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 <head>
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  <title>P1135R0: The C++20 Synchronization Library</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
 *
 * 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)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * 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
 *
 * 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)
 *
 ******************************************************************************/

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

	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;

		/* Colors */
		color: black;
		background: white 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-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		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;
		border-color: #c00;
	}

	/* 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: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* 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-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** 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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}
		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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}

		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;
		background: transparent;
	}

	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,
	#subtitle {
		/* #subtitle 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) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		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;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; 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: medium;
	}
	dfn var {
		font-style: normal;
	}

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

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** 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;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	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;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

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

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

	img {
		border-style: none;
	}

	/* 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;
	}
	.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 {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* 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;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* 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;
		background: #E9FBE9;
		overflow: auto;
	}

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

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

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

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

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

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** 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: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    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;
	}

	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-top-style: solid;
	}

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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 <body class="h-entry">
  <input id="hidedel" style="display:none" type="checkbox">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1135R0<br>The C++20 Synchronization Library</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-06-12">12 June 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P1135r0">http://wg21.link/P1135r0</a>
     <dt>Issue Tracking:
     <dd><a href="https://github.com/brycelelbach/wg21_p1135_cpp20_synchronization_library/issues">GitHub</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:brycelelbach@gmail.com">Bryce Adelstein Lelbach</a> (<span class="p-org org">NVIDIA</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ogiroux@nvidia.com">Olivier Giroux</a> (<span class="p-org org">NVIDIA</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dt>Audience:
     <dd>SG1, LEWG, LWG
     <dt>Toggle Diffs:
     <dd>
      <label for="hidedel" id="hidedel-label">Hide deleted text</label>
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </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="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#wording"><span class="secno">2</span> <span class="content">Wording</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>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>This paper is the unification of the wording for a series of related C++20
proposals for introducing new synchronization and thread coordination
facilities and enhancing existing ones:</p>
   <ul>
    <li data-md="">
     <p><span>[P0514]</span>: Efficient <code class="highlight"><span class="n">atomic</span></code> waiting and semaphores.</p>
    <li data-md="">
     <p><span>[P0666]</span>: Latches and barriers.</p>
    <li data-md="">
     <p><span>[P0995]</span>: <code class="highlight"><span class="n">atomic_flag</span><span class="o">::</span><span class="n">test</span></code> and lockfree integral types.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="wording"><span class="secno">2. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p class="note" role="note"><span>Note:</span> The � character is used to denote a placeholder number which shall be selected by the editor.</p>
   <p>Modify the header synopsis for <code class="highlight"><span class="o">&lt;</span><span class="n">atomic</span><span class="o">></span></code> in [<strong>atomics.syn</strong>] as follows:</p>
   <blockquote>
     <b>32.2 Header <code class="highlight"><span class="o">&lt;</span><span class="n">atomic</span><span class="o">></span></code> synopsis <span style="float: right;">[atomics.syn]</span></b> 
<pre class="highlight"><code class="highlight"><span class="k"><span class="k">namespace</span></span> <span class="n"><span class="n">std</span></span> <span class="p"><span class="p">{</span></span>
  <i><span class="c1"><span class="c1">// ...</span></span></i>
<span class="c1"></span><span class="c1"></span>
  <i><span class="c1"><span class="c1">// 32.3, type aliases</span></span></i>
<span class="c1"></span><span class="c1"></span>  <i><span class="c1"><span class="c1">// ...</span></span></i>
<span class="c1"></span><span class="c1"></span>   
  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_intptr_t</span></span>       <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">intptr_t</span></span><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span>
  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_uintptr_t</span></span>      <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">uintptr_t</span></span><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span>
  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_size_t</span></span>         <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">size_t</span></span><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span>
  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_ptrdiff_t</span></span>      <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span>
  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_intmax_t</span></span>       <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">intmax_t</span></span><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span>
  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_uintmax_t</span></span>      <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">uintmax_t</span></span><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span>
 
</code></pre>
<pre class="ins highlight"><code class="highlight">  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_int_fast_wait_t</span></span>  <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><i><span class="n"><span class="n">implementation</span></span><span class="o"><span class="o">-</span></span><span class="n"><span class="n">defined</span></span></i><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span></code></pre>
<pre class="ins highlight"><code class="highlight">  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_uint_fast_wait_t</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">atomic</span></span><span class="o"><span class="o">&lt;</span></span><i><span class="n"><span class="n">implementation</span></span><span class="o"><span class="o">-</span></span><span class="n"><span class="n">defined</span></span></i><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span></code></pre>
<pre class="ins highlight"><code class="highlight"></code></pre>
<pre class="ins highlight"><code class="highlight">  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_signed_lock_free</span></span>   <span class="o"><span class="o">=</span></span> <i><span class="n"><span class="n">see</span></span> <span class="n"><span class="n">below</span></span></i><span class="p"><span class="p">;</span></span></code></pre>
<pre class="ins highlight"><code class="highlight">  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">atomic_unsigned_lock_free</span></span> <span class="o"><span class="o">=</span></span> <i><span class="n"><span class="n">see</span></span> <span class="n"><span class="n">below</span></span></i><span class="p"><span class="p">;</span></span></code></pre>
<pre class="highlight"><code class="highlight"> 
  <i><span class="c1"><span class="c1">// ...</span></span></i>
<span class="c1"></span><span class="c1"></span>
  <i><span class="c1"><span class="c1">// 32.8, flag type and operations</span></span></i>
<span class="c1"></span><span class="c1"></span>  <span class="k"><span class="k">struct</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="p"><span class="p">;</span></span>
</code></pre>
    <ins>
<pre class="highlight">  <span class="kt">bool</span> <span class="nf">atomic_flag_test</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="kt">bool</span> <span class="nf">atomic_flag_test</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="kt">bool</span> <span class="nf">atomic_flag_test_explicit</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span> <span class="n">memory_order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="kt">bool</span> <span class="nf">atomic_flag_test_explicit</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span> <span class="n">memory_order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
    </ins>
<pre class="highlight"><code class="highlight">  <span class="kt"><span class="kt">bool</span></span> <span class="n"><span class="n">atomic_flag_test_and_set</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">atomic_flag_test_and_set</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">atomic_flag_test_and_set_explicit</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">atomic_flag_test_and_set_explicit</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear_explicit</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear_explicit</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="cp"><span class="cp">#define ATOMIC_FLAG_INIT </span></span><i><span class="cp"><span class="cp">see below</span></span></i>
<span class="cp"></span><span class="cp"></span>
  <i><span class="c1"><span class="c1">// 32.9, fences</span></span></i>
<span class="c1"></span><span class="c1"></span>  <span class="k"><span class="k">extern</span></span> <span class="s"><span class="s">"C"</span></span> <span class="kt"><span class="kt">void</span></span> <span class="n"><span class="n">atomic_thread_fence</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="k"><span class="k">extern</span></span> <span class="s"><span class="s">"C"</span></span> <span class="kt"><span class="kt">void</span></span> <span class="n"><span class="n">atomic_signal_fence</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>   
  
