https://wg21.link/dcl.init.list">[dcl.init.list]</a></span>/3.1
(conveniently, it’s 3.1 in both cases):</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">(3.1)</a></span>
If the <em>braced-init-list</em> contains a
<em>designated-initializer-list</em>,
<code class="sourceCode cpp">T</code> shall be an aggregate class. The
ordered identifiers in the designators of the
<em>designated-initializer-list</em> shall form a subsequence of the
ordered identifiers in the direct non-static data members of
<code class="sourceCode cpp">T</code>. Aggregate initialization is
performed ([dcl.init.aggr]).</p>
</blockquote>
</div>
<p>The proposal here is to extend both of these rules to cover not just
the direct non-static data members of
<code class="sourceCode cpp">T</code> but also all indirect members,
such that every interleaving base class is also an aggregate class. That
is:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> A <span class="op">{</span> <span class="dt">int</span> a; <span class="op">}</span>;</span>
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> B <span class="op">:</span> A <span class="op">{</span> <span class="dt">int</span> b; <span class="op">}</span>;</span>
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> C <span class="op">:</span> A <span class="op">{</span> C<span class="op">()</span>; <span class="dt">int</span> c; <span class="op">}</span>;</span>
<span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> D <span class="op">:</span> C <span class="op">{</span> <span class="dt">int</span> d; <span class="op">}</span>;</span>
<span id="cb9-5"><a href="#cb9-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb9-6"><a href="#cb9-6" aria-hidden="true" tabindex="-1"></a>A<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>;       <span class="co">// okay since C++17</span></span>
<span id="cb9-7"><a href="#cb9-7" aria-hidden="true" tabindex="-1"></a>B<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span>; <span class="co">// proposed okay, &#39;a&#39; is a direct member of an aggregate class</span></span>
<span id="cb9-8"><a href="#cb9-8" aria-hidden="true" tabindex="-1"></a>               <span class="co">// and A is a direct base</span></span>
<span id="cb9-9"><a href="#cb9-9" aria-hidden="true" tabindex="-1"></a>C<span class="op">{.</span>c<span class="op">=</span><span class="dv">1</span><span class="op">}</span>;       <span class="co">// error: C is not an aggregate</span></span>
<span id="cb9-10"><a href="#cb9-10" aria-hidden="true" tabindex="-1"></a>D<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span>;       <span class="co">// error: &#39;a&#39; is a direct member of an aggregate class</span></span>
<span id="cb9-11"><a href="#cb9-11" aria-hidden="true" tabindex="-1"></a>               <span class="co">// but an intermediate base class (C) is not an aggregate</span></span></code></pre></div>
</blockquote>
</div>
<p>Or, put differently, every identifier shall name a non-static data
member that is not a (direct or indirect) member of any base class that
is not an aggregate.</p>
<p>Also this is still based on lookup rules, so if the same name appears
in multiple base classes, then either it’s only the most derived one
that counts:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> X <span class="op">{</span> <span class="dt">int</span> x; <span class="op">}</span>;</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> Y <span class="op">:</span> X <span class="op">{</span> <span class="dt">int</span> x; <span class="op">}</span>;</span>
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a>Y<span class="op">{.</span>x<span class="op">=</span><span class="dv">1</span><span class="op">}</span>; <span class="co">// initializes Y::x</span></span></code></pre></div>
</blockquote>
</div>
<p>or is ambiguous:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb11"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> X <span class="op">{</span> <span class="dt">int</span> x; <span class="op">}</span>;</span>
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> Y <span class="op">{</span> <span class="dt">int</span> x; <span class="op">}</span>;</span>
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> Z <span class="op">:</span> X, Y <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a>Z<span class="op">{.</span>x<span class="op">=</span><span class="dv">1</span><span class="op">}</span>; <span class="co">// error:: ambiguous which X</span></span></code></pre></div>
</blockquote>
</div>
<p>would be ill-formed on the basis that
<code class="sourceCode cpp">Z<span class="op">::</span>x</code> is
ambiguous.</p>
<h2 data-number="3.3" id="impact-on-existing-code"><span class="header-section-number">3.3</span> Impact on Existing Code<a href="#impact-on-existing-code" class="self-link"></a></h2>
