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  <title>P0995R1: Improving atomic_flag</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,
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0995R1<br>Improving atomic_flag</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-06-22">22 June 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p0995r1">http://wg21.link/p0995r1</a>
     <dt>Authors:
     <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>)
     <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:andrewhhunter@gmail.com">Andrew Hunter</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>LWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
     <dt>Source:
     <dd><a href="https://github.com/jfbastien/papers/blob/master/source/p0995r1.bs">github.com/jfbastien/papers/blob/master/source/p0995r1.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>atomic_flag is marginally useful. Improve it in light of the new wait / notify APIs.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#rev"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r0r1"><span class="secno">1.1</span> <span class="content">r0 ➡ r1</span></a>
      <li><a href="#draftr0"><span class="secno">1.2</span> <span class="content">Draft ➡ r0</span></a>
     </ol>
    <li><a href="#intro"><span class="secno">2</span> <span class="content">Introduction</span></a>
    <li><a href="#word"><span class="secno">3</span> <span class="content">Wording</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <p class="note" role="note"><span>Note:</span> this paper is superseded by <a data-link-type="biblio" href="#biblio-p1135r0">[P1135r0]</a>, which takes the r1 version of
this paper and integrates it fully with other related papers which should be
voted into the working draft together.</p>
   <h2 class="heading settled" data-level="1" id="rev"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#rev"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="r0r1"><span class="secno">1.1. </span><span class="content">r0 ➡ r1</span><a class="self-link" href="#r0r1"></a></h3>
   <p>In Rapperswil, SG1 reviewed <a data-link-type="biblio" href="#biblio-p0995r0">[P0995R0]</a> at LEWG’s request.</p>
   <p>Potential ABI breakage to achieve implementation efficiency was considered. SG1
is unanimously comfortable with this. Implementations can choose to avoid
breakage and offer a less efficient implementation.</p>
   <p>LEWG was unhappy about <code class="highlight"><span class="n">atomic_signed_lock_free</span></code> / <code class="highlight"><span class="n">atomic_unsigned_lock_free</span></code> being optional. SG1 was worried that some platforms would be unable to implement
them as lock-free because they lack a compare-and-exchange instruction and might
not be able to disable interrupts on all cores. After discussion, SG1 agreed to
make these mandatory despite rare platforms potentially being unable to
implement these types.</p>
   <table class="def">
    <tbody>
     <tr>
      <th>
      <th><strong>SF</strong>
      <th><strong>F</strong>
      <th><strong>N</strong>
      <th><strong>A</strong>
      <th><strong>SA</strong>
     <tr>
      <th><code class="highlight"><span class="n">atomic_signed_lock_free</span></code> / <code class="highlight"><span class="n">atomic_unsigned_lock_free</span></code> should be mandatory
      <th>4
      <th>9
      <th>12
      <th>3
      <th>1
     <tr>
      <th>Move this paper to LEWG with intent to include in IS, either with or without the change above
      <th>20
      <th>7
      <th>2
      <th>0
      <th>0
   </table>
   <p>LEWG then saw the paper again, and there was unanimous consent to forward to LWG
for C++20.</p>
   <h3 class="heading settled" data-level="1.2" id="draftr0"><span class="secno">1.2. </span><span class="content">Draft ➡ r0</span><a class="self-link" href="#draftr0"></a></h3>
   <p>This paper was written in Jacksonville and presented to SG1, which unanimously
forwarded the paper to LEWG. LEWG looked at the paper and took the following
poll:</p>
   <table class="def">
    <tbody>
     <tr>
      <th>
      <th><strong>SF</strong>
      <th><strong>F</strong>
      <th><strong>N</strong>
      <th><strong>A</strong>
      <th><strong>SA</strong>
     <tr>
      <th>Make the type aliases non-optional.
