<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P3301R0: inplace_stoppable_base</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

	/* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    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://github.com/vasama/wg21" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="d8dc7e86ac9cf7f84fd1351b7bf30269a4646f07" name="document-revision">
<style>/* style-autolinks */
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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">P3301R0<br>inplace_stoppable_base</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-05-21">2024-05-21</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:wg21@vasama.org">Lauri Vasama</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
     <dt>Current draft:
     <dd><a href="https://vasama.github.io/wg21/D3301.html">vasama.github.io/wg21/D3301.html</a>
     <dt>Current draft source:
     <dd><a href="https://github.com/vasama/wg21/blob/main/D3301.bs">github.com/vasama/wg21/blob/main/D3301.bs</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>Introduce a CRTP base class template as a more efficient alternative to inplace_stop_callback.</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="#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li>
     <a href="#proposal"><span class="secno">2</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#proposal_name"><span class="secno">2.1</span> <span class="content">Naming of the derived member function</span></a>
      <li><a href="#proposal_for"><span class="secno">2.2</span> <span class="content">Interaction with <code class="highlight"><c- n>stop_callback_for_t</c-></code></span></a>
     </ol>
    <li>
     <a href="#alternatives"><span class="secno">3</span> <span class="content">Alternatives considered</span></a>
     <ol class="toc">
      <li><a href="#alternatives_parameter"><span class="secno">3.1</span> <span class="content"><code class="highlight"><c- n>inplace_stop_callback</c-></code> optional parameter</span></a>
      <li><a href="#alternatives_accessor"><span class="secno">3.2</span> <span class="content">Superobject accessor function</span></a>
     </ol>
    <li><a href="#implementation"><span class="secno">4</span> <span class="content">Implementation experience</span></a>
    <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>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
   <p>Any sender <a data-link-type="biblio" href="#biblio-p2300r9" title="`std::execution`">[P2300R9]</a> wishing to support cancellation using cancellation tokens, particularly <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>inplace_stop_token</c-></code>, must currently store as a subobject of its operation state a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-></code> object containing a reference to the operation state itself:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>my_sender</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>R</c-><c- o>></c->
  <c- k>class</c-> <c- nc>op_state</c-> <c- p>{</c->
    <c- n>R</c-> <c- n>receiver</c-><c- p>;</c->

    <c- k>class</c-> <c- nc>cb_t</c-> <c- p>{</c->
      <c- n>op_state</c-><c- o>&amp;</c-> <c- n>op</c-><c- p>;</c->
    <c- k>public</c-><c- o>:</c->
      <c- n>cb_t</c-><c- p>(</c-><c- n>op_state</c-><c- o>&amp;</c-> <c- n>op</c-><c- p>)</c->
        <c- o>:</c-> <c- n>op</c-><c- p>(</c-><c- n>op</c-><c- p>)</c-> <c- p>{}</c->
      <c- b>void</c-> <c- k>operator</c-><c- p>()()</c-> <c- k>const</c-> <c- p>{</c->
        <c- n>op</c-><c- p>.</c-><c- n>on_stop_requested</c-><c- p>();</c->
      <c- p>}</c->
    <c- p>};</c->
    <c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-><c- o>&lt;</c-><c- n>cb_t</c-><c- o>></c-> <c- n>cb</c-><c- p>;</c->

    <c- b>void</c-> <c- nf>on_stop_requested</c-><c- p>();</c->
  <c- p>};</c->
<c- p>};</c->
</pre>
   <p>This naturally comes with a small but non-zero cost in the space required for each operation state. While standard library implementations could likely eliminate this cost for any standard senders in just the manner described in this paper, that will not be possible for any user senders wishing to make use of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>inplace_stop_token</c-></code> as it is currently specified.</p>
