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  <title>P1144R8: std::is_trivially_relocatable</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 3383f409d, updated Fri Apr 28 15:49:22 2023 -0700" 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 {
    color: var(--a-normal-text);
    font-size: inherit;
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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">P1144R8<br>std::is_trivially_relocatable</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="2023-05-12">2023-05-12</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Issue Tracking:
     <dd><a href="#issues-index">Inline In Spec</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:arthur.j.odwyer@gmail.com">Arthur O'Dwyer</a>
     <dt>Audience:
     <dd>LEWG, EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Draft Revision:
     <dd>49
     <dt>Current Source:
     <dd><a href="https://github.com/Quuxplusone/draft/blob/gh-pages/d1144-object-relocation.bs">github.com/Quuxplusone/draft/blob/gh-pages/d1144-object-relocation.bs</a>
     <dt>Current:
     <dd><a href="https://rawgit.com/Quuxplusone/draft/gh-pages/d1144-object-relocation.html">rawgit.com/Quuxplusone/draft/gh-pages/d1144-object-relocation.html</a>
    </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 define a new verb, "relocate," equivalent to a move and a destroy.

  For many C++ types, this is implementable "trivially," as a single <code class="highlight"><c- n>memcpy</c-></code>.
  We propose a standard trait so library writers can detect such types.
  We propose a compiler rule that propagates trivial relocatability among Rule-of-Zero types.
  Finally, we propose a standard syntax for a user-defined type (e.g. <code class="highlight"><c- n>boost</c-><c- o>::</c-><c- n>shared_ptr</c-></code>) to warrant
  that it is trivially relocatable.
  P1144R3 was presented at C++Now 2019 as a 90-minute talk titled <a href="https://www.youtube.com/watch?v=SGdfPextuAU">"Trivially Relocatable."</a></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="#intro"><span class="secno">2</span> <span class="content">Introduction and motivation</span></a>
     <ol class="toc">
      <li>
       <a href="#applications"><span class="secno">2.1</span> <span class="content">Optimizations enabled by trivial relocatability</span></a>
       <ol class="toc">
        <li><a href="#benefit-resize"><span class="secno">2.1.1</span> <span class="content">Contiguous reallocation</span></a>
        <li><a href="#benefit-type-erasure"><span class="secno">2.1.2</span> <span class="content">Moving in-place/SBO type-erased types like <code class="highlight"><c- n>any</c-></code> and <code class="highlight"><c- n>function</c-></code></span></a>
        <li><a href="#benefit-fixed-capacity"><span class="secno">2.1.3</span> <span class="content">Moving fixed-capacity containers like <code class="highlight"><c- n>static_vector</c-></code> and <code class="highlight"><c- n>small_vector</c-></code></span></a>
        <li><a href="#benefit-insert-erase"><span class="secno">2.1.4</span> <span class="content">Contiguous insert and erase</span></a>
        <li><a href="#benefit-swap"><span class="secno">2.1.5</span> <span class="content">Swap</span></a>
       </ol>
      <li><a href="#benefit-assertions"><span class="secno">2.2</span> <span class="content">Assertions, not assumptions</span></a>
      <li><a href="#doing-it-wrong"><span class="secno">2.3</span> <span class="content">Eliminate manual warrants, improve safety</span></a>
     </ol>
    <li><a href="#design-goals"><span class="secno">3</span> <span class="content">Design goals</span></a>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Proposed wording for C++2b</span></a>
     <ol class="toc">
      <li><a href="#wording-algorithms.requirements"><span class="secno">4.1</span> <span class="content">Nothrow bidirectional iterator [algorithms.requirements]</span></a>
      <li><a href="#wording-defns.relocation"><span class="secno">4.2</span> <span class="content">Relocation operation [defns.relocation]</span></a>
      <li><a href="#wording-relocate"><span class="secno">4.3</span> <span class="content"><code class="highlight"><c- n>relocate_at</c-></code> and <code class="highlight"><c- n>relocate</c-></code> [specialized.relocate]</span></a>
      <li><a href="#wording-uninitialized.relocate"><span class="secno">4.4</span> <span class="content"><code class="highlight"><c- n>uninitialized_relocate</c-></code>, <code class="highlight"><c- n>uninitialized_relocate_n</c-></code>, <code class="highlight"><c- n>uninitialized_relocate_backward</c-></code> [uninitialized.relocate]</span></a>
      <li><a href="#wording-basic.types.general"><span class="secno">4.5</span> <span class="content">Trivially relocatable type [basic.types.general]</span></a>
      <li><a href="#wording-dcl.attr.trivreloc"><span class="secno">4.6</span> <span class="content"><code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> attribute [dcl.attr.trivreloc]</span></a>
      <li><a href="#wording-meta.unary.prop"><span class="secno">4.7</span> <span class="content">Type trait <code class="highlight"><c- n>is_trivially_relocatable</c-></code> [meta.unary.prop]</span></a>
      <li><a href="#wording-concept.relocatable"><span class="secno">4.8</span> <span class="content"><code class="highlight"><c- n>relocatable</c-></code> concept [concept.relocatable]</span></a>
     </ol>
    <li>
     <a href="#alternatives"><span class="secno">5</span> <span class="content">Rationale and alternatives</span></a>
     <ol class="toc">
      <li><a href="#maybe-trivially-relocatable"><span class="secno">5.1</span> <span class="content">Attribute <code class="highlight"><c- p>[[</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code></span></a>
      <li><a href="#pmr-concerns"><span class="secno">5.2</span> <span class="content">Confusing interactions with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-></code></span></a>
      <li><a href="#overlap-concerns"><span class="secno">5.3</span> <span class="content">Non-issue: Overlapping base-class subobjects</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">6</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#polls"><span class="secno"></span> <span class="content">Appendix A: Straw polls</span></a>
     <ol class="toc">
      <li><a href="#taken-polls-2023-02-10"><span class="secno"></span> <span class="content">Polls taken at EWGI at Issaquah on 2023-02-10</span></a>
      <li><a href="#taken-polls-2020-02-13"><span class="secno"></span> <span class="content">Polls taken at EWGI at Prague on 2020-02-13</span></a>
     </ol>
    <li><a href="#samples"><span class="secno"></span> <span class="content">Appendix B: Sample code</span></a>
    <li>
     <a href="#non-trivial-samples"><span class="secno"></span> <span class="content">Appendix C: Examples of non-trivially relocatable class types</span></a>
     <ol class="toc">
      <li><a href="#non-trivial-sample-string"><span class="secno"></span> <span class="content">Class contains pointer to self</span></a>
      <li><a href="#non-trivial-sample-list"><span class="secno"></span> <span class="content">Allocated memory contains pointer to self</span></a>
      <li><a href="#non-trivial-sample-offset-ptr"><span class="secno"></span> <span class="content">Class invariant depends on <code class="highlight"><c- k>this</c-></code></span></a>
      <li><a href="#non-trivial-sample-registry"><span class="secno"></span> <span class="content">Program invariant depends on <code class="highlight"><c- k>this</c-></code></span></a>
      <li><a href="#non-trivial-sample-polymorphic"><span class="secno"></span> <span class="content">Polymorphic downcast effectively relies on offset-into-self</span></a>
     </ol>
    <li><a href="#implementation"><span class="secno"></span> <span class="content">Appendix D: Implementation experience</span></a>
    <li><a href="#open-questions"><span class="secno"></span> <span class="content">Appendix E: Open questions</span></a>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </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>R8 (pre-Varna 2023):</p>
     <ul>
      <li data-md>
       <p><a href="#wording-dcl.attr.trivreloc">dcl.attr.trivreloc</a>: Removed "If a type <code class="highlight"><c- n>T</c-></code> is declared <code class="highlight"><c- n>trivially_relocatable</c-></code>,
    and <code class="highlight"><c- n>T</c-></code> is either not move-constructible or not destructible, the program is ill-formed."
    This should have been removed in R7 along with "move-constructible, destructible."</p>
      <li data-md>
       <p><a href="#wording-meta.unary.prop">meta.unary.prop</a>: Adjusted the precondition of <code class="highlight"><c- n>is_trivially_relocatable_v</c-></code> to match <code class="highlight"><c- n>is_trivially_copyable_v</c-></code>.
    Removed <code class="highlight"><c- n>is_relocatable_v</c-></code>, <code class="highlight"><c- n>is_nothrow_relocatable_v</c-></code> for insufficient motivation (but kept <code class="highlight"><c- k>concept</c-> <c- nc>relocatable</c-></code> because it expresses semantic requirements).</p>
      <li data-md>
       <p>Marked <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>relocate</c-></code> as <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code>; changed its return type to <code class="highlight"><c- n>remove_cv_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>.</p>
      <li data-md>
       <p>Resolved the <a href="#open-questions">Appendix E</a> question about potentially overlapping subobjects;
    we have implementation experience now that proves this is not a problem. Expanded discussion
    of <code class="highlight"><c- n>pmr</c-></code> types.</p>
      <li data-md>
       <p>Replaced <a href="#design-goals">§3 "Design goals"</a> with a shorter summary.</p>
     </ul>
    <li data-md>
     <p>R7 (post-Issaquah 2023):</p>
     <ul>
      <li data-md>
       <p><a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> debuted at Issaquah; it raised several open questions (see <a href="#open-questions">Appendix E: Open questions</a>).
    The authors of P1144 and P2786 plan a joint paper to resolve some of these design issues.</p>
      <li data-md>
       <p>Added <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uninitialized_relocate_backward</c-></code>, the building block of <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>insert</c-></code>.</p>
      <li data-md>
       <p>Removed "move-constructible, destructible" from the requirements in <a href="#wording-basic.types.general">basic.types.general</a>.
    Now trivially relocatable types, like trivially copyable types, can be non-relocatable.</p>
      <li data-md>
       <p>Added <a href="#applications">table §2.1</a> directly comparing this proposal to Folly, BSL, and Qt.</p>
      <li data-md>
       <p>Added straw poll results from Issaquah; removed much background and discussion
    (leaving only notes for the interested reader to consult <a data-link-type="biblio" href="#biblio-p1144r6" title="Object relocation in terms of move plus destroy">[P1144R6]</a>).</p>
      <li data-md>
       <p>Removed mentions of EASTL.</p>
      <li data-md>
       <p>Added <a href="#open-questions">Appendix E</a>.</p>
     </ul>
    <li data-md>
     <p>R6:</p>
     <ul>
      <li data-md>
       <p>Added <code class="highlight"><c- n>T</c-> <c- n>std</c-><c- o>::</c-><c- n>relocate</c-><c- p>(</c-><c- n>T</c-> <c- o>*</c-><c- n>source</c-><c- p>)</c-></code>.</p>
      <li data-md>
       <p>Changed the exception guarantee of <code class="highlight"><c- n>uninitialized_relocate</c-></code> and <code class="highlight"><c- n>uninitialized_relocate_n</c-></code> in accord with feedback from David Stone.</p>
      <li data-md>
       <p>Removed <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>relocate_at</c-></code> (but kept <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>relocate_at</c-></code>).</p>
     </ul>
    <li data-md>
     <p>R5 (post-Prague 2020):</p>
     <ul>
      <li data-md>
       <p>Polymorphic types (those with any vptr at all) inhibit "natural" trivial relocatability.
    David Stone gave the motivating example. See <a href="#non-trivial-sample-polymorphic">Polymorphic downcast effectively relies on offset-into-self</a>.</p>
      <li data-md>
       <p>Updated <code class="highlight"><c- k>concept</c-> <c- nc>relocatable</c-></code>. Added semantic requirements on assignment and some discussion
    of the issue with <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>insert</c-></code>.</p>
     </ul>
    <li data-md>
     <p>R3 (post-Kona 2019):</p>
     <ul>
      <li data-md>
       <p>User-provided copy constructors and copy/move assignment operators inhibit "natural" trivial relocatability.
    This permits optimizing <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>insert</c-></code> and <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>erase</c-></code>.</p>
      <li data-md>
       <p>Adopted <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>(</c-><c- b>bool</c-><c- p>)]]</c-></code>.</p>
      <li data-md>
       <p>Added <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>relocate_at</c-><c- p>(</c-><c- n>source</c-><c- p>,</c-> <c- n>dest</c-><c- p>)</c-></code>.</p>
      <li data-md>
       <p>Removed the core-language blanket wording that permitted "the implementation" to eliminate
    moves and destroys of trivially relocatable types. Instead, the existence of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>relocate_at</c-></code> and handwaviness of e.g. <a href="http://eel.is/c++draft/vector#capacity-4"> [vector.capacity</a>] combine to strongly imply
    (but still not to mandate) that library vendors will use <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>relocate_at</c-></code> wherever
    it helps with performance. We no longer propose to permit eliminating
    moves and destroys in e.g. <code class="highlight"><c- k>return</c-></code> statements, except as already permitted under the as-if rule.</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="intro"><span class="secno">2. </span><span class="content">Introduction and motivation</span><a class="self-link" href="#intro"></a></h2>
   <p>Given an object type <code class="highlight"><c- n>T</c-></code> and memory addresses <code class="highlight"><c- n>src</c-></code> and <code class="highlight"><c- n>dst</c-></code>,
the phrase "<b><i>relocate</i></b> a <code class="highlight"><c- n>T</c-></code> from <code class="highlight"><c- n>src</c-></code> to <code class="highlight"><c- n>dst</c-></code>" means
"<em>move-construct</em> <code class="highlight"><c- n>dst</c-></code> from <code class="highlight"><c- n>src</c-></code>, and then immediately <em>destroy</em> the object at <code class="highlight"><c- n>src</c-></code>."</p>
   <p>Any type which is both move-constructible and destructible is <b><i>relocatable</i></b>.
A type is <b><i>nothrow relocatable</i></b> if its relocation operation is noexcept, and a type
is <b><i><a data-link-type="abstract-op" href="#abstract-opdef-trivially-relocatable" id="ref-for-abstract-opdef-trivially-relocatable">trivially relocatable</a></i></b> if its relocation operation is trivial (which,
just like trivial move-construction and trivial copy-construction, means
"tantamount to <code class="highlight"><c- n>memcpy</c-></code>").</p>
   <p>In practice, almost all relocatable types are trivially relocatable: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> (on libc++), <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> (on MSVC).
Examples of non-trivially relocatable types include <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> (on libstdc++ and libc++), <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> (on libstdc++ and MSVC), <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>set</c-></code> (everywhere). See <a href="#non-trivial-samples">Appendix C: Examples of non-trivially relocatable class types</a>.</p>
   <p>P1144 allows the library programmer
to <b><i>warrant</i></b> to the compiler that a resource-management type is trivially relocatable.
Explicit warrants are rarely needed because the compiler can infer trivial relocatability for
Rule-of-Zero types. See <a href="#wording-basic.types.general">§ 4.5 Trivially relocatable type [basic.types.general]</a>.</p>
   <p>The most important thing about P1144 relocation is that it is backward compatible and does not
break either API or ABI. My intention is simply to legalize the well-understood tricks
that many industry codebases (<a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>, <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>, <a data-link-type="biblio" href="#biblio-deque" title="Improve relocation ... (__is_trivially_relocatable): Specialize for deque">[Deque]</a>) are already doing in practice.
P1144 is not intended to change the behavior of any existing source code (except to speed it up),
nor does it require major work from standard library vendors.</p>
   <h3 class="heading settled" data-level="2.1" id="applications"><span class="secno">2.1. </span><span class="content">Optimizations enabled by trivial relocatability</span><a class="self-link" href="#applications"></a></h3>
   <p>The following optimizations are possible according to P1144R8’s notion of
trivial relocatability. Here’s who does these optimizations in practice:</p>
   <table>
    <tbody>
     <tr>
      <td>
      <td> vector realloc 
      <td> type erasure 
      <td> fixed-cap move 
      <td> vector <code class="highlight"><c- n>erase</c-></code> 
      <td> vector <code class="highlight"><c- n>insert</c-></code> 
      <td> <code class="highlight"><c- n>rotate</c-></code> 
      <td> <code class="highlight"><c- n>swap</c-></code> 
     <tr>
      <td> Arthur’s libc++ <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> 
      <td> <a href="https://github.com/Quuxplusone/llvm-project/blob/d0c5d3639e/libcxx/include/vector#L936-L942"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code></a> 
      <td> no 
      <td> N/A 
      <td> <a href="https://github.com/Quuxplusone/llvm-project/blob/e8b61de07/libcxx/include/vector#L1663-L1669"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code></a><br> (<a href="https://godbolt.org/z/18hvvxjE4">Godbolt</a>) 
      <td> <a href="https://github.com/Quuxplusone/llvm-project/blob/e8b61de07/libcxx/include/vector#L1722-L1732"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code></a><br> (<a href="https://godbolt.org/z/18hvvxjE4">Godbolt</a>) 
      <td> yes, uses <code class="highlight"><c- n>swap_ranges</c-></code><br> (<a href="https://godbolt.org/z/onxrEce6G">Godbolt</a>) 
      <td> <a href="https://github.com/Quuxplusone/llvm-project/blob/e8b61de07/libcxx/include/__algorithm/swap_ranges.h#L59-L75"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap_ranges</c-></code></a><br> (<a href="https://godbolt.org/z/onxrEce6G">Godbolt</a>) 
     <tr>
      <td> <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a> <code class="highlight"><c- n>bslmf</c-><c- o>::</c-><c- n>IsBitwiseMoveable</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> 
      <td> <a href="https://github.com/bloomberg/bde/blob/e15f05be6/groups/bsl/bslstl/bslstl_vector.h#L3374-L3398"><code class="highlight"><c- n>bsl</c-><c- o>::</c-><c- n>vector</c-></code></a> 
      <td> <a href="https://github.com/bloomberg/bde/blob/e15f05be6/groups/bsl/bslstl/bslstl_function.h#L116-L131">no</a> 
      <td> ? 
      <td> <a href="https://github.com/bloomberg/bde/blob/e15f05be6/groups/bsl/bslalg/bslalg_arrayprimitives.h#L3769-L3800"><code class="highlight"><c- n>bsl</c-><c- o>::</c-><c- n>vector</c-></code></a> 
      <td> <a href="https://github.com/bloomberg/bde/blob/e15f05be6/groups/bsl/bslalg/bslalg_arrayprimitives.h#L3606-L3643"><code class="highlight"><c- n>bsl</c-><c- o>::</c-><c- n>vector</c-></code></a> 
      <td> <a href="https://github.com/bloomberg/bde/blob/e15f05be6/groups/bsl/bslalg/bslalg_arrayprimitives.h#L1707-L1722"><code class="highlight"><c- n>ArrayPrimitives_Imp</c-><c- o>::</c-><c- n>rotate</c-></code></a>, unused by <code class="highlight"><c- n>bsl</c-><c- o>::</c-><c- n>rotate</c-></code> 
      <td> no 
     <tr>
      <td> <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a> <code class="highlight"><c- n>folly</c-><c- o>::</c-><c- n>IsRelocatable</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> 
      <td> <a href="https://github.com/facebook/folly/blob/1d4690d0a3/folly/FBVector.h#L945-L966"><code class="highlight"><c- n>fbvector</c-></code></a> 
      <td> <a href="https://github.com/facebook/folly/blob/1d4690d0a3/folly/Function.h#L749-L755">no</a> 
      <td> <a href="https://github.com/facebook/folly/pull/1934">proposed</a> for <code class="highlight"><c- n>small_vector</c-></code> 
      <td> <a href="https://github.com/facebook/folly/blob/1d4690d0a3/folly/FBVector.h#L1232-L1259"><code class="highlight"><c- n>fbvector</c-></code></a> 
      <td> <a href="https://github.com/facebook/folly/blob/1d4690d0a3/folly/FBVector.h#L1273-L1292"><code class="highlight"><c- n>fbvector</c-></code></a> 
      <td> N/A 
      <td> N/A 
     <tr>
      <td> <a data-link-type="biblio" href="#biblio-qt" title="Qt Base">[Qt]</a> <code class="highlight"><c- n>QTypeInfo</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>isRelocatable</c-></code> 
      <td> <a href="https://github.com/qt/qtbase/blob/fbfee2d/src/corelib/tools/qarraydataops.h#L865-L872"><code class="highlight"><c- n>QList</c-></code></a> 
      <td> <a href="https://github.com/qt/qtbase/blob/fbfee2d/src/corelib/kernel/qvariant.h#L73-L94"><code class="highlight"><c- n>QVariant</c-></code></a> 
      <td> <a href="https://github.com/qt/qtbase/blob/fbfee2d/src/corelib/tools/qvarlengtharray.h#L312-L328"><code class="highlight"><c- n>QVarLengthArray</c-></code></a> 
      <td> <a href="https://github.com/qt/qtbase/blob/fbfee2d/src/corelib/tools/qarraydataops.h#L842-L863"><code class="highlight"><c- n>QList</c-></code></a> 
      <td> <a href="https://github.com/qt/qtbase/blob/fbfee2d/src/corelib/tools/qarraydataops.h#L679-L730"><code class="highlight"><c- n>QList</c-></code></a> 
      <td> <a href="https://github.com/qt/qtbase/blob/7db28fb/src/corelib/tools/qcontainertools_impl.h#L89-L107"><code class="highlight"><c- n>q_rotate</c-></code></a> 
      <td> N/A 
   </table>
   <h4 class="heading settled" data-level="2.1.1" id="benefit-resize"><span class="secno">2.1.1. </span><span class="content">Contiguous reallocation</span><a class="self-link" href="#benefit-resize"></a></h4>
   <p>Trivial relocation can be used anywhere we do the moral equivalent of <code class="highlight"><c- n>realloc</c-></code>,
such as in <code class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>::</c-><c- n>reserve</c-></code>.</p>
   <p><a data-link-type="biblio" href="#biblio-bench" title="Benchmark code from &quot;The Best Type Traits C++ Doesn&apos;t Have&quot;">[Bench]</a> (presented at C++Now 2018) shows a 3x speedup on <code class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>>::</c-><c- n>resize</c-></code>. <a href="https://www.reddit.com/r/cpp/comments/9wj4vt/trivially_relocatable_in_san_diego_call_for/e9p76i4/">This Reddit thread</a> demonstrates a similar 3x speedup using the online tool Quick-Bench.</p>
   <h4 class="heading settled" data-level="2.1.2" id="benefit-type-erasure"><span class="secno">2.1.2. </span><span class="content">Moving in-place/SBO type-erased types like <code class="highlight"><c- n>any</c-></code> and <code class="highlight"><c- n>function</c-></code></span><a class="self-link" href="#benefit-type-erasure"></a></h4>
   <p>Trivial relocation can be used to de-duplicate the code generated by type-erasing wrappers
like <code class="highlight"><c- n>any</c-></code>, <code class="highlight"><c- n>function</c-></code>, and <code class="highlight"><c- n>move_only_function</c-></code>.
For these types, a <em>move</em> of the wrapper object is implemented in terms of a <em>relocation</em> of the contained object. (See for example <a href="https://github.com/llvm-mirror/libcxx/blob/8fdc4918/include/any#L389-L394">libc++'s <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>any</c-></code></a>.)
In general, the <em>relocate</em> operation must have a different instantiation for each
different contained type <code class="highlight"><c- n>C</c-></code>, leading to code bloat. But every trivially relocatable <code class="highlight"><c- n>C</c-></code> of a given size can share the same instantiation.</p>
   <h4 class="heading settled" data-level="2.1.3" id="benefit-fixed-capacity"><span class="secno">2.1.3. </span><span class="content">Moving fixed-capacity containers like <code class="highlight"><c- n>static_vector</c-></code> and <code class="highlight"><c- n>small_vector</c-></code></span><a class="self-link" href="#benefit-fixed-capacity"></a></h4>
   <p>The move-constructor of <code class="highlight"><c- n>fixed_capacity_vector</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-><c- n>N</c-><c- o>></c-></code> can be implemented naïvely as an element-by-element <em>move</em> (leaving the source vector’s elements in their moved-from state),
or efficiently as an element-by-element <em>relocate</em> (leaving the source vector empty).</p>
   <p>For a detailed analysis of this case, see <a data-link-type="biblio" href="#biblio-fixedcapacityvector" title="P1144 case study: Moving a `fixed_capacity_vector`">[FixedCapacityVector]</a>.</p>
   <p><code class="highlight"><c- n>boost</c-><c- o>::</c-><c- n>container</c-><c- o>::</c-><c- n>static_vector</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-><c- n>N</c-><c- o>></c-></code> currently implements the
naïve element-by-element-move strategy, but after LEWG feedback, <a data-link-type="biblio" href="#biblio-p0843r5" title="static_vector">[P0843R5]</a> <code class="highlight"><c- n>static_vector</c-></code> does <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0843r5.html#Move-semantics">permit</a> the faster relocation strategy.</p>
   <h4 class="heading settled" data-level="2.1.4" id="benefit-insert-erase"><span class="secno">2.1.4. </span><span class="content">Contiguous insert and erase</span><a class="self-link" href="#benefit-insert-erase"></a></h4>
   <p>Trivial relocation can be used anywhere we shift a contiguous range left or right,
such as in <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>erase</c-></code> (i.e., destroy the erased elements and "close the window"
by memmoving left) and <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>insert</c-></code> (i.e., "make a window" by memmoving right and
then construct the new elements in place). <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>'s <code class="highlight"><c- n>fbvector</c-></code> does
this optimization; see <a href="https://github.com/facebook/folly/blob/1d4690d0a3/folly/FBVector.h#L1273-L1308"><code class="highlight"><c- n>fbvector</c-><c- o>::</c-><c- n>make_window</c-></code></a>.
Bloomberg’s <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a> also does this optimization.</p>
   <p>But <a href="#pmr-concerns">§ 5.2 Confusing interactions with std::pmr</a> determines whether this optimization will be possible for certain types.</p>
   <h4 class="heading settled" data-level="2.1.5" id="benefit-swap"><span class="secno">2.1.5. </span><span class="content">Swap</span><a class="self-link" href="#benefit-swap"></a></h4>
   <p>Given a reliable way of detecting trivial relocatability,
we can optimize any routine that uses the moral equivalent of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap</c-></code>, such as <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rotate</c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>partition</c-></code>, or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sort</c-></code>.</p>
   <p>Optimizing <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap</c-></code> produces massive code-size improvements for all swap-based
algorithms, including <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sort</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rotate</c-></code>. 
See <a href="https://www.youtube.com/watch?v=SGdfPextuAU&amp;t=19m56s">@19:56–21:22</a> in my C++Now 2019 talk,
and see <a href="https://p1144.godbolt.org/z/PPYhcYd8d">this Godbolt</a>. <a href="https://quick-bench.com/q/suAss7ElmwgLDbOJQGnlEseheSQ">This Quick-Bench</a> shows a 25% speedup
in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rotate</c-></code> when it is allowed to use bitwise swap on a Rule-of-Zero type.</p>
   <p>But <a href="#pmr-concerns">§ 5.2 Confusing interactions with std::pmr</a> determines whether this optimization will be possible for certain types.</p>
   <h3 class="heading settled" data-level="2.2" id="benefit-assertions"><span class="secno">2.2. </span><span class="content">Assertions, not assumptions</span><a class="self-link" href="#benefit-assertions"></a></h3>
   <p>Some data structures might reasonably assert the trivial relocatability of
their elements, just as they sometimes assert the stronger property of trivial <em>copyability</em> today.
This might help them guarantee reasonable performance, or guarantee exception-safety.</p>
   <h3 class="heading settled" data-level="2.3" id="doing-it-wrong"><span class="secno">2.3. </span><span class="content">Eliminate manual warrants, improve safety</span><a class="self-link" href="#doing-it-wrong"></a></h3>
   <p>Many real-world codebases contain templates which require
trivial relocatability of their template parameters, but cannot today <em>verify</em> trivial relocatability. For example, <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a> requires the programmer to warrant the trivial
relocatability of any type stored in a <code class="highlight"><c- n>folly</c-><c- o>::</c-><c- n>fbvector</c-></code>:</p>
<pre class="language-c++ highlight"><c- n>class</c-> <c- n>Widget</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>lst_</c-><c- p>;</c->
<c- p>};</c->