  <i><span class="c1"><span class="c1">// 32.�, waiting and notifying functions</span></span></i>
</code></pre>
    <ins>
<pre class="highlight">  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">,</span> 
                     <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span><span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">,</span>
                     <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span>
                   <span class="kt">bool</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span>
                   <span class="kt">bool</span><span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">,</span> 
                              <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span><span class="p">,</span> 
                              <span class="n">memory_order</span><span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
    <span class="kt">void</span> <span class="n">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span><span class="p">,</span> 
                              <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span><span class="p">,</span>
                              <span class="n">memory_order</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span>
                            <span class="kt">bool</span><span class="p">,</span>
                            <span class="n">memory_order</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span>
                            <span class="kt">bool</span><span class="p">,</span>
                            <span class="n">memory_order</span><span class="p">);</span>
</pre>
    </ins>
<pre class="highlight"><span class="p">}</span>
</pre>
   </blockquote>
   <p>Modify [<strong>atomics.alias</strong>] as follows:</p>
   <blockquote>
     <b>32.3 Type aliases <span style="float: right;">[atomics.alias]</span></b> 
    <div class="numbered"> The type aliases <code class="highlight"><span class="n">atomic_intN_t</span></code>, <code class="highlight"><span class="n">atomic_uintN_t</span></code>, <code class="highlight"><span class="n">atomic_intptr_t</span></code>, and <code class="highlight"><span class="n">atomic_uintptr_t</span></code> are defined if and only if <code class="highlight"><span class="n">intN_t</span></code>, <code class="highlight"><span class="n">uintN_t</span></code>, <code class="highlight"><span class="kt">intptr_t</span></code>, and <code class="highlight"><span class="kt">uintptr_t</span></code> are defined, respectively. </div>
    <div class="ins">
     <div class="numbered"> The type aliases <code class="highlight"><span class="n">atomic_signed_lock_free</span></code> and <code class="highlight"><span class="n">atomic_unsigned_lock_free</span></code> are defined to be specializations of <code class="highlight"><span class="n">atomic</span></code> whose template arguments are integral types, respectively signed and unsigned, other than <code class="highlight"><span class="kt">bool</span></code>. <code class="highlight"><span class="n">is_always_lock_free</span></code> shall be <code class="highlight">true</code> for <code class="highlight"><span class="n">atomic_signed_lock_free</span></code> and <code class="highlight"><span class="n">atomic_unsigned_lock_free</span></code>. An implementation should choose the integral specialization of <code class="highlight"><span class="n">atomic</span></code> for which the waiting and notifying functions are most efficient. </div>
     <div class="numbered"> The type aliases <code class="highlight"><span class="n">atomic_int_fast_wait_t</span></code> and <code class="highlight"><span class="n">atomic_uint_fast_wait_t</span></code> are integral atomic types.
Implementations should ensure that invocations of waiting and notifying functions (32.�) with these types have the lowest performance overhead among integer types. </div>
    </div>
   </blockquote>
   <p class="note" role="note"><span>Note:</span> The reference to "waiting and notifying functions" in the above change should refer to the new [<strong>atomic.wait</strong>] subclause.</p>
   <p>Modify [<strong>atomics.flag</strong>] as follows:</p>
   <blockquote>
     <b>32.8 Flag type and operations <span style="float: right;">[atomics.flag]</span></b> 
<pre class="highlight"><span class="k">namespace</span> <span class="n">std</span> <span class="p">{</span>
  <span class="k">struct</span> <span class="n">atomic_flag</span> <span class="p">{</span>
</pre>
    <ins>
<pre class="highlight">    <span class="kt">bool</span> <span class="nf">test</span><span class="p">(</span><span class="n">memory_order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">volatile</span> <span class="k">noexcept</span><span class="p">;</span>
    <span class="kt">bool</span> <span class="nf">test</span><span class="p">(</span><span class="n">memory_order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
    </ins>
<pre class="highlight"><code class="highlight">    <span class="kt"><span class="kt">bool</span></span> <span class="n"><span class="n">test_and_set</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">memory_order</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">memory_order_seq_cst</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">volatile</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
    <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">test_and_set</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">memory_order</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">memory_order_seq_cst</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">clear</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">memory_order</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">memory_order_seq_cst</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">volatile</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">clear</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">memory_order</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">memory_order_seq_cst</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>

    <span class="n"><span class="n">atomic_flag</span></span><span class="p"><span class="p">()</span></span> <span class="k"><span class="k">noexcept</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">default</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">atomic_flag</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">volatile</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
  <span class="p"><span class="p">};</span></span>
 
