<!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>P2782R0: A proposal for a type trait to detect if value initialization can be achieved by zero-filling</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;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom: 3px solid #054572;
			border-bottom: 3px solid var(--toclink-underline);
			margin-bottom: -3px;
		}
		#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>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    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 }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    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 859efb4bd, updated Fri Jan 27 16:04:01 2023 -0800" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
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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">P2782R0<br>A proposal for a type trait to detect if value initialization can be achieved by zero-filling</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="2023-01-30">2023-01-30</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:giuseppe.dangelo@kdab.com">Giuseppe D'Angelo</a>
     <dt>Audience:
     <dd>EWG, LEWG, SG14
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </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>We propose a new type trait that checks whether an instance of a

    trivially default constructible type can be value-initialized by 
    setting all the bytes in its object representation to 0. 
    The trait helps with detecting implementation-specific behavior:
    the object representation of floating-point types, pointer types, pointer to non-static data members
    etc. is not mandated by the Standard and ultimately depends on the platform.
    We therefore need compiler hooks in order to implement this detection.
    The detection offered by this trait can be used to optimize
    the implementation of certain functions and algorithms.</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>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivation and Scope</span></a>
     <ol class="toc">
      <li><a href="#priorart"><span class="secno">2.1</span> <span class="content">Prior art</span></a>
      <li><a href="#applications"><span class="secno">2.2</span> <span class="content">Further applications</span></a>
     </ol>
    <li>
     <a href="#design"><span class="secno">3</span> <span class="content">Design Decisions</span></a>
     <ol class="toc">
      <li><a href="#usercode"><span class="secno">3.1</span> <span class="content">Do we need this trait in the Standard Library? Can it be implemented entirely in user code?</span></a>
      <li><a href="#padding"><span class="secno">3.2</span> <span class="content">What about padding bits?</span></a>
      <li><a href="#naming"><span class="secno">3.3</span> <span class="content">Bikeshedding: naming</span></a>
      <li><a href="#future"><span class="secno">3.4</span> <span class="content">Future work</span></a>
     </ol>
    <li><a href="#impact"><span class="secno">4</span> <span class="content">Impact on the Standard</span></a>
    <li><a href="#technicalspecifications"><span class="secno">5</span> <span class="content">Technical Specifications</span></a>
    <li><a href="#proposedwording"><span class="secno">6</span> <span class="content">Proposed wording</span></a>
    <li><a href="#acknowledgements"><span class="secno">7</span> <span class="content">Acknowledgements</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>
   <ul>
    <li data-md>
     <p>R0</p>
     <ul>
      <li data-md>
       <p>First submission</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation and Scope</span><a class="self-link" href="#motivation"></a></h2>
   <p>Consider the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uninitialized_value_construct</c-></code> algorithm. This algorithm constructs a number of objects into uninitialized storage, value-initializing each object. A crude implementation looks like this:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ForwardIt</c-><c- o>></c->
<c- b>void</c-> <c- n>uninitialized_value_construct</c-><c- p>(</c-><c- n>ForwardIt</c-> <c- n>first</c-><c- p>,</c-> <c- n>ForwardIt</c-> <c- n>last</c-><c- p>)</c->
