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  <title>P2889R0: Distributed Arrays</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

	/* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 3383f409d, updated Fri Apr 28 15:49:22 2023 -0700" name="generator">
  <link href="https://github.com/vasama/wg21" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>/* style-autolinks */
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 </head><body class="h-entry toc-sidebar"><p id="toc-nav"><a id="toc-jump" href="https://vasama.github.io/wg21/P2889R0.html#toc"><span aria-hidden="true">↑</span> <span>Jump to Table of Contents</span></a><a id="toc-toggle" href="https://vasama.github.io/wg21/P2889R0.html#toc"><span aria-hidden="true">←</span> <span>Collapse Sidebar</span></a></p>
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2889R0<br>Distributed Arrays</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2023-05-15">2023-05-15</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     </dt><dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:wg21@vasama.org">Lauri Vasama</a>
     </dd><dt>Audience:
     </dt><dd>EWG
     </dd><dt>Project:
     </dt><dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     </dd><dt>Current draft:
     </dt><dd><a href="https://vasama.github.io/wg21/D2889.html">vasama.github.io/wg21/D2889.html</a>
     </dd><dt>Current draft source:
     </dt><dd><a href="https://github.com/vasama/wg21/blob/main/D2889.bs">github.com/vasama/wg21/blob/main/D2889.bs</a>
    </dd></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>A new method of defining global arrays by merging definitions from multiple translation units.</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="https://vasama.github.io/wg21/P2889R0.html#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
     <ol class="toc">
      <li><a href="https://vasama.github.io/wg21/P2889R0.html#introduction_comparison"><span class="secno">1.1</span> <span class="content">Comparison to state of the art</span></a>
     </li></ol>
    </li><li>
     <a href="https://vasama.github.io/wg21/P2889R0.html#existing"><span class="secno">2</span> <span class="content">Existing practice</span></a>
     <ol class="toc">
      <li><a href="https://vasama.github.io/wg21/P2889R0.html#existing_ccpp"><span class="secno">2.1</span> <span class="content">Examples of existing usage in C and C++</span></a>
      </li><li>
       <a href="https://vasama.github.io/wg21/P2889R0.html#existing_other"><span class="secno">2.2</span> <span class="content">Prior art in other languages</span></a>
       <ol class="toc">
        <li><a href="https://vasama.github.io/wg21/P2889R0.html#existing_other_linkme"><span class="secno">2.2.1</span> <span class="content"><span>github.com/dtolnay/linkme</span></span></a>
        </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#existing_other_reflection"><span class="secno">2.2.2</span> <span class="content">Runtime reflection</span></a>
       </li></ol>
     </li></ol>
    </li><li>
     <a href="https://vasama.github.io/wg21/P2889R0.html#alternatives"><span class="secno">3</span> <span class="content">Alternatives considered</span></a>
     <ol class="toc">
      <li><a href="https://vasama.github.io/wg21/P2889R0.html#alternatives_static"><span class="secno">3.1</span> <span class="content">Static initialiser side effects</span></a>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#alternatives_linker"><span class="secno">3.2</span> <span class="content">Linker sections</span></a>
     </li></ol>
    </li><li>
     <a href="https://vasama.github.io/wg21/P2889R0.html#design"><span class="secno">4</span> <span class="content">Design considerations</span></a>
     <ol class="toc">
      <li><a href="https://vasama.github.io/wg21/P2889R0.html#design_declaration"><span class="secno">4.1</span> <span class="content">Declaring distributed arrays</span></a>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_type"><span class="secno">4.2</span> <span class="content">Distributed array type</span></a>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_definition"><span class="secno">4.3</span> <span class="content">Defining elements of distributed arrays</span></a>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_empty"><span class="secno">4.4</span> <span class="content">Empty distributed arrays</span></a>
      </li><li>
       <a href="https://vasama.github.io/wg21/P2889R0.html#design_size"><span class="secno">4.5</span> <span class="content">Accessing the size of the distributed array</span></a>
       <ol class="toc">
        <li><a href="https://vasama.github.io/wg21/P2889R0.html#design_size_sizeof"><span class="secno">4.5.1</span> <span class="content"><code class="highlight"><c- k="">sizeof</c-></code> operator</span></a>
        </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_size_operator"><span class="secno">4.5.2</span> <span class="content">Function-like operator</span></a>