      <th>1
      <th>4
      <th>4
      <th>2
      <th>2
   </table>
   <p>The types were made optional in case an architecture, such as PA-RISC, cannot
support always-lock-free integral types because no compare-and-exchange
instruction is available. There was no consensus for making aliases required,
though concern was expressed that LEWG doesn’t usually make functionality
optional. In the C++ standard library optionality is present as follows:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="n">intN_t</span></code> / <code class="highlight"><span class="n">uintN_t</span></code> are mandated by C, "if an implementation provides integer
types with widths of 8, 16, 32, or 64 bits, no padding bits, and (for the
signed types) that have a two’s complement representation";</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">abs</span></code> and <code class="highlight"><span class="n">div</span></code> overloads "if and only if the type <code class="highlight"><span class="kt">intmax_t</span></code> designates an
extended integer type";</p>
    <li data-md="">
     <p>The library <code class="highlight"><span class="n">allocator_traits</span></code> template has optional requirements.</p>
   </ul>
   <p>There was also discussion about ABI breakage to <code class="highlight"><span class="n">atomic_flag</span></code>. An argument was
made that <code class="highlight"><span class="n">atomic_flag</span></code> should also be sized such that waiting on them is most
efficient (which would be an ABI breakage), and if that breakage doesn’t occur
then adding wait / notify overloads is actively misleading. LEWG want SG1 to
reconsider whether the overloads should be provided.</p>
   <h2 class="heading settled" data-level="2" id="intro"><span class="secno">2. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>C++11 added <code class="highlight"><span class="n">atomic_flag</span></code> to the language as the minimally-required class which
could be used to implement <code class="highlight"><span class="n">atomic</span><span class="o">&lt;></span></code> on hardware which seemed relevant at the
time. Detailed <code class="highlight"><span class="n">atomic_flag</span></code> history can be found in <a data-link-type="biblio" href="#biblio-n2145">[N2145]</a>, <a data-link-type="biblio" href="#biblio-n2324">[N2324]</a>, and <a data-link-type="biblio" href="#biblio-n2393">[N2393]</a>. The specification was quite successful at minimalism—the only member
functions of <code class="highlight"><span class="n">atomic_flag</span></code> are <code class="highlight"><span class="n">test_and_set</span></code> and <code class="highlight"><span class="n">clear</span></code>—but <code class="highlight"><span class="n">atomic</span><span class="o">&lt;></span></code> was
wildly more successful and to our knowledge has always been implemented with
compiler support instead of with the very inefficient (but beautifully simple) <code class="highlight"><span class="n">atomic_flag</span></code>. Our experience is that <code class="highlight"><span class="n">atomic_flag</span></code>'s interface is so minimal as
to be mostly useless, in particular it doesn’t have a method which can load the
flag’s value without modifying it.</p>
   <p>We’ve heard of it being used as:</p>
   <ul>
    <li data-md="">
     <p>A questionable spinloop (as was originally intended);</p>
    <li data-md="">
     <p>A "check-in" flag used to know when at least one thread has reached a
program location.</p>
   </ul>
   <p>The one special power <code class="highlight"><span class="n">atomic_flag</span></code> has is in being the only type which is
guaranteed to be lock-free, albeit a mostly powerless one.</p>
   <p>SG1 tried to salvage <code class="highlight"><span class="n">atomic_flag</span></code> in <a data-link-type="biblio" href="#biblio-p0514r0">[P0514R0]</a> by adding <code class="highlight"><span class="n">set</span></code>, <code class="highlight"><span class="n">test</span></code>, <code class="highlight"><span class="n">wait</span></code>, <code class="highlight"><span class="n">wait_until</span></code>, and <code class="highlight"><span class="n">wait_for</span></code> methods but decided to leave it as-is and
implement efficient waiting differently, eventually going for <a data-link-type="biblio" href="#biblio-p0514r3">[P0514R3]</a>.</p>
   <p>The time has come to thank <code class="highlight"><span class="n">atomic_flag</span></code> for serving its purpose as an
implementability stand-in, and help it find its true purpose. We propose:</p>
   <ul>
    <li data-md="">
     <p>Adding a <code class="highlight"><span class="n">test</span></code> method to it as <a data-link-type="biblio" href="#biblio-p0514r0">[P0514R0]</a> did. This could technically
forbids some ancestral processors from implementing modern C++, but these
platforms already don’t support any C++.</p>
    <li data-md="">
     <p>Add <code class="highlight"><span class="n">atomic_flag</span></code> overloads to <a data-link-type="biblio" href="#biblio-p0514r3">[P0514R3]</a>'s waiting and notify functions.</p>
    <li data-md="">
     <p>Add always-lock-free integral type aliases, which are encouraged to
be sized such that waiting on them is most efficient.</p>
   </ul>