<c- p>{</c->
    <c- k>using</c-> <c- n>Value</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>iterator_traits</c-><c- o>&lt;</c-><c- n>ForwardIt</c-><c- o>>::</c-><c- n>value_type</c-><c- p>;</c->
    <c- n>ForwardIt</c-> <c- n>current</c-> <c- o>=</c-> <c- n>first</c-><c- p>;</c->
    
    <c- k>try</c-> <c- p>{</c->
        <c- k>for</c-> <c- p>(;</c-> <c- n>current</c-> <c- o>!=</c-> <c- n>last</c-><c- p>;</c-> <c- o>++</c-><c- n>current</c-><c- p>)</c->
            <c- o>::</c-><c- k>new</c-> <c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- o>*</c-><c- n>current</c-><c- p>)</c-> <c- n>Value</c-><c- p>();</c->
    <c- p>}</c-> <c- k>catch</c-> <c- p>(...)</c-> <c- p>{</c->
        <c- n>std</c-><c- o>::</c-><c- n>destroy</c-><c- p>(</c-><c- n>first</c-><c- p>,</c-> <c- n>current</c-><c- p>);</c->
        <c- k>throw</c-><c- p>;</c->
    <c- p>}</c->
<c- p>}</c->
</pre>
   <p>This algorithm has a natural application in containers, since they have operations that require to value-initialize elements in bulk (such as <code class="highlight"><c- n>resize</c-><c- p>(</c-><c- n>N</c-><c- p>)</c-></code>, or a constructor such as <code class="highlight"><c- n>container</c-><c- o>&lt;</c-><c- n>X</c-><c- o>></c-> <c- n>obj</c-><c- p>(</c-><c- n>N</c-><c- p>)</c-></code>).</p>
   <p>(Technically speaking, allocator-aware containers cannot use the algorithm directly, because they are supposed to use <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocator_traits</c-><c- o>&lt;</c-><c- n>Alloc</c-><c- o>>::</c-><c- n>construct</c-></code>; we’ll come back to this in a second.)</p>
   <p>There’s an enormous performance improvement possible in case we need to value-initialize objects of a "simple" datatype, for instance <code class="highlight"><c- b>int</c-></code>, and we’re constructing over contiguous storage (f.i. <code class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>). In this case, the <code class="highlight"><c- k>for</c-></code> loop can be entirely replaced by a completely equivalent call to <code class="highlight"><c- n>memset</c-><c- p>(</c-><c- n>ptr</c-><c- p>,</c-> <c- mi>0</c-><c- p>,</c-> <c- n>bytes_size</c-><c- p>);</c-></code>. In case we’re acquiring brand new storage, it could be acquired using <code class="highlight"><c- n>calloc</c-></code> instead of <code class="highlight"><c- n>malloc</c-></code>.</p>
   <p>It turns out that optimizing <em>compilers</em> already do this transformation. For instance, GCC 12 has this codegen on X86-64:</p>
   <table>
    <thead>
    <tbody>
     <tr>
      <td>
<pre class="highlight"><c- cp>#include</c-> &lt;new>

<c- k>using</c-> <c- n>T</c-> <c- o>=</c-> <c- b>int</c-><c- p>;</c->
<c- k>extern</c-> <c- n>T</c-> <c- n>buffer</c-><c- p>[];</c->
<c- k>extern</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>N</c-><c- p>;</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- k>for</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>)</c->
        <c- o>::</c-><c- k>new</c-> <c- p>(</c-><c- n>buffer</c-> <c- o>+</c-> <c- n>i</c-><c- p>)</c-> <c- n>T</c-><c- p>();</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- nl>main</c-><c- p>:</c->
        <c- n>mov</c->     <c- n>rdx</c-><c- p>,</c-> <c- n>QWORD</c-> <c- n>PTR</c-> <c- n>N</c-><c- p>[</c-><c- n>rip</c-><c- p>]</c->
        <c- n>test</c->    <c- n>rdx</c-><c- p>,</c-> <c- n>rdx</c->
        <c- n>je</c->      <c- p>.</c-><c- n>L7</c->
        <c- n>sub</c->     <c- n>rsp</c-><c- p>,</c-> <c- mi>8</c->
        <c- n>sal</c->     <c- n>rdx</c-><c- p>,</c-> <c- mi>2</c->
        <c- n>xor</c->     <c- n>esi</c-><c- p>,</c-> <c- n>esi</c->
        <c- n>mov</c->     <c- n>edi</c-><c- p>,</c-> <c- n>OFFSET</c-> <c- n>FLAT</c-><c- o>:</c-><c- n>buffer</c->
        <c- n>call</c->    <c- n>memset</c->
        <c- n>xor</c->     <c- n>eax</c-><c- p>,</c-> <c- n>eax</c->
        <c- n>add</c->     <c- n>rsp</c-><c- p>,</c-> <c- mi>8</c->
        <c- n>ret</c->
<c- p>.</c-><c- n>L7</c-><c- o>:</c->
        <c- n>xor</c->     <c- n>eax</c-><c- p>,</c-> <c- n>eax</c->
        <c- n>ret</c->
</pre>
   </table>