        </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_size_template"><span class="secno">4.5.3</span> <span class="content">Library template</span></a>
        </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_size_function"><span class="secno">4.5.4</span> <span class="content">Library function</span></a>
       </li></ol>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_ordering"><span class="secno">4.6</span> <span class="content">Ordering of distributed array elements</span></a>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_trivial"><span class="secno">4.7</span> <span class="content">Non-trivial element types</span></a>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#design_thread"><span class="secno">4.8</span> <span class="content">Thread storage duration</span></a>
      </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#implementation_dynamic"><span class="secno">4.9</span> <span class="content">Dynamic linking</span></a>
     </li></ol>
    </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#implementation"><span class="secno">5</span> <span class="content">Implementation experience</span></a>
    </li><li>
     <a href="https://vasama.github.io/wg21/P2889R0.html#examples"><span class="secno">6</span> <span class="content">Examples</span></a>
     <ol class="toc">
      <li>
       <a href="https://vasama.github.io/wg21/P2889R0.html#examples_test"><span class="secno">6.1</span> <span class="content">Unit test framework</span></a>
       <ol class="toc">
        <li><a href="https://vasama.github.io/wg21/P2889R0.html#examples_test_header"><span class="secno">6.1.1</span> <span class="content">Framework header</span></a>
        </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#examples_test_user"><span class="secno">6.1.2</span> <span class="content">User test code</span></a>
        </li><li><a href="https://vasama.github.io/wg21/P2889R0.html#examples_test_source"><span class="secno">6.1.3</span> <span class="content">Framework source</span></a>
       </li></ol>
     </li></ol>
    </li><li>
     <a href="https://vasama.github.io/wg21/P2889R0.html#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="https://vasama.github.io/wg21/P2889R0.html#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </li></ol>
   </li></ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#introduction"></a></h2>
   <p>It is often desirable to define sets of static objects in a distributed manner across multiple translation units without explicitly and manually maintaining the full list of elements in any single location. This improves software maintainability when the entire set must ultimately be available at runtime.</p>
   <p>Is this really worth the committee’s time? I believe so. While the feature may seem novel, it is arguably standardising existing practice that is being widely used out in the field. In fact the facility being proposed already is already in use in all major C++ implementations as an implementation strategy for static initialisers, thread local storage and exception handling tables. This paper proposes to expose that existing facility to the programmer.</p>
   <p>This feature was previously introduced in <a data-link-type="biblio" href="https://vasama.github.io/wg21/P2889R0.html#biblio-p2268r0" title="Freestanding Roadmap">[P2268R0]</a> under the name "link-time arrays" with hopes of future standardisation.</p>
   <h3 class="heading settled" data-level="1.1" id="introduction_comparison"><span class="secno">1.1. </span><span class="content">Comparison to state of the art</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#introduction_comparison"></a></h3>
   <p>Here is a quick comparison to static initialiser side effects, which seems to be the solution most often employed by users out in the field today:</p>
   <table>
    <tbody>
     <tr>
      <th>Static intialisers
      </th><th>Distributed array
     </th></tr><tr>
      <td>
<pre class="highlight"><c- n="">std</c-><c- o="">::</c-><c- n="">vector</c-><c- o="">&lt;</c-><c- b="">int</c-><c- o="">&gt;&amp;</c-> <c- n="">get_array_storage</c-><c- p="">()</c-> <c- p="">{</c->
  <c- k="">static</c-> <c- n="">std</c-><c- o="">::</c-><c- n="">vector</c-><c- o="">&lt;</c-><c- n="">T</c-><c- o="">&gt;</c-> <c- n="">vector</c-><c- p="">;</c->
  <c- k="">return</c-> <c- n="">vector</c-><c- p="">;</c->
<c- p="">}</c->

<c- n="">std</c-><c- o="">::</c-><c- n="">span</c-><c- o="">&lt;</c-><c- b="">int</c-> <c- k="">const</c-><c- o="">&gt;</c-> <c- n="">get_array</c-><c- p="">()</c-> <c- p="">{</c->
  <c- k="">return</c-> <c- n="">get_array_storage</c-><c- p="">();</c->
<c- p="">}</c->


<c- c1="">// In another translation unit.</c->
<c- p="">[[</c-><c- n="">maybed_unused</c-><c- p="">]]</c->
<c- k="">static</c-> <c- b="">char</c-> <c- k="">const</c-> <c- n="">elements_pusher</c-> <c- o="">=</c-> <c- p="">(</c->