<c- n>folly</c-><c- o>::</c-><c- n>fbvector</c-><c- o>&lt;</c-><c- n>Widget</c-><c- o>></c-> <c- n>vec</c-><c- p>;</c->  <c- c1>// FAILS AT COMPILE TIME for lack of warrant</c->
</pre>
   <p>This merely encourages the programmer to add the warrant and continue. An incorrect
warrant will be discovered only at runtime, via undefined behavior. See <a href="#non-trivial-sample-list">Allocated memory contains pointer to self</a>, <a data-link-type="biblio" href="#biblio-follyissue889" title="Traits.h marks std::list as trivially relocatable, but in fact it is not">[FollyIssue889]</a>, and (most importantly) <a href="https://www.youtube.com/watch?v=SGdfPextuAU&amp;t=27m26s">@27:26–31:47</a> in my C++Now 2019 talk.</p>
<pre class="language-c++ highlight"><c- n>class</c-> <c- n>Gadget</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>lst_</c-><c- p>;</c->
<c- p>};</c->
<c- c1>// sigh, add the warrant on autopilot</c->
<c- n>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>folly</c-><c- o>::</c-><c- n>IsRelocatable</c-><c- o>&lt;</c-><c- n>Gadget</c-><c- o>></c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>true_type</c-> <c- p>{};</c->

<c- n>folly</c-><c- o>::</c-><c- n>fbvector</c-><c- o>&lt;</c-><c- n>Gadget</c-><c- o>></c-> <c- n>vec</c-><c- p>;</c->  <c- c1>// CRASHES AT RUNTIME due to fraudulent warrant</c->
</pre>
   <p>If this proposal is adopted, then Folly can start using <code class="highlight"><c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>)</c-></code> in the implementation of <code class="highlight"><c- n>fbvector</c-></code>, and the programmer can stop writing explicit warrants.
Finally, the programmer can start writing assertions of correctness, which aids maintainability and
can even find real bugs. <a href="https://p1144.godbolt.org/z/7b8893sjr">Example:</a></p>
<pre class="language-c++ highlight"><c- n>class</c-> <c- n>Widget</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>lst_</c-><c- p>;</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>Widget</c-><c- o>></c-><c- p>);</c->  <c- c1>// correctly SUCCEEDS</c->