   <p><a href="https://gcc.godbolt.org/z/rvo7WG1Mj">Compiler Explorer</a> shows that GCC 12, Clang 15, MSVC "latest" all implement this optimization.</p>
   <p>The branch in the generated code exists to avoid potentially passing <code class="highlight"><c- k>nullptr</c-></code> to <code class="highlight"><c- n>memset</c-></code>, which is undefined behavior (even if <code class="highlight"><c- n>N</c-></code> is 0). Adding a compiler assumption on <code class="highlight"><c- n>N</c-> <c- o>></c-> <c- mi>0</c-></code> makes the branch disappear.</p>
   <p>This optimization is extremely advantageous; amongst other things, as mentioned before, an allocator-aware container needs to add a further indirection to construct each element. An optimizer can "see through" all the relevant code and replace a <code class="highlight"><c- n>construct</c-></code> loop with much more efficient code.</p>
   <p>However, relying on the optimizer comes with the usual set of problems:</p>
   <ul>
    <li data-md>
     <p>compilers sometimes miss the transformation and leave the loop in the code (<a href="https://gcc.godbolt.org/z/K765v5Mro">example</a> of GCC missing the optimization);</p>
    <li data-md>
     <p>one needs aggressive optimizations turned on (under GCC, at least -O2 is necessary);</p>
    <li data-md>
     <p>optimizations hurt the debugging experience, and disabling optimizations leads to very inefficient code generation, which <em>also</em> hurts debugging (lose/lose scenario);</p>
    <li data-md>
     <p>optimizations increase compilation times, and this hurts the development cycle.</p>
   </ul>
   <p>For these reasons, many libraries manually implement the optimization above in their source code. In other words, if they can detect that it’s "safe" to zero-fill memory in order to perform value initialization for a given type <code class="highlight"><c- n>T</c-></code>, then they will explictly call <code class="highlight"><c- n>memset</c-></code>.</p>
   <p><strong>What this paper proposes is a type trait that implements this detection so that it is correct and complete.</strong> Such a trait is currently lacking from the Standard Library.</p>
   <h3 class="heading settled" data-level="2.1" id="priorart"><span class="secno">2.1. </span><span class="content">Prior art</span><a class="self-link" href="#priorart"></a></h3>
   <p>Usage of <code class="highlight"><c- n>memset</c-></code> to zero-fill in order to achieve value initialization happens for instance in <a data-link-type="biblio" href="#biblio-boost-container" title="Boost.Container 1.81.0: Non-standard value initialization using std::memset">Boost.Container</a> (in spite of the containers being allocator-aware!); in <a data-link-type="biblio" href="#biblio-folly" title="IsZeroInitializable type trait">FBVector from Folly</a> through the <code class="highlight"><c- n>IsZeroInitializable</c-></code> type trait; and used to happen in Qt container classes (which are not allocator-aware).</p>
   <p>Since there isn’t a standard type trait that detects if zero-filling is possible for a type <code class="highlight"><c- n>T</c-></code>, all of these libraries use an ad-hoc detection, which is incomplete and, in many cases, incorrect. Specifically:</p>
   <ul>
    <li data-md>
     <p>Boost.Container uses zero-filling by default on integer types, floating point types, pointers to object and functions (à la <code class="highlight"><c- n>is_pointer</c-></code> -- excluding pointers to data members / member functions), and before Boost 1.82, also POD types. There are opt-outs available using certain preprocessor macros.</p>
    <li data-md>
     <p>Folly uses zero-filling on non-class types by default. There is an opt-in available (specializing the <code class="highlight"><c- n>IsZeroInitializable</c-></code> trait).</p>
    <li data-md>
     <p>Qt’s contiguous containers (e.g. QVector) used to zero-fill trivial types, and an opt-in was offered through a type trait (<code class="highlight"><c- n>Q_PRIMITIVE_TYPE</c-></code>). Today, the containers do not longer zero-fill (for the reasons discussed below); certain type erasure facilities zero-fill only scalar types that aren’t pointers to members.</p>
   </ul>
   <p>This detection is clearly <em>incomplete</em>:</p>