  <c- n="">get_array_storage</c-><c- p="">().</c-><c- n="">push_back</c-><c- p="">(</c-><c- n="">value_0</c-><c- p="">),</c->
  <c- n="">get_array_storage</c-><c- p="">().</c-><c- n="">push_back</c-><c- p="">(</c-><c- n="">value_1</c-><c- p="">),</c->
  <c- mi="">0</c-> <c- d="">/* elements_pusher value. */</c-><c- p="">);</c->
</pre>
      </td><td>
<pre class="highlight"><c- k="">extern</c-> <c- b="">int</c-> <c- k="">const</c-> <c- n="">array</c-><c- p="">[];</c->

<c- n="">std</c-><c- o="">::</c-><c- n="">span</c-><c- o="">&lt;</c-><c- b="">int</c-> <c- k="">const</c-><c- o="">&gt;</c-> <c- n="">get_array</c-><c- p="">()</c-> <c- p="">{</c->
  <c- k="">return</c-> <c- n="">std</c-><c- o="">::</c-><c- n="">span</c-><c- p="">(</c->
    <c- n="">array</c-><c- p="">,</c->
    <c- n="">std</c-><c- o="">::</c-><c- n="">distributed_array_size</c-><c- p="">(</c-><c- n="">array</c-><c- p="">));</c->
<c- p="">}</c->


<c- c1="">// In another translation unit.</c->
<c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">array</c-><c- p="">)</c->
<c- k="">static</c-> <c- b="">int</c-> <c- k="">const</c-> <c- n="">elements</c-><c- p="">[]</c-> <c- o="">=</c-> <c- p="">{</c->
  <c- n="">value_0</c-><c- p="">,</c->
  <c- n="">value_1</c-><c- p="">,</c->
<c- p="">};</c->
</pre>
   </td></tr></tbody></table>
   <h2 class="heading settled" data-level="2" id="existing"><span class="secno">2. </span><span class="content">Existing practice</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#existing"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="existing_ccpp"><span class="secno">2.1. </span><span class="content">Examples of existing usage in C and C++</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#existing_ccpp"></a></h3>
   <ul>
    <li data-md="">
     <p>Test cases in all major C++ unit test frameworks (Catch2 on <a href="https://github.com/catchorg/Catch2/blob/0dc82e08dfec25faf6c6c145f3592984db7276cc/src/catch2/internal/catch_test_registry.hpp">GitHub</a>)</p>
    </li><li data-md="">
     <p>Optimisation passes in LLVM (<a href="https://github.com/llvm/llvm-project/blob/d526e2ec957fc7bc31d7be3670cd1673c2b4389b/llvm/include/llvm/PassSupport.h">GitHub</a>)</p>
    </li><li data-md="">
     <p>Type information in Unreal Engine (Source available on <a href="https://github.com/EpicGames/UnrealEngine/blob/5ca9da84c694c6eee288c30a547fcaa1a40aed9b/Engine/Source/Runtime/CoreUObject/Public/UObject/UObjectBase.h">GitHub</a> through <a href="https://www.unrealengine.com/en-US/ue-on-github">Epic Games</a>)</p>
    </li><li data-md="">
     <p>Type information in Boost.Serialization (<a href="https://github.com/boostorg/serialization/blob/3f322d4adc3c88a667751ad66ce19217a3bba1f9/include/boost/serialization/void_cast.hpp">GitHub</a>)</p>
    </li><li data-md="">
     <p>Multiple types of objects in Qt Framework (<a href="https://github.com/qt/qtbase/blob/278acae61146fae4811bc3643516ff41cdff24fc/src/corelib/kernel/qcoreapplication.h">GitHub</a>)</p>
    </li><li data-md="">
     <p>Kernels and operations in TensorFlow (<a href="https://github.com/tensorflow/tensorflow/blob/5a06771ea43a1574a3f9a1a5673b339aa4076f6f/tensorflow/core/framework/op.h">GitHub</a>)</p>
    </li><li data-md="">
     <p>Generators in Apache Thrift (<a href="https://github.com/apache/thrift/blob/3880a09565a9a1dad028b3679746eafac268c819/compiler/cpp/src/thrift/generate/t_generator_registry.h">GitHub</a>)</p>
    </li><li data-md="">
     <p>Initialisation callbacks in the Linux kernel (C language, freestanding) (<a href="https://github.com/torvalds/linux/blob/16a8829130ca22666ac6236178a6233208d425c3/include/linux/init.h">GitHub</a>)</p>
    </li><li data-md="">
     <p>Multiple tables defined using linker sections in iPXE (C language, freestanding) (<a href="https://github.com/ipxe/ipxe/blob/03eea19c19b52851002654c2818b765d4aa42894/src/include/ipxe/tables.h">GitHub</a>)</p>
    </li><li data-md="">
     <p>The author is aware of more usage in proprietary code bases.</p>
   </li></ul>
   <h3 class="heading settled" data-level="2.2" id="existing_other"><span class="secno">2.2. </span><span class="content">Prior art in other languages</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#existing_other"></a></h3>
   <h4 class="heading settled" data-level="2.2.1" id="existing_other_linkme"><span class="secno">2.2.1. </span><span class="content"><a href="https://github.com/dtolnay/linkme">github.com/dtolnay/linkme</a></span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#existing_other_linkme"></a></h4>
    Linkme is a Rust library implementing distributed arrays using linker sections. From the readme: 
   <blockquote>
     A distributed slice is a collection of static elements that are gathered into a contiguous section of the binary by the linker. Slice elements may be defined individually from anywhere in the dependency graph of the final binary. 
