<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P3405R0: Out-of-order designated initializers</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

/******************************************************************************/
/*                                    Body                                    */
/******************************************************************************/

	html {
	}

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;			  /* limit line length to 50em for readability   */
		margin: 0 auto;				/* center text within page                    */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag    */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em; }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small; }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;	/* Inherit the font family. */
		line-height: 1.2;		/* Keep wrapped headings compact */
		hyphens: manual;		/* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#profile-and-date {
		/* #profile-and-date is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	border-left: 0.5em solid #DEF;
	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 border-radius: .5em;
	 margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	 margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	 margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	margin-left: -2em;
	}
	dl.switch {
	padding-left: 2em;
	}
	dl.switch > dt {
	text-indent: -1.5em;
	margin-top: 1em;
	}
	dl.switch > dt + dt {
	margin-top: 0;
	}
	dl.switch > dt::before {
	content: '\21AA';
	padding: 0 0.5em 0 0;
	display: inline-block;
	width: 1em;
	text-align: right;
	line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: inherit;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

	/* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

	/* For autogen numbers, add
	  .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,	.sidefigure img,	figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
		height: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em;
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em; }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote,
	.amendment, .correction, .addition {
		margin: 1em auto;
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	  or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #e0cb52;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	  or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52e052;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary {
		color: hsl(120, 70%, 30%);
		color: var(--noteheading-text);
	}
	/* Add .note::before { content: "Note "; } for autogen label,
	  or use class="marker" to mark up the label in source. */

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
		border-bottom: 1px solid var(--defrow-border);
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def td.footnote {
		padding-top: 0.6em;
	}
	table.def td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		<table class="data"> highlights structural relationships in a table
		when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		Use class="complex data" for particularly complicated tables --
		(This will draw more lines: busier, but clearer.)

		Use class="long" on table cells with paragraph-like contents
		(This will adjust text alignment accordingly.)
		Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-color: var(--datacell-border);
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		border-top: 1px solid var(--datacell-border);
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
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    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
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    blockquote c-[b] { color: inherit; } /* Keyword.Type */
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    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
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    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 4416b18d5, updated Tue Jan 2 15:52:39 2024 -0800" name="generator">
  <link href="http://wg21.link/P3405R0.html" rel="canonical">
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<style>
table.p3405-table {
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P3405R0<br>Out-of-order designated initializers</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-09-28">2024-09-28</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P3405R0.html">http://wg21.link/P3405R0.html</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:isocpp@eliaskosunen.com">Elias Kosunen</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper proposes relaxing the rules regarding designated initializers
by allowing the initializers to appear out of order.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#changelog"><span class="secno">1</span> <span class="content">Changelog</span></a>
     <ol class="toc">
      <li><a href="#r0"><span class="secno">1.1</span> <span class="content">R0 (Pre-Wrocław, 10/2024)</span></a>
     </ol>
    <li><a href="#intro"><span class="secno">2</span> <span class="content">Introduction</span></a>
    <li><a href="#motivation"><span class="secno">3</span> <span class="content">Motivation</span></a>
    <li><a href="#background"><span class="secno">4</span> <span class="content">Background</span></a>
    <li>
     <a href="#proposal"><span class="secno">5</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#proposal-a"><span class="secno">5.1</span> <span class="content">(A): Trivially destructible types</span></a>
      <li><a href="#proposal-b"><span class="secno">5.2</span> <span class="content">(B): Initialization through temporaries</span></a>
      <li><a href="#proposal-c"><span class="secno">5.3</span> <span class="content">(C): Out-of-order list initialization</span></a>
      <li><a href="#proposal-mess"><span class="secno">5.4</span> <span class="content">Messing with the order of construction and destruction</span></a>
      <li><a href="#proposal-summary"><span class="secno">5.5</span> <span class="content">Summary</span></a>
     </ol>
    <li><a href="#impl-and-wording"><span class="secno">6</span> <span class="content">Implementation and wording</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Changelog</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="r0"><span class="secno">1.1. </span><span class="content">R0 (Pre-Wrocław, 10/2024)</span><a class="self-link" href="#r0"></a></h3>
   <p>Initial revision.</p>
   <h2 class="heading settled" data-level="2" id="intro"><span class="secno">2. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>Designated initializers were introduced in C++20, by <a data-link-type="biblio" href="#biblio-p0329r4" title="Designated Initialization Wording">[P0329R4]</a> (design in <a data-link-type="biblio" href="#biblio-p0329r0" title="Designated Initialization">[P0329R0]</a>), adapted from C.
However, in C, designated initializers are arguably more powerful than in C++.
This is due to some deliberate limitations placed on said syntax in C++.
While there are valid technical reasons for these limitations,
to this day, this remains as a common point of surprise,
and is often cited by a pain point by the community at large.</p>
<pre class="language-cpp highlight"><c- c1>// Example adapted from Annex C ([diff.dcl] p9)</c->
<c- k>struct</c-> <c- nc>A</c-> <c- p>{</c-> <c- b>int</c-> <c- n>x</c-><c- p>,</c-> <c- n>y</c-><c- p>;</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>B</c-> <c- p>{</c-> <c- k>struct</c-> <c- nc>A</c-> <c- n>a</c-><c- p>;</c-> <c- p>}</c->