   <ul>
    <li data-md>
     <p>a non-POD type (e.g. a non-standard-layout one) such as <code class="highlight"><c- k>class</c-> <c- nc>C</c-> <c- p>{</c-> <c- k>public</c-><c- o>:</c-> <c- b>int</c-> <c- n>x</c-><c- p>;</c-> <c- k>private</c-><c- o>:</c-> <c- b>int</c-> <c- n>y</c-><c- p>;</c-> <c- p>};</c-></code> is in principle zero-fillable, but Boost.Container classes won’t use <code class="highlight"><c- n>memset</c-></code> on it;</p>
    <li data-md>
     <p>the same type won’t be zero-filled automatically by <code class="highlight"><c- n>FBVector</c-></code>, unless one enables the corresponding trait (<code class="highlight"><c- n>C</c-></code> is a class type);</p>
    <li data-md>
     <p>and finally the same type won’t be zero-filled by Qt, which no longer considers any user-defined type as zero-fillable.</p>
   </ul>
   <p>Is the detection even correct?</p>
   <ul>
    <li data-md>
     <p>All three libraries correctly detect integers (a zero-filled representation must give the value 0, which matches value initialization); floating point types (same, assuming IEEE 754 representation); and pointers to objects and pointers to functions (on any common ABI).</p>
    <li data-md>
     <p>Pointers to data members and member functions are more problematic. For instance, on the Itanium C++ ABI, a pointer to data member <em>cannot</em> be zero filled in order to be value initialized; it must instead be initialized with the value -1 (cf. <a data-link-type="biblio" href="#biblio-itanium" title="Itanium C++ ABI, 2.3.1 Data Member Pointers">the specification</a>). Boost and Qt correctly handle this case, but <strong>Folly does not</strong>.</p>
    <li data-md>
     <p>Folly also zero-fills <em>union</em> types, even when their value initialization cannot be achieved this way (upstream <a href="https://github.com/facebook/folly/issues/1271">bug</a>).</p>
    <li data-md>
     <p>It is impossible to know if a class type can be zero-filled, as a class may contain such a pointer to data member. <strong>Boost erroneously did not exclude class types</strong>, and in fact considered a POD type such as <code class="highlight"><c- k>struct</c-> <c- nc>S</c-> <c- p>{</c-> <c- b>int</c-> <c- n>S</c-><c- o>::*</c-><c- n>ptr</c-><c- p>;</c-> <c- p>};</c-></code> as zero-fillable (upstream <a href="https://github.com/boostorg/container/issues/238">bug</a>, fixed in Boost 1.82). On Itanium, as we have just discussed, <code class="highlight"><c- n>S</c-></code> is not zero-fillable; the result is that the elements in e.g. a <code class="highlight"><c- n>boost</c-><c- o>::</c-><c- n>container</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>S</c-><c- o>></c-> <c- n>v</c-><c- p>(</c-><c- mi>10</c-><c- p>);</c-></code> object have not been correctly value initialized -- their <code class="highlight"><c- n>ptr</c-></code> members are <em>not</em> null pointers (!).</p>
    <li data-md>
     <p><strong>Qt has also historically had the very same bug</strong>: the <code class="highlight"><c- n>S</c-></code> type above is trivial, and Qt used to consider trivial types as zero-fillable. This problem was fixed in general only very recently (December 2022, see <a href="https://codereview.qt-project.org/c/qt/qtbase/+/450639">here</a>, <a href="https://bugreports.qt.io/browse/QTBUG-109594">here</a>, <a href="https://codereview.qt-project.org/c/qt/qtbase/+/321764">here</a>). In principle, Qt could reintroduce zero-filling in containers using a limited detection.</p>
   </ul>
   <p>The conclusion is that creating an ad-hoc detection is incomplete and extremely error prone. <em>Expert</em> C++ developers from three major C++ libraries have consistently got it wrong. Moreover, we do not believe that this trait can be fully implemented in user code without some form of compiler support (cf. <a href="#usercode">§ 3.1 Do we need this trait in the Standard Library? Can it be implemented entirely in user code?</a>).</p>
   <p>These considerations call for adding this trait to the Standard Library.</p>
   <h3 class="heading settled" data-level="2.2" id="applications"><span class="secno">2.2. </span><span class="content">Further applications</span><a class="self-link" href="#applications"></a></h3>