   <h2 class="heading settled" data-level="2" id="proposal"><span class="secno">2. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>We propose to introduce a new standard library CRTP class template for use as a base subobject of classes which would otherwise contain as a subobject a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-></code> with a reference to the enclosing object:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>inplace_stoppable_base</c-><c- p>;</c->
</pre>
   <p>As is usual with CRTP, it is required that the class type <code class="highlight"><c- n>T</c-></code> is derived (directly or indirectly) from <code class="highlight"><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> and that the conversion from <code class="highlight"><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-></code> to <code class="highlight"><c- n>T</c-><c- o>*</c-></code> is accessible to <code class="highlight"><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>.</p>
   <p>Unlike <code class="highlight"><c- n>inplace_stop_callback</c-></code>, the only constructor of <code class="highlight"><c- n>inplace_stoppable_base</c-></code> has <code class="highlight"><c- k>protected</c-></code> access and takes a single parameter of type <code class="highlight"><c- n>inplace_stop_token</c-></code>.</p>
   <p>Upon request for cancellation of the <code class="highlight"><c- n>inplace_stop_token</c-></code> to which a <code class="highlight"><c- n>inplace_stoppable_base</c-></code> object is registered, the member function <code class="highlight"><c- n>on_stop_requested</c-><c- p>()</c-></code> is invoked on the derived object of type <code class="highlight"><c- n>T</c-></code>.</p>
   <p>Using this class template, the previous example can not only be optimised, but vastly simplified:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>my_sender</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>R</c-><c- o>></c->
  <c- k>class</c-> <c- nc>op_state</c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>op_state</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-> <c- p>{</c->
    <c- n>R</c-> <c- n>receiver</c-><c- p>;</c->
    <c- c1>// invoked upon request for cancellation.</c->
    <c- b>void</c-> <c- nf>on_stop_requested</c-><c- p>();</c->
  <c- p>};</c->
<c- p>};</c->
</pre>
   <p>Note that it is possible to implement <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-></code> in terms of <code class="highlight"><c- n>inplace_stoppable_base</c-></code>, while the reverse is not true.</p>
   <h3 class="heading settled" data-level="2.1" id="proposal_name"><span class="secno">2.1. </span><span class="content">Naming of the derived member function</span><a class="self-link" href="#proposal_name"></a></h3>
   <p>We propose the name <code class="highlight"><c- n>on_stop_requested</c-></code> which has much precedent in libraries in the wild and in other languages, but little directly applicable precedent in the standard library. The standard library does include a few things named similarly:</p>
   <ul>
    <li data-md>
     <p>Allocator propagation traits such as <code class="highlight"><c- n>propagate_on_container_move_assignment</c-></code>.</p>
    <li data-md>
     <p>A few functions related to <code class="highlight"><c- n>iostream</c-></code> such as <code class="highlight"><c- n>set_emit_on_sync</c-></code>.</p>
   </ul>
   <p>Other possible names include:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>stop_requested</c-></code> <br> This is a reasonable name, but <code class="highlight"><c- n>inplace_stop_token</c-></code> itself has a function with the same name used for a different but related purpose.</p>
    <li data-md>
     <p><code class="highlight"><c- n>at_stop_requested</c-></code> <br> This pattern has existing precedent in the standard library for callback functions:</p>
     <ul>
      <li data-md>
       <p><code class="highlight"><c- n>atexit</c-></code></p>
      <li data-md>
       <p><code class="highlight"><c- n>at_quick_exit</c-></code>.</p>
     </ul>
   </ol>
   <p>Note that it is fairly trivial for users to create a thin wrapper which uses their desired naming:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>InplaceStoppableBase</c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>InplaceStoppableBase</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>InplaceStoppableBase</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>;</c->
<c- k>protected</c-><c- o>:</c->