<c- n>class</c-> <c- n>Gadget</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>lst_</c-><c- p>;</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>Gadget</c-><c- o>></c-><c- p>);</c->  <c- c1>// correctly ERRORS</c->
</pre>
   <p>The improvement in user experience and safety in real-world codebases (<a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>, <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>, <a data-link-type="biblio" href="#biblio-qt" title="Qt Base">[Qt]</a>, etc.)
is the most important benefit to be gained by this proposal.</p>
   <h2 class="heading settled" data-level="3" id="design-goals"><span class="secno">3. </span><span class="content">Design goals</span><a class="self-link" href="#design-goals"></a></h2>
   <p>For the former, longer, contents of this section, see <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p1144r7.html#design-goals">P1144R7 §3</a>.</p>
   <p>Briefly: We want to support all five of the following use-cases (<a href="https://godbolt.org/z/4WGq7ddWh">Godbolt</a>).</p>
<pre class="language-c++ highlight"><c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-><c- p>);</c-> <c- c1>// #1</c->

<c- k>struct</c-> <c- nc>RuleOfZero</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>p_</c-><c- p>;</c-> <c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>RuleOfZero</c-><c- o>></c-><c- p>);</c-> <c- c1>// #2</c->

<c- k>struct</c-> <c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-> <c- n>RuleOf3</c-> <c- p>{</c->
    <c- n>RuleOf3</c-><c- p>(</c-><c- n>RuleOf3</c-><c- o>&amp;&amp;</c-><c- p>);</c->
    <c- n>RuleOf3</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- n>RuleOf3</c-><c- o>&amp;&amp;</c-><c- p>);</c->
    <c- o>~</c-><c- n>RuleOf3</c-><c- p>();</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>RuleOf3</c-><c- o>></c-><c- p>);</c-> <c- c1>// #3</c->

<c- k>struct</c-> <c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-> <c- n>Wrap0</c-> <c- p>{</c->
    <c- n>boost</c-><c- o>::</c-><c- n>movelib</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>p_</c-><c- p>;</c->
    <c- k>static_assert</c-><c- p>(</c-><c- o>!</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>decltype</c-><c- p>(</c-><c- n>p_</c-><c- p>)</c-><c- o>></c-><c- p>);</c->
        <c- c1>// it’s not annotated, but we know it’s actually trivial</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>Wrap0</c-><c- o>></c-><c- p>);</c-> <c- c1>// #4</c->