   <p>The trait that we are proposing can also be used as an optimization for type-erased factories.</p>
   <p>In order to build a value-initialized instance of a type <code class="highlight"><c- n>T</c-></code> (identified by some means by the factory -- the name, an id, etc.), the factory would normally need to store a pointer to a "construction function" that performs value initialization for <code class="highlight"><c- n>T</c-></code> in some storage space. If the factory can detect that <code class="highlight"><c- n>T</c-></code> can be value-initialized by zero filling, it could store that information somewhere (e.g. alongside <code class="highlight"><c- n>T</c-></code>'s other metadata such as size, alignment, etc.) and simply use <code class="highlight"><c- n>memset</c-></code> instead. The construction function for <code class="highlight"><c- n>T</c-></code> would then <em>not</em> be generated at all, and this would reduce code bloat (by generating less code). Qt uses this optimization in <a href="https://github.com/qt/qtbase/blob/6.5/src/corelib/kernel/qmetatype.h#L2395">QMetaType</a>.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design Decisions</span><a class="self-link" href="#design"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="usercode"><span class="secno">3.1. </span><span class="content">Do we need this trait in the Standard Library? Can it be implemented entirely in user code?</span><a class="self-link" href="#usercode"></a></h3>
   <p>At the time of this writing we believe that it is <em>not possible</em> to implement this trait in a way that is correct and complete without using private compiler hooks. Basically, if a trivially default constructible type <code class="highlight"><c- n>T</c-></code> contains a pointer to data member, we cannot zero-fill it on Itanium, but there is no way to know if this is the case "from the outside". It is certainly an interesting application of the capabilities of a static reflection system, should C++ gain one.</p>
   <p>An interesting idea (many thanks to Ed Catmur) is to try to <code class="highlight"><c- n>bit_cast</c-></code> a value-initialized instance of type <code class="highlight"><c- n>T</c-></code> to an array of bytes of suitable size (e.g. <code class="highlight"><c- n>array</c-><c- o>&lt;</c-><c- b>unsigned</c-> <c- b>char</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-><c- o>></c-></code>). The result can then be checked for bits different from zero, for instance by comparing it against a zero-filled array:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_value_initialized_to_zero_v</c-> <c- o>=</c-> <c- p>[]</c->
<c- p>{</c->
    <c- k>using</c-> <c- n>A</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- b>unsigned</c-> <c- b>char</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-><c- o>></c-><c- p>;</c->
    <c- k>return</c-> <c- n>A</c-><c- p>{}</c-> <c- o>==</c-> <c- n>std</c-><c- o>::</c-><c- n>bit_cast</c-><c- o>&lt;</c-><c- n>A</c-><c- o>></c-><c- p>(</c-><c- n>T</c-><c- p>());</c->
<c- p>}();</c->
</pre>
   <p>This detection can then be combined with checking whether <code class="highlight"><c- n>T</c-></code> is trivially default constructible.</p>
   <p>Note that trivial default constructability implies that <code class="highlight"><c- n>T</c-></code> has not a user-provided default constructor ([class.default.ctor]/3), which also implies that value initialization performs zero initialization ([dcl.init.general]/9.1.1 and 9.3). If <code class="highlight"><c- n>T</c-></code> has padding bits, then the provision in [dcl.init.general]/6.2 ensures that they are set to 0 when performing zero initialization. This means that comparing against a zero-filled buffer will work correctly even in the presence of padding bits.</p>