<c- c1>// Valid C, valid C++</c->
<c- k>struct</c-> <c- nc>A</c-> <c- n>a</c-> <c- o>=</c-> <c- p>{.</c-><c- n>x</c-> <c- o>=</c-> <c- mi>1</c-><c- p>,</c-> <c- p>.</c-><c- n>y</c-> <c- o>=</c-> <c- mi>2</c-><c- p>};</c->
<c- c1>// Valid C, invalid C++ (out-of-order initializers)</c->
<c- k>struct</c-> <c- nc>A</c-> <c- n>a</c-> <c- o>=</c-> <c- p>{.</c-><c- n>y</c-> <c- o>=</c-> <c- mi>1</c-><c- p>,</c-> <c- p>.</c-><c- n>x</c-> <c- o>=</c-> <c- mi>2</c-><c- p>};</c->
<c- c1>// Valid C, invalid C++ (array initializers)</c->
<c- b>int</c-> <c- n>arr</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- o>=</c-> <c- p>{[</c-><c- mi>1</c-><c- p>]</c-> <c- o>=</c-> <c- mi>5</c-><c- p>};</c->
<c- c1>// Valid C, invalid C++ (nested initializers)</c->
<c- k>struct</c-> <c- nc>B</c-> <c- n>b</c-> <c- o>=</c-> <c- p>{.</c-><c- n>a</c-><c- p>.</c-><c- n>x</c-> <c- o>=</c-> <c- mi>0</c-><c- p>};</c->
<c- c1>// Valid C, invalid C++ (mixed initializers)</c->
<c- k>struct</c-> <c- nc>A</c-> <c- n>a</c-> <c- o>=</c-> <c- p>{.</c-><c- n>x</c-> <c- o>=</c-> <c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>};</c->
</pre>
   <p>This paper proposes making some out-of-order designated initializers well-formed,
while leaving the other omissions be.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>A</c-> <c- p>{</c-> <c- b>int</c-> <c- n>x</c-><c- p>,</c-> <c- n>y</c-><c- p>;</c-> <c- p>};</c->
<c- c1>// Valid C, proposed to be valid C++</c->
<c- k>struct</c-> <c- nc>A</c-> <c- n>a</c-> <c- o>=</c-> <c- p>{.</c-><c- n>y</c-> <c- o>=</c-> <c- mi>1</c-><c- p>,</c-> <c- p>.</c-><c- n>x</c-> <c- o>=</c-> <c- mi>2</c-><c- p>};</c->
</pre>
   <h2 class="heading settled" data-level="3" id="motivation"><span class="secno">3. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>As mentioned above, the fact that out-of-order designated initializers are not supported is surprising to users.</p>
   <p>Using designated initializers in code increases readability.
It makes it explicit which specific fields are being initialized, and with which values.
Initializing a struct with multiple fields without designated initializers can be difficult to grok
without external tooling support (like IDEs) or handcrafted comments.</p>
   <p>Aggregates that are initialized with designated initializers are a common pattern for emulating named function parameters.
However, the utility of this pattern is diminished by the fact that these "named parameters" have to appear in order.
It’s not difficult to imagine a function taking a number of named arguments, with the user having difficulty remembering
the order they appear within the intermediary struct, which will just cause the compiler to yell at the user for no clear reason.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>named_arguments</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>foo</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>option</c-><c- p>;</c->
    <c- b>bool</c-> <c- n>flag</c-><c- p>;</c->
    <c- b>int</c-> <c- n>bar</c-><c- p>;</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>func</c-><c- p>(</c-><c- n>named_arguments</c-><c- p>);</c->

<c- c1>// `.bar` appears before `.flag`, error</c->
<c- n>func</c-><c- p>({</c-> <c- p>.</c-><c- n>foo</c-><c- o>=</c-><c- mi>123</c-><c- p>,</c-> <c- p>.</c-><c- n>bar</c-><c- o>=</c-><c- mi>456</c-><c- p>,</c-> <c- p>.</c-><c- n>flag</c-><c- o>=</c->true <c- p>});</c->
</pre>
   <p>As a variant of the above, designated initializers can be used as a form of the builder pattern,
where a resulting object is constructed based on the state of an intermediate (builder) object.
These builders are often assigned state using chained member function calls.
Crucially, this can be done in arbitrary order, while also assigning names to each of the values given to the builder,
increasing legibility.
Allowing out-of-order designated initializers would make simple aggregates usable as builders.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>my_type</c-><c- p>;</c->

<c- c1>// Before:</c->
<c- k>struct</c-> <c- nc>my_type_builder</c-> <c- p>{</c->
    <c- n>my_type_builder</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

    <c- n>my_type_builder</c-> <c- o>&amp;</c-> <c- nf>foo</c-><c- p>(</c-><c- b>int</c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c->
        <c- n>assert</c-><c- p>(</c-><c- k>not</c-> <c- n>foo_</c-><c- p>);</c->
        <c- n>foo_</c-> <c- o>=</c-> <c- n>v</c-><c- p>;</c->
        <c- k>return</c-> <c- o>*</c-><c- k>this</c-><c- p>;</c->
    <c- p>}</c->
    <c- n>my_type_builder</c-> <c- o>&amp;</c-> <c- nf>bar</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c->
        <c- n>assert</c-><c- p>(</c-><c- k>not</c-> <c- n>bar_</c-><c- p>);</c->
        <c- n>bar_</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>v</c-><c- p>);</c->
        <c- k>return</c-> <c- o>*</c-><c- k>this</c-><c- p>;</c->
    <c- p>}</c->

    <c- n>my_type</c-> <c- nf>done</c-><c- p>()</c-> <c- o>&amp;&amp;</c-><c- p>;</c->

<c- k>private</c-><c- o>:</c->
    <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>foo_</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- o>></c-> <c- n>bar_</c-><c- p>;</c->
<c- p>};</c->