<p>There is one case I can think of where code would change meaning:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> A <span class="op">{</span> <span class="dt">int</span> a; <span class="op">}</span>;</span>
<span id="cb12-2"><a href="#cb12-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> B <span class="op">:</span> A <span class="op">{</span> <span class="dt">int</span> b; <span class="op">}</span>;</span>
<span id="cb12-3"><a href="#cb12-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb12-4"><a href="#cb12-4" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> f<span class="op">(</span>A<span class="op">)</span>; <span class="co">// #1</span></span>
<span id="cb12-5"><a href="#cb12-5" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> f<span class="op">(</span>B<span class="op">)</span>; <span class="co">// #2</span></span>
<span id="cb12-6"><a href="#cb12-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb12-7"><a href="#cb12-7" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> g<span class="op">()</span> <span class="op">{</span></span>
<span id="cb12-8"><a href="#cb12-8" aria-hidden="true" tabindex="-1"></a>    f<span class="op">({.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">})</span>;</span>
<span id="cb12-9"><a href="#cb12-9" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
</div>
<p>In C++23, <code class="sourceCode cpp">f<span class="op">({.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">})</span></code>
calls <code class="sourceCode cpp"><span class="pp">#1</span></code>, as
it’s the only viable candidate. But with this change,
<code class="sourceCode cpp"><span class="pp">#2</span></code> also
becomes a viable candidate, so this call becomes ambiguous. I have no
idea how much such code exists. This is, at least, easy to fix.</p>
<p>I don’t think there’s a case where code would change from one valid
meaning to a different valid meaning - just from valid to ambiguous.</p>
<h2 data-number="3.4" id="implementation-experience"><span class="header-section-number">3.4</span> Implementation Experience<a href="#implementation-experience" class="self-link"></a></h2>
<p>I implemented this <a href="https://github.com/llvm/llvm-project/compare/main...brevzin:llvm-project:p2287?expand=1">in
clang</a> in a very literal way — by synthesizing a new
designated-initializer-list to initialize the base classes in the
situations where that comes up. There is probably a more straightforward
way to do this, but all the examples in the paper work.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<h2 data-number="4.1" id="strategy"><span class="header-section-number">4.1</span> Strategy<a href="#strategy" class="self-link"></a></h2>
<p>The wording strategy here is as follows. Let’s say we have this
simple case:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb13"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> A <span class="op">{</span> <span class="dt">int</span> a; <span class="op">}</span>;</span>
<span id="cb13-2"><a href="#cb13-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> B <span class="op">:</span> A <span class="op">{</span> <span class="dt">int</span> b; <span class="op">}</span>;</span></code></pre></div>
</blockquote>
</div>
<p>In the current wording, <code class="sourceCode cpp">B</code> has two
elements (the <code class="sourceCode cpp">A</code> direct base class
and then the <code class="sourceCode cpp">b</code> member). The
initialization <code class="sourceCode cpp">B<span class="op">{.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span></code>
is considered to have one explicitly initialized element (the
<code class="sourceCode cpp">b</code>, initialized with
<code class="sourceCode cpp"><span class="dv">2</span></code>) and then
the <code class="sourceCode cpp">A</code> is not explicitly initialized
and cannot have a default member initializer, so it is copy-initialized
from <code class="sourceCode cpp"><span class="op">{}</span></code>.</p>
<p>The strategy to handle <code class="sourceCode cpp">B<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span>, <span class="op">.</span>b<span class="op">=</span><span class="dv">2</span><span class="op">}</span></code>
is to group the indirect non-static data members under their
corresponding direct base class and to treat those base class elements
as being explicitly initialized. So here, the
<code class="sourceCode cpp">A</code> element is explicitly initialized
from <code class="sourceCode cpp"><span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span></code>
and the <code class="sourceCode cpp">b</code> element continues to be
explicitly initialized from
<code class="sourceCode cpp"><span class="dv">2</span></code>. And then
this applies recursively, so given:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> C <span class="op">:</span> B <span class="op">{</span> <span class="dt">int</span> c; <span class="op">}</span>;</span></code></pre></div>