   <p>The above snippet however does not work in case <code class="highlight"><c- n>T</c-></code> contains pointers, as <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bit_cast</c-></code> is not <code class="highlight"><c- k>constexpr</c-></code> in that context ([bit.cast]/3.2 and 3.3). Moreover, and pending <a data-link-type="biblio" href="#biblio-lwg2827" title="is_trivially_constructible and non-trivial destructors">[LWG2827]</a> resolution, in general <code class="highlight"><c- n>T</c-></code> should not be required to be trivially <em>copyable</em> (a constraint of <code class="highlight"><c- n>bit_cast</c-></code>, [bit.cast]/1.3); in fact, <code class="highlight"><c- n>T</c-></code> should not be required to be trivially destructible at all, but only trivially default constructible.</p>
   <p>In principle, the restriction of <code class="highlight"><c- n>bit_cast</c-></code> on pointers could be relaxed so that constant evaluation works if one asks to cast a <em>null pointer value</em>. Assuming we also solve the problem that we don’t want to require trivial copiability, we would still be left with a somehow tricky/clever/"experts-only" implementation; wrapping it in a standardized type trait would definitely increase its usability and discoverability.</p>
   <h3 class="heading settled" data-level="3.2" id="padding"><span class="secno">3.2. </span><span class="content">What about padding bits?</span><a class="self-link" href="#padding"></a></h3>
   <p>See the remark in <a href="#usercode">§ 3.1 Do we need this trait in the Standard Library? Can it be implemented entirely in user code?</a>.</p>
   <h3 class="heading settled" data-level="3.3" id="naming"><span class="secno">3.3. </span><span class="content">Bikeshedding: naming</span><a class="self-link" href="#naming"></a></h3>
   <p>The trait that we are proposing describes a type property which does not have a pre-existing name in the Standard. We must therefore introduce a new name.</p>
   <p>For the moment being, we are going to propose the (quite verbose) "trivially value-initializable by zero-filling" name. This describes all the characteristics that we are looking for:</p>
   <ul>
    <li data-md>
     <p>we want to achieve value initialization;</p>
    <li data-md>
     <p>this is done "trivially", in the sense that there is no "specialized" code to run;</p>
    <li data-md>
     <p>and specifically, it’s done by zero-filling storage.</p>
   </ul>
   <p>Another possible wording would be "trivially zero-initializable"; for trivially default constructible classes, value initialization always boils down to zero initialization. This could clash with possible <a href="#future">future extensions</a> of this trait (in case it is extended to types where value initialization does not perform zero initialization). In general, given that "zero initialization" does not imply "zero filling" (and vice-versa), we would prefer to highlight the latter name and avoid any possible confusion on the intended semantics.</p>
   <h3 class="heading settled" data-level="3.4" id="future"><span class="secno">3.4. </span><span class="content">Future work</span><a class="self-link" href="#future"></a></h3>
   <p>A possible future extension to this paper would be to also cover implicit-lifetime types, which are not necessarily trivially <em>default</em> constructible. For instance, consider a type like <code class="highlight"><c- n>string_view</c-></code>:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>string_view</c->
<c- p>{</c->
    <c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- n>begin</c-><c- p>,</c-> <c- o>*</c-><c- n>end</c-><c- p>;</c->
    
<c- k>public</c-><c- o>:</c->
    <c- c1>// not trivial</c->
    <c- k>constexpr</c-> <c- n>string_view</c-><c- p>()</c-> <c- k>noexcept</c-> <c- o>:</c-> <c- n>begin</c-><c- p>(</c-><c- k>nullptr</c-><c- p>),</c-> <c- n>end</c-><c- p>(</c-><c- k>nullptr</c-><c- p>)</c-> <c- p>{}</c->
<c- p>};</c->
</pre>
   <p>On all common implementations such as a class is value initializable via zero-filling. <code class="highlight"><c- n>string_view</c-></code> is also implicit-lifetime: it has a trivial copy constructor and a trivial non-deleted destructor. One can therefore use facilities such as <code class="highlight"><c- n>start_lifetime_as</c-></code> on a zero-filled storage to create <code class="highlight"><c- n>string_view</c-></code> objects.</p>