<c- k>auto</c-> <c- n>val</c-> <c- o>=</c-> <c- n>my_type_builder</c-><c- p>{}</c->
    <c- p>.</c-><c- n>foo</c-><c- p>(</c-><c- mi>123</c-><c- p>)</c->
    <c- p>.</c-><c- n>bar</c-><c- p>(</c-><c- s>"Hello world!"</c-><c- p>)</c->
    <c- p>.</c-><c- n>done</c-><c- p>();</c->

<c- c1>// After:</c->
<c- k>struct</c-> <c- nc>my_type_builder</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>foo</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>bar</c-><c- p>;</c->

    <c- n>my_type</c-> <c- nf>done</c-><c- p>()</c-> <c- o>&amp;&amp;</c-><c- p>;</c->
<c- p>};</c->

<c- k>auto</c-> <c- n>val</c-> <c- o>=</c-> <c- n>my_type_builder</c-><c- p>{</c->
    <c- p>.</c-><c- n>foo</c-> <c- o>=</c-> <c- mi>123</c-><c- p>,</c->
    <c- p>.</c-><c- n>bar</c-> <c- o>=</c-> <c- s>"Hello world!"</c->
<c- p>}.</c-><c- n>done</c-><c- p>();</c->
</pre>
   <p>Take note, how in the "After"-example, initializing the same field multiple times is prevented statically by-construction,
whereas runtime asserts (or some other error checking mechanisms) are required for the "Before"-case.
Similarly, required fields could be implemented with a per-field wrapper class that doesn’t have a default constructor,
whereas runtime checking is again required here for the "Before"-case.</p>
   <p>In addition, this is a point of incompatibility with C and C++.
In an idealistic sense, it seems silly that C has a language feature, while C++ only has a cut-down version of that same feature.
In the least, we should look into supporting this syntax for types for which C already supports it, i.e., POD types.
Although, the actually relevant type category here seems to be whether a type has a trivial destructor:
we don’t need to require full PODness (which is also no longer a thing).</p>
   <h2 class="heading settled" data-level="4" id="background"><span class="secno">4. </span><span class="content">Background</span><a class="self-link" href="#background"></a></h2>
   <p>The core of the issue lies within the rules for the order of construction and destruction of objects.
In C++, objects are constructed in the order they are declared, and destructed in the reverse order of construction.
This guarantee holds for all types and constructs within the language.
This is also somewhat present in C: the fields of a struct are initialized in order, while destructors are not a thing in C.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>C</c-> <c- p>{</c->
    <c- n>T</c-> <c- n>a</c-><c- p>{};</c->
    <c- n>T</c-> <c- n>b</c-><c- p>{};</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// c.a is constructed first, followed by c.b</c->
    <c- n>C</c-> <c- n>c</c-><c- p>{};</c->
    <c- c1>// c.b is destructed first, followed by c.a</c->
<c- p>}</c->
</pre>
   <p>In addition to the order of (de)initialization, there’s also the issue of the order of evaluation for the initializers.
Currently, with all <i>list-initialization</i> (including aggregate initialization, which includes designated initializers),
the initializers are guaranteed to be evaluated in lexical order, i.e., in the order they appear.
This is different from C, where the order of evaluation of the initializers of an aggregate are unspecified.</p>
   <blockquote>
     [dcl.init.list] p4 
    <p>Within the <i>initializer-list</i> of a <i>braced-init-list</i>, the <i>initializer-clauses</i>,
including any that result from pack expansions ([temp.variadic]),
are evaluated in the order in which they appear.
That is, every value computation and side effect associated with a given <i>initializer-clause</i> is sequenced before every value computation and side effect associated with any <i>initializer-clause</i> that follows it in the comma-separated list of the <i>initializer-list</i>.</p>
    <p>[<i>Note</i> 4: This evaluation ordering holds regardless of the semantics of the initialization;
for example, it applies when the elements of the <i>initializer-list</i> are interpreted as arguments of a constructor call,
even though ordinarily there are no sequencing constraints on the arguments of a call.
— <i>end note</i>]</p>
   </blockquote>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>D</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>a</c-><c- p>;</c->
    <c- b>int</c-> <c- n>b</c-><c- p>;</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// `123` is evaluated first, followed by `456`</c->
    <c- n>D</c-> <c- n>d</c-><c- p>{</c-><c- mi>123</c-><c- p>,</c-> <c- mi>456</c-><c- p>};</c->
<c- p>}</c->
</pre>
   <p>In summary, there’s two different orderings to consider here:</p>
   <ul>
    <li data-md>
     <p>the order to evaluate the initializers</p>
    <li data-md>
     <p>the order of construction and destruction</p>
   </ul>
   <p>Currently, for consistency with list-initialization, and with the rest of the language in general,
these orderings are equivalent (= lexical / left-to-right, up-to-down).
In order to support out of order designated initializers, either:</p>
   <ol>
    <li data-md>
     <p>one or both of these orderings need to be made different from the rest of the language, or</p>
    <li data-md>
     <p>intermediary temporaries need to be created for out-of-order fields.</p>
   </ol>
   <p>Below, the options (A) and (C) do 1., while option (B) does 2,
although option (A) has a trick up its sleeve.</p>
   <h2 class="heading settled" data-level="5" id="proposal"><span class="secno">5. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="proposal-a"><span class="secno">5.1. </span><span class="content">(A): Trivially destructible types</span><a class="self-link" href="#proposal-a"></a></h3>
   <p>There’s a category of types where achieving the objective of this proposal
could be considered to be relatively easy.
That category is types that have a trivial destructor (= trivially destructible types).</p>
   <p>Trivial destructors do nothing, their behavior isn’t observable,
and they don’t even need to be called on object destruction.
Therefore, the order in which the fields are constructed doesn’t actually matter,
as the order of destruction is in no way observable.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>D</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>a</c-><c- p>;</c->
    <c- b>int</c-> <c- n>b</c-><c- p>;</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// Order of construction is d.b followed by d.a:</c->
    <c- c1>// lexical order in list-initialization is still respected</c->
    <c- n>D</c-> <c- n>d</c-><c- p>{.</c-><c- n>b</c-> <c- o>=</c-> <c- mi>456</c-><c- p>,</c-> <c- p>.</c-><c- n>a</c-> <c- o>=</c-> <c- mi>123</c-><c- p>};</c->
    <c- c1>// Order of destruction doesn't matter (it isn't observable),</c->
    <c- c1>// because `D` is trivially destructible</c->
<c- p>}</c->
</pre>
   <p>It should be noted, that because the members are still initialized
in lexical order, the common pitfalls with member initializer lists
in class constructors can be avoided: the order of field initialization here is
predictable, unsurprising, and can be determined solely by looking at the initializer.
Also, because in aggregate initialization, fields cannot be initialized using the values of other fields,
changing the order of construction doesn’t have an effect on the actual initialization.</p>
   <h3 class="heading settled" data-level="5.2" id="proposal-b"><span class="secno">5.2. </span><span class="content">(B): Initialization through temporaries</span><a class="self-link" href="#proposal-b"></a></h3>
   <p>For types that don’t have a trivial destructor, the matter is slightly more complicated.
If we want to both guarantee lexical order of evaluation for the initializers,
and to maintain ordered construction and destruction of fields,
the only option is for the compiler to insert temporaries,
and essentially perform the initialization in two stages.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>E</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>a</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>b</c-><c- p>;</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- n>E</c-> <c- n>e</c-><c- p>{.</c-><c- n>b</c-> <c- o>=</c-> <c- s>"bbb"</c-><c- p>,</c-> <c- p>.</c-><c- n>a</c-> <c- o>=</c-> <c- s>"aaa"</c-><c- p>};</c->