</blockquote>
</div>
<p>With <code class="sourceCode cpp">C<span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span>, <span class="op">.</span>c<span class="op">=</span><span class="dv">2</span><span class="op">}</span></code>,
we have:</p>
<ul>
<li>the <code class="sourceCode cpp">B</code> element is explicitly
initialized from <code class="sourceCode cpp"><span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span></code>,
which leads to:
<ul>
<li>the <code class="sourceCode cpp">A</code> element is explicitly
initialized from <code class="sourceCode cpp"><span class="op">{.</span>a<span class="op">=</span><span class="dv">1</span><span class="op">}</span></code>,
which leads to:
<ul>
<li>the <code class="sourceCode cpp">a</code> element is explicitly
initialized from
<code class="sourceCode cpp"><span class="dv">1</span></code></li>
</ul></li>
<li>the <code class="sourceCode cpp">b</code> element is not explicitly
initialized and has no default member initializer, so it is
copy-initialized from
<code class="sourceCode cpp"><span class="op">{}</span></code></li>
</ul></li>
<li>the <code class="sourceCode cpp">c</code> element is explicitly
initialized from
<code class="sourceCode cpp"><span class="dv">2</span></code></li>
</ul>
<h2 data-number="4.2" id="actual-wording"><span class="header-section-number">4.2</span> Actual Wording<a href="#actual-wording" class="self-link"></a></h2>
<p>Change the grammar in <span>9.4.1 <a href="https://wg21.link/dcl.init.general">[dcl.init.general]</a></span>/1
to allow a
<code class="sourceCode cpp"><em>designated-initializer-list</em></code>
to start with an
<code class="sourceCode cpp"><em>initializer-list</em></code>:</p>
<div class="std">
<blockquote>
<div>
<div class="sourceCode" id="cb15"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb15-1"><a href="#cb15-1" aria-hidden="true" tabindex="-1"></a>  <em>designated-initializer-list</em>:</span>
<span id="cb15-2"><a href="#cb15-2" aria-hidden="true" tabindex="-1"></a><span class="va">+   <em>designated-only-initializer-list</em></span></span>
<span id="cb15-3"><a href="#cb15-3" aria-hidden="true" tabindex="-1"></a><span class="va">+   <em>initializer-list</em> , <em>designated-only-initializer-list</em></span></span>
<span id="cb15-4"><a href="#cb15-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb15-5"><a href="#cb15-5" aria-hidden="true" tabindex="-1"></a><span class="va">+ <em>designated-only-initializer-list</em>:</span></span>
<span id="cb15-6"><a href="#cb15-6" aria-hidden="true" tabindex="-1"></a>    <em>designated-initializer-clause</em></span>
<span id="cb15-7"><a href="#cb15-7" aria-hidden="true" tabindex="-1"></a><span class="st">-   <em>designated-initializer-list</em> , <em>designated-initializer-clause</em></span></span>
<span id="cb15-8"><a href="#cb15-8" aria-hidden="true" tabindex="-1"></a><span class="va">+   <em>designated-only-initializer-list</em> , <em>designated-initializer-clause</em></span></span></code></pre></div>
</div>
</blockquote>
</div>
<p>Add a new term after we define what an aggregate and the elements of
an aggregate are:</p>
<div class="std">
<blockquote>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">*</a></span>
The <em>designatable members</em> of an aggregate
<code class="sourceCode cpp">T</code> are:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(*.1)</a></span>
For each direct base class <code class="sourceCode cpp">C</code> of
<code class="sourceCode cpp">T</code> that is an aggregate class, the
designatable members of <code class="sourceCode cpp">C</code> for which
lookup for that member in <code class="sourceCode cpp">T</code> finds
the member of <code class="sourceCode cpp">C</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">(*.2)</a></span>
The ordered <em>identifiers</em> in the direct non-static members of
<code class="sourceCode cpp">T</code>.</li>
</ul>
</div>
</blockquote>
</div>
<p>Extend <span>9.4.2 <a href="https://wg21.link/dcl.init.aggr">[dcl.init.aggr]</a></span>/3.1:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">(3.1)</a></span>
If the initializer list is a brace-enclosed
<em>designated-initializer-list</em>, the aggregate shall be of class
type<span class="rm" style="color: #bf0303"><del>, the identifier in
each designator shall name a direct non-static data member of the class,
and the explicitly initialized elements of the aggregate are the
elements that are, or contain, those members.</del></span> <span class="addu"><code class="sourceCode cpp">C</code> .</span></p>
<div class="addu">
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">(3.1.1)</a></span>
If the initializer list contains a
<code class="sourceCode cpp"><em>initializer-list</em></code>, then each
<code class="sourceCode cpp"><em>initializer-clause</em></code> in the