   <h2 class="heading settled" data-level="3" id="word"><span class="secno">3. </span><span class="content">Wording</span><a class="self-link" href="#word"></a></h2>
   <p>Under Header <code class="highlight"><span class="o">&lt;</span><span class="n">atomic</span><span class="o">></span></code> synopsis [<strong>atomics.syn</strong>] edit as follows:</p>
   <blockquote>
<pre class="highlight"><span class="c1">// 32.3, type aliases</span>
<span class="c1"></span>
<span class="c1">// ...</span>
</pre>
    <ins>
<pre class="highlight"><span class="k">using</span> <span class="n">atomic_signed_lock_free</span>   <span class="o">=</span> <span class="n">see</span> <span class="n">below</span><span class="p">;</span>
<span class="k">using</span> <span class="n">atomic_unsigned_lock_free</span> <span class="o">=</span> <span class="n">see</span> <span class="n">below</span><span class="p">;</span>
</pre>
    </ins>
<pre class="highlight"><span class="c1">// 32.8, flag type and operations</span>
<span class="c1"></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">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"><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="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="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="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_and_set_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>
<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="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="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="p">,</span> <span class="n">memory_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="p">,</span> <span class="n">memory_order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="cp">#define ATOMIC_FLAG_INIT see below</span>
</pre>
<pre class="highlight"><span class="c1">// 32.10, waiting and notifying functions</span>
<span class="c1"></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="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>
</pre>
    <ins>
<pre class="highlight"><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>
</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_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>
</pre>
    <ins>
<pre class="highlight"><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>
</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_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>
</pre>
    <ins>
<pre class="highlight"><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>
</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_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>
</pre>
    <ins>
<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="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>
   </blockquote>
   <p>In Atomic operations library [<strong>atomics</strong>], under Type aliases
[<strong>atomics.alias</strong>], edit as follows:</p>
   <blockquote>
    <p>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.</p>
    <ins>
     <p>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.</p>
    </ins>
   </blockquote>
   <p>In Atomic operations library [<strong>atomics</strong>], under Flag type and operations
[<strong>atomics.flag</strong>], edit as follows:</p>
   <blockquote>
<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">  <span class="kt">bool</span> <span class="nf">test_and_set</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_and_set</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>
  <span class="kt">void</span> <span class="nf">clear</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">void</span> <span class="nf">clear</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>
  <span class="n">atomic_flag</span><span class="p">()</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="n">atomic_flag</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">&amp;</span><span class="p">)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span>
  <span class="n">atomic_flag</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">&amp;</span><span class="p">)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span>
  <span class="n">atomic_flag</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">atomic_flag</span><span class="o">&amp;</span><span class="p">)</span> <span class="k">volatile</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span>
<span class="p">};</span>
</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">  <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="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="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="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_and_set_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>
  <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="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="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="p">,</span> <span class="n">memory_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="p">,</span> <span class="n">memory_order</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="cp">#define ATOMIC_FLAG_INIT see below</span>
<span class="cp"></span><span class="p">}</span>
</pre>
    <p>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.</p>