<c- k>struct</c-> <c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-> <c- n>Wrap3</c-> <c- p>{</c->
    <c- n>Wrap3</c-><c- p>(</c-><c- n>Wrap3</c-><c- o>&amp;&amp;</c-><c- p>);</c->
    <c- n>Wrap3</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- n>Wrap3</c-><c- o>&amp;&amp;</c-><c- p>);</c->
    <c- o>~</c-><c- n>Wrap3</c-><c- p>();</c->
    <c- b>int</c-> <c- n>i_</c-><c- p>;</c->
    <c- n>boost</c-><c- o>::</c-><c- n>interprocess</c-><c- o>::</c-><c- n>offset_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>p_</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>i_</c-><c- p>;</c->
    <c- k>static_assert</c-><c- p>(</c-><c- o>!</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>decltype</c-><c- p>(</c-><c- n>p_</c-><c- p>)</c-><c- o>></c-><c- p>);</c->
        <c- c1>// it’s not trivial, but we preserve our invariant correctly</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>Wrap3</c-><c- o>></c-><c- p>);</c-> <c- c1>// #5</c->
</pre>
   <h2 class="heading settled" data-level="4" id="wording"><span class="secno">4. </span><span class="content">Proposed wording for C++2b</span><a class="self-link" href="#wording"></a></h2>
   <p>The wording in this section is relative to the current working draft (<a data-link-type="biblio" href="#biblio-n4944" title="Working Draft, Standard for Programming Language C++">[N4944]</a>).</p>
   <p class="note" role="note"><span class="marker">Note:</span> There is no difficulty in changing the attribute syntax to a contextual-keyword syntax;
the only downsides are aesthetic. We can defer that decision to the last minute, according
to CWG’s feedback on the final wording.</p>
   <h3 class="heading settled" data-level="4.1" id="wording-algorithms.requirements"><span class="secno">4.1. </span><span class="content">Nothrow bidirectional iterator [algorithms.requirements]</span><a class="self-link" href="#wording-algorithms.requirements"></a></h3>
   <p>Modify <a href="https://eel.is/c++draft/algorithms.requirements">[algorithms.requirements]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <ul><li data-md><p>If an algorithm’s template parameter is named <code class="highlight"><c- n>ForwardIterator</c-></code>, <code class="highlight"><c- n>ForwardIterator1</c-></code>, <code class="highlight"><c- n>ForwardIterator2</c-></code>, or <code class="highlight"><c- n>NoThrowForwardIterator</c-></code>,
the template argument shall meet the <em>Cpp17ForwardIterator</em> requirements ([forward.iterators]) if it is required to be a mutable iterator,
or model <code class="highlight"><c- n>forward_iterator</c-></code> ([iterator.concept.forward]) otherwise.</p> </li><li data-md><p>If an algorithm’s template parameter is named <code class="highlight"><c- n>NoThrowForwardIterator</c-></code>, the template argument is also required to have the property
that no exceptions are thrown from increment, assignment, or comparison of, or indirection through, valid iterators.</p> </li><li data-md><p>If an algorithm’s template parameter is named <code class="highlight"><c- n>BidirectionalIterator</c-></code>, <code class="highlight"><c- n>BidirectionalIterator1</c-></code>, <del>or</del> <code class="highlight"><c- n>BidirectionalIterator2</c-></code>, <ins>or <code class="highlight"><c- n>NoThrowBidirectionalIterator</c-></code>,</ins> the template argument shall meet the <em>Cpp17BidirectionalIterator</em> requirements ([bidirectional.iterators])
if it is required to be a mutable iterator, or model <code class="highlight"><c- n>bidirectional_iterator</c-></code> ([iterator.concept.bidir]) otherwise.</p> </li><li data-md><ins>If an algorithm’s template parameter is named <code class="highlight"><c- n>NoThrowBidirectionalIterator</c-></code>, the template argument is also required to have the property
that no exceptions are thrown from increment, decrement, assignment, or comparison of, or indirection through, valid iterators.</ins> </li></ul> </small>
   </blockquote>
   <h3 class="heading settled" data-level="4.2" id="wording-defns.relocation"><span class="secno">4.2. </span><span class="content">Relocation operation [defns.relocation]</span><a class="self-link" href="#wording-defns.relocation"></a></h3>
   <p>Add a new section in <a href="http://eel.is/c++draft/definitions">[intro.defs]</a>:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <dl><dt data-md><dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export id="abstract-opdef-relocation-operation">relocation operation</dfn></dt><dd data-md><p>the homogeneous binary operation performed by <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>relocate_at</c-></code>, consisting of a move construction immediately followed by a destruction of the source object</p> </dd></dl> </small>
   </blockquote>
   <h3 class="heading settled" data-level="4.3" id="wording-relocate"><span class="secno">4.3. </span><span class="content"><code class="highlight"><c- n>relocate_at</c-></code> and <code class="highlight"><c- n>relocate</c-></code> [specialized.relocate]</span><a class="self-link" href="#wording-relocate"></a></h3>
   <p>Add a new section after <a href="http://eel.is/c++draft/specialized.destroy">[specialized.destroy]</a>:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- n>T</c-> <c- o>*</c-><c- n>relocate_at</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>source</c-><c- p>,</c-> <c- n>T</c-><c- o>*</c-> <c- n>dest</c-><c- p>);</c->
</pre> <p><em>Mandates:</em> <code class="highlight"><c- n>T</c-></code> is a complete non-array object type.</p> <p><em>Effects:</em> Equivalent to:</p> <pre class="highlight"><c- k>struct</c-> <c- nc>guard</c-> <c- p>{</c-> <c- n>T</c-> <c- o>*</c-><c- n>t</c-><c- p>;</c-> <c- o>~</c-><c- n>guard</c-><c- p>()</c-> <c- p>{</c-> <c- n>destroy_at</c-><c- p>(</c-><c- n>t</c-><c- p>);</c-> <c- p>}</c-> <c- p>}</c-> <c- n>g</c-><c- p>(</c-><c- n>source</c-><c- p>);</c->
<c- k>return</c-> <c- o>::</c-><c- k>new</c-> <c- p>(</c-><c- n>voidify</c-><c- p>(</c-><c- o>*</c-><c- n>dest</c-><c- p>))</c-> <c- n>T</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- o>*</c-><c- n>source</c-><c- p>));</c->
</pre><p>except that if <code class="highlight"><c- n>T</c-></code> is trivially relocatable <i>[basic.types]</i>, side effects
associated with the relocation of the value of <code class="highlight"><c- o>*</c-><c- n>source</c-></code> might not happen.</p> <pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>remove_cv_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>relocate</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>source</c-><c- p>);</c->
</pre> <p><em>Mandates:</em> <code class="highlight"><c- n>T</c-></code> is a complete non-array object type.</p> <p><em>Effects:</em> Equivalent to:</p> <pre class="highlight"><c- n>remove_cv_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>t</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>source</c-><c- p>);</c->
<c- n>destroy_at</c-><c- p>(</c-><c- n>source</c-><c- p>);</c->
<c- k>return</c-> <c- n>t</c-><c- p>;</c->
</pre><p>except that if <code class="highlight"><c- n>T</c-></code> is trivially relocatable <i>[basic.types]</i>, side effects
associated with the relocation of the object’s value might not happen.</p> </small>
   </blockquote>
   <p class="note" role="note"><span class="marker">Note:</span> These functions have both been implemented in my libc++ fork; for <code class="highlight"><c- n>relocate</c-></code>, see <a href="https://p1144.godbolt.org/z/cqPP4oeE9">godbolt.org/z/cqPP4oeE9</a> and <a data-link-type="biblio" href="#biblio-stdrelocateiscute" title="std::relocate&apos;s implementation is cute">[StdRelocateIsCute]</a>. My implementation of their "as-if-by-memcpy" codepaths relies on
Clang’s <code class="highlight"><c- n>__builtin_memmove</c-></code>; vendors can use any vendor-specific means to implement them.
The wording also deliberately permits a low-quality implementation with no such codepath at all.
See <a href="https://www.youtube.com/watch?v=SGdfPextuAU&amp;t=45m23s">@45:23–48:39</a> in my C++Now 2019 talk.</p>
   <h3 class="heading settled" data-level="4.4" id="wording-uninitialized.relocate"><span class="secno">4.4. </span><span class="content"><code class="highlight"><c- n>uninitialized_relocate</c-></code>, <code class="highlight"><c- n>uninitialized_relocate_n</c-></code>, <code class="highlight"><c- n>uninitialized_relocate_backward</c-></code> [uninitialized.relocate]</span><a class="self-link" href="#wording-uninitialized.relocate"></a></h3>
   <p>Add a new section after <a href="http://eel.is/c++draft/uninitialized.move">[uninitialized.move]</a>:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>NoThrowForwardIterator</c-><c- o>></c->
<c- n>NoThrowForwardIterator</c-> <c- n>uninitialized_relocate</c-><c- p>(</c-><c- n>InputIterator</c-> <c- n>first</c-><c- p>,</c-> <c- n>InputIterator</c-> <c- n>last</c-><c- p>,</c->
                                              <c- n>NoThrowForwardIterator</c-> <c- n>result</c-><c- p>);</c->
</pre> <p><em>Effects:</em> Equivalent to:</p> <pre class="highlight"><c- k>try</c-> <c- p>{</c->
  <c- k>for</c-> <c- p>(;</c-> <c- n>first</c-> <c- o>!=</c-> <c- n>last</c-><c- p>;</c-> <c- o>++</c-><c- n>result</c-><c- p>,</c-> <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- o>++</c-><c- n>first</c-><c- p>)</c-> <c- p>{</c->
    <c- o>::</c-><c- k>new</c-> <c- p>(</c-><c- n>voidify</c-><c- p>(</c-><c- o>*</c-><c- n>result</c-><c- p>))</c->
      <c- k>typename</c-> <c- nc>iterator_traits</c-><c- o>&lt;</c-><c- n>NoThrowForwardIterator</c-><c- o>>::</c-><c- n>value_type</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>));</c->
    <c- n>destroy_at</c-><c- p>(</c-><c- n>addressof</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>));</c->
  <c- p>}</c->
  <c- k>return</c-> <c- n>result</c-><c- p>;</c->
<c- p>}</c-> <c- k>catch</c-> <c- p>(...)</c-> <c- p>{</c->
  <c- n>destroy</c-><c- p>(</c-><c- o>++</c-><c- n>first</c-><c- p>,</c-> <c- n>last</c-><c- p>);</c->
  <c- k>throw</c-><c- p>;</c->
<c- p>}</c->
</pre><p>except that if the iterators' common value type is trivially relocatable, side effects
associated with the relocation of the object’s value might not happen.</p> <p><em>Remarks:</em> If an exception is thrown, all objects in both the source and destination
ranges are destroyed.</p> <pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Size</c-><c- p>,</c-> <c- k>class</c-> <c- nc>NoThrowForwardIterator</c-><c- o>></c->
  <c- n>pair</c-><c- o>&lt;</c-><c- n>InputIterator</c-><c- p>,</c-> <c- n>NoThrowForwardIterator</c-><c- o>></c->
    <c- n>uninitialized_relocate_n</c-><c- p>(</c-><c- n>InputIterator</c-> <c- n>first</c-><c- p>,</c-> <c- n>Size</c-> <c- n>n</c-><c- p>,</c-> <c- n>NoThrowForwardIterator</c-> <c- n>result</c-><c- p>);</c->
</pre> <p><em>Effects:</em> Equivalent to:</p> <pre class="highlight"><c- k>try</c-> <c- p>{</c->
  <c- k>for</c-> <c- p>(;</c-> <c- n>n</c-> <c- o>></c-> <c- mi>0</c-><c- p>;</c-> <c- o>++</c-><c- n>result</c-><c- p>,</c-> <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- o>++</c-><c- n>first</c-><c- p>,</c-> <c- o>--</c-><c- n>n</c-><c- p>)</c-> <c- p>{</c->
    <c- o>::</c-><c- k>new</c-> <c- p>(</c-><c- n>voidify</c-><c- p>(</c-><c- o>*</c-><c- n>result</c-><c- p>))</c->
      <c- k>typename</c-> <c- nc>iterator_traits</c-><c- o>&lt;</c-><c- n>NoThrowForwardIterator</c-><c- o>>::</c-><c- n>value_type</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>));</c->
    <c- n>destroy_at</c-><c- p>(</c-><c- n>addressof</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>));</c->
  <c- p>}</c->
  <c- k>return</c-> <c- p>{</c-><c- n>first</c-><c- p>,</c-> <c- n>result</c-><c- p>};</c->
<c- p>}</c-> <c- k>catch</c-> <c- p>(...)</c-> <c- p>{</c->
  <c- n>destroy_n</c-><c- p>(</c-><c- o>++</c-><c- n>first</c-><c- p>,</c-> <c- o>--</c-><c- n>n</c-><c- p>);</c->
  <c- k>throw</c-><c- p>;</c->
<c- p>}</c->
</pre><p>except that if the iterators' common value type is trivially relocatable, side effects
associated with the relocation of the object’s value might not happen.</p> <p><em>Remarks:</em> If an exception is thrown, all objects in both the source and destination
ranges are destroyed.</p> <pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>BidirectionalIterator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>NoThrowBidirectionalIterator</c-><c- o>></c->
  <c- n>NoThrowBidirectionalIterator</c->
    <c- n>uninitialized_relocate_backward</c-><c- p>(</c-><c- n>BidirectionalIterator</c-> <c- n>first</c-><c- p>,</c-> <c- n>BidirectionalIterator</c-> <c- n>last</c-><c- p>,</c->
                                    <c- n>NoThrowBidirectionalIterator</c-> <c- n>result</c-><c- p>);</c->
</pre> <p><em>Effects:</em> Equivalent to:</p> <pre class="highlight"><c- k>try</c-> <c- p>{</c->
  <c- k>for</c-> <c- p>(;</c-> <c- n>last</c-> <c- o>!=</c-> <c- n>first</c-><c- p>;</c-> <c- p>)</c-> <c- p>{</c->
    <c- o>--</c-><c- n>last</c-><c- p>;</c->
    <c- o>--</c-><c- n>result</c-><c- p>;</c->
    <c- o>::</c-><c- k>new</c-> <c- p>(</c-><c- n>voidify</c-><c- p>(</c-><c- o>*</c-><c- n>result</c-><c- p>))</c->
      <c- k>typename</c-> <c- nc>iterator_traits</c-><c- o>&lt;</c-><c- n>NoThrowBidirectionalIterator</c-><c- o>>::</c-><c- n>value_type</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- o>*</c-><c- n>last</c-><c- p>));</c->
    <c- n>destroy_at</c-><c- p>(</c-><c- n>addressof</c-><c- p>(</c-><c- o>*</c-><c- n>last</c-><c- p>));</c->
  <c- p>}</c->
  <c- k>return</c-> <c- n>result</c-><c- p>;</c->
<c- p>}</c-> <c- k>catch</c-> <c- p>(...)</c-> <c- p>{</c->
  <c- n>destroy</c-><c- p>(</c-><c- n>first</c-><c- p>,</c-> <c- o>++</c-><c- n>last</c-><c- p>);</c->
  <c- k>throw</c-><c- p>;</c->
<c- p>}</c->
</pre><p>except that if the iterators' common value type is trivially relocatable, side effects
associated with the relocation of the object’s value might not happen.</p> <p><em>Remarks:</em> If an exception is thrown, all objects in both the source and destination
ranges are destroyed.</p> </small>
   </blockquote>
   <p class="note" role="note"><span class="marker">Note:</span> The Remarks allude to blanket wording in <a href="http://eel.is/c++draft/specialized.algorithms#general-2">[specialized.algorithms.general]/2</a>.</p>
   <h3 class="heading settled" data-level="4.5" id="wording-basic.types.general"><span class="secno">4.5. </span><span class="content">Trivially relocatable type [basic.types.general]</span><a class="self-link" href="#wording-basic.types.general"></a></h3>
   <p>Add a new section in <a href="http://eel.is/c++draft/basic.types.general">[basic.types.general]</a>:</p>
   <p><small></small></p>
   <blockquote><small> An object type <code class="highlight"><c- n>T</c-></code> is a <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export id="abstract-opdef-trivially-relocatable">trivially relocatable</dfn> type if it is:<p></p> <ul><li data-md><p>a trivially copyable type, or</p> </li><li data-md><p>an array of trivially relocatable type, or</p> </li><li data-md><p>a (possibly cv-qualified) class type declared with a <code class="highlight"><c- n>trivially_relocatable</c-></code> attribute
with value <code class="highlight">true</code> <a href="#wording-dcl.attr.trivreloc">[dcl.attr.trivreloc]</a>, or</p> </li><li data-md><p>a (possibly cv-qualified) class type which:</p> <ul><li data-md><p>has no user-provided move constructors or move assignment operators,</p> </li><li data-md><p>has no user-provided copy constructors or copy assignment operators,</p> </li><li data-md><p>has no user-provided destructors,</p> </li><li data-md><p>has no virtual member functions,</p> </li><li data-md><p>has no virtual base classes,</p> </li><li data-md><p>all of whose members are either of reference type or of trivially relocatable type, and</p> </li><li data-md><p>all of whose base classes are of trivially relocatable type.</p> </li></ul></li></ul> <p>[<em>Note:</em> For a trivially relocatable type, the <a data-link-type="abstract-op" href="#abstract-opdef-relocation-operation" id="ref-for-abstract-opdef-relocation-operation">relocation operation</a> (as performed by,
for example, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap_ranges</c-></code> or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>::</c-><c- n>reserve</c-></code>) is tantamount
to a simple copy of the underlying bytes. <em>—end note</em>]</p> <p>[<em>Note:</em> It is intended that most standard library types be trivially relocatable types. <em>—end note</em>]</p> </small></blockquote>
   <p class="note" role="note"><span class="marker">Note:</span> Polymorphic types are disallowed from "natural" trivial relocatability.
See <a href="#non-trivial-sample-polymorphic">Appendix C, example 5</a>.
Volatile members are not disallowed. See <a data-link-type="biblio" href="#biblio-subobjects" title="When is a trivially copyable object not trivially copyable?">[Subobjects]</a>.</p>
   <h3 class="heading settled" data-level="4.6" id="wording-dcl.attr.trivreloc"><span class="secno">4.6. </span><span class="content"><code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> attribute [dcl.attr.trivreloc]</span><a class="self-link" href="#wording-dcl.attr.trivreloc"></a></h3>
   <p>Add a new section after <a href="http://eel.is/c++draft/dcl.attr.nouniqueaddr">[dcl.attr.nouniqueaddr]</a>:</p>
   <p><small></small></p>
   <blockquote><small> The <em>attribute-token</em> <code class="highlight"><c- n>trivially_relocatable</c-></code> specifies that a class type’s relocation operation has no
visible side-effects other than a copy of the underlying bytes, as if by the library function <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>memcpy</c-></code>.
It may be applied to the definition of a class. It shall appear at most once in each <em>attribute-list</em>.
An <em>attribute-argument-clause</em> may be present and, if present, shall have the form<p></p> <pre class="highlight"><c- p>(</c-> <c- n>constant</c-><c- o>-</c-><c- n>expression</c-> <c- p>)</c->
</pre>The <em>constant-expression</em> shall be an integral constant expression of type <code class="highlight"><c- b>bool</c-></code>.
If no <em>attribute-argument-clause</em> is present, it has the same effect as an <em>attribute-argument-clause</em> of <code class="highlight"><c- p>(</c->true<c- p>)</c-></code>. <p>If any definition of a class type has a <code class="highlight"><c- n>trivially_relocatable</c-></code> attribute with value <em>V</em>, then each
definition of the same class type shall have a <code class="highlight"><c- n>trivially_relocatable</c-></code> attribute with value <em>V</em>.
No diagnostic is required if definitions in different translation units have
mismatched <code class="highlight"><c- n>trivially_relocatable</c-></code> attributes.</p> <p>If a class type is declared with the <code class="highlight"><c- n>trivially_relocatable</c-></code> attribute, and the program relies on
observable side-effects of its relocation other than a copy of the underlying bytes, the behavior is undefined.</p> </small></blockquote>
   <h3 class="heading settled" data-level="4.7" id="wording-meta.unary.prop"><span class="secno">4.7. </span><span class="content">Type trait <code class="highlight"><c- n>is_trivially_relocatable</c-></code> [meta.unary.prop]</span><a class="self-link" href="#wording-meta.unary.prop"></a></h3>
   <p>Add a new entry to Table 47 in <a href="http://eel.is/c++draft/meta.unary.prop">[meta.unary.prop]</a>:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <table> <tbody><tr><th>Template</th><th>Condition</th><th>Preconditions</th></tr> <tr> <td><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_relocatable</c-><c- p>;</c-></code></td> <td><code class="highlight"><c- n>T</c-></code> is a <a data-link-type="abstract-op" href="#abstract-opdef-trivially-relocatable" id="ref-for-abstract-opdef-trivially-relocatable①">trivially relocatable</a> type.</td> <td><code class="highlight"><c- n>remove_all_extents_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> shall be a complete type or <i>cv</i> <code class="highlight"><c- b>void</c-></code>.</td> </tr> </tbody></table> </small>
   </blockquote>
   <h3 class="heading settled" data-level="4.8" id="wording-concept.relocatable"><span class="secno">4.8. </span><span class="content"><code class="highlight"><c- n>relocatable</c-></code> concept [concept.relocatable]</span><a class="self-link" href="#wording-concept.relocatable"></a></h3>
   <p>Add a new section after <a href="http://eel.is/c++draft/concept.copyconstructible">[concept.copyconstructible]</a>:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>concept</c-> <c- nc>relocatable</c-> <c- o>=</c-> <c- n>move_constructible</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c->
</pre> <p>If <code class="highlight"><c- n>T</c-></code> is an object type, then let <code class="highlight"><c- n>rv</c-></code> be an rvalue of type <code class="highlight"><c- n>T</c-></code>, <code class="highlight"><c- n>lv</c-></code> an lvalue of type <code class="highlight"><c- n>T</c-></code> equal to <code class="highlight"><c- n>rv</c-></code>,
and <code class="highlight"><c- n>u2</c-></code> a distinct object of type <code class="highlight"><c- n>T</c-></code> equal to <code class="highlight"><c- n>rv</c-></code>. <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>relocatable</c-></code> only if</p> <ul><li data-md><p>After the definition <code class="highlight"><c- n>T</c-> <c- n>u</c-> <c- o>=</c-> <c- n>rv</c-><c- p>;</c-></code>, <code class="highlight"><c- n>u</c-></code> is equal to <code class="highlight"><c- n>u2</c-></code>.</p> </li><li data-md><p><code class="highlight"><c- n>T</c-><c- p>(</c-><c- n>rv</c-><c- p>)</c-></code> is equal to <code class="highlight"><c- n>u2</c-></code>.</p> </li><li data-md><p>If the expression <code class="highlight"><c- n>u2</c-> <c- o>=</c-> <c- n>rv</c-></code> is well-formed, then the expression has the same semantics as <code class="highlight"><c- n>u2</c-><c- p>.</c-><c- o>~</c-><c- n>T</c-><c- p>();</c-> <c- o>::</c-><c- k>new</c-> <c- p>((</c-><c- b>void</c-><c- o>*</c-><c- p>)</c-><c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>u2</c-><c- p>))</c-> <c- n>T</c-><c- p>(</c-><c- n>rv</c-><c- p>);</c-></code></p> </li><li data-md><p>If the definition <code class="highlight"><c- n>T</c-> <c- n>u</c-> <c- o>=</c-> <c- n>lv</c-><c- p>;</c-></code> is well-formed, then after the definition <code class="highlight"><c- n>u</c-></code> is equal to <code class="highlight"><c- n>u2</c-></code>.</p> </li><li data-md><p>If the expression <code class="highlight"><c- n>T</c-><c- p>(</c-><c- n>lv</c-><c- p>)</c-></code> is well-formed, then the expression’s result is equal to <code class="highlight"><c- n>u2</c-></code>.</p> </li><li data-md><p>If the expression <code class="highlight"><c- n>u2</c-> <c- o>=</c-> <c- n>lv</c-></code> is well-formed, then the expression has the same semantics as <code class="highlight"><c- n>u2</c-><c- p>.</c-><c- o>~</c-><c- n>T</c-><c- p>();</c-> <c- o>::</c-><c- k>new</c-> <c- p>((</c-><c- b>void</c-><c- o>*</c-><c- p>)</c-><c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>u2</c-><c- p>))</c-> <c- n>T</c-><c- p>(</c-><c- n>lv</c-><c- p>);</c-></code></p> </li></ul> </small>
   </blockquote>
   <p class="note" role="note"><span class="marker">Note:</span> We intend that a type may be relocatable
regardless of whether it is copy-constructible; but, if it is copy-constructible then copy-and-destroy
must have the same semantics as move-and-destroy. We intend that a type may be relocatable regardless of
whether it is assignable; but, if it is assignable then assignment must have the same semantics as
destroy-and-copy or destroy-and-move.
The semantic requirements on assignment help us optimize <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>insert</c-></code> and <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>erase</c-></code>. <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>forward_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> satisfies <code class="highlight"><c- n>relocatable</c-></code>, but it models <code class="highlight"><c- n>relocatable</c-></code> only when all relevant objects have equal allocators.</p>
   <h2 class="heading settled" data-level="5" id="alternatives"><span class="secno">5. </span><span class="content">Rationale and alternatives</span><a class="self-link" href="#alternatives"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="maybe-trivially-relocatable"><span class="secno">5.1. </span><span class="content">Attribute <code class="highlight"><c- p>[[</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code></span><a class="self-link" href="#maybe-trivially-relocatable"></a></h3>
   <p>My Clang implementation, currently available on Godbolt, supports both <code class="highlight"><c- p>[[</c-><c- n>clang</c-><c- o>::</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> and another attribute called <code class="highlight"><c- p>[[</c-><c- n>clang</c-><c- o>::</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code>,
which John McCall requested that I explore.</p>
   <p>In Issaquah (February 2023), <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> suggested a very similar design to <code class="highlight"><c- p>[[</c-><c- n>clang</c-><c- o>::</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code>.
EWGI took a three-way straw poll on <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> versus <code class="highlight"><c- p>[[</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code>,
with an inconclusive 7–5–6 vote (the author of P1144 voting "For" and the three representatives of P2786
presumably voting "Against").</p>
   <p>For discussion of <code class="highlight"><c- p>[[</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code> and why I don’t think it’s the right thing
to standardize, see</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- p>[[</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code> by itself cannot support <a href="#design-goals">§ 3 Design goals</a> examples #4 and #5</p>
    <li data-md>
     <p><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1144r4.html#maybe-trivially-relocatable">P1144R4 section 6.2</a></p>
    <li data-md>
     <p><a href="https://quuxplusone.github.io/blog/2023/03/10/sharp-knife-dull-knife/">"Should the compiler sometimes reject a <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> warrant?"</a> (2023-03-10)</p>
   </ul>
   <h3 class="heading settled" data-level="5.2" id="pmr-concerns"><span class="secno">5.2. </span><span class="content">Confusing interactions with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-></code></span><a class="self-link" href="#pmr-concerns"></a></h3>
   <p class="note" role="note"><span class="marker">Note:</span> This section was added in P1144R5, revised in P1144R7, and again in P1144R8.
Here I assume libc++, where <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> is trivially relocatable.
Feel free to substitute your favorite trivially relocatable container type;
I’m sticking with <code class="highlight"><c- n>string</c-></code> for these examples because it is short
and easy to spell.</p>
   <p>P1144 proposes a "trivial relocatability" very similar to "trivial copyability": if the user
provides any special member, we assume that the user wants control over this type’s value
semantics, and we won’t implicitly infer triviality. This includes the assignment operator.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>A</c-> <c- p>{</c->
    <c- n>A</c-><c- p>(</c-><c- n>A</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>A</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- n>A</c-><c- o>&amp;&amp;</c-><c- p>);</c->
    <c- o>~</c-><c- n>A</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- o>!</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>A</c-><c- o>></c-><c- p>);</c->
  <c- c1>// P2786 would say the opposite here</c->
</pre>
   <p>P1144 treats move as an optimization of copy; thus we assume that it always ought to be
safe to replace "copy and destroy" with "move and destroy" (and thus with "relocate").
This is informed by Arthur’s experience standardizing "implicit move" in <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1155r3.html">P1155</a> (C++20) and <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2266r3.html">P2266</a> (C++23).
"Implicit move" affects <code class="highlight"><c- n>pmr</c-></code> types because their copy differs from their move. <a href="https://godbolt.org/z/sGKhxbsrK">Example</a>:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>ByValueSink</c-> <c- p>{</c->
    <c- n>ByValueSink</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>v</c-><c- p>)</c-> <c- o>:</c-> <c- n>v_</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>v</c-><c- p>))</c-> <c- p>{}</c->
    <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>v_</c-><c- p>;</c->
<c- p>};</c->