</code></pre>
    <ins>
<pre class="highlight">  <span class="kt">bool</span> <span class="nf">atomic_flag_test</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="kt">bool</span> <span class="nf">atomic_flag_test</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="kt">bool</span> <span class="nf">atomic_flag_test_explicit</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span> <span class="n">memory_order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="kt">bool</span> <span class="nf">atomic_flag_test_explicit</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span><span class="p">,</span> <span class="n">memory_order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
    </ins>
<pre class="highlight"><code class="highlight">  <span class="kt"><span class="kt">bool</span></span> <span class="n"><span class="n">atomic_flag_test_and_set</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">atomic_flag_test_and_set</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">atomic_flag_test_and_set_explicit</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">atomic_flag_test_and_set_explicit</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear_explicit</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">volatile</span></span> <span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
  <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">atomic_flag_clear_explicit</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">atomic_flag</span></span><span class="o"><span class="o">*</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">memory_order</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
</code></pre>
<pre class="highlight"><code class="highlight">  <span class="cp"><span class="cp">#define ATOMIC_FLAG_INIT </span></span><i><span class="cp"><span class="cp">see below</span></span></i></code></pre>
<pre class="highlight"><span class="p">}</span>
</pre>
    <div class="numbered"> The <code class="highlight"><span class="n">atomic_flag</span></code> type provides the classic test-and-set functionality. It has two states, set and clear. </div>
    <div class="numbered"> Operations on an object of type <code class="highlight"><span class="n">atomic_flag</span></code> shall be lock-free.
[ <em>Note:</em> Hence the operations should also be address-free. — <em>end note</em> ] </div>
    <div class="numbered"> The <code class="highlight"><span class="n">atomic_flag</span></code> type is a standard-layout struct. It has a trivial default
constructor and a trivial destructor. </div>
    <div class="numbered">
      The macro <code class="highlight"><span class="n">ATOMIC_FLAG_INIT</span></code> shall be defined in such a way that it can be used to initialize an object of type <code class="highlight"><span class="n">atomic_flag</span></code> to the clear state.
The macro can be used in the form: 
<pre class="highlight"><span class="n">atomic_flag</span> <span class="n">guard</span> <span class="o">=</span> <span class="n">ATOMIC_FLAG_INIT</span><span class="p">;</span></pre>
     <p>It is unspecified whether the macro can be used in other initialization contexts.
For a complete static-duration object, that initialization shall be static.
Unless initialized with <code class="highlight"><span class="n">ATOMIC_FLAG_INIT</span></code>, it is unspecified whether an <code class="highlight"><span class="n">atomic_flag</span></code> object has an initial state of set or clear.</p>
    </div>
    <div class="ins">
<pre class="highlight"><span class="kt">bool</span> <span class="nf">atomic_flag_test</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="nf">atomic_flag_test</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="nf">atomic_flag_test_explicit</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span> <span class="n">memory_order</span> <span class="n">order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="nf">atomic_flag_test_explicit</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span> <span class="n">memory_order</span> <span class="n">order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="n">atomic_flag</span><span class="o">::</span><span class="n">test</span><span class="p">(</span><span class="n">memory_order</span> <span class="n">order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">volatile</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="n">atomic_flag</span><span class="o">::</span><span class="n">test</span><span class="p">(</span><span class="n">memory_order</span> <span class="n">order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <div class="newnumbered"> <em>Requires:</em> The <code class="highlight"><span class="n">order</span></code> argument shall not be <code class="highlight"><span class="n">memory_order_release</span></code> nor <code class="highlight"><span class="n">memory_order_acq_rel</span></code>. </div>
     <div class="newnumbered"> <em>Effects:</em> Memory is affected according to the value of <code class="highlight"><span class="n">order</span></code>. </div>
     <div class="newnumbered"> <em>Returns:</em> Atomically returns the value pointed to by <code class="highlight"><span class="n">object</span></code> or <code class="highlight"><span class="k">this</span></code>. </div>
    </div>
<pre class="highlight"><span class="kt">bool</span> <span class="nf">atomic_flag_test_and_set</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="nf">atomic_flag_test_and_set</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="nf">atomic_flag_test_and_set_explicit</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span>
                                       <span class="n">memory_order</span> <span class="n">order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="nf">atomic_flag_test_and_set_explicit</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span> <span class="n">memory_order</span> <span class="n">order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="n">atomic_flag</span><span class="o">::</span><span class="n">test_and_set</span><span class="p">(</span><span class="n">memory_order</span> <span class="n">order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">volatile</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">bool</span> <span class="n">atomic_flag</span><span class="o">::</span><span class="n">test_and_set</span><span class="p">(</span><span class="n">memory_order</span> <span class="n">order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
    <div class="numbered"> <em>Effects:</em> Atomically sets the value pointed to by <code class="highlight"><span class="n">object</span></code> or by <code class="highlight"><span class="k">this</span></code> to <code class="highlight">true</code>.
Memory is affected according to the value of <code class="highlight"><span class="n">order</span></code>.
These operations are atomic read-modify-write operations (4.7). </div>
    <div class="numbered"> <em>Returns:</em> Atomically, the value of the object immediately before the effects. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">atomic_flag_clear</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">void</span> <span class="nf">atomic_flag_clear</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">void</span> <span class="nf">atomic_flag_clear_explicit</span><span class="p">(</span><span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span>
                                <span class="n">memory_order</span> <span class="n">order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">void</span> <span class="nf">atomic_flag_clear_explicit</span><span class="p">(</span><span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span> <span class="n">memory_order</span> <span class="n">order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">void</span> <span class="n">atomic_flag</span><span class="o">::</span><span class="n">clear</span><span class="p">(</span><span class="n">memory_order</span> <span class="n">order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">volatile</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="kt">void</span> <span class="n">atomic_flag</span><span class="o">::</span><span class="n">clear</span><span class="p">(</span><span class="n">memory_order</span> <span class="n">order</span> <span class="o">=</span> <span class="n">memory_order_seq_cst</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
    <p><em>Requires:</em> The <code class="highlight"><span class="n">order</span></code> argument shall not be <code class="highlight"><span class="n">memory_order_consume</span></code>, <code class="highlight"><span class="n">memory_order_acquire</span></code>, nor <code class="highlight"><span class="n">memory_order_acq_rel</span></code>.</p>
    <p><em>Effects:</em> Atomically sets the value pointed to by <code class="highlight"><span class="n">object</span></code> or by <code class="highlight"><span class="k">this</span></code> to <code class="highlight">false</code>. Memory is affected according to the value of <code class="highlight"><span class="n">order</span></code>.</p>
   </blockquote>
   <p>Add a new subclause after [<strong>atomics.fences</strong>]:</p>
   <blockquote class="ins">
     <b>32.� Waiting and notifying functions <span style="float: right;">[atomics.wait]</span></b> 
    <div class="numbered"> The functions in this subclause provide a mechanism to wait for the value of an atomic object to change, more efficiently than can be achieved with polling.
Waiting functions in this facility may block until they are unblocked by notifying functions, according to each function’s effects.
[ <em>Note:</em> Programs are not guaranteed to observe transient atomic values, an issue known as the A-B-A problem, resulting in continued blocking if a condition is only temporarily met. – <em>End note</em> ] </div>