    <p>The implementation is based on link_section attributes and platform-specific linker support. It does not involve life-before-main or any other runtime initialization on any platform. This is a zero-cost safe abstraction that operates entirely during compilation and linking.</p>
   </blockquote>
   <h4 class="heading settled" data-level="2.2.2" id="existing_other_reflection"><span class="secno">2.2.2. </span><span class="content">Runtime reflection</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#existing_other_reflection"></a></h4>
   <p>In languages such as Java and C# with support for runtime reflection, similar results are achieved by reflecting over the program at runtime and picking out types or functions decorated with specific attributes. In these languages the technique seems especially popular for service discovery paired with dependency injection.</p>
   <p>In the case of C++, collation of type information for the purposes of runtime reflection may itself become an important use case in the future if static reflection is added to the language. A runtime reflection library may register descriptions of user types and other entities in distributed arrays for runtime use. This is already being done out in the field to some extent as seen in Unreal Engine and Boost.Serialization.</p>
   <h2 class="heading settled" data-level="3" id="alternatives"><span class="secno">3. </span><span class="content">Alternatives considered</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#alternatives"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="alternatives_static"><span class="secno">3.1. </span><span class="content">Static initialiser side effects</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#alternatives_static"></a></h3>
   <p>In standard C++ the only way to achieve the desired result is by building the set at runtime using static initialisers. This approach is widely utilised, but comes with many problems:</p>
   <ul>
    <li data-md="">
     <p>Accessing specific elements adds additional complexity. For example by storing indices in global variables during static initialisation.</p>
    </li><li data-md="">
     <p>Is subject to the static initialisation order fiasco if other static initialisers should access the set under construction, even when the actual data is known statically and could in theory have been constant initialised.</p>
    </li><li data-md="">
     <p>Use in freestanding software poses challenges: Dynamic allocation can only be avoided using linked lists or other non-contiguous data structures.</p>
    </li><li data-md="">
     <p>Is made fragile due to the fact that static initialisers, even when they have side effects, are not always executed if the objects they initialise are never used. In practice this happens when a static library containing translation units with such static initialisers is linked into the program. Any such translation unit containing no externally used definitions is discarded in its entirety. See <a href="https://eel.is/c++draft/basic.start#dynamic-5">[basic.start.dynamic] 6.9.3.3.5</a>.</p>
    </li><li data-md="">
     <p>Conversely static initialisers with side effects cannot be elided when their effects, i.e. the set they build at runtime, are not actually used.</p>
    </li><li data-md="">
     <p>Adds unnecessary overhead in both time and memory at either initialisation due to dynamic allocation and data copying or during data access due to non-contiguous storage.</p>
    </li><li data-md="">
     <p>Impedes optimisation as the data is always stored in writable memory even when its mutation after static initialisation is not necessary.</p>
   </li></ul>
   <h3 class="heading settled" data-level="3.2" id="alternatives_linker"><span class="secno">3.2. </span><span class="content">Linker sections</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#alternatives_linker"></a></h3>
   <p>Outside of standard C++ there exist implementation specific methods to achieve the desired result using linker sections. The variables making up the array elements are all placed into a single linker section using implementation specific pragmas, attributes or linker scripts. The data is accessed at runtime using special symbols placed by the linker at the beginning and end of the section. This is also how static initialisers themselves are generally implemented. The problems with relying on non-portable linker based solutions should be obvious. Apart from the non-portability, this solution also offers poor type safety as variables of differing types may be placed into the section.</p>
   <h2 class="heading settled" data-level="4" id="design"><span class="secno">4. </span><span class="content">Design considerations</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="design_declaration"><span class="secno">4.1. </span><span class="content">Declaring distributed arrays</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_declaration"></a></h3>