    <c- c1>// Equivalent to:</c->
    <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>__b_tmp</c-> <c- o>=</c-> <c- s>"bbb"</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>__a_tmp</c-> <c- o>=</c-> <c- s>"aaa"</c-><c- p>;</c->
    <c- n>E</c-> <c- n>e</c-><c- p>{.</c-><c- n>a</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>__a_tmp</c-><c- p>),</c-> <c- p>.</c-><c- n>b</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>__b_tmp</c-><c- p>)};</c->
    <c- c1>// Initializers evaluated left-to-right,</c->
    <c- c1>// fields constructed in order of declaration,</c->
    <c- c1>// and destructed in reverse order</c->
<c- p>}</c->
</pre>
   <p>This would only be required for out-of-order initializers: existing code using ordered designated initializers
would exhibit no change in behavior or generated code.
It should be noted, that the trivial destructability of a single field doesn’t matter here,
what matters is whether the type as a whole has a trivial destructor.</p>
   <p>This option obviously would require the fields to be move constructible.
If that operation isn’t cheap, it could add non-trivial amounts of overhead.</p>
   <h4 class="heading settled" data-level="5.2.1" id="proposal-b-semantics"><span class="secno">5.2.1. </span><span class="content">Semantics of initialization through temporaries</span><a class="self-link" href="#proposal-b-semantics"></a></h4>
   <p>The pattern showed above, i.e., initialize each out-of-order field by first constructing a temporary
of the field type, and then move constructing the fields from these temporaries, only works in the simple case.
References and cv-qualifiers cause complications.
Let’s go over each of the cases to refine the correct semantics.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>F</c-> <c- p>{</c->
    <c- n>T</c->          <c- n>value</c-><c- p>;</c->
    <c- n>T</c-> <c- o>&amp;</c->        <c- n>lvalue_ref</c-><c- p>;</c->
    <c- n>T</c-> <c- k>const</c-> <c- o>&amp;</c->  <c- n>const_lvalue_ref</c-><c- p>;</c->
    <c- n>T</c-> <c- o>&amp;&amp;</c->       <c- n>rvalue_ref</c-><c- p>;</c->
    <c- n>T</c-> <c- k>const</c-> <c- o>&amp;&amp;</c-> <c- n>const_rvalue_ref</c-><c- p>;</c->
    <c- c1>// skipping `volatile`,</c->
    <c- c1>// it'll fall out naturally once we handle `const` properly</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// Construct everything out-of-order to force creation of temporaries</c->
    <c- n>F</c-> <c- n>f</c-><c- p>{</c->
        <c- p>.</c-><c- n>const_rvalue_ref</c-> <c- o>=</c-> <c- p>...,</c->
        <c- p>.</c-><c- n>rvalue_ref</c-> <c- o>=</c-> <c- p>...,</c->
        <c- p>.</c-><c- n>const_lvalue_ref</c-> <c- o>=</c-> <c- p>...,</c->
        <c- p>.</c-><c- n>lvalue_ref</c-> <c- o>=</c-> <c- p>...,</c->
        <c- p>.</c-><c- n>value</c-> <c- o>=</c-> <c- p>...</c->
    <c- p>};</c->

    <c- c1>// All the temporaries have the same type as the fields:</c->
    <c- c1>// references and cv-qualifiers included</c->
    <c- n>T</c-> <c- k>const</c-> <c- o>&amp;&amp;</c-> <c- n>__const_rvalue_ref_tmp</c-> <c- o>=</c-> <c- p>...;</c->
    <c- n>T</c-> <c- o>&amp;&amp;</c-> <c- n>__rvalue_ref_tmp</c-> <c- o>=</c-> <c- p>...;</c->
    <c- n>T</c-> <c- k>const</c-> <c- o>&amp;</c-> <c- n>__const_lvalue_ref_tmp</c-> <c- o>=</c-> <c- p>...;</c->
    <c- n>T</c-> <c- o>&amp;</c-> <c- n>__lvalue_ref_tmp</c-> <c- o>=</c-> <c- p>...;</c->
    <c- n>T</c-> <c- n>__value_tmp</c-> <c- o>=</c-> <c- p>...;</c->