    <div class="numbered"> The functions <code class="highlight"><span class="n">atomic_wait</span></code> and <code class="highlight"><span class="n">atomic_wait_explicit</span></code> are waiting functions.
The functions <code class="highlight"><span class="n">atomic_notify_one</span></code> and <code class="highlight"><span class="n">atomic_notify_all</span></code> are notifying functions. </div>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">);</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">atomic_notify_one</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Effects:</em> Unblocks up to execution of a waiting function that blocked after observing the result of an atomic operation <code class="highlight"><span class="n">X</span></code>, if there exists another atomic operation <code class="highlight"><span class="n">Y</span></code>, such that <code class="highlight"><span class="n">X</span></code> precedes <code class="highlight"><span class="n">Y</span></code> in the modification order of <code class="highlight"><span class="o">*</span><span class="n">object</span></code>, and <code class="highlight"><span class="n">Y</span></code> happens before this call. </div>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">);</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">atomic_notify_all</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Effects:</em> Unblocks each execution of a waiting function that blocked after observing the result of an atomic operation <code class="highlight"><span class="n">X</span></code>, if there exists another atomic operation <code class="highlight"><span class="n">Y</span></code>, such that <code class="highlight"><span class="n">X</span></code> precedes <code class="highlight"><span class="n">Y</span></code> in the modification order of <code class="highlight"><span class="o">*</span><span class="n">object</span></code>, and <code class="highlight"><span class="n">Y</span></code> happens before this call. </div>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">,</span> 
                   <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span> <span class="n">old</span><span class="p">);</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">,</span> 
                   <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span> <span class="n">old</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span>
                 <span class="kt">bool</span> <span class="n">old</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">atomic_wait</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span>
                 <span class="kt">bool</span> <span class="n">old</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Effects:</em> Equivalent to <code class="highlight"><span class="n">atomic_wait_explicit</span><span class="p">(</span><span class="n">object</span><span class="p">,</span> <span class="n">old</span><span class="p">,</span> <span class="n">memory_order_seq_cst</span><span class="p">);</span></code> </div>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">,</span> 
                            <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span> <span class="n">old</span><span class="p">,</span> 
                            <span class="n">memory_order</span> <span class="n">order</span><span class="p">);</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> 
  <span class="kt">void</span> <span class="n">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>*</span> <span class="n">object</span><span class="p">,</span> 
                            <span class="k">typename</span> <span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value_type</span> <span class="n">old</span><span class="p">,</span> 
                            <span class="n">memory_order</span> <span class="n">order</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> The order argument shall not be <code class="highlight"><span class="n">memory_order_release</span></code> nor <code class="highlight"><span class="n">memory_order_acq_rel</span></code>. </div>
    <div class="numbered">
      <em>Effects:</em> Repeatedly performs the following steps, in order: 
     <ul>
      <li data-md="">
       <p>Evaluates <code class="highlight"><span class="n">object</span><span class="o">-></span><span class="n">load</span><span class="p">(</span><span class="n">order</span><span class="p">)</span> <span class="o">!=</span> <span class="n">old</span></code> then, if the result is <code class="highlight">true</code>, returns.</p>
      <li data-md="">
       <p>Blocks until an implementation-defined condition has been met.
[ <em>Note:</em> Consequently, it may unblock for reasons other than a call to a notifying function. — <em>end note</em> ]</p>
     </ul>
    </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="k">volatile</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span> 
                          <span class="kt">bool</span> <span class="n">old</span><span class="p">,</span>
                          <span class="n">memory_order</span> <span class="n">order</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">atomic_wait_explicit</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">*</span> <span class="n">object</span><span class="p">,</span> 
                          <span class="kt">bool</span> <span class="n">old</span><span class="p">,</span>
                          <span class="n">memory_order</span> <span class="n">order</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> The order argument shall not be <code class="highlight"><span class="n">memory_order_release</span></code> nor <code class="highlight"><span class="n">memory_order_acq_rel</span></code>. </div>
    <div class="numbered">
      <em>Effects:</em> Repeatedly performs the following steps, in order: 
     <ul>
      <li data-md="">
       <p>Evaluates <code class="highlight"><span class="n">object</span><span class="o">-></span><span class="n">test</span><span class="p">(</span><span class="n">order</span><span class="p">)</span> <span class="o">!=</span> <span class="n">old</span></code> then, if the result is <code class="highlight">true</code>, returns.</p>
      <li data-md="">
       <p>Blocks until an implementation-defined condition has been met.
[ <em>Note:</em> Consequently, it may unblock for reasons other than a call to a notifying function. — <em>end note</em> ]</p>
     </ul>
    </div>
   </blockquote>
   <p>Modify Table 140 "Thread support library summary" in [<strong>thread.general</strong>] as follows:</p>
   <blockquote>
    <div style="text-align: center;"> Table 140 — Thread support library summary </div>
    <table border="1" style="margin: auto;">
     <tbody>
      <tr>
       <th>
       <th>Subclause
       <th>Header(s)
      <tr>
       <td>33.2
       <td>Requirements
       <td>
      <tr>
       <td>33.3
       <td>Threads
       <td><code class="highlight"><span class="o">&lt;</span><span class="kr">thread</span><span class="o">></span></code>
      <tr>
       <td>33.4
       <td>Mutual exclusion
       <td><code class="highlight"><span class="o">&lt;</span><span class="n">mutex</span><span class="o">></span></code> <code class="highlight"><span class="o">&lt;</span><span class="n">shared_mutex</span><span class="o">></span></code>
      <tr>
       <td>33.5
       <td>Condition variables
       <td><code class="highlight"><span class="o">&lt;</span><span class="n">condition_variable</span><span class="o">></span></code>
      <tr class="ins">
       <td>33.�
       <td>Semaphores
       <td><code class="highlight"><span class="o">&lt;</span><span class="n">semaphore</span><span class="o">></span></code>
      <tr class="ins">
       <td>33.�
       <td>Latches and barriers
       <td><code class="highlight"><span class="o">&lt;</span><span class="n">latch</span><span class="o">></span></code> <code class="highlight"><span class="o">&lt;</span><span class="n">barrier</span><span class="o">></span></code>
      <tr>
       <td>33.6
       <td>Futures
       <td><code class="highlight"><span class="o">&lt;</span><span class="n">future</span><span class="o">></span></code>
    </table>
   </blockquote>
   <p>Add two new subclauses after [<strong>thread.condition</strong>]:</p>
   <blockquote class="ins">
     <b>33.� Semaphores <span style="float: right;">[thread.semaphore]</span></b> 
    <div class="numbered"> Semaphores are lightweight synchronization primitives used to constrain concurrent access to a shared resource. They are widely used to implement other synchronization primitives and, whenever both are applicable, can be more efficient than condition variables. </div>
    <div class="numbered"> A counting semaphore is a semaphore object that models a non-negative resource count. A binary semaphore is a semaphore object that has only two states, also known as available and unavailable.
[ <em>Note:</em> A binary semaphore should be more efficient than a counting semaphore with a unit magnitude count. – <em>end note</em> ] </div>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.1 Header <code class="highlight"><span class="o">&lt;</span><span class="n">semaphore</span><span class="o">></span></code> synopsis <span style="float: right;">[thread.semaphore.syn]</span></b> 
<pre class="highlight"><code class="highlight"><span class="k"><span class="k">namespace</span></span> <span class="n"><span class="n">std</span></span> <span class="p"><span class="p">{</span></span>  
  <span class="k"><span class="k">template</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">least_max_value</span></span> <span class="o"><span class="o">=</span></span> <i><span class="n"><span class="n">implementation</span></span><span class="o"><span class="o">-</span></span><span class="n"><span class="n">defined</span></span></i><span class="o"><span class="o">></span></span>
    <span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">counting_semaphore</span></span><span class="p"><span class="p">;</span></span>