<c- n>ByValueSink</c-> <c- nf>f</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>memory_resource</c-> <c- o>*</c-><c- n>mr</c-><c- p>)</c-> <c- p>{</c->
    <c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>mr</c-><c- p>);</c->
    <c- k>return</c-> <c- n>v</c-><c- p>;</c->
      <c- c1>// C++17, Clang 12-: Copies (changes allocator)</c->
      <c- c1>// C++20, GCC, Clang 13+: Moves (preserves allocator)</c->
<c- p>}</c->
</pre>
   <p>We didn’t care about this when fiddling with implicit move; P1144 implicitly believes we shouldn’t
care about it now. (This position might be debatable, but it’s definitely part of the worldview that
led to P1144’s specific proposed semantics, so it’s good to have this background in mind.)</p>
   <p>P1144 wants to be able to optimize not just vector reserve, but also vector insert and erase
(see <a href="#applications">table §2.1</a>). Today, vector insert and erase use move-assignment, not relocation.
(In fact, <a href="https://eel.is/c++draft/vector#modifiers-5">[vector.modifiers]/5</a> specifies
the exact number of times "the assignment operator of <code class="highlight"><c- n>T</c-></code> is called." We might want to fix that,
but that’s out of scope for this paper.)
We can optimize vector insert to use (trivial) relocation for element types that model <code class="highlight"><c- n>relocatable</c-></code> (<a href="#wording-concept.relocatable">[concept.relocatable</a>]).</p>
   <p>Some types invariably model <code class="highlight"><c- n>relocatable</c-></code>; that is, their assignment is naturally tantamount to construct-and-destroy.
But for <code class="highlight"><c- n>pmr</c-></code> types and polymorphic types, assignment is <em>not</em> tantamount to construct-and-destroy, <em>except</em> under certain conditions. For <code class="highlight"><c- n>pmr</c-></code> types, the condition is "both objects have the same
allocator." For polymorphic types, the condition is "both objects have the same dynamic type."</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-> <c- n>dv</c-> <c- o>=</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>};</c->
<c- n>dv</c-><c- p>.</c-><c- n>erase</c-><c- p>(</c-><c- n>dv</c-><c- p>.</c-><c- n>begin</c-><c- p>());</c->
  <c- c1>// Every object in the vector certainly has the same dynamic type.</c->

<c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>string</c-><c- o>></c-> <c- n>pv</c-> <c- o>=</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>};</c->
<c- n>pv</c-><c- p>.</c-><c- n>erase</c-><c- p>(</c-><c- n>pv</c-><c- p>.</c-><c- n>begin</c-><c- p>());</c->
  <c- c1>// Every string in the vector certainly has the same allocator.</c->

<c- n>Derived</c-> <c- n>da</c-><c- p>[</c-><c- mi>5</c-><c- p>]</c-> <c- o>=</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>rotate</c-><c- p>(</c-><c- n>da</c-><c- p>,</c-> <c- n>da</c-><c- o>+</c-><c- mi>2</c-><c- p>,</c-> <c- n>da</c-><c- o>+</c-><c- mi>5</c-><c- p>);</c->
  <c- c1>// Every object in the contiguous range certainly has the same dynamic type.</c->

<c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>string</c-> <c- n>pa</c-><c- p>[</c-><c- mi>5</c-><c- p>]</c-> <c- o>=</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>rotate</c-><c- p>(</c-><c- n>pa</c-><c- p>,</c-> <c- n>pa</c-><c- o>+</c-><c- mi>2</c-><c- p>,</c-> <c- n>pa</c-><c- o>+</c-><c- mi>5</c-><c- p>);</c->
  <c- c1>// Objects in the contiguous range might have different allocators?</c->
</pre>
   <p>P1144 aims to permit efficient insertion and erasure in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code>. <a href="https://p1144.godbolt.org/z/vv5q69q44">Example:</a></p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- o>></c-> <c- n>vs</c-><c- p>(</c-><c- mi>501</c-><c- p>);</c->
<c- n>vs</c-><c- p>.</c-><c- n>erase</c-><c- p>(</c-><c- n>vs</c-><c- p>.</c-><c- n>begin</c-><c- p>());</c->
</pre>
   <p>This <code class="highlight"><c- n>erase</c-></code> requires us to shift down 500 <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> objects, which can be done either
by 500 calls to <code class="highlight"><c- n>string</c-><c- o>::</c-><c- k>operator</c-><c- o>=</c-></code> followed by one call to <code class="highlight"><c- o>~</c-><c- n>string</c-></code>, or by
one call to <code class="highlight"><c- o>~</c-><c- n>string</c-></code> followed by a <code class="highlight"><c- n>memmove</c-></code> (as seen for example in <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a> and BSL;
see <a href="#applications">table §2.1</a>). We want to permit BSL’s implementation.
That’s why since P1144R5, the definition of <code class="highlight"><c- k>concept</c-> <c- nc>relocatable</c-></code> in <a href="#wording-concept.relocatable">§4.8</a> places semantic requirements on <code class="highlight"><c- n>T</c-></code>'s assignment operators (if they exist) as well as on <code class="highlight"><c- n>T</c-></code>'s constructors and destructors.</p>
   <p>If <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>string</c-></code> is considered trivially relocatable, this will trickle down into all Rule-of-Zero
types with a <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>string</c-></code> member. (<a href="https://godbolt.org/z/nhf38n33G">Godbolt.</a>)</p>
<pre class="language-c++ highlight"><c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>string</c-><c- o>></c-><c- p>);</c->
  <c- c1>// Before: not true, of course</c->
  <c- c1>// After: suppose this is true</c->
<c- k>struct</c-> <c- nc>A</c-> <c- p>{</c->
  <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>string</c-> <c- n>ps</c-><c- p>;</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>A</c-><c- o>></c-><c- p>);</c->
  <c- c1>// After: then this will also be true, by the Rule of Zero</c->
<c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>A</c-><c- o>></c-> <c- n>v</c-><c- p>;</c->
<c- n>v</c-><c- p>.</c-><c- n>push_back</c-><c- p>({</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>string</c-><c- p>(</c-><c- s>"one"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>mr1</c-><c- p>)});</c->
<c- n>v</c-><c- p>.</c-><c- n>push_back</c-><c- p>({</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>string</c-><c- p>(</c-><c- s>"two"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>mr2</c-><c- p>)});</c->
<c- n>v</c-><c- p>.</c-><c- n>erase</c-><c- p>(</c-><c- n>v</c-><c- p>.</c-><c- n>begin</c-><c- p>());</c->
  <c- c1>// Before: Well-defined behavior, acts as-if by assignment</c->
  <c- c1>// After: Do we somehow make this a precondition violation?</c->
<c- n>assert</c-><c- p>(</c-><c- n>v</c-><c- p>[</c-><c- mi>0</c-><c- p>].</c-><c- n>ps</c-><c- p>.</c-><c- n>get_allocator</c-><c- p>().</c-><c- n>resource</c-><c- p>()</c-> <c- o>==</c-> <c- o>&amp;</c-><c- n>mr1</c-><c- p>);</c->
  <c- c1>// Before: Succeeds</c->
  <c- c1>// After: Might fail (and on a quality implementation, *will* fail)</c->
</pre>
   <p>If we (1) advertise <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>string</c-></code> as <code class="highlight"><c- n>is_trivially_relocatable</c-></code>; (2) propagate
trivial relocatability in the core language as both P1144 and P2786 propose to do; and
(3) optimize vector insert and erase for trivially relocatable types; then we inevitably
arrive here. Arthur’s solution is to impose preconditions on user-defined types
used within certain parts of the STL: Just as their destructors are forbidden to (dynamically)
throw exceptions, and their copy constructors are forbidden to (dynamically) make things that aren’t copies,
their assignment operators ought to be forbidden to (dynamically) violate <code class="highlight"><c- k>concept</c-> <c- nc>relocatable</c-></code>'s semantic requirements.</p>
   <p class="issue" id="issue-97f1a6a8"><a class="self-link" href="#issue-97f1a6a8"></a> What new wording is needed to achieve this?</p>
   <h3 class="heading settled" data-level="5.3" id="overlap-concerns"><span class="secno">5.3. </span><span class="content">Non-issue: Overlapping base-class subobjects</span><a class="self-link" href="#overlap-concerns"></a></h3>
   <p class="note" role="note"><span class="marker">Note:</span> This section was added in P1144R7 and resolved in P1144R8.</p>
   <p>In the following snippet, <code class="highlight"><c- k>struct</c-> <c- nc>Dangerous</c-></code> is trivially copyable, so we
might think that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap</c-><c- p>(</c-><c- n>ra</c-><c- p>,</c-> <c- n>rb</c-><c- p>)</c-></code> should be allowed to swap the bytes of <code class="highlight"><c- n>ra</c-></code> with the bytes of <code class="highlight"><c- n>rb</c-></code>.
A naïve approach is to swap <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- n>Dangerous</c-><c- p>)</c-> <c- o>==</c-> <c- mi>8</c-></code> bytes, but that’s unsafe,
because the tail padding of <code class="highlight"><c- n>Dangerous</c-></code> is allowed to contain the value of <code class="highlight"><c- n>Derived</c-><c- o>::</c-><c- n>k</c-></code>. If <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap</c-></code> is to do "trivial swapping," it must account
for potentially overlapping subobjects.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>Dangerous</c-> <c- p>{</c->
    <c- n>Dangerous</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- b>int</c-> <c- n>j</c-><c- p>)</c-> <c- o>:</c-> <c- n>i</c-><c- p>(</c-><c- n>i</c-><c- p>),</c-> <c- n>j</c-><c- p>(</c-><c- n>j</c-><c- p>)</c-> <c- p>{}</c->
    <c- b>int</c-> <c- n>i</c-><c- p>;</c->
    <c- b>short</c-> <c- n>j</c-><c- p>;</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_standard_layout_v</c-><c- o>&lt;</c-><c- n>Dangerous</c-><c- o>></c-><c- p>);</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_copyable_v</c-><c- o>&lt;</c-><c- n>Dangerous</c-><c- o>></c-><c- p>);</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>Dangerous</c-><c- o>></c-><c- p>);</c->