<code class="sourceCode cpp"><em>initializer-list</em></code> shall
appertain (see below) to a base class subobject of
<code class="sourceCode cpp">C</code> and the
<code class="sourceCode cpp"><em>identifier</em></code> in each
<code class="sourceCode cpp"><em>designator</em></code> shall name a
direct non-static data member of <code class="sourceCode cpp">C</code>,
and the explicitly initialized elements of the aggregate are those base
class subobjects and direct members.</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">(3.1.2)</a></span>
Otherwise, the <code class="sourceCode cpp"><em>identifier</em></code>
in each <code class="sourceCode cpp"><em>designator</em></code> shall
name a designatable member of the class. The explicitly initialized
elements of the aggregate are the elements that are, or contain (in the
case of an anonymous union or of a base class) those members.</li>
</ul>
</div>
</blockquote>
</div>
<p>And extend <span>9.4.2 <a href="https://wg21.link/dcl.init.aggr">[dcl.init.aggr]</a></span>/4 to
cover base class elements:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">4</a></span>
For each explicitly initialized element:</p>
<div class="addu">
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">(4.0)</a></span>
If the initializer list is a brace-enclosed
<em>designated-initializer-list</em> with no
<code class="sourceCode cpp"><em>initializer-list</em></code> and the
element is a direct base class, then let
<code class="sourceCode cpp">C</code> denote that direct base class and
let <code class="sourceCode cpp">T</code> denote the class. The element
is initialized from a synthesized brace-enclosed
<em>designated-initializer-list</em> containing each <em>designator</em>
for which lookup in <code class="sourceCode cpp">T</code> names a direct
or indirect non-static data member of
<code class="sourceCode cpp">C</code> in the same order as in the
original <em>designated-initializer-list</em>.</li>
</ul>
<div class="example">
<span>[ <em>Example 1:</em> </span>
<div class="sourceCode" id="cb16"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb16-1"><a href="#cb16-1" aria-hidden="true" tabindex="-1"></a>struct A { int a; };</span>
<span id="cb16-2"><a href="#cb16-2" aria-hidden="true" tabindex="-1"></a>struct B : A { int b; };</span>
<span id="cb16-3"><a href="#cb16-3" aria-hidden="true" tabindex="-1"></a>struct C : B { int c; };</span>
<span id="cb16-4"><a href="#cb16-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb16-5"><a href="#cb16-5" aria-hidden="true" tabindex="-1"></a>// the A element is intialized from {.a=1}</span>
<span id="cb16-6"><a href="#cb16-6" aria-hidden="true" tabindex="-1"></a>B x = B{.a=1};</span>
<span id="cb16-7"><a href="#cb16-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb16-8"><a href="#cb16-8" aria-hidden="true" tabindex="-1"></a>// the B element is initialized from {.a=2, .b=3}</span>
<span id="cb16-9"><a href="#cb16-9" aria-hidden="true" tabindex="-1"></a>// which leads to its A element being initialized from {.a=2}</span>
<span id="cb16-10"><a href="#cb16-10" aria-hidden="true" tabindex="-1"></a>C y = C{.a=2, .b=3, .c=4};</span>
<span id="cb16-11"><a href="#cb16-11" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb16-12"><a href="#cb16-12" aria-hidden="true" tabindex="-1"></a>struct A2 : A { int a; };</span>
<span id="cb16-13"><a href="#cb16-13" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb16-14"><a href="#cb16-14" aria-hidden="true" tabindex="-1"></a>// the A element is not explicitly initialized</span>
<span id="cb16-15"><a href="#cb16-15" aria-hidden="true" tabindex="-1"></a>A2 z = {.a=1};</span></code></pre></div>
<span> — <em>end example</em> ]</span>
</div>
</div>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">(4.1)</a></span>
<span class="rm" style="color: #bf0303"><del>If</del></span> <span class="addu">Otherwise, if</span> the element is an anonymous union
[…]</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">(4.2)</a></span>
Otherwise, if the initializer list is a brace-enclosed
<code class="sourceCode cpp"><em>designated-initializer-list</em></code>
<span class="addu">and the element is not a base class subobject</span>,
the element is initialized with the
<code class="sourceCode cpp"><em>brace-or-equal-initializer</em></code>
of the corresponding
<code class="sourceCode cpp"><em>designated-initializer-clause</em></code>
[…]</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_13" id="pnum_13">(4.3)</a></span>
Otherwise, <span class="addu">either</span> the initializer list is a
brace-enclosed
<code class="sourceCode cpp"><em>initializer-list</em></code> <span class="addu">or the element is a base class subobject and the
initializer list is a brace-enclosed
<code class="sourceCode cpp"><em>designated-initializer-list</em></code>
which contains an
<code class="sourceCode cpp"><em>initializer-list</em></code></span>.