  <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">binary_semaphore</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">counting_semaphore</span></span><span class="o"><span class="o">&lt;</span></span><span class="mi"><span class="mi">1</span></span><span class="o"><span class="o">></span></span><span class="p"><span class="p">;</span></span>
<span class="p"><span class="p">}</span></span>
</code></pre>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.2 Class template <code class="highlight"><span class="n">counting_semaphore</span></code> <span style="float: right;">[thread.semaphore.counting.class]</span></b> 
<pre class="highlight"><code class="highlight"><span class="k"><span class="k">namespace</span></span> <span class="n"><span class="n">std</span></span> <span class="p"><span class="p">{</span></span>
  <span class="k"><span class="k">template</span></span><span class="o"><span class="o">&lt;</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">least_max_value</span></span><span class="o"><span class="o">></span></span>
  <span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">counting_semaphore</span></span> <span class="p"><span class="p">{</span></span>
  <span class="k"><span class="k">public</span></span><span class="o"><span class="o">:</span></span>
    <span class="k"><span class="k">static</span></span> <span class="k"><span class="k">constexpr</span></span> <span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">max</span></span><span class="p"><span class="p">()</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>

    <span class="k"><span class="k">explicit</span></span> <span class="k"><span class="k">constexpr</span></span> <span class="nf"><span class="nf">counting_semaphore</span></span><span class="p"><span class="p">(</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span><span class="p"><span class="p">);</span></span>
    <span class="o"><span class="o">~</span></span><span class="n"><span class="n">counting_semaphore</span></span><span class="p"><span class="p">();</span></span>

    <span class="n"><span class="n">counting_semaphore</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">basic_semaphore</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">counting_semaphore</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">basic_semaphore</span></span><span class="o"><span class="o">&amp;&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">counting_semaphore</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">basic_semaphore</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">counting_semaphore</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">basic_semaphore</span></span><span class="o"><span class="o">&amp;&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>