    <p>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>]</p>
    <p>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.</p>
    <p>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:</p>
<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>
    <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>
     <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_release</span></code> nor <code class="highlight"><span class="n">memory_order_acq_rel</span></code>.</p>
     <p><em>Effects:</em> Memory is affected according to the value of <code class="highlight"><span class="n">order</span></code>.</p>
     <p><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>.</p>
    </ins>
<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>
    <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">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).</p>
    <p><em>Returns:</em> Atomically, the value of the object immediately before the effects.</p>
<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>In Atomic operations library [<strong>atomics</strong>], under Waiting and notifying
functions [<strong>atomics.wait</strong>], edit as follows:</p>
   <blockquote>
    <p>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>]</p>
    <p>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.</p>
<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>
</pre>
    <ins>
<pre class="highlight"><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>
    </ins>
    <p><em>Effects:</em> unblocks up to execution of a waiting function that blocked after
observing the result of an atomic operation X, if there exists another atomic
operation Y, such that X precedes Y in the modification order of <code class="highlight"><span class="o">*</span><span class="n">object</span></code>, and
Y happens-before this call.</p>
<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>
</pre>
    <ins>
<pre class="highlight"><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>
    </ins>
    <p><em>Effects:</em> unblocks each execution of a waiting function that blocked after
observing the result of an atomic operation X, if there exists another atomic
operation Y, such that X precedes Y in the modification order of <code class="highlight"><span class="o">*</span><span class="n">object</span></code>, and
Y happens-before this call.</p>
<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>
    <p><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>.</p>
    <p><em>Effects:</em> Repeatedly performs the following steps, in order:</p>
    <ol>
     <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>
    </ol>
    <ins>
<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>
     <p><em>Effects:</em> Repeatedly performs the following steps, in order:</p>
     <ol>
      <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>
     </ol>
    </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_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>
</pre>
    <ins>
<pre class="highlight"><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>
    </ins>
    <p><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></p>
   </blockquote>
   <p>Two feature test macros should be added:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="n">__cpp_lib_atomic_flag_test</span></code> implies the <code class="highlight"><span class="n">test</span></code> methods for <code class="highlight"><span class="n">atomic_flag</span></code> and free functions, as well as the notify and wait overloads 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_lock_free_type_aliases</span></code> implies <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 defined.</p>
   </ul>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-n2145">[N2145]
   <dd>H.-J. Boehm, L. Crowl. <a href="https://wg21.link/n2145">C++ Atomic Types and Operations</a>. 12 January 2007. URL: <a href="https://wg21.link/n2145">https://wg21.link/n2145</a>
   <dt id="biblio-n2324">[N2324]
   <dd>H.-J. Boehm, L. Crowl. <a href="https://wg21.link/n2324">C++ Atomic Types and Operations</a>. 24 June 2007. URL: <a href="https://wg21.link/n2324">https://wg21.link/n2324</a>
   <dt id="biblio-n2393">[N2393]
   <dd>H.-J. Boehm, L. Crowl. <a href="https://wg21.link/n2393">C++ Atomic Types and Operations</a>. 10 September 2007. URL: <a href="https://wg21.link/n2393">https://wg21.link/n2393</a>
   <dt id="biblio-p0514r0">[P0514R0]
   <dd>Olivier Giroux. <a href="https://wg21.link/p0514r0">Enhancing std::atomic_flag for waiting</a>. 15 November 2016. URL: <a href="https://wg21.link/p0514r0">https://wg21.link/p0514r0</a>
   <dt id="biblio-p0514r3">[P0514R3]
   <dd>Olivier Giroux. <a href="https://wg21.link/p0514r3">Efficient concurrent waiting for C++20</a>. 10 February 2018. URL: <a href="https://wg21.link/p0514r3">https://wg21.link/p0514r3</a>
   <dt id="biblio-p0995r0">[P0995R0]
   <dd>JF Bastien, Olivier Giroux, Andrew Hunter. <a href="https://wg21.link/p0995r0">Improving atomic_flag</a>. 17 March 2018. URL: <a href="https://wg21.link/p0995r0">https://wg21.link/p0995r0</a>
   <dt id="biblio-p1135r0">[P1135r0]
   <dd><a href="https://wg21.link/p1135r0">The C++20 Concurrency Library</a>. URL: <a href="https://wg21.link/p1135r0">https://wg21.link/p1135r0</a>
  </dl>