[…]</li>
</ul>
<div class="example2">
<span>[ <em>Example 2:</em> </span>
<div>
<div class="sourceCode" id="cb17"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb17-1"><a href="#cb17-1" aria-hidden="true" tabindex="-1"></a>  struct base1 { int b1, b2 = 42; };</span>
<span id="cb17-2"><a href="#cb17-2" aria-hidden="true" tabindex="-1"></a>  struct base2 {</span>
<span id="cb17-3"><a href="#cb17-3" aria-hidden="true" tabindex="-1"></a>    base2() {</span>
<span id="cb17-4"><a href="#cb17-4" aria-hidden="true" tabindex="-1"></a>      b3 = 42;</span>
<span id="cb17-5"><a href="#cb17-5" aria-hidden="true" tabindex="-1"></a>    }</span>
<span id="cb17-6"><a href="#cb17-6" aria-hidden="true" tabindex="-1"></a>    int b3;</span>
<span id="cb17-7"><a href="#cb17-7" aria-hidden="true" tabindex="-1"></a>  };</span>
<span id="cb17-8"><a href="#cb17-8" aria-hidden="true" tabindex="-1"></a>  struct derived : base1, base2 {</span>
<span id="cb17-9"><a href="#cb17-9" aria-hidden="true" tabindex="-1"></a>    int d;</span>
<span id="cb17-10"><a href="#cb17-10" aria-hidden="true" tabindex="-1"></a>  };</span>
<span id="cb17-11"><a href="#cb17-11" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb17-12"><a href="#cb17-12" aria-hidden="true" tabindex="-1"></a>  derived d1{{1, 2}, {}, 4};</span>
<span id="cb17-13"><a href="#cb17-13" aria-hidden="true" tabindex="-1"></a>  derived d2{{}, {}, 4};</span>
<span id="cb17-14"><a href="#cb17-14" aria-hidden="true" tabindex="-1"></a><span class="va">+ derived d3{{1, 2}, {}, .d=4};</span></span></code></pre></div>
</div>
<p>initializes:</p>
<ul>
<li><code class="sourceCode cpp">d1<span class="op">.</span>b1</code>
with <code class="sourceCode cpp"><span class="dv">1</span></code>,
<code class="sourceCode cpp">d1<span class="op">.</span>b2</code> with
<code class="sourceCode cpp"><span class="dv">2</span></code>,
<code class="sourceCode cpp">d1<span class="op">.</span>b3</code> with
<code class="sourceCode cpp"><span class="dv">42</span></code>,
<code class="sourceCode cpp">d1<span class="op">.</span>d</code> with
<code class="sourceCode cpp"><span class="dv">4</span></code>, <span class="rm" style="color: #bf0303"><del>and</del></span></li>
<li><code class="sourceCode cpp">d2<span class="op">.</span>b1</code>
with <code class="sourceCode cpp"><span class="dv">0</span></code>,
<code class="sourceCode cpp">d2<span class="op">.</span>b2</code> with
<code class="sourceCode cpp"><span class="dv">42</span></code>,
<code class="sourceCode cpp">d2<span class="op">.</span>b3</code> with
<code class="sourceCode cpp"><span class="dv">42</span></code>,
<code class="sourceCode cpp">d2<span class="op">.</span>d</code> with
<code class="sourceCode cpp"><span class="dv">4</span></code><span class="addu">, and</span></li>
<li><span class="addu"><code class="sourceCode cpp">d3<span class="op">.</span>b1</code>
with <code class="sourceCode cpp"><span class="dv">1</span></code>,
<code class="sourceCode cpp">d3<span class="op">.</span>b2</code> with
<code class="sourceCode cpp"><span class="dv">2</span></code>,
<code class="sourceCode cpp">d3<span class="op">.</span>b3</code> with
<code class="sourceCode cpp"><span class="dv">42</span></code>,
<code class="sourceCode cpp">d3<span class="op">.</span>d</code> with
<code class="sourceCode cpp"><span class="dv">4</span></code></span>.</li>
</ul>
<span> — <em>end example</em> ]</span>
</div>
</blockquote>
</div>