    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">release</span></span><span class="p"><span class="p">(</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">update</span></span> <span class="o"><span class="o">=</span></span> <span class="mi"><span class="mi">1</span></span><span class="p"><span class="p">);</span></span>
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">acquire</span></span><span class="p"><span class="p">();</span></span>
    <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">try_acquire</span></span><span class="p"><span class="p">();</span></span>
    <span class="k"><span class="k">template</span></span><span class="o"><span class="o">&lt;</span></span><span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">Clock</span></span><span class="p"><span class="p">,</span></span> <span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">Duration</span></span><span class="o"><span class="o">></span></span>
      <span class="kt"><span class="kt">bool</span></span> <span class="n"><span class="n">try_acquire_until</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">chrono</span></span><span class="o"><span class="o">::</span></span><span class="n"><span class="n">time_point</span></span><span class="o"><span class="o">&lt;</span></span><span class="n"><span class="n">Clock</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">Duration</span></span><span class="o"><span class="o">>&amp;</span></span><span class="p"><span class="p">);</span></span>
    <span class="k"><span class="k">template</span></span><span class="o"><span class="o">&lt;</span></span><span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">Rep</span></span><span class="p"><span class="p">,</span></span> <span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">Period</span></span><span class="o"><span class="o">></span></span>
      <span class="kt"><span class="kt">bool</span></span> <span class="n"><span class="n">try_acquire_for</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">chrono</span></span><span class="o"><span class="o">::</span></span><span class="n"><span class="n">duration</span></span><span class="o"><span class="o">&lt;</span></span><span class="n"><span class="n">Rep</span></span><span class="p"><span class="p">,</span></span> <span class="n"><span class="n">Period</span></span><span class="o"><span class="o">>&amp;</span></span><span class="p"><span class="p">);</span></span>

  <span class="k"><span class="k">private</span></span><span class="o"><span class="o">:</span></span>
    <span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">counter</span></span><span class="p"><span class="p">;</span></span> <i><span class="c1"><span class="c1">// exposition only</span></span></i>
<span class="c1"></span><span class="c1"></span>  <span class="p"><span class="p">};</span></span>
<span class="p"><span class="p">}</span></span>
</code></pre>
    <div class="numbered"> Class <code class="highlight"><span class="n">counting_semaphore</span></code> maintains an internal counter that is initialized when the semaphore is created. Threads may block waiting until <code class="highlight"><span class="n">counter</span> <span class="o">>=</span> <span class="mi">1</span></code>. </div>
    <div class="numbered"> Semaphores permit concurrent invocation of the <code class="highlight"><span class="n">release</span></code>, <code class="highlight"><span class="n">acquire</span></code>, <code class="highlight"><span class="n">try_acquire</span></code>, <code class="highlight"><span class="n">try_acquire_for</span></code>, and <code class="highlight"><span class="n">try_acquire_until</span></code> member functions. </div>
<pre class="highlight"><span class="k">static</span> <span class="k">constexpr</span> <span class="kt">ptrdiff_t</span> <span class="nf">max</span><span class="p">()</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
    <div class="numbered"> <em>Returns:</em> The maximum value of <code class="highlight"><span class="n">counter</span></code>. This value shall not be less than that of the template argument <code class="highlight"><span class="n">least_max_value</span></code>.
[ <em>Note:</em> The value may exceed least_max_value. – <em>end note</em> ] </div>
<pre class="highlight"><span class="k">explicit</span> <span class="k">constexpr</span> <span class="nf">counting_semaphore</span><span class="p">(</span><span class="kt">ptrdiff_t</span> <span class="n">desired</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> <code class="highlight"><span class="n">desired</span> <span class="o">>=</span> <span class="mi">0</span></code> and <code class="highlight"><span class="n">desired</span> <span class="o">&lt;=</span> <span class="n">max</span><span class="p">()</span></code>. </div>
    <div class="numbered"> <em>Effects:</em> <code class="highlight"><span class="n">counter</span> <span class="o">=</span> <span class="n">desired</span></code>. </div>
<pre class="highlight"><span class="o">~</span><span class="n">counting_semaphore</span><span class="p">();</span>
</pre>
    <div class="numbered"> <em>Requires:</em> For every function call that blocks on <code class="highlight"><span class="n">counter</span></code>, a function call that will cause it to unblock and return shall happen before this call.
[ <em>Note:</em> This relaxes the usual rules, which would have required all wait calls to happen before destruction. — <em>end note</em> ] </div>
    <div class="numbered"> <em>Effects:</em> Destroys the object. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">release</span><span class="p">(</span><span class="kt">ptrdiff_t</span> <span class="n">update</span> <span class="o">=</span> <span class="mi">1</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> <code class="highlight"><span class="n">update</span> <span class="o">>=</span> <span class="mi">0</span></code>, and <code class="highlight"><span class="n">counter</span> <span class="o">+</span> <span class="n">update</span> <span class="o">&lt;=</span> <span class="n">max</span><span class="p">()</span></code>. </div>
    <div class="numbered"> <em>Effects:</em> <code class="highlight"><span class="n">counter</span> <span class="o">+=</span> <span class="n">update</span></code>, executed atomically. If any threads are blocked on counter, unblocks them. </div>
    <div class="numbered"> <em>Synchronization:</em> Strongly happens before invocations of <code class="highlight"><span class="n">try_acquire</span></code> that observe the result of the effects. </div>
<pre class="highlight"><span class="kt">bool</span> <span class="nf">try_acquire</span><span class="p">();</span>
</pre>
    <div class="numbered">
      <em>Effects:</em> 
     <ul>
      <li data-md="">
       <p>With low probability, returns immediately. [ <em>Note:</em> An implementation should ensure that <code class="highlight"><span class="n">try_acquire</span></code> does not consistently return <code class="highlight">false</code> in the absence of contending acquisitions. — <em>end note</em> ]</p>
      <li data-md="">
       <p>Otherwise, if <code class="highlight"><span class="n">counter</span> <span class="o">>=</span> <span class="mi">1</span></code>, then <code class="highlight"><span class="n">counter</span> <span class="o">-=</span> <span class="mi">1</span></code> is executed atomically.</p>
     </ul>
    </div>
    <div class="numbered"> <em>Returns:</em> <code class="highlight">true</code> if <code class="highlight"><span class="n">counter</span></code> was decremented, otherwise <code class="highlight">false</code>. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">acquire</span><span class="p">();</span>
</pre>
    <div class="numbered">
      <em>Effects:</em> Repeatedly performs the following steps, in order: 
     <ul>
      <li data-md="">
       <p>Evaluates <code class="highlight"><span class="n">try_acquire</span></code>, then, if the result is <code class="highlight">true</code>, returns.</p>
      <li data-md="">
       <p>Blocks until <code class="highlight"><span class="n">counter</span> <span class="o">>=</span> <span class="mi">1</span></code>.</p>
     </ul>
    </div>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">Clock</span><span class="p">,</span> <span class="k">class</span> <span class="nc">Duration</span><span class="o">></span>
  <span class="kt">bool</span> <span class="n">try_acquire_until</span><span class="p">(</span><span class="k">const</span> <span class="n">chrono</span><span class="o">::</span><span class="n">time_point</span><span class="o">&lt;</span><span class="n">Clock</span><span class="p">,</span> <span class="n">Duration</span><span class="o">>&amp;</span> <span class="n">abs_time</span><span class="p">);</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">Rep</span><span class="p">,</span> <span class="k">class</span> <span class="nc">Period</span><span class="o">></span>
  <span class="kt">bool</span> <span class="n">try_acquire_for</span><span class="p">(</span><span class="k">const</span> <span class="n">chrono</span><span class="o">::</span><span class="n">duration</span><span class="o">&lt;</span><span class="n">Rep</span><span class="p">,</span> <span class="n">Period</span><span class="o">>&amp;</span> <span class="n">rel_time</span><span class="p">);</span>
</pre>
    <div class="numbered">
      <em>Effects:</em> Repeatedly performs the following steps, in order: 
     <ul>
      <li data-md="">
       <p>Evaluates <code class="highlight"><span class="n">try_acquire</span></code>. If the result is <code class="highlight">true</code>, returns <code class="highlight">true</code>.</p>
      <li data-md="">
       <p>Blocks until the timeout expires or <code class="highlight"><span class="n">counter</span> <span class="o">>=</span> <span class="mi">1</span></code>. If the timeout expired, returns <code class="highlight">false</code>.</p>
     </ul>
    </div>
    <div class="numbered"> <em>Throws:</em> Timeout-related exceptions (33.2.4). </div>
   </blockquote>
   <blockquote class="ins">
     <b>33.� Coordination Types <span style="float: right;">[thread.coord]</span></b> 
    <div class="numbered"> This section describes various concepts related to thread coordination, and defines the <dfn data-dfn-type="dfn" data-noexport="" id="coordination-types">coordination types<a class="self-link" href="#coordination-types"></a></dfn> <code class="highlight"><span class="n">latch</span></code> and <code class="highlight"><span class="n">barrier</span></code>. These types facilitate concurrent computation performed by a number of threads, in one or more phases. </div>
    <div class="numbered"> In this subclause, a synchronization point represents a condition that a thread may contribute to or wait for, potentially blocking until it is satisfied. A thread arrives at the synchronization point when it has an effect on the state of the condition, even if it does not cause it to become satisfied. </div>
    <div class="numbered"> Concurrent invocations of the member functions of coordination types, other than their destructors, do not introduce data races. </div>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.1 Latches <span style="float: right;">[thread.coord.latch]</span></b> 
    <div class="numbered"> A latch is a thread coordination mechanism that allows any number of threads to block until an expected count is summed (exactly) by threads that arrived at the latch. The expected count is set when the latch is constructed. An individual latch is a single-use object; once the count has been reached, the latch cannot be reused. </div>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.1.1 Header <code class="highlight"><span class="o">&lt;</span><span class="n">latch</span><span class="o">></span></code> synopsis <span style="float: right;">[thread.coord.latch.syn]</span></b> 
<pre class="highlight"><span class="k">namespace</span> <span class="n">std</span> <span class="p">{</span>
  <span class="k">class</span> <span class="nc">latch</span>
<span class="p">}</span>
</pre>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.1.2 Class <code class="highlight"><span class="n">latch</span></code> <span style="float: right;">[thread.coord.latch.class]</span></b> 
<pre class="highlight"><code class="highlight"><span class="k"><span class="k">namespace</span></span> <span class="n"><span class="n">std</span></span> <span class="p"><span class="p">{</span></span>
  <span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">latch</span></span> <span class="p"><span class="p">{</span></span>
  <span class="k"><span class="k">public</span></span><span class="o"><span class="o">:</span></span>
    <span class="k"><span class="k">explicit</span></span> <span class="k"><span class="k">constexpr</span></span> <span class="n"><span class="n">latch</span></span><span class="p"><span class="p">(</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">expected</span></span><span class="p"><span class="p">);</span></span>
    <span class="o"><span class="o">~</span></span><span class="n"><span class="n">latch</span></span><span class="p"><span class="p">();</span></span>