   <p>The problem here is that such a detection cannot be automatically done by the compiler, as it can’t "see" into the body of a non-trivial default constructor. Therefore, we will necessarily need an opt-in mechanism, such as a type trait or an attribute. This will necessarily complicate the language aspects, with implications similar to e.g. <a data-link-type="biblio" href="#biblio-p1144r6" title="Object relocation in terms of move plus destroy">[P1144R6]</a>'s <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> attribute.</p>
   <p>While we are not proposing such an extension at the moment, it is our belief that this paper should not impede it either.</p>
   <h2 class="heading settled" data-level="4" id="impact"><span class="secno">4. </span><span class="content">Impact on the Standard</span><a class="self-link" href="#impact"></a></h2>
   <p>This proposal adds a new property for types to the C++ language, and a corresponding type trait for this property to <code class="highlight"><c- o>&lt;</c-><c- n>type_traits</c-><c- o>></c-></code>. Vendors are expected to implement the trait through internal compiler hooks.</p>
   <p>It is expected that the results of the trait are implementation-specific, as it requires an implementation to consider object representations that are mandated by the architecture/ABI.</p>
   <h2 class="heading settled" data-level="5" id="technicalspecifications"><span class="secno">5. </span><span class="content">Technical Specifications</span><a class="self-link" href="#technicalspecifications"></a></h2>
   <p>All the proposed changes are relative to <a data-link-type="biblio" href="#biblio-n4892" title="Working Draft, Standard for Programming Language C++">[N4892]</a>.</p>
   <h2 class="heading settled" data-level="6" id="proposedwording"><span class="secno">6. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposedwording"></a></h2>
   <p>Add to the list in [version.syn]:</p>
   <blockquote>
<pre class="highlight"><ins><c- cp>#define __cpp_lib_is_trivially_value_initializable_by_zero_filling YYYYMML </c-><c- c1>// also in &lt;type_traits></c->
</ins></pre>
   </blockquote>
   <p>Add at the end of [basic.types.general]:</p>
   <blockquote>
    <ins>12 A type is <em>trivially value-initializabile by zero-filling</em> if it is:</ins>
    <br> 
    <ul>
     <li data-md>
      <ins>12.1 an integer type; or</ins>
     <li data-md>
      <ins>12.2 an enumeration type; or</ins>
     <li data-md>
      <ins>12.3 any other scalar type for which it is implementation-defined that it is trivially value-initializabile by zero-filling; or</ins>
     <li data-md>
      <ins>12.4 an array of trivially value-initializabile by zero-filling type; or</ins>
     <li data-md>
      <ins>12.5 a (possibly cv-qualified) trivially value-initializabile by zero-filling class type ([class.prop]).</ins>
    </ul>
    <ins>[<em>Note 6</em>: The object representation ([basic.types.general]) of a value-initialized object ([dcl.init.general]) of a trivially value-initializabile by zero-filling type <code class="highlight"><c- n>T</c-></code> consists of <em>N</em> <code class="highlight"><c- b>unsigned</c-> <c- b>char</c-></code> objects all equal to <code class="highlight"><c- mi>0</c-></code>, where <em>N</em> equals <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code>. Conversely, it is possible to value-initialize an object of type <code class="highlight"><c- n>T</c-></code> by filling <code class="highlight"><c- n>N</c-></code> bytes of suitable storage with zeroes, and starting the lifetime of the <code class="highlight"><c- n>T</c-></code> object in that storage ([basic.life]). — <em>end note</em>]</ins>
   </blockquote>
   <p>Add at the end of [class.prop]:</p>
   <blockquote>
    <ins>10 A class <code class="highlight"><c- n>S</c-></code> is a <em>trivially value-initializabile by zero-filling class</em> if:</ins>
    <ul>
     <li data-md>
      <ins>10.1 it has an eligible trivial default constructor ([class.default.ctor]), and</ins>
     <li data-md>
      <ins>10.2 all the non-static data members and base classes of <code class="highlight"><c- n>S</c-></code> are of trivially value-initializabile by zero-filling type ([basic.types.general]).</ins>
    </ul>
   </blockquote>
   <p>Modify [meta.type.synop] as shown. At the end of the first [meta.unary.prop] block:</p>
   <blockquote>