<p>Extend <span>9.4.5 <a href="https://wg21.link/dcl.init.list">[dcl.init.list]</a></span>/3.1:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_14" id="pnum_14">(3.1)</a></span>
If the <em>braced-init-list</em> contains a
<em>designated-initializer-list</em>,
<code class="sourceCode cpp">T</code> shall be an aggregate class. The
ordered <em>identifier</em>s in the designators of the
<em>designated-initializer-list</em> shall form a subsequence of <span class="addu">designatable members [(dcl.init.aggr)] of
<code class="sourceCode cpp">T</code>.</span></p>
<p>Aggregate initialization is performed ([dcl.init.aggr]).</p>
<div class="example">
<span>[ <em>Example 3:</em> </span>
<div>
<div class="sourceCode" id="cb18"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb18-1"><a href="#cb18-1" aria-hidden="true" tabindex="-1"></a>    struct A { int x; int y; int z; };</span>
<span id="cb18-2"><a href="#cb18-2" aria-hidden="true" tabindex="-1"></a>    A a{.y = 2, .x = 1};                // error: designator order does not match declaration order</span>
<span id="cb18-3"><a href="#cb18-3" aria-hidden="true" tabindex="-1"></a>    A b{.x = 1, .z = 2};                // OK, b.y initialized to 0</span>
<span id="cb18-4"><a href="#cb18-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb18-5"><a href="#cb18-5" aria-hidden="true" tabindex="-1"></a><span class="va">+   struct B : A { int q; };</span></span>
<span id="cb18-6"><a href="#cb18-6" aria-hidden="true" tabindex="-1"></a><span class="va">+   B e{.x = 1, .q = 3};                // OK, e.y and e.z initialized to 0</span></span>
<span id="cb18-7"><a href="#cb18-7" aria-hidden="true" tabindex="-1"></a><span class="va">+   B f{.q = 3, .x = 1};                // error: designator order does not match declaration order</span></span>
<span id="cb18-8"><a href="#cb18-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb18-9"><a href="#cb18-9" aria-hidden="true" tabindex="-1"></a><span class="va">+   struct C { int p; int x; };</span></span>
<span id="cb18-10"><a href="#cb18-10" aria-hidden="true" tabindex="-1"></a><span class="va">+   struct D : A, C { };</span></span>
<span id="cb18-11"><a href="#cb18-11" aria-hidden="true" tabindex="-1"></a><span class="va">+   D g{.y=1, .p=2};                    // OK</span></span>
<span id="cb18-12"><a href="#cb18-12" aria-hidden="true" tabindex="-1"></a><span class="va">+   D h{.x=2};                          // error: x is not a designatable member</span></span>
<span id="cb18-13"><a href="#cb18-13" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb18-14"><a href="#cb18-14" aria-hidden="true" tabindex="-1"></a><span class="va">+   struct NonAggr { int na; NonAggr(int); };</span></span>
<span id="cb18-15"><a href="#cb18-15" aria-hidden="true" tabindex="-1"></a><span class="va">+   struct E : NonAggr { int e; };</span></span>
<span id="cb18-16"><a href="#cb18-16" aria-hidden="true" tabindex="-1"></a><span class="va">+   E i{.na=1, .e=2};                   // error: na is not a designatable member</span></span></code></pre></div>
</div>
<span> — <em>end example</em> ]</span>
</div>
</blockquote>
</div>
<p>Add an Annex C entry:</p>
<div class="std">
<blockquote>
<div class="addu">
<p><strong>Affected sublcause</strong>: [dcl.init] <br />
<strong>Change</strong>: Support for designated initialization of base
classes of aggregates. <br /> <strong>Rationale</strong>: New
functionality. <br /> <strong>Effect on original feature</strong>: Some
valid C++23 code may fail to compile. For example:</p>