    <span class="n"><span class="n">latch</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">latch</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">latch</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">latch</span></span><span class="o"><span class="o">&amp;&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">latch</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">latch</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">latch</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">latch</span></span><span class="o"><span class="o">&amp;&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">arrive</span></span><span class="p"><span class="p">(</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">update</span></span> <span class="o"><span class="o">=</span></span> <span class="mi"><span class="mi">1</span></span><span class="p"><span class="p">);</span></span>
    <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">try_wait</span></span><span class="p"><span class="p">()</span></span> <span class="k"><span class="k">const</span></span> <span class="k"><span class="k">noexcept</span></span><span class="p"><span class="p">;</span></span>
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">wait</span></span><span class="p"><span class="p">()</span></span> <span class="k"><span class="k">const</span></span><span class="p"><span class="p">;</span></span>
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">arrive_and_wait</span></span><span class="p"><span class="p">(</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">n</span></span> <span class="n"><span class="n">update</span></span> <span class="o"><span class="o">=</span></span> <span class="mi"><span class="mi">1</span></span><span class="p"><span class="p">);</span></span>

  <span class="k"><span class="k">private</span></span><span class="o"><span class="o">:</span></span>
    <span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">counter</span></span><span class="p"><span class="p">;</span></span> <i><span class="c1"><span class="c1">// exposition only</span></span></i>
<span class="c1"></span><span class="c1"></span>  <span class="p"><span class="p">};</span></span>
<span class="p"><span class="p">}</span></span> 
</code></pre>
    <div class="numbered"> A <code class="highlight"><span class="n">latch</span></code> maintains an internal counter that is initialized when the <code class="highlight"><span class="n">latch</span></code> is created. Threads may block at the <code class="highlight"><span class="n">latch</span></code>’s synchronization point, waiting for <code class="highlight"><span class="n">counter</span></code> to be decremented to <code class="highlight"><span class="mi">0</span></code>. </div>
<pre class="highlight"><span class="k">explicit</span> <span class="k">constexpr</span> <span class="nf">latch</span><span class="p">(</span><span class="kt">ptrdiff_t</span> <span class="n">expected</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> <code class="highlight"><span class="n">expected</span> <span class="o">>=</span> <span class="mi">0</span></code>. </div>
    <div class="numbered"> <em>Effects:</em> <code class="highlight"><span class="n">counter</span> <span class="o">=</span> <span class="n">expected</span></code>. </div>
<pre class="highlight"><span class="o">~</span><span class="n">latch</span><span class="p">();</span>
</pre>
    <div class="numbered"> <em>Requires:</em> No threads are blocked at the synchronization point. </div>
    <div class="numbered"> <em>Effects:</em> Destroys the latch. </div>
    <div class="numbered"> <em>Remarks:</em> May be called even if some threads have not yet returned from functions that block at the synchronization point, provided that they are unblocked. [ <em>Note:</em> The destructor may block until all threads have exited invocations of <code class="highlight"><span class="n">wait</span></code> on this object. — <em>end note</em> ] </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">arrive</span><span class="p">(</span><span class="kt">ptrdiff_t</span> <span class="n">update</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> <code class="highlight"><span class="n">counter</span> <span class="o">>=</span> <span class="n">update</span></code> and <code class="highlight"><span class="n">update</span> <span class="o">>=</span> <span class="mi">0</span></code>. </div>
    <div class="numbered"> <em>Effects:</em> Atomically decrements <code class="highlight"><span class="n">counter</span></code> by <code class="highlight"><span class="n">update</span></code>. </div>
    <div class="numbered"> <em>Synchronization:</em> Synchronizes with the returns from all calls unblocked by the effects. </div>
    <div class="numbered"> <em>Remarks:</em> Arrives at the synchronization point with <code class="highlight"><span class="n">update</span></code> count. </div>
<pre class="highlight"><span class="kt">bool</span> <span class="nf">try_wait</span><span class="p">()</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
    <div class="numbered"> <em>Returns:</em> <code class="highlight"><span class="n">counter</span> <span class="o">==</span> <span class="mi">0</span></code>. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">wait</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>
</pre>
    <div class="numbered"> <em>Effects:</em> If <code class="highlight"><span class="n">counter</span> <span class="o">==</span> <span class="mi">0</span></code>, returns immediately. Otherwise, blocks the calling thread at the synchronization point until <code class="highlight"><span class="n">counter</span> <span class="o">==</span> <span class="mi">0</span></code>. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">arrive_and_wait</span><span class="p">(</span><span class="kt">ptrdiff_t</span> <span class="n">update</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Effects:</em> Equivalent to <code class="highlight"><span class="n">arrive</span><span class="p">(</span><span class="n">update</span><span class="p">);</span> <span class="n">wait</span><span class="p">();</span></code>. </div>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.2 Barriers <span style="float: right;">[thread.coord.barrier]</span></b> 
    <div class="numbered"> A barrier is a thread coordination mechanism that allows at most an expected count of threads to block until that count is summed (exactly) by threads that arrived at the barrier in each of its successive phases. Once threads are released from blocking at the synchronization point for a phase, they can reuse the same barrier immediately in its next phase. [ <em>Note:</em> It is thus useful for managing repeated tasks, or phases of a larger task, that are handled by multiple threads. — <em>end note</em> ] </div>
    <div class="numbered">
      A barrier has a completion step that is a (possibly empty) set of effects associated with a phase of the barrier. When the member functions defined in this subclause arrive at the barrier, they have the following effects: 
     <ul>
      <li data-md="">
       <p>When the expected number of threads for this phase have arrived at the barrier, one of those threads executes the barrier type’s completion step.</p>
      <li data-md="">
       <p>When the completion step is completed, all threads blocked at the synchronization point for this phase are unblocked and the barrier enters its next phase. The end of the completion step strongly happens before the returns from all calls unblocked by its completion.</p>
     </ul>
    </div>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.2.1 Header <code class="highlight"><span class="o">&lt;</span><span class="n">barrier</span><span class="o">></span></code> synopsis <span style="float: right;">[thread.coord.barrier.syn]</span></b> 
<pre class="highlight"><code class="highlight"><span class="k"><span class="k">namespace</span></span> <span class="n"><span class="n">std</span></span> <span class="p"><span class="p">{</span></span>
  <span class="k"><span class="k">template</span></span><span class="o"><span class="o">&lt;</span></span><span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">CompletionFunction</span></span> <span class="o"><span class="o">=</span></span> <i><span class="n"><span class="n">implementation</span></span><span class="o"><span class="o">-</span></span><span class="n"><span class="n">defined</span></span></i><span class="o"><span class="o">></span></span>
    <span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">barrier</span></span><span class="p"><span class="p">;</span></span>
<span class="p"><span class="p">}</span></span>
</code></pre>
   </blockquote>
   <blockquote class="ins">
     <b>33.�.2.2 Class template <code class="highlight"><span class="n">barrier</span></code> <span style="float: right;">[thread.coord.barrier.class]</span></b> 
<pre class="highlight"><code class="highlight"><span class="k"><span class="k">namespace</span></span> <span class="n"><span class="n">std</span></span> <span class="p"><span class="p">{</span></span>
  <span class="k"><span class="k">template</span></span><span class="o"><span class="o">&lt;</span></span><span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">CompletionFunction</span></span><span class="o"><span class="o">></span></span>
  <span class="k"><span class="k">class</span></span> <span class="nc"><span class="nc">barrier</span></span> <span class="p"><span class="p">{</span></span>
  <span class="k"><span class="k">public</span></span><span class="o"><span class="o">:</span></span>
    <span class="k"><span class="k">using</span></span> <span class="n"><span class="n">arrival_token</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">implementation</span></span><span class="o"><span class="o">-</span></span><span class="n"><span class="n">defined</span></span><span class="p"><span class="p">;</span></span>

    <span class="k"><span class="k">explicit</span></span> <span class="nf"><span class="nf">barrier</span></span><span class="p"><span class="p">(</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">expected</span></span><span class="p"><span class="p">,</span></span>
                     <span class="n"><span class="n">CompletionFunction</span></span> <span class="n"><span class="n">f</span></span> <span class="o"><span class="o">=</span></span> <span class="n"><span class="n">CompletionFunction</span></span><span class="p"><span class="p">());</span></span>
    <span class="o"><span class="o">~</span></span><span class="n"><span class="n">barrier</span></span><span class="p"><span class="p">();</span></span>

    <span class="n"><span class="n">barrier</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">barrier</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">barrier</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">barrier</span></span><span class="o"><span class="o">&amp;&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">barrier</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="k"><span class="k">const</span></span> <span class="n"><span class="n">barrier</span></span><span class="o"><span class="o">&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>
    <span class="n"><span class="n">barrier</span></span><span class="o"><span class="o">&amp;</span></span> <span class="k"><span class="k">operator</span></span><span class="o"><span class="o">=</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">barrier</span></span><span class="o"><span class="o">&amp;&amp;</span></span><span class="p"><span class="p">)</span></span> <span class="o"><span class="o">=</span></span> <span class="k"><span class="k">delete</span></span><span class="p"><span class="p">;</span></span>

    <span class="p"><span class="p">[[</span></span><span class="n"><span class="n">nodiscard</span></span><span class="p"><span class="p">]]</span></span> <span class="n"><span class="n">arrival_token</span></span> <span class="n"><span class="n">arrive</span></span><span class="p"><span class="p">(</span></span><span class="kt"><span class="kt">ptrdiff_t</span></span> <span class="n"><span class="n">update</span></span> <span class="o"><span class="o">=</span></span> <span class="mi"><span class="mi">1</span></span><span class="p"><span class="p">);</span></span>
    <span class="kt"><span class="kt">bool</span></span> <span class="nf"><span class="nf">try_wait</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">arrival_token</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">const</span></span><span class="p"><span class="p">;</span></span>
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">wait</span></span><span class="p"><span class="p">(</span></span><span class="n"><span class="n">arrival_token</span></span><span class="p"><span class="p">)</span></span> <span class="k"><span class="k">const</span></span><span class="p"><span class="p">;</span></span>

    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">arrive_and_wait</span></span><span class="p"><span class="p">();</span></span>
    <span class="kt"><span class="kt">void</span></span> <span class="nf"><span class="nf">arrive_and_drop</span></span><span class="p"><span class="p">();</span></span>