<c- k>struct</c-> <c- nc>Derived</c-> <c- o>:</c-> <c- n>Dangerous</c-> <c- p>{</c->
    <c- n>Derived</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- b>int</c-> <c- n>j</c-><c- p>,</c-> <c- b>int</c-> <c- n>k</c-><c- p>)</c-> <c- o>:</c-> <c- n>Dangerous</c-><c- p>(</c-><c- n>i</c-><c- p>,</c-> <c- n>j</c-><c- p>),</c-> <c- n>k</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-> <c- p>{}</c->
    <c- b>short</c-> <c- n>k</c-><c- p>;</c->
<c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- o>!</c-><c- n>std</c-><c- o>::</c-><c- n>is_standard_layout_v</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-><c- p>);</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_copyable_v</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-><c- p>);</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-><c- p>);</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- n>Derived</c-> <c- n>a</c-> <c- o>=</c-> <c- n>Derived</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>);</c->
    <c- n>Derived</c-> <c- n>b</c-> <c- o>=</c-> <c- n>Derived</c-><c- p>(</c-><c- mi>4</c-><c- p>,</c-><c- mi>5</c-><c- p>,</c-><c- mi>6</c-><c- p>);</c->
    <c- n>Dangerous</c-> <c- o>&amp;</c-><c- n>ra</c-> <c- o>=</c-> <c- n>a</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>rb</c-> <c- o>=</c-> <c- n>b</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>swap</c-><c- p>(</c-><c- n>ra</c-><c- p>,</c-> <c- n>rb</c-><c- p>);</c->  <c- c1>// must swap only 6 bytes,</c->
                        <c- c1>// not sizeof(Dangerous) = 8 bytes</c->
    <c- n>assert</c-><c- p>(</c-><c- n>a</c-><c- p>.</c-><c- n>k</c-> <c- o>==</c-> <c- mi>3</c-> <c- o>&amp;&amp;</c-> <c- n>b</c-><c- p>.</c-><c- n>k</c-> <c- o>==</c-> <c- mi>6</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <p>Arthur’s libc++ fork implements this solution (P1144R7’s "possible solution #4"): <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap</c-></code> is allowed to bytewise-swap trivially copyable and/or trivially relocatable types
only when they are definitely not potentially overlapping subobjects. In practice, this means:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap</c-></code> can swap bytes as long as the type has no
occupied padding. On the Itanium ABI, this means we can swap <a href="https://danakj.github.io/2023/01/15/trivially-relocatable.html"><code class="highlight"><c- n>datasizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code></a> bytes: non-tail padding bytes are never occupied. On an unknown ABI, we can still swap <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code> bytes if we know that <code class="highlight"><c- n>has_unique_object_representations_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, because such a <code class="highlight"><c- n>T</c-></code> cannot have
any padding at all. Worst case, we simply treat the type as non-trivial.</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>swap_ranges</c-></code> can swap bytes as long as <code class="highlight"><c- n>distance</c-><c- p>(</c-><c- n>first</c-><c- p>,</c-> <c- n>last</c-><c- p>)</c-> <c- o>>=</c-> <c- mi>2</c-></code>.
A range of length 1 might be a single object, which might have padding that cannot be swapped.
Implementors already must do this logic in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>copy</c-></code> etc.
(see <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108846">libstdc++ bug #108846</a>)
so doing it also in <code class="highlight"><c- n>swap_ranges</c-></code> is no hardship.</p>
   </ul>
   <h2 class="heading settled" data-level="6" id="acknowledgements"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to Pablo Halpern for <a data-link-type="biblio" href="#biblio-n4158" title="N4158: Destructive Move (rev 1)">[N4158]</a>, to which this paper bears a striking resemblance —<wbr>including the meaning assigned to the word "trivial," and the library-algorithm approach to avoiding the
problems with "lame duck objects" discussed in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1377.htm#Alternative%20move%20designs">the final section</a> of <a data-link-type="biblio" href="#biblio-n1377" title="N1377: A Proposal to Add Move Semantics Support to the C++ Language">[N1377]</a>. See <a href="http://wiki.edg.com/bin/view/Wg21rapperswil2014/N4034">discussion of N4034 at Rapperswil</a> (June 2014)
and <a href="http://wiki.edg.com/bin/view/Wg21urbana-champaign/EvolutionWorkingGroup#N4158_Destructive_Move_Rev_1">discussion
of N4158 at Urbana</a> (November 2014).</p>
   <p>Significantly different approaches to this problem have previously appeared in Rodrigo Castro Campos’s <a data-link-type="biblio" href="#biblio-n2754" title="N2754: TriviallyDestructibleAfterMove and TriviallyReallocatable (rev 3)">[N2754]</a>, Denis Bider’s <a data-link-type="biblio" href="#biblio-p0023r0" title="P0023R0: Relocator: Efficiently Moving Objects">[P0023R0]</a> (introducing a core-language "relocation" operator), and
Niall Douglas’s <a data-link-type="biblio" href="#biblio-p1029r3" title="P1029R3: move = bitcopies">[P1029R3]</a> (treating trivial relocatability as an aspect of move-construction in isolation,
rather than an aspect of the class type as a whole). A less different approach is taken
in Mungo Gill &amp; Alisdair Meredith’s <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a>.</p>
   <p>Thanks to Elias Kosunen, Niall Douglas, John Bandela, and Nicolas Lesser for their feedback on early drafts of P1144R0.</p>
   <p>Thanks to Nicolas Lesser and John McCall for their review comments on the Clang implementation <a data-link-type="biblio" href="#biblio-d50119" title="Compiler support for P1144R0 __is_trivially_relocatable(T)">[D50119]</a>.</p>
   <p>Many thanks to Matt Godbolt for allowing me to install my Clang fork on Compiler Explorer
(<a href="https://p1144.godbolt.org/z/oWEd_X">godbolt.org</a>). See also <a data-link-type="biblio" href="#biblio-announcing" title="Announcing &quot;trivially relocatable&quot;">[Announcing]</a>.</p>
   <p>Thanks to Howard Hinnant for appearing with me on <a data-link-type="biblio" href="#biblio-cppchat" title="cpp.chat episode 40: It works but it&apos;s undefined behavior">[CppChat]</a>, and to Jon Kalb and Phil Nash for hosting us.</p>
   <p>Thanks to Marc Glisse for his work integrating a "trivially relocatable" trait into GNU libstdc++
and for answering my questions on <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87106">GCC bug 87106</a>.</p>
   <p>Thanks to Jens Maurer for his feedback on P1144R3 at Kona 2019, and to Corentin Jabot for championing
P1144R4 at Prague 2020.</p>
   <p>Thanks to Dana Jansens for her contributions re overlapping and non-standard-layout types (see <a data-link-type="biblio" href="#biblio-subspace" title="Trivially Relocatable Types in C++/Subspace">[Subspace]</a>),
to Alisdair Meredith for our extensive discussions during the February 2023 drafting of <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a>,
to Giuseppe D’Angelo and Thiago Maceira for contributing the Qt entries in <a href="#applications">table §2.1</a>,
and to Giuseppe D’Angelo for extensive review comments and discussion.</p>
   <p>Thanks to Mungo Gill and Alisdair Meredith for <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> and <a data-link-type="biblio" href="#biblio-p2814r0" title="Trivial relocatability — comparing P2786 with P1144">[P2814R0]</a>.</p>
   <h2 class="heading settled" id="polls"><span class="content">Appendix A: Straw polls</span><a class="self-link" href="#polls"></a></h2>
   <h3 class="heading settled" id="taken-polls-2023-02-10"><span class="content">Polls taken at EWGI at Issaquah on 2023-02-10</span><a class="self-link" href="#taken-polls-2023-02-10"></a></h3>
   <p>Arthur O’Dwyer presented <a data-link-type="biblio" href="#biblio-p1144r6" title="Object relocation in terms of move plus destroy">[P1144R6]</a>. Alisdair Meredith presented <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> (which proposed a <code class="highlight"><c- p>[[</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code>-style facility, and expressed it as a contextual keyword instead
of an attribute). EWGI took the following <a href="https://wiki.edg.com/bin/view/Wg21issaquah2023/EWGIP1144R6">straw polls</a> (as well as polls on attribute syntax and on both papers' readiness for EWG).</p>
   <table class="def">
    <tbody>
     <tr>
      <th style="width: 70%;">
      <th><strong>SF</strong>
      <th><strong>F</strong>
      <th><strong>N</strong>
      <th><strong>A</strong>
      <th><strong>SA</strong>
     <tr>
      <th><small>The problem presented in P1144/P2786 is worth solving.</small>
      <th> 10 
      <th> 8 
      <th> 0 
      <th> 0 
      <th> 0 
     <tr>
      <th><small>The problem being introduced in P1144/P2786 should be solved in a more general way instead of as proposed.</small>
      <th> 3 
      <th> 0 
      <th> 5 
      <th> 6 
      <th> 4 
     <tr>
      <th><small>The annotation should "trust the user" as in P1144R6’s <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> ("sharp knife"),
               instead of diagnosing as in P1144R6’s <code class="highlight"><c- p>[[</c-><c- n>clang</c-><c- o>::</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code> and P2786R0’s <code class="highlight"><c- n>trivially_relocatable</c-></code> ("dull knife"). Three-way poll.</small>
      <th> — 
      <th> 7 
      <th> 5 
      <th> 6 
      <th> — 
   </table>
   <h3 class="heading settled" id="taken-polls-2020-02-13"><span class="content">Polls taken at EWGI at Prague on 2020-02-13</span><a class="self-link" href="#taken-polls-2020-02-13"></a></h3>
   <p>Corentin Jabot championed P1144R4. EWGI discussed P1144R4 and Niall Douglas’s <a data-link-type="biblio" href="#biblio-p1029r3" title="P1029R3: move = bitcopies">[P1029R3]</a> consecutively,
then took the following <a href="https://wiki.edg.com/bin/view/Wg21prague/P1144R4SG17">straw polls</a> (as well as a poll on the attribute syntax).</p>
   <table class="def">
    <tbody>
     <tr>
      <th style="width: 70%;">
      <th><strong>SF</strong>
      <th><strong>F</strong>
      <th><strong>N</strong>
      <th><strong>A</strong>
      <th><strong>SA</strong>
     <tr>
      <th><small>We believe that P1029 and P1144 are sufficiently different that they should be advanced separately.</small>
      <th> 7 
      <th> 3 
      <th> 2 
      <th> 0 
      <th> 0 
     <tr>
      <th><small>EWGI is ok to have the spelling as an attribute with an expression argument.</small>
      <th> 3 
      <th> 5 
      <th> 1 
      <th> 1 
      <th> 0 
     <tr>
      <th><small>EWGI thinks the author should explore P1144 as a customizable type trait.</small>
      <th> 0 
      <th> 0 
      <th> 0 
      <th> 9 
      <th> 2 
     <tr>
      <th><small>Forward P1144 to EWG.</small>
      <th> 1 
      <th> 3 
      <th> 4 
      <th> 1 
      <th> 0 
   </table>
   <p>For polls taken September–November 2018, see <a data-link-type="biblio" href="#biblio-p1144r6" title="Object relocation in terms of move plus destroy">[P1144R6]</a>.</p>
   <h2 class="heading settled" id="samples"><span class="content">Appendix B: Sample code</span><a class="self-link" href="#samples"></a></h2>
   <p>See <a data-link-type="biblio" href="#biblio-p1144r6" title="Object relocation in terms of move plus destroy">[P1144R6]</a>'s Appendix B for reference implementations of <code class="highlight"><c- n>relocate</c-></code>, <code class="highlight"><c- n>relocate_at</c-></code>, and
P1144R6’s version of the <code class="highlight"><c- n>uninitialized_relocate</c-></code> library algorithm, plus
a conditionally trivially relocatable <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>.</p>
   <p>I have implemented the entire Standard Library using the <code class="highlight"><c- p>[[</c-><c- n>clang</c-><c- o>::</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> attribute; you can find the source code <a href="https://github.com/Quuxplusone/libcxx/tree/trivially-relocatable">on my GitHub</a> and explore the resulting codegen <a href="https://p1144.godbolt.org/z/t28saC">on Godbolt Compiler Explorer</a>.</p>
   <h2 class="heading settled" id="non-trivial-samples"><span class="content">Appendix C: Examples of non-trivially relocatable class types</span><a class="self-link" href="#non-trivial-samples"></a></h2>
   <h3 class="heading settled" id="non-trivial-sample-string"><span class="content">Class contains pointer to self</span><a class="self-link" href="#non-trivial-sample-string"></a></h3>
   <p>This fictional <code class="highlight"><c- n>short_string</c-></code> illustrates a mechanism that can apply
to any small-buffer-optimized class. <a href="https://github.com/llvm-mirror/libcxx/blob/4e7ffcaa/include/functional#L1719-L1734">libc++'s <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>function</c-></code></a> uses this mechanism and is thus not trivially relocatable.</p>
   <p>However, different mechanisms for small-buffer optimization exist. <a href="https://github.com/llvm-mirror/libcxx/blob/8fdc4918/include/any#L389-L394">libc++'s <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>any</c-></code></a> achieves small-buffer optimization
on a 24-byte buffer, without (necessarily) sacrificing trivial relocatability.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>short_string</c-> <c- p>{</c->
    <c- b>char</c-> <c- o>*</c-><c- n>data_</c-> <c- o>=</c-> <c- n>buffer_</c-><c- p>;</c->
    <c- b>size_t</c-> <c- n>size_</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
    <c- b>char</c-> <c- n>buffer_</c-><c- p>[</c-><c- mi>8</c-><c- p>]</c-> <c- o>=</c-> <c- p>{};</c->

    <c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- nf>data</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- n>data_</c-><c- p>;</c-> <c- p>}</c->

    <c- n>short_string</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>short_string</c-><c- p>(</c-><c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- n>s</c-><c- p>)</c-> <c- o>:</c-> <c- n>size_</c-><c- p>(</c-><c- n>strlen</c-><c- p>(</c-><c- n>s</c-><c- p>))</c-> <c- p>{</c->
        <c- k>if</c-> <c- p>(</c-><c- n>size_</c-> <c- o>&lt;</c-> <c- k>sizeof</c-> <c- n>buffer_</c-><c- p>)</c->
            <c- n>strcpy</c-><c- p>(</c-><c- n>buffer_</c-><c- p>,</c-> <c- n>s</c-><c- p>);</c->
        <c- k>else</c->
            <c- n>data_</c-> <c- o>=</c-> <c- n>strdup</c-><c- p>(</c-><c- n>s</c-><c- p>);</c->
    <c- p>}</c->
    <c- n>short_string</c-><c- p>(</c-><c- n>short_string</c-><c- o>&amp;&amp;</c-> <c- n>s</c-><c- p>)</c-> <c- p>{</c->
        <c- n>memcpy</c-><c- p>(</c-><c- n>this</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>s</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>(</c-><c- o>*</c-><c- n>this</c-><c- p>));</c->
        <c- k>if</c-> <c- p>(</c-><c- n>s</c-><c- p>.</c-><c- n>data_</c-> <c- o>==</c-> <c- n>s</c-><c- p>.</c-><c- n>buffer_</c-><c- p>)</c->
            <c- n>data_</c-> <c- o>=</c-> <c- n>buffer_</c-><c- p>;</c->
        <c- k>else</c->
            <c- n>s</c-><c- p>.</c-><c- n>data_</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->
    <c- p>}</c->
    <c- o>~</c-><c- n>short_string</c-><c- p>()</c-> <c- p>{</c->
        <c- k>if</c-> <c- p>(</c-><c- n>data_</c-> <c- o>!=</c-> <c- n>buffer_</c-><c- p>)</c->
            <c- n>free</c-><c- p>(</c-><c- n>data_</c-><c- p>);</c->
    <c- p>}</c->
<c- p>};</c->
</pre></small> 
   <h3 class="heading settled" id="non-trivial-sample-list"><span class="content">Allocated memory contains pointer to self</span><a class="self-link" href="#non-trivial-sample-list"></a></h3>
   <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>list</c-></code> needs somewhere to store its "past-the-end" node, commonly referred to
as the "sentinel node," whose <code class="highlight"><c- n>prev</c-></code> pointer points to the list’s last node.
If the sentinel node is allocated on the heap, then <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>list</c-></code> can be trivially
relocatable; but if the sentinel node is placed within the <code class="highlight"><c- n>list</c-></code> object itself
(as happens on libc++ and libstdc++), then relocating the <code class="highlight"><c- n>list</c-></code> object requires
fixing up the list’s last node’s <code class="highlight"><c- n>next</c-></code> pointer so that it points to the
new sentinel node inside the destination <code class="highlight"><c- n>list</c-></code> object. This fixup of an arbitrary
heap object cannot be simulated by <code class="highlight"><c- n>memcpy</c-></code>.</p>
   <p>Traditional implementations of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>set</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>map</c-></code> also store a "past-the-end"
node inside themselves and thus also fall into this category.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>node</c-> <c- p>{</c->
    <c- n>node</c-> <c- o>*</c-><c- n>prev_</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->
    <c- n>node</c-> <c- o>*</c-><c- n>next_</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->
<c- p>};</c->
<c- k>struct</c-> <c- nc>list</c-> <c- p>{</c->
    <c- n>node</c-> <c- n>n_</c-><c- p>;</c->
    <c- n>iterator</c-> <c- nf>begin</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>iterator</c-><c- p>(</c-><c- n>n_</c-><c- p>.</c-><c- n>next_</c-><c- p>);</c-> <c- p>}</c->
    <c- n>iterator</c-> <c- nf>end</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>iterator</c-><c- p>(</c-><c- o>&amp;</c-><c- n>n_</c-><c- p>);</c-> <c- p>}</c->
    <c- n>list</c-><c- p>(</c-><c- n>list</c-><c- o>&amp;&amp;</c-> <c- n>l</c-><c- p>)</c-> <c- p>{</c->
        <c- k>if</c-> <c- p>(</c-><c- n>l</c-><c- p>.</c-><c- n>n_</c-><c- p>.</c-><c- n>next_</c-><c- p>)</c-> <c- n>l</c-><c- p>.</c-><c- n>n_</c-><c- p>.</c-><c- n>next_</c-><c- o>-></c-><c- n>prev_</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>n_</c-><c- p>;</c->  <c- c1>// fixup</c->
        <c- k>if</c-> <c- p>(</c-><c- n>l</c-><c- p>.</c-><c- n>n_</c-><c- p>.</c-><c- n>prev_</c-><c- p>)</c-> <c- n>l</c-><c- p>.</c-><c- n>n_</c-><c- p>.</c-><c- n>prev_</c-><c- o>-></c-><c- n>next_</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>n_</c-><c- p>;</c->  <c- c1>// fixup</c->
        <c- n>n_</c-> <c- o>=</c-> <c- n>l</c-><c- p>.</c-><c- n>n_</c-><c- p>;</c->
        <c- n>l</c-><c- p>.</c-><c- n>n_</c-> <c- o>=</c-> <c- n>node</c-><c- p>{};</c->
    <c- p>}</c->
    <c- c1>// ...</c->
<c- p>};</c->
</pre></small> 
   <h3 class="heading settled" id="non-trivial-sample-offset-ptr"><span class="content">Class invariant depends on <code class="highlight"><c- k>this</c-></code></span><a class="self-link" href="#non-trivial-sample-offset-ptr"></a></h3>
   <p>The <code class="highlight"><c- n>offset_ptr</c-></code> provided by <a data-link-type="biblio" href="#biblio-boostinterprocess" title="Mapping Address Independent Pointer: offset_ptr">[Boost.Interprocess]</a> is an example of this category.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>offset_ptr</c-> <c- p>{</c->
    <c- b>uintptr_t</c-> <c- n>value_</c-><c- p>;</c->