  <c- k>using</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>InplaceStoppableBase</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>>::</c-><c- n>inplace_stoppable_base</c-><c- p>;</c->
<c- k>private</c-><c- o>:</c->
  <c- b>void</c-> <c- n>on_stop_requested</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c->
    <c- k>static_cast</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-><c- p>(</c-><c- k>this</c-><c- p>)</c-><c- o>-></c-><c- n>OnStopRequested</c-><c- p>();</c->
  <c- p>}</c->
<c- p>};</c->
</pre>
   <h3 class="heading settled" data-level="2.2" id="proposal_for"><span class="secno">2.2. </span><span class="content">Interaction with <code class="highlight"><c- n>stop_callback_for_t</c-></code></span><a class="self-link" href="#proposal_for"></a></h3>
   <p>This proposal does not include any changes to <code class="highlight"><c- n>stop_callback_for_t</c-></code> or the stop token concepts.</p>
   <p>It is possible for users to handle arbitrary stop token types by specializing for types known to support CRTP and falling back to the callback type otherwise:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>Token</c-><c- o>></c->
<c- k>class</c-> <c- nc>my_stoppable_base_for</c-> <c- p>{</c->
  <c- k>class</c-> <c- nc>cb_t</c-> <c- p>{</c->
    <c- n>my_stoppable_base_for</c-><c- o>&amp;</c-> <c- n>base</c-><c- p>;</c->
  <c- k>public</c-><c- o>:</c->
    <c- k>explicit</c-> <c- n>cb_t</c-><c- p>(</c-><c- n>my_stoppable_base_for</c-><c- o>&amp;</c-> <c- n>base</c-><c- p>)</c->
      <c- o>:</c-> <c- n>base</c-><c- p>(</c-><c- n>base</c-><c- p>)</c-> <c- p>{}</c->
    <c- b>void</c-> <c- k>operator</c-><c- p>()()</c-> <c- p>{</c->
      <c- k>static_cast</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;></c-><c- p>(</c-><c- n>base</c-><c- p>).</c-><c- n>on_stop_requested</c-><c- p>();</c->
    <c- p>}</c->
  <c- p>};</c->
  <c- n>std</c-><c- o>::</c-><c- n>stop_callback_for_t</c-><c- o>&lt;</c-><c- n>Token</c-><c- p>,</c-> <c- n>cb_t</c-><c- o>></c-> <c- n>cb</c-><c- p>;</c->

<c- k>protected</c-><c- o>:</c->
  <c- k>explicit</c-> <c- n>my_stoppable_base_for</c-><c- p>(</c-><c- n>Token</c-> <c- n>token</c-><c- p>)</c->
    <c- o>:</c-> <c- n>cb</c-><c- p>(</c-><c- n>token</c-><c- p>)</c-> <c- p>{}</c->
<c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>my_stoppable_base_for</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stop_token</c-><c- o>></c->
  <c- o>:</c-> <c- k>public</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c->
<c- k>protected</c-><c- o>:</c->
  <c- k>using</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- o>::</c-><c- n>inplace_stoppable_base</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <h2 class="heading settled" data-level="3" id="alternatives"><span class="secno">3. </span><span class="content">Alternatives considered</span><a class="self-link" href="#alternatives"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="alternatives_parameter"><span class="secno">3.1. </span><span class="content"><code class="highlight"><c- n>inplace_stop_callback</c-></code> optional parameter</span><a class="self-link" href="#alternatives_parameter"></a></h3>
   <p>Instead of introducing a new class template, <code class="highlight"><c- n>inplace_stop_callback</c-></code> could be modified such that a reference to the <code class="highlight"><c- n>inplace_stop_callback</c-><c- o>&lt;</c-><c- n>CB</c-><c- o>></c-></code> itself is passed in the invocation of its <i>stop-callback</i> member, if such an invocation is viable. In addition to no new standard library template, this avoids choosing a name to be imposed on the user operation state.</p>
   <p>We prefer the proposed <code class="highlight"><c- n>inplace_stoppable_base</c-></code> over this approach mainly due to the increased complexity of user code in the common case:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>my_sender</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- n>OpState</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>cb_t</c-> <c- p>{</c->
    <c- k>static</c-> <c- b>void</c-> <c- nf>operator</c-><c- p>()(</c-><c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-><c- o>&lt;</c-><c- n>cb_t</c-><c- o>&lt;</c-><c- n>OpState</c-><c- o>>>&amp;</c-> <c- n>cb</c-><c- p>)</c-> <c- p>{</c->
      <c- k>static_cast</c-><c- o>&lt;</c-><c- n>OpState</c-><c- o>&amp;></c-><c- p>(</c-><c- n>cb</c-><c- p>).</c-><c- n>on_stop_requested</c-><c- p>();</c->
    <c- p>}</c->