<pre class="highlight">  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- k>struct</c-> <c- nc>reference_converts_from_temporary</c-><c- p>;</c->
  
<ins>  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_trivially_value_initializable_by_zero_filling</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <p>And at the end of the second:</p>
   <blockquote>
<pre class="highlight">  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
    <c- k>constexpr</c-> <c- b>bool</c-> <c- n>reference_converts_from_temporary_v</c->
      <c- o>=</c-> <c- n>reference_converts_from_temporary</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c->

<ins>  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
    <c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_trivially_value_initializable_by_zero_filling_v</c->
      <c- o>=</c-> <c- n>is_trivially_value_initializable_by_zero_filling</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <p>Add a new row at the end of Table 48 in [meta.unary.prop]:</p>
   <blockquote>
    <table>
     <thead>
     <tbody>
      <tr>
       <td>
        <ins><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_trivially_value_initializable_by_zero_filling</c-><c- p>;</c-></code></ins>
       <td>
        <ins><code class="highlight"><c- n>T</c-></code> is a trivially value-initializabile by zero-filling type ([basic.types.general]).</ins>
       <td>
        <ins><code class="highlight"><c- n>T</c-></code> shall be a complete type, <em>cv</em> <code class="highlight"><c- b>void</c-></code>, or an array of unknown bound.</ins>
    </table>
   </blockquote>
   <h2 class="heading settled" data-level="7" id="acknowledgements"><span class="secno">7. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to KDAB for supporting this work.</p>
   <p>Thanks to Ed Catmur for the discussions and drafting a proposal to allow <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bit_cast</c-></code> of null pointer values during constant evaluation.</p>
   <p>Thanks to Thiago Macieira and Arthur O’Dwyer for the discussions.</p>
   <p>All remaining errors are ours and ours only.</p>
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  <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-boost-container">[BOOST-CONTAINER]
   <dd><a href="https://www.boost.org/doc/libs/1_81_0/doc/html/container/cpp_conformance.html#container.cpp_conformance.non_standard_memset_initialization"><cite>Boost.Container 1.81.0: Non-standard value initialization using std::memset</cite></a>. URL: <a href="https://www.boost.org/doc/libs/1_81_0/doc/html/container/cpp_conformance.html#container.cpp_conformance.non_standard_memset_initialization">https://www.boost.org/doc/libs/1_81_0/doc/html/container/cpp_conformance.html#container.cpp_conformance.non_standard_memset_initialization</a>
   <dt id="biblio-folly">[FOLLY]
   <dd><a href="https://github.com/facebook/folly/blob/main/folly/Traits.h#L446"><cite>IsZeroInitializable type trait</cite></a>. URL: <a href="https://github.com/facebook/folly/blob/main/folly/Traits.h#L446">https://github.com/facebook/folly/blob/main/folly/Traits.h#L446</a>
   <dt id="biblio-itanium">[ITANIUM]
   <dd><a href="https://itanium-cxx-abi.github.io/cxx-abi/abi.html#data-member-pointers"><cite>Itanium C++ ABI, 2.3.1 Data Member Pointers</cite></a>. URL: <a href="https://itanium-cxx-abi.github.io/cxx-abi/abi.html#data-member-pointers">https://itanium-cxx-abi.github.io/cxx-abi/abi.html#data-member-pointers</a>
   <dt id="biblio-lwg2827">[LWG2827]
   <dd>Richard Smith. <a href="https://wg21.link/lwg2827"><cite>is_trivially_constructible and non-trivial destructors</cite></a>. New. URL: <a href="https://wg21.link/lwg2827">https://wg21.link/lwg2827</a>
   <dt id="biblio-n4892">[N4892]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4892"><cite>Working Draft, Standard for Programming Language C++</cite></a>. 18 June 2021. URL: <a href="https://wg21.link/n4892">https://wg21.link/n4892</a>
   <dt id="biblio-p1144r6">[P1144R6]
   <dd>Arthur O'Dwyer. <a href="https://wg21.link/p1144r6"><cite>Object relocation in terms of move plus destroy</cite></a>. 10 June 2022. URL: <a href="https://wg21.link/p1144r6">https://wg21.link/p1144r6</a>
  </dl>