    <c- n>F</c-> <c- n>f</c-><c- p>{</c->
        <c- c1>// We want to move the value</c->
        <c- p>.</c-><c- n>value</c-> <c- o>=</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>T</c-> <c- o>&amp;&amp;></c-><c- p>(</c-><c- n>__value_tmp</c-><c- p>),</c->
        <c- c1>// The lvalue reference can be initialized directly</c->
        <c- p>.</c-><c- n>lvalue_ref</c-> <c- o>=</c-> <c- n>__lvalue_ref_tmp</c-><c- p>,</c->
        <c- c1>// Likewise for the lvalue reference to const</c->
        <c- p>.</c-><c- n>const_lvalue_ref</c-> <c- o>=</c-> <c- n>__const_lvalue_ref_tmp</c-><c- p>,</c->
        <c- c1>// `__rvalue_ref_tmp` is an lvalue,</c->
        <c- c1>// we need to cast it to an rvalue again</c->
        <c- p>.</c-><c- n>rvalue_ref</c-> <c- o>=</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>T</c-> <c- o>&amp;&amp;></c-><c- p>(</c-><c- n>__rvalue_ref_tmp</c-><c- p>),</c->
        <c- c1>// Likewise for the rvalue reference to const</c->
        <c- p>.</c-><c- n>const_rvalue_ref</c-> <c- o>=</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>T</c-> <c- k>const</c-> <c- o>&amp;&amp;></c-><c- p>(</c-><c- n>__const_rvalue_ref_tmp</c-><c- p>)</c->
    <c- p>};</c->
<c- p>}</c->
</pre>
   <p>Since <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>foo</c-><c- p>)</c-></code> is equivalent to <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><c- n>foo</c-><c- p>)</c-><c- o>></c-> <c- o>&amp;&amp;></c-><c- p>(</c-><c- n>foo</c-><c- p>)</c-></code>,
in the example above, all the casts are actually equivalent to calls to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-></code>. <code class="highlight"><c- k>const</c-></code> (and therefore, <code class="highlight"><c- k>volatile</c-></code>) don’t seem to have an effect here.</p>
   <p>Therefore, the rule is as follows:</p>
   <ul>
    <li data-md>
     <p>Construct a temporary variable, of type <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>field</c-><c- p>)</c-></code>,
with the initializer given in the list-initialization</p>
    <li data-md>
     <p>If <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>field</c-><c- p>)</c-></code> is an lvalue reference, initialize the field directly from the temporary,</p>
    <li data-md>
     <p>otherwise, initialize the field as-if by a call to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-></code></p>
   </ul>
   <p>Because of this, strictly speaking,
we don’t actually need to require the out-of-order fields to be move constructible.
Only the fields of a non-reference type need to be move constructible.
Admittedly, that’s not a large difference in practice, given the general avoidance for
references as non-static data members.</p>
   <h3 class="heading settled" data-level="5.3" id="proposal-c"><span class="secno">5.3. </span><span class="content">(C): Out-of-order list initialization</span><a class="self-link" href="#proposal-c"></a></h3>
   <p>An alternative to the option detailed above, would be to take a look at how member initializer lists of
constructors do things. It is a semi-well known fact, that the order of member initializers in a constructor
doesn’t matter: the fields are always constructed in the order of declaration within the class.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>G</c-> <c- p>{</c->
    <c- c1>// In reality,</c->
    <c- c1>// a_ is constructed before b_,</c->
    <c- c1>// even though b_ appears first, here</c->
    <c- n>G</c-><c- p>(</c-><c- b>int</c-> <c- n>a</c-><c- p>,</c-> <c- b>int</c-> <c- n>b</c-><c- p>)</c-> <c- o>:</c-> <c- n>b_</c-><c- p>(</c-><c- n>b</c-><c- p>),</c-> <c- n>a_</c-><c- p>(</c-><c- n>a</c-><c- p>)</c-> <c- p>{}</c->

    <c- b>int</c-> <c- n>a_</c-><c- p>;</c->
    <c- b>int</c-> <c- n>b_</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>In C++11, the rules for the order of evaluation in a expression were tightened.
Before that, the order of evaluation within aggregate initialization was actually unspecified,
which is still the case in C. Since C++11, however, the rules have been clear:
list-initialization is always performed left-to-right.</p>
   <p>We could consider changing this, to sort of follow what member initializers in constructors do.
The initializers could be evaluated in the order the members appear in the class.
Since aggregate initialization requires for the type to be complete,
the compiler has knowledge of the order of the members in the class, so this could be technically achievable.</p>
<pre class="language-cpp highlight"><c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// `a` is the first member of the class, so its initializer is evaluated first,</c->
    <c- c1>// and it's constructed first.</c->
    <c- c1>// `b`'s initializer is evaluated and is constructed second.</c->
    <c- n>E</c-> <c- n>e</c-><c- p>{.</c-><c- n>b</c-> <c- o>=</c-> <c- s>"bbb"</c-><c- p>,</c-> <c- p>.</c-><c- n>a</c-> <c- o>=</c-> <c- s>"aaa"</c-><c- p>};</c->