  <c- p>};</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>R</c-><c- o>></c->
  <c- k>class</c-> <c- nc>op_state</c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-><c- o>&lt;</c-><c- n>cb_t</c-><c- o>&lt;</c-><c- n>op_state</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>>></c-> <c- p>{</c->
    <c- k>friend</c-> <c- n>my_sender</c-><c- p>;</c->
    <c- n>R</c-> <c- n>receiver</c-><c- p>;</c->
    <c- b>void</c-> <c- nf>on_stop_requested</c-><c- p>();</c->
  <c- p>};</c->
<c- p>};</c->
</pre>
   <p>Note that both this modified <code class="highlight"><c- n>inplace_stop_callback</c-></code> (under another user-chosen name) and <code class="highlight"><c- n>inplace_stoppable_base</c-></code> are implementable in terms of the other.</p>
   <h3 class="heading settled" data-level="3.2" id="alternatives_accessor"><span class="secno">3.2. </span><span class="content">Superobject accessor function</span><a class="self-link" href="#alternatives_accessor"></a></h3>
   <p>A new standard library function template <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get_superobject</c-></code> which, given a reference to the <i>stop-callback</i> object, returns a reference to the enclosing <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-><c- o>&lt;</c-><c- n>CB</c-><c- o>></c-></code> object:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>my_sender</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- n>OpState</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>cb_t</c-> <c- p>{</c->
    <c- b>void</c-> <c- nf>operator</c-><c- p>()()</c-> <c- p>{</c->
      <c- k>using</c-> <c- n>super_type</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-><c- o>&lt;</c-><c- n>cb_t</c-><c- o>&lt;</c-><c- n>OpState</c-><c- o>>></c-><c- p>;</c->
      <c- k>static_cast</c-><c- o>&lt;</c-><c- n>OpState</c-><c- o>&amp;></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>get_superobject</c-><c- o>&lt;</c-><c- n>super_type</c-><c- o>&amp;></c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)).</c-><c- n>on_stop_requested</c-><c- p>();</c->
    <c- p>}</c->
  <c- p>};</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>R</c-><c- o>></c->
  <c- k>class</c-> <c- nc>op_state</c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>inplace_stop_callback</c-><c- o>&lt;</c-><c- n>cb_t</c-><c- o>&lt;</c-><c- n>op_state</c-><c- o>>></c-> <c- p>{</c->
    <c- k>friend</c-> <c- n>my_sender</c-><c- p>;</c->
    <c- n>R</c-> <c- n>receiver</c-><c- p>;</c->
    <c- b>void</c-> <c- nf>on_stop_requested</c-><c- p>();</c->
  <c- p>};</c->
<c- p>};</c->
</pre>
   <p>The other alternatives are implementable in terms of this option, while the reverse is not true. In that sense this the most general solution. However, apart from introducing this new function template and all the bikeshedding that comes with that, there is one major downside: In order to derive a reference to the <code class="highlight"><c- n>inplace_stop_callback</c-></code> from a reference to its <i>stop-callback</i> member, one of three conditions must be fulfilled:</p>
   <ol>
    <li data-md>
     <p>The <i>stop-callback</i> must be a base subobject of the <code class="highlight"><c- n>inplace_stop_callback</c-></code>, which in turn requires the user callback type to be non-final.</p>
    <li data-md>
     <p>The <code class="highlight"><c- n>inplace_stop_callback</c-></code> must be a standard layout type, which in turn requires the user callback type itself to also be a standard layout type.</p>
    <li data-md>
     <p>The standard library must use compiler magic not accessible to the user in the implementation of <code class="highlight"><c- n>inplace_stop_callback</c-></code>.</p>
   </ol>
   <h2 class="heading settled" data-level="4" id="implementation"><span class="secno">4. </span><span class="content">Implementation experience</span><a class="self-link" href="#implementation"></a></h2>
   <p>This proposal has been implemented at <a href="https://github.com/vasama/stdexec/tree/inplace_stoppable_base">github.com/vasama/stdexec</a>.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-p2300r9">[P2300R9]
   <dd>Eric Niebler, Michał Dominiak, Georgy Evtushenko, Lewis Baker, Lucian Radu Teodorescu, Lee Howes, Kirk Shoop, Michael Garland, Bryce Adelstein Lelbach. <a href="https://wg21.link/p2300r9"><cite>`std::execution`</cite></a>. 2 April 2024. URL: <a href="https://wg21.link/p2300r9">https://wg21.link/p2300r9</a>
  </dl>