  <span class="k"><span class="k">private</span></span><span class="o"><span class="o">:</span></span>
    <span class="n"><span class="n">CompletionFunction</span></span> <span class="n"><span class="n">completion</span></span><span class="p"><span class="p">;</span></span> <i><span class="c1"><span class="c1">// exposition only</span></span></i>
<span class="c1"></span><span class="c1"></span>  <span class="p"><span class="p">};</span></span>
<span class="p"><span class="p">}</span></span>
</code></pre>
    <div class="numbered"> A <code class="highlight"><span class="n">barrier</span></code> is a barrier type with a completion step controlled by a function object. The completion step calls <code class="highlight"><span class="n">completion</span></code>. Threads may block at the barrier’s synchronization point for a phase, waiting for the expected sum contributions by threads that arrive in that phase. </div>
    <div class="numbered"> <code class="highlight"><span class="n">CompletionFunction</span></code> shall be <code class="highlight"><span class="n">CopyConstructible</span></code> and <code class="highlight"><span class="n">is_invocable_r_v</span><span class="o">&lt;</span><span class="kt">void</span><span class="p">,</span> <span class="n">CompletionFunction</span><span class="o">></span></code> shall be <code class="highlight">true</code>. </div>
    <div class="numbered"> <code class="highlight"><span class="n">barrier</span><span class="o">::</span><span class="n">arrival_token</span></code> is an implementation-defined type that is <code class="highlight"><span class="n">MoveConstructible</span></code> and <code class="highlight"><span class="n">MoveAssignable</span></code>. </div>
<pre class="highlight"><span class="k">explicit</span> <span class="nf">barrier</span><span class="p">(</span><span class="kt">ptrdiff_t</span> <span class="n">expected</span><span class="p">,</span> <span class="n">CompletionFunction</span> <span class="n">f</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> <code class="highlight"><span class="n">expected</span> <span class="o">>=</span> <span class="mi">0</span></code>, and <code class="highlight"><span class="k">noexcept</span><span class="p">(</span><span class="n">f</span><span class="p">())</span></code> shall be <code class="highlight">true</code>. </div>
    <div class="numbered"> <em>Effects:</em> Initializes the barrier for <code class="highlight"><span class="n">expected</span></code> number of threads in the first phase, and initializes <code class="highlight"><span class="n">completion</span></code> with <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">f</span><span class="p">)</span></code>. [ <em>Note:</em> If <code class="highlight"><span class="n">expected</span></code> is <code class="highlight"><span class="mi">0</span></code> this object may only be destroyed. — <em>end note</em> ] </div>
<pre class="highlight"><span class="o">~</span><span class="n">barrier</span><span class="p">();</span>
</pre>
    <div class="numbered"> <em>Requires:</em> No threads are blocked at a synchronization point for any phase. </div>
    <div class="numbered"> <em>Effects:</em> Destroys the barrier. </div>
    <div class="numbered"> <em>Remarks:</em> May be called even if some threads have not yet returned from functions that block at a synchronization point, provided that they have unblocked. [ <em>Note:</em> The destructor may block until all threads have exited invocations of wait() on this object. — <em>end note</em> ] </div>
<pre class="highlight"><span class="p">[[</span><span class="n">nodiscard</span><span class="p">]]</span> <span class="n">arrival_token</span> <span class="n">arrive</span><span class="p">(</span><span class="kt">ptrdiff_t</span> <span class="n">update</span><span class="p">);</span>
</pre>
    <div class="numbered"> <em>Requires:</em> The expected count is not less than <code class="highlight"><span class="n">update</span></code>. </div>
    <div class="numbered"> <em>Effects:</em> Constructs an object of type <code class="highlight"><span class="n">arrival_token</span></code> that is associated with the <code class="highlight"><span class="n">barrier</span></code>'s synchronization point for the current phase, then arrives <code class="highlight"><span class="n">update</span></code> times at the synchronization point for the current phase. </div>
    <div class="numbered"> <em>Synchronization:</em> The call to <code class="highlight"><span class="n">arrive</span></code> strongly happens before the start of the completion step for the current phase. </div>
    <div class="numbered"> <em>Returns:</em> The constructed object. </div>
    <div class="numbered"> <em>Remarks:</em> This may cause the completion step to start. </div>
<pre class="highlight"><span class="kt">bool</span> <span class="nf">try_wait</span><span class="p">(</span><span class="n">arrival_token</span> <span class="n">arrival</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
    <div class="numbered"> <em>Requires:</em> <code class="highlight"><span class="n">arrival</span></code> is associated with a synchronization point for the current or the immediately preceding phases of the <code class="highlight"><span class="n">barrier</span></code>. </div>
    <div class="numbered"> <em>Returns:</em> <code class="highlight">true</code> if the synchronization point condition associated with <code class="highlight"><span class="n">arrival</span></code> is satisfied, otherwise <code class="highlight">false</code>. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">wait</span><span class="p">(</span><span class="n">arrival_token</span> <span class="n">arrival</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
    <div class="numbered"> <em>Requires:</em> <code class="highlight"><span class="n">arrival</span></code> is associated with a synchronization point for the current or the immediately preceding phases of the <code class="highlight"><span class="n">barrier</span></code>. </div>
    <div class="numbered"> <em>Effects:</em> Blocks at the synchronization point associated with <code class="highlight"><span class="n">arrival</span></code> until the condition is satisfied. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">arrive_and_wait</span><span class="p">();</span>
</pre>
    <div class="numbered"> <em>Effects:</em> Equivalent to <code class="highlight"><span class="n">wait</span><span class="p">(</span><span class="n">arrive</span><span class="p">())</span></code>. </div>
<pre class="highlight"><span class="kt">void</span> <span class="nf">arrive_and_drop</span><span class="p">();</span>
</pre>
    <div class="numbered"> <em>Requires:</em> The expected number of threads for the current phase is not <code class="highlight"><span class="mi">0</span></code>. </div>
    <div class="numbered"> <em>Effects:</em> Decrements the expected number of threads for subsequent phases by <code class="highlight"><span class="mi">1</span></code>, then arrives at the synchronization point for the current phase. </div>
    <div class="numbered"> <em>Synchronization:</em> The call to <code class="highlight"><span class="n">arrive_and_drop</span></code> strongly happens before the start of the completion step for the current phase. </div>
    <div class="numbered"> <em>Remarks:</em> This may cause the completion phase to start. </div>
   </blockquote>
   <p>Create the following feature test macros:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="n">__cpp_lib_atomic_lock_free_type_aliases</span></code>, which implies that <code class="highlight"><span class="n">atomic_signed_lock_free</span></code> and <code class="highlight"><span class="n">atomic_unsigned_lock_free</span></code> types are available.</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">__cpp_lib_atomic_flag_test</span></code>, which implies the <code class="highlight"><span class="n">test</span></code> methods and free functions for <code class="highlight"><span class="n">atomic_flag</span></code> are available.</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">__cpp_lib_atomic_wait</span></code>, which implies the <code class="highlight"><span class="n">notify_</span><span class="o">*</span></code> and <code class="highlight"><span class="n">wait</span></code> methods and free functions for <code class="highlight"><span class="n">atomic</span></code> and <code class="highlight"><span class="n">atomic_flag</span></code> and the <code class="highlight"><span class="n">atomic_int_fast_wait_t</span></code> and <code class="highlight"><span class="n">atomic_uint_fast_wait_t</span></code> types are available.</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">__cpp_lib_semaphore</span></code>, which implies that <code class="highlight"><span class="n">counting_semaphore</span></code> and <code class="highlight"><span class="n">binary_semaphore</span></code> are available.</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">__cpp_lib_latch</span></code>, which implies that <code class="highlight"><span class="n">latch</span></code> is available.</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">__cpp_lib_barrier</span></code>, which implies that <code class="highlight"><span class="n">barrier</span></code> is available.</p>
   </ul>
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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="#coordination-types">coordination types</a><span>, in §2</span>
  </ul>