    <c- c1>// Strictly equivalent to:</c->
    <c- n>E</c-> <c- n>e</c-><c- p>{.</c-><c- n>a</c-> <c- o>=</c-> <c- s>"aaa"</c-><c- p>,</c-> <c- p>.</c-><c- n>b</c-> <c- o>=</c-> <c- s>"bbb"</c-><c- p>};</c->
<c- p>}</c->
</pre>
   <p>This ordering property of member initializers is however often citen as a common pitfall in C++,
and can lead to potentially surprising behavior.
Because designated initializers cannot refer to other (potentially uninitialized) fields of the aggregate,
it wouldn’t be as big of an issue here, but the altered order of evaluation could still be unexpected,
especially if the fields themselves have sufficiently adventurous constructors.
Because of this, this is given here as an alternative to option (B) <a href="#proposal-b">§ 5.2 (B): Initialization through temporaries</a> above,
but not as the primary option.</p>
<pre class="language-cpp highlight"><c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
    <c- c1>// d.a is initialized first, to 0</c->
    <c- c1>// d.b is initialized second, to 1</c->
    <c- n>D</c-> <c- n>d</c-><c- p>{.</c-><c- n>b</c-> <c- o>=</c-> <c- n>i</c-><c- o>++</c-><c- p>,</c-> <c- p>.</c-><c- n>a</c-> <c- o>=</c-> <c- n>i</c-><c- o>++</c-><c- p>};</c->
<c- p>}</c->

<c- b>int</c-> <c- n>global_state</c-> <c- o>=</c-> <c- mi>1</c-><c- p>;</c->

<c- k>struct</c-> <c- nc>H</c-> <c- p>{</c->
    <c- k>struct</c-> <c- nc>Inner</c-> <c- p>{</c->
        <c- n>Inner</c-><c- p>(</c-><c- b>int</c-> <c- n>val</c-><c- p>)</c-> <c- o>:</c-> <c- n>value</c-><c- p>(</c-><c- n>global_state</c-> <c- o>*</c-> <c- n>val</c-><c- p>)</c-> <c- p>{</c->
            <c- n>global_state</c-> <c- o>*=</c-> <c- mi>10</c-><c- p>;</c->
        <c- p>}</c->

        <c- b>int</c-> <c- n>value</c-><c- p>;</c->
    <c- p>};</c->

    <c- n>Inner</c-> <c- n>a</c-><c- p>;</c->
    <c- n>Inner</c-> <c- n>b</c-><c- p>;</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>bar</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// h.a initialized first, followed by h.b</c->
    <c- n>H</c-> <c- n>h</c-><c- p>{.</c-><c- n>b</c-> <c- o>=</c-> <c- mi>456</c-><c- p>,</c-> <c- p>.</c-><c- n>a</c-> <c- o>=</c-> <c- mi>123</c-><c- p>};</c->
    <c- c1>// h.a is 123, h.b is 4560</c->
<c- p>}</c->
</pre>
   <p>It should be noted, that this wouldn’t cause existing standard-compliant code to change its meaning.
This change would only apply to out-of-order designated initializers, which are currently ill-formed.</p>
   <p>(C) is the behavior currently implemented in Clang (as of v19.1.0), even when compiling C++,
although it does issue a warning: <a href="https://godbolt.org/z/rrejKf7bx">Compiler Explorer</a>.
GCC currently (as of v14.2) does not allow this code, even when compiling with extensions enabled (<code class="highlight"><c- o>-</c-><c- n>std</c-><c- o>=</c-><c- n>gnu</c-><c- o>++</c-><c- mi>20</c-></code>).</p>
<pre class="language-cpp highlight"><c- cp>#include</c-> &lt;iostream>

<c- k>struct</c-> <c- nc>Field</c-> <c- p>{</c->
    <c- n>Field</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- o>:</c-> <c- n>value</c-><c- p>(</c-><c- n>i</c-><c- p>)</c-> <c- p>{</c->
        <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"Field("</c-> <c- o>&lt;&lt;</c-> <c- n>value</c-> <c- o>&lt;&lt;</c-> <c- s>")</c-><c- se>\n</c-><c- s>"</c-><c- p>;</c->
    <c- p>}</c->

    <c- o>~</c-><c- n>Field</c-><c- p>()</c-> <c- p>{</c->
        <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"~Field("</c-> <c- o>&lt;&lt;</c-> <c- n>value</c-> <c- o>&lt;&lt;</c-> <c- s>")</c-><c- se>\n</c-><c- s>"</c-><c- p>;</c->
    <c- p>}</c->

    <c- b>int</c-> <c- n>value</c-><c- p>;</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
    <c- n>Field</c-> <c- n>a</c-><c- p>;</c->
    <c- n>Field</c-> <c- n>b</c-><c- p>;</c->
<c- p>};</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
    <c- n>S</c-> <c- n>s</c-> <c- o>=</c-> <c- p>{.</c-><c- n>b</c-> <c- o>=</c-> <c- n>i</c-><c- o>++</c-><c- p>,</c-> <c- p>.</c-><c- n>a</c-> <c- o>=</c-> <c- n>i</c-><c- o>++</c-><c- p>};</c->
    <c- c1>// s.a.value is 0:</c->
    <c- c1>// The initializer for .a is evaluated first</c->
    <c- c1>//</c->
    <c- c1>// Field(0) is called before Field(1):</c->
    <c- c1>// fields are initialized in the order of declaration</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>".a = "</c-> <c- o>&lt;&lt;</c-> <c- n>s</c-><c- p>.</c-><c- n>a</c-><c- p>.</c-><c- n>value</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>".b = "</c-> <c- o>&lt;&lt;</c-> <c- n>s</c-><c- p>.</c-><c- n>b</c-><c- p>.</c-><c- n>value</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c->
<c- p>}</c->