<div class="sourceCode" id="cb19"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb19-1"><a href="#cb19-1" aria-hidden="true" tabindex="-1"></a>struct A { int a; };</span>
<span id="cb19-2"><a href="#cb19-2" aria-hidden="true" tabindex="-1"></a>struct B : A { int b; };</span>
<span id="cb19-3"><a href="#cb19-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb19-4"><a href="#cb19-4" aria-hidden="true" tabindex="-1"></a>void f(A); // #1</span>
<span id="cb19-5"><a href="#cb19-5" aria-hidden="true" tabindex="-1"></a>void f(B); // #2</span>
<span id="cb19-6"><a href="#cb19-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb19-7"><a href="#cb19-7" aria-hidden="true" tabindex="-1"></a>void g() {</span>
<span id="cb19-8"><a href="#cb19-8" aria-hidden="true" tabindex="-1"></a>    f({.a=1}); // OK (calls #1) in C++23, now ill-formed (ambiguous)</span>
<span id="cb19-9"><a href="#cb19-9" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
</div>
</blockquote>
</div>
<h2 data-number="4.3" id="feature-test-macro"><span class="header-section-number">4.3</span> Feature-test Macro<a href="#feature-test-macro" class="self-link"></a></h2>
<p>Bump
<code class="sourceCode cpp">__cpp_­designated_­initializers</code> in
<span>15.11 <a href="https://wg21.link/cpp.predefined">[cpp.predefined]</a></span>:</p>
<div class="std">
<blockquote>
<div>
<div class="sourceCode" id="cb20"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb20-1"><a href="#cb20-1" aria-hidden="true" tabindex="-1"></a><span class="st">- __cpp_­designated_­initializers <span class="diffdel">201707L</span></span></span>
<span id="cb20-2"><a href="#cb20-2" aria-hidden="true" tabindex="-1"></a><span class="va">+ __cpp_­designated_­initializers <span class="diffins">2024XXL</span></span></span></code></pre></div>
</div>
</blockquote>
</div>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="acknowledgements"><span class="header-section-number">5</span>
Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>Thanks to Matthias Stearn for, basically, the proposal. Thanks to Tim
Song for helping with design questions and wording.</p>
<h1 data-number="6" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">6</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">
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[P0017R1] Oleg Smolsky. 2015-10-24. Extension to aggregate
initialization. <a href="https://wg21.link/p0017r1"><div class="csl-block">https://wg21.link/p0017r1</div></a>
</div>
<div id="ref-P0329R4" class="csl-entry" role="doc-biblioentry">
[P0329R4] Tim Shen, Richard Smith. 2017-07-12. Designated Initialization
Wording. <a href="https://wg21.link/p0329r4"><div class="csl-block">https://wg21.link/p0329r4</div></a>
</div>
<div id="ref-P2287R0" class="csl-entry" role="doc-biblioentry">
[P2287R0] Barry Revzin. 2021-01-21. Designated-initializers for base
classes. <a href="https://wg21.link/p2287r0"><div class="csl-block">https://wg21.link/p2287r0</div></a>
</div>
<div id="ref-P2287R1" class="csl-entry" role="doc-biblioentry">
[P2287R1] Barry Revzin. 2021-02-15. Designated-initializers for base
classes. <a href="https://wg21.link/p2287r1"><div class="csl-block">https://wg21.link/p2287r1</div></a>
</div>
<div id="ref-P2287R2" class="csl-entry" role="doc-biblioentry">
[P2287R2] Barry Revzin. 2023-03-12. Designated-initializers for base
classes. <a href="https://wg21.link/p2287r2"><div class="csl-block">https://wg21.link/p2287r2</div></a>
</div>
</div>
</div>
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