    <c- b>uintptr_t</c-> <c- nf>here</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- b>uintptr_t</c-><c- p>(</c-><c- n>this</c-><c- p>);</c-> <c- p>}</c->
    <c- b>uintptr_t</c-> <c- nf>distance_to</c-><c- p>(</c-><c- b>void</c-> <c- o>*</c-><c- n>p</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- b>uintptr_t</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-> <c- o>-</c-> <c- n>here</c-><c- p>();</c-> <c- p>}</c->
    <c- b>void</c-> <c- o>*</c-><c- nf>get</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- p>(</c-><c- b>void</c-><c- o>*</c-><c- p>)(</c-><c- n>here</c-><c- p>()</c-> <c- o>+</c-> <c- n>value_</c-><c- p>);</c-> <c- p>}</c->

    <c- n>offset_ptr</c-><c- p>()</c-> <c- o>:</c-> <c- n>value_</c-><c- p>(</c-><c- n>distance_to</c-><c- p>(</c-><c- n>nullptr</c-><c- p>))</c-> <c- p>{}</c->
    <c- n>offset_ptr</c-><c- p>(</c-><c- b>void</c-> <c- o>*</c-><c- n>p</c-><c- p>)</c-> <c- o>:</c-> <c- n>value_</c-><c- p>(</c-><c- n>distance_to</c-><c- p>(</c-><c- n>p</c-><c- p>))</c-> <c- p>{}</c->
    <c- n>offset_ptr</c-><c- p>(</c-><c- k>const</c-> <c- n>offset_ptr</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- o>:</c-> <c- n>value_</c-><c- p>(</c-><c- n>distance_to</c-><c- p>(</c-><c- n>rhs</c-><c- p>.</c-><c- n>get</c-><c- p>()))</c-> <c- p>{}</c->
    <c- n>offset_ptr</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>offset_ptr</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- p>{</c->
        <c- n>value_</c-> <c- o>=</c-> <c- n>distance_to</c-><c- p>(</c-><c- n>rhs</c-><c- p>.</c-><c- n>get</c-><c- p>());</c->
        <c- k>return</c-> <c- o>*</c-><c- n>this</c-><c- p>;</c->
    <c- p>}</c->
    <c- o>~</c-><c- n>offset_ptr</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
<c- p>};</c->
</pre></small> 
   <h3 class="heading settled" id="non-trivial-sample-registry"><span class="content">Program invariant depends on <code class="highlight"><c- k>this</c-></code></span><a class="self-link" href="#non-trivial-sample-registry"></a></h3>
   <p>In the following snippet, <code class="highlight"><c- k>struct</c-> <c- nc>Widget</c-></code> is relocatable, but not
trivially relocatable, because the relocation operation of destroying a <code class="highlight"><c- n>Widget</c-></code> at point A
and constructing a new <code class="highlight"><c- n>Widget</c-></code> at point B has behavior that is observably different
from a simple <code class="highlight"><c- n>memcpy</c-></code>.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>set</c-><c- o>&lt;</c-><c- b>void</c-> <c- o>*></c-> <c- n>registry</c-><c- p>;</c->