<c- c1>// Output:</c->
<c- c1>// Field(0)</c->
<c- c1>// Field(1)</c->
<c- c1>// .a = 0</c->
<c- c1>// .b = 1</c->
<c- c1>// ~Field(1)</c->
<c- c1>// ~Field(0)</c->
</pre>
   <h3 class="heading settled" data-level="5.4" id="proposal-mess"><span class="secno">5.4. </span><span class="content">Messing with the order of construction and destruction</span><a class="self-link" href="#proposal-mess"></a></h3>
   <p>This paper doesn’t propose altering the order of destruction
(except for types with trivial destructors, with option (A) <a href="#proposal-a">§ 5.1 (A): Trivially destructible types</a>).
Practically speaking, the order of destruction needs to be statically determinable,
or otherwise some sort of hidden run-time tracking would be required.
This would be insane, and is therefore not an option.
The order of destruction needs to remain the same,
i.e., non-static member variables need to be destructed in the reverse order they were declared.</p>
<pre class="language-cpp highlight"><c- c1>// Not proposed!</c->

<c- n>E</c-> <c- nf>get_e</c-><c- p>();</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// Opaque function call,</c->
    <c- c1>// how will we know in what order to destruct the non-static members?</c->
    <c- k>auto</c-> <c- n>e</c-> <c- o>=</c-> <c- n>get_e</c-><c- p>();</c->
<c- p>}</c->
</pre>
   <p>There’s a slightly less outlandish idea, where the order of destruction would remain the same,
but the order of construction and destruction could be made to be out of sync with each other,
by not always destructing objects in the reverse order of construction.
However, this would break an extremely long-standing guarantee within the C++ object model,
and for that reason, it’s not reasonable.</p>
<pre class="language-cpp highlight"><c- c1>// Not proposed!</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// `e.b` constructed before `e.a`:</c->
    <c- c1>// it appears first in the initializer,</c->
    <c- c1>// even though it's not first in the class</c->
    <c- n>E</c-> <c- n>e</c-><c- p>{.</c-><c- n>b</c-> <c- o>=</c-> <c- s>"bbb"</c-><c- p>,</c-> <c- p>.</c-><c- n>a</c-> <c- o>=</c-> <c- s>"aaa"</c-><c- p>};</c->

    <c- c1>// `e.b` destructed before `e.a`:</c->
    <c- c1>// fields are destructed in the reverse order of declaration</c->
<c- p>}</c->
</pre>
   <h3 class="heading settled" data-level="5.5" id="proposal-summary"><span class="secno">5.5. </span><span class="content">Summary</span><a class="self-link" href="#proposal-summary"></a></h3>
   <p>To summarize, in addition to the status quo (no out-of-order designated initializers),
the following options are presented by the proposal above:</p>
   <table class="p3405-table">
    <tbody>
     <tr>
      <td> (A) (for trivially destructible types) + (B) (for other aggregates)<br> <a href="#proposal-a">§ 5.1 (A): Trivially destructible types</a><br><a href="#proposal-b">§ 5.2 (B): Initialization through temporaries</a> 
      <td>
       <ul>
        <li data-md>
         <p>Enables out-of-order designated initializers for struct types with a trivial destructor</p>
        <li data-md>
         <p>Enables out-of-order designated initializers for fields of a non-reference type that are move constructible</p>
        <li data-md>
         <p>Enables out-of-order designated initializers for all fields of a reference type</p>
        <li data-md>
         <p>Changes order of field construction when it’s not observable:
creates temporaries otherwise</p>
        <li data-md>
         <p>No changes to list-initialization (lexical order maintained)</p>
        <li data-md>
         <p>Destructors called in reverse order of construction, if observable</p>
       </ul>
     <tr>
      <td> Only (A), only for trivially destructible types<br> <a href="#proposal-a">§ 5.1 (A): Trivially destructible types</a> 
      <td>
       <ul>
        <li data-md>
         <p>Enables out-of-order designated initializers for struct types with a trivial destructor</p>
        <li data-md>
         <p>No changes to list-initialization (lexical order maintained)</p>
        <li data-md>
         <p>Changes order of field construction</p>
        <li data-md>
         <p>Destructor calling order changed (not observable)</p>
       </ul>
     <tr>
      <td> Only (B), extend to all aggregate types<br> <a href="#proposal-b">§ 5.2 (B): Initialization through temporaries</a> 
      <td>
       <ul>
        <li data-md>
         <p>Enables out-of-order designated initializers for fields of a non-reference type that are move constructible</p>
        <li data-md>
         <p>Enables out-of-order designated initializers for all fields of a reference type</p>
        <li data-md>
         <p>Causes additional copying and temporaries (for out-of-order initializers)</p>
        <li data-md>
         <p>No changes to list-initialization (lexical order maintained)</p>
        <li data-md>
         <p>No changes to order of field construction or destruction</p>
       </ul>
     <tr>
      <td> Only (C), extend to all aggregate types<br> <a href="#proposal-c">§ 5.3 (C): Out-of-order list initialization</a> 
      <td>
       <ul>
        <li data-md>
         <p>Enables out-of-order designated initializers for all types</p>
        <li data-md>
         <p>Initializers no longer evaluated in lexical order</p>
        <li data-md>
         <p>No changes to order of field construction or destruction</p>
        <li data-md>
         <p>Implemented in Clang for C++</p>
       </ul>
   </table>
   <p>(B) and (C) are mutually exclusive.
Only one of them is needed to support this proposal,
and doing them both would be unnecessary, and only needlessly complicate things.</p>
   <p>(A) and (C) in conjunction would be undesirable,
as that would make the order of evaluation in designated initializers 
different based on the type of the object that’s being initialized.
There’s no potential performance penalty associated with (C) (like with (B) with its temporaries),
so doing an "optimization" with (A) is not needed.</p>
   <p>The order of preference of the author is (A)+(B), (A), (B), (C).
The author finds (A)+(B) the most attractive of the bunch,
as it enables the use of out-of-order designated initializers for all aggregates,
with no observable change in semantics (apart from the possible loss of copy elision through creation of temporaries).
(C) does however have a clear advantage through the fact that it’s already implemented in Clang.</p>
   <p>It should be noted, that this is not really something we can change later, if and once it has been specified,
as that would induce change in behavior in well-formed code.
As this is currently specified to be ill-formed, we can choose the behavior we want quite freely.</p>
   <p>Below is a table comparing these options, but organized in a slightly different manner:</p>
   <table class="p3405-table">
    <thead>
     <tr>
      <th>
      <th>(A)+(B)
      <th>(A)
      <th>(B)
      <th>(C)
    <tbody>
     <tr>
      <td>Out-of-order designated initializers for all types
      <td style="text-align:center">⚠️<sup>1 or 2</sup>
      <td style="text-align:center">⚠️<sup>2</sup>
      <td style="text-align:center">⚠️<sup>1</sup>
      <td style="text-align:center">✅
     <tr>
      <td>Maintains order of destruction of fields as the reverse order of declaration
      <td style="text-align:center">✅
      <td style="text-align:center">✅
      <td style="text-align:center">✅
      <td style="text-align:center">✅
     <tr>
      <td>Maintains order of destruction of fields as the reverse order of construction
      <td style="text-align:center">✅<sup>3</sup>
      <td style="text-align:center">✅<sup>3</sup>
      <td style="text-align:center">✅
      <td style="text-align:center">✅
     <tr>
      <td>Maintains order of construction as the order of declaration
      <td style="text-align:center">✅<sup>3</sup>
      <td style="text-align:center">✅<sup>3</sup>
      <td style="text-align:center">✅
      <td style="text-align:center">✅
     <tr>
      <td>Maintains lexical order of evaluation for list-initialization
      <td style="text-align:center">✅
      <td style="text-align:center">✅
      <td style="text-align:center">✅
      <td style="text-align:center">❌
     <tr>
      <td>Never creates temporaries for out-of-order initializers
      <td style="text-align:center">️⚠️<sup>4</sup>
      <td style="text-align:center">✅
      <td style="text-align:center">❌
      <td style="text-align:center">✅
     <tr>
      <td>Implemented in Clang
      <td style="text-align:center">❌️
      <td style="text-align:center">❌️
      <td style="text-align:center">❌
      <td style="text-align:center">✅<sup>5</sup>
   </table>
   <p><sup>1</sup>: As long as all out-of-order fields of non-reference type are move constructible<br> <sup>2</sup>: As long as the whole structure is trivially destructible<br> <sup>3</sup>: Changed for trivially destructible types, which is not observable<br> <sup>4</sup>: Only for non-trivially destructible types<br> <sup>5</sup>: As an extension: currently issues a warning in C++</p>
   <h2 class="heading settled" data-level="6" id="impl-and-wording"><span class="secno">6. </span><span class="content">Implementation and wording</span><a class="self-link" href="#impl-and-wording"></a></h2>
   <p>As of now, this paper specifically hasn’t been implemented, and no wording is provided.
Option (C) <a href="#proposal-c">§ 5.3 (C): Out-of-order list initialization</a> is implemented in Clang as an extension.
This paper is intended to explore the design space, and to gauge potential interest in the feature.</p>
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                          + '<span>Jump to Table of Contents</span>';

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  var autoToggle   = function(e){ toggleSidebar(e.matches) };
  if(sidebarMedia.addListener) {
    sidebarMedia.addListener(autoToggle);
  }

  function toggleSidebar(on) {
    if (on == undefined) {
      on = !document.body.classList.contains('toc-sidebar');
    }

    /* Don't scroll to compensate for the ToC if we're above it already. */
    var headY = 0;
    var head = document.querySelector('.head');
    if (head) {
      // terrible approx of "top of ToC"
      headY += head.offsetTop + head.offsetHeight;
    }
    var skipScroll = window.scrollY < headY;

    var toggle = document.getElementById('toc-toggle');
    var tocNav = document.getElementById('toc');
    if (on) {
      var tocHeight = tocNav.offsetHeight;
      document.body.classList.add('toc-sidebar');
      document.body.classList.remove('toc-inline');
      toggle.innerHTML = collapseSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, 0 - tocHeight);
      }
      tocNav.focus();
      sidebarMedia.addListener(autoToggle); // auto-collapse when out of room
    }
    else {
      document.body.classList.add('toc-inline');
      document.body.classList.remove('toc-sidebar');
      toggle.innerHTML = expandSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, tocNav.offsetHeight);
      }
      if (toggle.matches(':hover')) {
        /* Unfocus button when not using keyboard navigation,
           because I don't know where else to send the focus. */
        toggle.blur();
      }
    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn't exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn't standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don't have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we're at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can't find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-p0329r0">[P0329R0]
   <dd>Tim Shen, Richard Smith, Zhihao Yuan, Chandler Carruth. <a href="https://wg21.link/p0329r0"><cite>Designated Initialization</cite></a>. 9 May 2016. URL: <a href="https://wg21.link/p0329r0">https://wg21.link/p0329r0</a>
   <dt id="biblio-p0329r4">[P0329R4]
   <dd>Tim Shen, Richard Smith. <a href="https://wg21.link/p0329r4"><cite>Designated Initialization Wording</cite></a>. 12 July 2017. URL: <a href="https://wg21.link/p0329r4">https://wg21.link/p0329r4</a>
  </dl>