<c- k>struct</c-> <c- nc>registered_object</c-> <c- p>{</c->
    <c- n>registered_object</c-><c- p>()</c-> <c- p>{</c-> <c- n>registry</c-><c- p>.</c-><c- n>insert</c-><c- p>(</c-><c- n>this</c-><c- p>);</c-> <c- p>}</c->
    <c- n>registered_object</c-><c- p>(</c-><c- n>registered_object</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>registered_object</c-><c- p>(</c-><c- k>const</c-> <c- n>registered_object</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>registered_object</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- n>registered_object</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>registered_object</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>registered_object</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- o>~</c-><c- n>registered_object</c-><c- p>()</c-> <c- p>{</c-> <c- n>registry</c-><c- p>.</c-><c- n>erase</c-><c- p>(</c-><c- n>this</c-><c- p>);</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>Widget</c-> <c- o>:</c-> <c- n>registered_object</c-> <c- p>{};</c->
</pre></small> 
   <h3 class="heading settled" id="non-trivial-sample-polymorphic"><span class="content">Polymorphic downcast effectively relies on offset-into-self</span><a class="self-link" href="#non-trivial-sample-polymorphic"></a></h3>
   <p>Thanks to David Stone for this example.
In the following snippet, <code class="highlight"><c- k>struct</c-> <c- nc>Base</c-></code> is relocatable, but not trivially relocatable,
because its copy constructor and assignment operator do not copy the entire state of the
right-hand object. (Notice that <code class="highlight"><c- n>pf</c-></code> is initialized with <code class="highlight"><c- n>f</c-></code>, not with a copy of <code class="highlight"><c- n>o</c-><c- p>.</c-><c- n>pf</c-></code>.)</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>Base</c-> <c- p>{</c->
    <c- k>static</c-> <c- b>int</c-> <c- nf>f</c-><c- p>(</c-><c- n>Base</c-><c- o>*</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>21</c-><c- p>;</c-> <c- p>}</c->
    <c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>pf</c-><c- p>)(</c-><c- n>Base</c-><c- o>*</c-><c- p>);</c->
    <c- n>Base</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>pf</c-><c- p>)(</c-><c- n>Base</c-><c- o>*</c-><c- p>)</c-> <c- o>=</c-> <c- n>f</c-><c- p>)</c-> <c- o>:</c-> <c- n>pf</c-><c- p>(</c-><c- n>pf</c-><c- p>)</c-> <c- p>{}</c->
    <c- n>Base</c-><c- p>(</c-><c- k>const</c-> <c- n>Base</c-><c- o>&amp;</c-> <c- n>o</c-><c- p>)</c-> <c- o>:</c-> <c- n>pf</c-><c- p>(</c-><c- n>f</c-><c- p>)</c-> <c- p>{}</c->
    <c- n>Base</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>Base</c-><c- o>&amp;</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- o>*</c-><c- n>this</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->
<c- k>struct</c-> <c- nc>Derived</c-> <c- o>:</c-> <c- n>Base</c-> <c- p>{</c->
    <c- k>static</c-> <c- b>int</c-> <c- nf>f</c-><c- p>(</c-><c- n>Base</c-> <c- o>*</c-><c- n>self</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- p>((</c-><c- n>Derived</c-><c- o>*</c-><c- p>)</c-><c- n>self</c-><c- p>)</c-><c- o>-></c-><c- n>x</c-><c- p>;</c-> <c- p>}</c->
    <c- n>Derived</c-><c- p>()</c-> <c- o>:</c-> <c- n>Base</c-><c- p>(</c-><c- n>f</c-><c- p>)</c-> <c- p>{}</c->
    <c- n>Derived</c-><c- p>(</c-><c- k>const</c-> <c- n>Derived</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>Derived</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>Derived</c-><c- o>&amp;</c-> <c- n>o</c-><c- p>)</c-> <c- p>{</c-> <c- n>x</c-> <c- o>=</c-> <c- n>o</c-><c- p>.</c-><c- n>x</c-><c- p>;</c-> <c- k>return</c-> <c- o>*</c-><c- n>this</c-><c- p>;</c-> <c- p>}</c->
    <c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- mi>42</c-><c- p>;</c->
<c- p>};</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- n>Base</c-><c- o>&amp;&amp;</c-> <c- n>d</c-> <c- o>=</c-> <c- n>Derived</c-><c- p>();</c->
    <c- n>Base</c-><c- o>&amp;&amp;</c-> <c- n>b</c-> <c- o>=</c-> <c- n>Base</c-><c- p>();</c->
    <c- n>std</c-><c- o>::</c-><c- n>swap</c-><c- p>(</c-><c- n>b</c-><c- p>,</c-> <c- n>d</c-><c- p>);</c->
    <c- n>printf</c-><c- p>(</c-><c- s>"%d</c-><c- se>\n</c-><c- s>"</c-><c- p>,</c-> <c- n>b</c-><c- p>.</c-><c- n>pf</c-><c- p>(</c-><c- o>&amp;</c-><c- n>b</c-><c- p>));</c->
<c- p>}</c->
</pre>
   <p>The above snippet is isomorphic to a classically polymorphic hierarchy
with virtual methods. Here is the same snippet using <code class="highlight"><c- k>virtual</c-></code>:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>Base</c-> <c- p>{</c->
    <c- n>virtual</c-> <c- b>int</c-> <c- n>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>21</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->
<c- k>struct</c-> <c- nc>Derived</c-> <c- o>:</c-> <c- n>Base</c-> <c- p>{</c->
    <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- n>override</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- p>;</c-> <c- p>}</c->
    <c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- mi>42</c-><c- p>;</c->
<c- p>};</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- n>Base</c-><c- o>&amp;&amp;</c-> <c- n>b</c-> <c- o>=</c-> <c- n>Base</c-><c- p>();</c->
    <c- n>Base</c-><c- o>&amp;&amp;</c-> <c- n>d</c-> <c- o>=</c-> <c- n>Derived</c-><c- p>();</c->
    <c- n>std</c-><c- o>::</c-><c- n>swap</c-><c- p>(</c-><c- n>b</c-><c- p>,</c-> <c- n>d</c-><c- p>);</c->
    <c- n>printf</c-><c- p>(</c-><c- s>"%d</c-><c- se>\n</c-><c- s>"</c-><c- p>,</c-> <c- n>b</c-><c- p>.</c-><c- n>f</c-><c- p>());</c->
<c- p>}</c->
</pre>
   <p>This is why (since P1144R5) the compiler will not consider types with
virtual methods to be "naturally" trivially relocatable.</p>
   <h2 class="heading settled" id="implementation"><span class="content">Appendix D: Implementation experience</span><a class="self-link" href="#implementation"></a></h2>
   <p>A prototype Clang/libc++ implementation is at</p>
   <ul>
    <li data-md>
     <p><a href="https://github.com/Quuxplusone/llvm-project/tree/trivially-relocatable">github.com/Quuxplusone/llvm-project/tree/trivially-relocatable</a></p>
    <li data-md>
     <p><a href="https://p1144.godbolt.org/z/oWEd_X">godbolt.org</a>, under the name "x86-64 clang (experimental P1144)"</p>
   </ul>
   <p>Side-by-side case studies of <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code>, <code class="highlight"><c- p>[[</c-><c- n>maybe_trivially_relocatable</c-><c- p>]]</c-></code>,
and <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>(</c-><c- b>bool</c-><c- p>)]]</c-></code> are given in <a data-link-type="biblio" href="#biblio-p1144r6" title="Object relocation in terms of move plus destroy">[P1144R6]</a>.</p>
   <p>As of November 2018, libstdc++ performs the <code class="highlight"><c- n>vector</c-><c- o>::</c-><c- n>resize</c-></code> optimization
for any type which has manually specialized <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>__is_bitwise_relocatable</c-></code>.
(See <a href="https://godbolt.org/z/f7971hEcG">this Godbolt</a>.)
Manual specialization is also the approach used by <a data-link-type="biblio" href="#biblio-qt" title="Qt Base">[Qt]</a>, <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>, and <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>.
As of May 2023, the only libstdc++ library type for which <code class="highlight"><c- n>__is_bitwise_relocatable</c-></code> has
been specialized is <code class="highlight"><c- n>deque</c-></code>; see <a data-link-type="biblio" href="#biblio-deque" title="Improve relocation ... (__is_trivially_relocatable): Specialize for deque">[Deque]</a>.</p>
   <p>Clang trunk provides a compiler builtin type trait <code class="highlight"><c- n>__is_trivially_relocatable</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code> (see <a data-link-type="biblio" href="#biblio-d114732" title="[clang] Mark trivial_abi types as trivially relocatable">[D114732]</a>), which is largely the same as the trait proposed in this paper. (There are slight
differences; <a href="https://p1144.godbolt.org/z/MhncMxrdj">e.g.</a>, Clang reports polymorphic types, reference types, and incomplete types
as trivially relocatable, whereas P1144 does not. I’m not aware that any of Clang’s differences were intentional.)
Clang trunk has no equivalent of the <code class="highlight"><c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>]]</c-></code> attribute, so <code class="highlight"><c- n>__is_trivially_relocatable</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code> is true only for
trivially copyable types and types marked with the <code class="highlight"><c- p>[[</c-><c- n>clang</c-><c- o>::</c-><c- n>trivial_abi</c-><c- p>]]</c-></code> attribute. As of May 2023, Clang trunk has no conception of a type which is non-trivial for purposes of calls
and yet is trivially relocatable. But Clang’s current status is compatible with P1144 (modulo the
few unintentional differences in <code class="highlight"><c- n>__is_trivially_relocatable</c-></code> mentioned above).</p>
   <h2 class="heading settled" id="open-questions"><span class="content">Appendix E: Open questions</span><a class="self-link" href="#open-questions"></a></h2>
   <p class="issue" id="issue-142cf072"><a class="self-link" href="#issue-142cf072"></a> In P1144R8, types with vptrs are never naturally trivially relocatable.
Arthur claims that types with vptrs should be used only with inheritance, which means they never do value semantics
(they slice instead), which means relocating one is always a bug, just like assigning or copying one. But <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> proposes to make polymorphic types naturally trivially relocatable. Is there any use-case for relocating polymorphic
types? Is P1144 being too conservative here?</p>
   <p class="issue" id="issue-89b71674"><a class="self-link" href="#issue-89b71674"></a> In P1144R8, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>>::</c-><c- n>erase</c-><c- p>(</c-><c- n>pos</c-><c- p>)</c-></code> can use trivial relocation to
"close up the window" in the vector; this is also done in <a data-link-type="biblio" href="#biblio-qt" title="Qt Base">[Qt]</a>, <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>, and <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>. The difference between
closing-the-window-by-move-assignment and closing-the-window-by-memmove is detectable in a conforming C++ program.
Therefore <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> proposes that relocation should not be allowed here.
Is there any implementation experience of P2786R0’s conservative position here?
(<a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>'s <code class="highlight"><c- n>bsl</c-><c- o>::</c-><c- n>vector</c-></code> follows P1144R8’s liberal position already.)</p>
   <p class="issue" id="issue-39dfb60e"><a class="self-link" href="#issue-39dfb60e"></a> In P1144R8, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rotate</c-><c- p>(</c-><c- n>a</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>2</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>8</c-><c- p>)</c-></code> on an array <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>a</c-><c- p>[</c-><c- mi>10</c-><c- p>]</c-></code> can use trivial relocation to
swap the elements. The difference between swap-by-move-assignment and swap-by-trivial-relocation is not detectable
in a conforming C++ program, because swapping <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-></code>s with unequal allocators is UB. <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> conservatively proposes that relocation should not be allowed here, anyway.
Is there any usage experience in <a data-link-type="biblio" href="#biblio-qt" title="Qt Base">[Qt]</a>, <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>, <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>, or elsewhere that bears on this question?
(Arthur’s libc++ fork does relocation here, but doesn’t really count as usage experience since he’s the only one using it.)</p>
   <p class="issue" id="issue-c41eb6cd"><a class="self-link" href="#issue-c41eb6cd"></a> If <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> is considered trivially relocatable, then so would be a Rule-of-Zero type <code class="highlight"><c- n>Widget</c-></code> with a <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-></code> data member. This means that we optimize <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rotate</c-><c- p>(</c-><c- n>Widget</c-><c- o>*</c-><c- p>,</c-> <c- p>...)</c-></code> (<a href="https://godbolt.org/z/4zGPcoejc">Godbolt</a>),
which is a huge performance win but again is <a href="https://godbolt.org/z/fY1fjs3Mn">observably</a> different behavior
from the old way — and in this particular case the old behavior is <em>not</em> undefined!
Still, it would be sad to give up this massive performance improvement for <code class="highlight"><c- n>pmr</c-></code> types.
Can we modify the preconditions of {<code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-></code>, <code class="highlight"><c- n>swap</c-></code>, <code class="highlight"><c- n>swap_ranges</c-></code>, <code class="highlight"><c- n>rotate</c-></code>} to make this optimization conforming?</p>
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  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#abstract-opdef-relocation-operation">relocation operation</a><span>, in § 4.2</span>
   <li><a href="#abstract-opdef-trivially-relocatable">trivially relocatable</a><span>, in § 4.5</span>
  </ul>
  <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="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-n4944">[N4944]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4944"><cite>Working Draft, Standard for Programming Language C++</cite></a>. 22 March 2023. URL: <a href="https://wg21.link/n4944">https://wg21.link/n4944</a>
  </dl>
  <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-announcing">[Announcing]
   <dd>Arthur O'Dwyer. <a href="https://quuxplusone.github.io/blog/2018/07/18/announcing-trivially-relocatable/"><cite>Announcing "trivially relocatable"</cite></a>. July 2018. URL: <a href="https://quuxplusone.github.io/blog/2018/07/18/announcing-trivially-relocatable/">https://quuxplusone.github.io/blog/2018/07/18/announcing-trivially-relocatable/</a>
   <dt id="biblio-bench">[Bench]
   <dd>Arthur O'Dwyer. <a href="https://github.com/Quuxplusone/from-scratch/blob/095b246d/cppnow2018/benchmark-relocatable.cc"><cite>Benchmark code from "The Best Type Traits C++ Doesn't Have"</cite></a>. April 2018. URL: <a href="https://github.com/Quuxplusone/from-scratch/blob/095b246d/cppnow2018/benchmark-relocatable.cc">https://github.com/Quuxplusone/from-scratch/blob/095b246d/cppnow2018/benchmark-relocatable.cc</a>
   <dt id="biblio-boostinterprocess">[Boost.Interprocess]
   <dd>Ion Gaztañaga. <a href="https://www.boost.org/doc/libs/1_67_0/doc/html/interprocess/offset_ptr.html"><cite>Mapping Address Independent Pointer: offset_ptr</cite></a>. 2005. URL: <a href="https://www.boost.org/doc/libs/1_67_0/doc/html/interprocess/offset_ptr.html">https://www.boost.org/doc/libs/1_67_0/doc/html/interprocess/offset_ptr.html</a>
   <dt id="biblio-bsl">[BSL]
   <dd>Bloomberg. <a href="https://github.com/bloomberg/bde/blob/962f7aa/groups/bsl/bslmf/bslmf_isbitwisemoveable.h#L8-L48"><cite>bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction</cite></a>. 2013–2022. URL: <a href="https://github.com/bloomberg/bde/blob/962f7aa/groups/bsl/bslmf/bslmf_isbitwisemoveable.h#L8-L48">https://github.com/bloomberg/bde/blob/962f7aa/groups/bsl/bslmf/bslmf_isbitwisemoveable.h#L8-L48</a>
   <dt id="biblio-cppchat">[CppChat]
   <dd>Howard Hinnant; Arthur O'Dwyer. <a href="https://www.youtube.com/watch?v=8u5Qi4FgTP8"><cite>cpp.chat episode 40: It works but it's undefined behavior</cite></a>. August 2018. URL: <a href="https://www.youtube.com/watch?v=8u5Qi4FgTP8">https://www.youtube.com/watch?v=8u5Qi4FgTP8</a>
   <dt id="biblio-d114732">[D114732]
   <dd>Devin Jeanpierre. <a href="https://reviews.llvm.org/D114732"><cite>[clang] Mark trivial_abi types as trivially relocatable</cite></a>. November 2021. URL: <a href="https://reviews.llvm.org/D114732">https://reviews.llvm.org/D114732</a>
   <dt id="biblio-d50119">[D50119]
   <dd>Arthur O'Dwyer; Nicolas Lesser; John McCall. <a href="https://reviews.llvm.org/D50119"><cite>Compiler support for P1144R0 __is_trivially_relocatable(T)</cite></a>. July 2018. URL: <a href="https://reviews.llvm.org/D50119">https://reviews.llvm.org/D50119</a>
   <dt id="biblio-deque">[Deque]
   <dd>Marc Glisse. <a href="https://github.com/gcc-mirror/gcc/commit/a9b9381580de611126c9888c1a6c12a77d9b682e"><cite>Improve relocation ... (__is_trivially_relocatable): Specialize for deque</cite></a>. November 2018. URL: <a href="https://github.com/gcc-mirror/gcc/commit/a9b9381580de611126c9888c1a6c12a77d9b682e">https://github.com/gcc-mirror/gcc/commit/a9b9381580de611126c9888c1a6c12a77d9b682e</a>
   <dt id="biblio-fixedcapacityvector">[FixedCapacityVector]
   <dd>Arthur O'Dwyer. <a href="https://quuxplusone.github.io/blog/2019/02/22/p1144-fixed-capacity-vector/"><cite>P1144 case study: Moving a `fixed_capacity_vector`</cite></a>. URL: <a href="https://quuxplusone.github.io/blog/2019/02/22/p1144-fixed-capacity-vector/">https://quuxplusone.github.io/blog/2019/02/22/p1144-fixed-capacity-vector/</a>
   <dt id="biblio-folly">[Folly]
   <dd>Facebook. <a href="https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md#object-relocation"><cite>Folly documentation on "Object Relocation"</cite></a>. URL: <a href="https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md#object-relocation">https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md#object-relocation</a>
   <dt id="biblio-follyissue889">[FollyIssue889]
   <dd>Arthur O'Dwyer. <a href="https://github.com/facebook/folly/issues/889"><cite>Traits.h marks std::list as trivially relocatable, but in fact it is not</cite></a>. URL: <a href="https://github.com/facebook/folly/issues/889">https://github.com/facebook/folly/issues/889</a>
   <dt id="biblio-n1377">[N1377]
   <dd>Howard Hinnant; Peter Dimov; Dave Abrahams. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1377.htm"><cite>N1377: A Proposal to Add Move Semantics Support to the C++ Language</cite></a>. September 2002. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1377.htm">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1377.htm</a>
   <dt id="biblio-n2754">[N2754]
   <dd>Rodrigo Castro Campos. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2754.html"><cite>N2754: TriviallyDestructibleAfterMove and TriviallyReallocatable (rev 3)</cite></a>. September 2008. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2754.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2754.html</a>
   <dt id="biblio-n4158">[N4158]
   <dd>Pablo Halpern. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4158.pdf"><cite>N4158: Destructive Move (rev 1)</cite></a>. October 2014. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4158.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4158.pdf</a>
   <dt id="biblio-p0023r0">[P0023R0]
   <dd>Denis Bider. <a href="http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0023r0.pdf"><cite>P0023R0: Relocator: Efficiently Moving Objects</cite></a>. April 2016. URL: <a href="http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0023r0.pdf">http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0023r0.pdf</a>
   <dt id="biblio-p0843r5">[P0843R5]
   <dd>Gonzalo Brito Gadeschi. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0843r5.html"><cite>static_vector</cite></a>. July 2022. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0843r5.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0843r5.html</a>
   <dt id="biblio-p1029r3">[P1029R3]
   <dd>Niall Douglas. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1029r3.pdf"><cite>P1029R3: move = bitcopies</cite></a>. January 2020. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1029r3.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1029r3.pdf</a>
   <dt id="biblio-p1144r6">[P1144R6]
   <dd>Arthur O'Dwyer. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1144r6.html"><cite>Object relocation in terms of move plus destroy</cite></a>. June 2022. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1144r6.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1144r6.html</a>
   <dt id="biblio-p2786r0">[P2786R0]
   <dd>Mungo Gill; Alisdair Meredith. <a href="https://wg21.link/p2786r0"><cite>Trivial relocatability options</cite></a>. February 2023. URL: <a href="https://wg21.link/p2786r0">https://wg21.link/p2786r0</a>
   <dt id="biblio-p2814r0">[P2814R0]
   <dd>Mungo Gill; Alisdair Meredith. <a href="https://wg21.link/p2814r0"><cite>Trivial relocatability — comparing P2786 with P1144</cite></a>. May 2023. URL: <a href="https://wg21.link/p2814r0">https://wg21.link/p2814r0</a>
   <dt id="biblio-qt">[Qt]
   <dd><a href="https://github.com/qt/qtbase/"><cite>Qt Base</cite></a>. February 2023. URL: <a href="https://github.com/qt/qtbase/">https://github.com/qt/qtbase/</a>
   <dt id="biblio-stdrelocateiscute">[StdRelocateIsCute]
   <dd>Arthur O'Dwyer. <a href="https://quuxplusone.github.io/blog/2022/05/18/std-relocate/"><cite>std::relocate's implementation is cute</cite></a>. May 2022. URL: <a href="https://quuxplusone.github.io/blog/2022/05/18/std-relocate/">https://quuxplusone.github.io/blog/2022/05/18/std-relocate/</a>
   <dt id="biblio-subobjects">[Subobjects]
   <dd>Arthur O'Dwyer. <a href="https://quuxplusone.github.io/blog/2018/07/13/trivially-copyable-corner-cases/"><cite>When is a trivially copyable object not trivially copyable?</cite></a>. July 2018. URL: <a href="https://quuxplusone.github.io/blog/2018/07/13/trivially-copyable-corner-cases/">https://quuxplusone.github.io/blog/2018/07/13/trivially-copyable-corner-cases/</a>
   <dt id="biblio-subspace">[Subspace]
   <dd>Dana Jansens. <a href="https://danakj.github.io/2023/01/15/trivially-relocatable.html"><cite>Trivially Relocatable Types in C++/Subspace</cite></a>. January 2023. URL: <a href="https://danakj.github.io/2023/01/15/trivially-relocatable.html">https://danakj.github.io/2023/01/15/trivially-relocatable.html</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> What new wording is needed to achieve this? <a class="issue-return" href="#issue-97f1a6a8" title="Jump to section">↵</a></div>
   <div class="issue"> In P1144R8, types with vptrs are never naturally trivially relocatable.
Arthur claims that types with vptrs should be used only with inheritance, which means they never do value semantics
(they slice instead), which means relocating one is always a bug, just like assigning or copying one. But <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> proposes to make polymorphic types naturally trivially relocatable. Is there any use-case for relocating polymorphic
types? Is P1144 being too conservative here? <a class="issue-return" href="#issue-142cf072" title="Jump to section">↵</a></div>
   <div class="issue"> In P1144R8, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>>::</c-><c- n>erase</c-><c- p>(</c-><c- n>pos</c-><c- p>)</c-></code> can use trivial relocation to
"close up the window" in the vector; this is also done in <a data-link-type="biblio" href="#biblio-qt" title="Qt Base">[Qt]</a>, <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>, and <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>. The difference between
closing-the-window-by-move-assignment and closing-the-window-by-memmove is detectable in a conforming C++ program.
Therefore <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> proposes that relocation should not be allowed here.
Is there any implementation experience of P2786R0’s conservative position here?
(<a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>'s <code class="highlight"><c- n>bsl</c-><c- o>::</c-><c- n>vector</c-></code> follows P1144R8’s liberal position already.) <a class="issue-return" href="#issue-89b71674" title="Jump to section">↵</a></div>
   <div class="issue"> In P1144R8, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rotate</c-><c- p>(</c-><c- n>a</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>2</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>8</c-><c- p>)</c-></code> on an array <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>a</c-><c- p>[</c-><c- mi>10</c-><c- p>]</c-></code> can use trivial relocation to
swap the elements. The difference between swap-by-move-assignment and swap-by-trivial-relocation is not detectable
in a conforming C++ program, because swapping <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-></code>s with unequal allocators is UB. <a data-link-type="biblio" href="#biblio-p2786r0" title="Trivial relocatability options">[P2786R0]</a> conservatively proposes that relocation should not be allowed here, anyway.
Is there any usage experience in <a data-link-type="biblio" href="#biblio-qt" title="Qt Base">[Qt]</a>, <a data-link-type="biblio" href="#biblio-bsl" title="bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction">[BSL]</a>, <a data-link-type="biblio" href="#biblio-folly" title="Folly documentation on &quot;Object Relocation&quot;">[Folly]</a>, or elsewhere that bears on this question?
(Arthur’s libc++ fork does relocation here, but doesn’t really count as usage experience since he’s the only one using it.) <a class="issue-return" href="#issue-39dfb60e" title="Jump to section">↵</a></div>
   <div class="issue"> If <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> is considered trivially relocatable, then so would be a Rule-of-Zero type <code class="highlight"><c- n>Widget</c-></code> with a <code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-></code> data member. This means that we optimize <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rotate</c-><c- p>(</c-><c- n>Widget</c-><c- o>*</c-><c- p>,</c-> <c- p>...)</c-></code> (<a href="https://godbolt.org/z/4zGPcoejc">Godbolt</a>),
which is a huge performance win but again is <a href="https://godbolt.org/z/fY1fjs3Mn">observably</a> different behavior
from the old way — and in this particular case the old behavior is <em>not</em> undefined!
Still, it would be sad to give up this massive performance improvement for <code class="highlight"><c- n>pmr</c-></code> types.
Can we modify the preconditions of {<code class="highlight"><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-></code>, <code class="highlight"><c- n>swap</c-></code>, <code class="highlight"><c- n>swap_ranges</c-></code>, <code class="highlight"><c- n>rotate</c-></code>} to make this optimization conforming? <a class="issue-return" href="#issue-c41eb6cd" title="Jump to section">↵</a></div>
  </div>
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