   <p>The current revision of this proposal works by providing definitions for unbounded array declarations (<code class="highlight"><c- k="">extern</c-> <c- n="">T</c-> <c- n="">array</c-><c- p="">[];</c-></code>) through one or more distributed array definitions. This approach was chosen primarily to minimise impact on the existing standard. However introducing a new kind of declaration for distributed arrays is a viable alternative. The strawman syntax <code class="highlight"><c- k="">extern</c-> <c- n="">T</c-> <c- n="">array</c-><c- p="">[</c-><c- k="">register</c-><c- p="">]</c-></code> was used in <a data-link-type="biblio" href="https://vasama.github.io/wg21/P2889R0.html#biblio-p2268r0" title="Freestanding Roadmap">[P2268R0]</a> for such declarations.</p>
   <h3 class="heading settled" data-level="4.2" id="design_type"><span class="secno">4.2. </span><span class="content">Distributed array type</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_type"></a></h3>
   <p>A new kind of array declaration would be most powerful if combined with a new kind of array type (strawman syntax <code class="highlight"><c- n="">T</c-><c- p="">[</c-><c- k="">register</c-><c- p="">]</c-></code>, <code class="highlight"><c- n="">T</c-><c- p="">(</c-><c- o="">&amp;</c-><c- p="">)[</c-><c- k="">register</c-><c- p="">]</c-></code>, <code class="highlight"><c- n="">T</c-><c- p="">(</c-><c- o="">*</c-><c- p="">)[</c-><c- k="">register</c-><c- p="">]</c-></code>, etc.). This presents a larger change to the language, but comes with some benefits:</p>
   <ul>
    <li data-md="">
     <p>Better and earlier diagnostics due to more clearly stated programmer intent.</p>
    </li><li data-md="">
     <p>Range-based for loops can be applied to distributed arrays directly.</p>
    </li><li data-md="">
     <p><code class="highlight"><c- n="">std</c-><c- o="">::</c-><c- n="">ranges</c-></code> utilities and algorithms can be applied to distributed arrays directly.</p>
   </li></ul>
   <p>Such new distributed array types would behave similarly to the existing unbounded array types, with the addition of a mechanism for querying the size of a distributed array object (see <a href="https://vasama.github.io/wg21/P2889R0.html#design_size">§ 4.5 Accessing the size of the distributed array</a>).</p>
   <h3 class="heading settled" data-level="4.3" id="design_definition"><span class="secno">4.3. </span><span class="content">Defining elements of distributed arrays</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_definition"></a></h3>
   <p>This proposal uses the strawman syntax <code class="highlight"><c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c-></code> to specify that a variable definition should also act as a partial definition for a previously declared array <code class="highlight"><c- n="">A</c-></code>. All such partial definitions are merged to form a definition for the array <code class="highlight"><c- n="">A</c-></code>. This definition would conflict with any other definition of <code class="highlight"><c- n="">A</c-></code> which is not a distributed array definition.</p>
   <p>Given the declaration <code class="highlight"><c- k="">extern</c-> <c- n="">cv</c-> <c- n="">T</c-> <c- n="">A</c-><c- p="">[];</c-></code>, <code class="highlight"><c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c-></code> may be applied to any namespace scope variable or static member variable declaration <code class="highlight"><c- n="">D</c-></code> with static storage duration, provided that <code class="highlight"><c- n="">D</c-></code> declares a variable of type <code class="highlight"><c- n="">cv</c-> <c- n="">T</c-></code> or an array with element type <code class="highlight"><c- n="">cv</c-> <c- n="">T</c-></code>. If <code class="highlight"><c- n="">A</c-></code> is declared <code class="highlight"><c- k="">constinit</c-></code>, <code class="highlight"><c- n="">D</c-></code> must be declared <code class="highlight"><c- k="">constinit</c-></code>. If <code class="highlight"><c- n="">A</c-></code> is declared <code class="highlight"><c- k="">const</c-></code>, <code class="highlight"><c- n="">D</c-></code> may be declared <code class="highlight"><c- k="">constexpr</c-></code>.</p>
   <p>It is notable that this proposal makes it possible for the first time in standard C++ to access a global object through two different symbols. There are some use cases for this: consider a graph of initialisation callbacks where nodes refer to other nodes representing dependencies. Those use cases can generally be solved in other ways using additional indirection. The more important reason for this choice however, is distributed array element templates. In order to contribute to a distributed array through a template, a name is naturally required for that template. Additionally this mechanism maps very directly to how linkers actually work in practice. Existing implementations do currently assume that two distinct global variables declared <code class="highlight"><c- k="">extern</c-> <c- b="">int</c-> <c- n="">a</c-><c- p="">;</c-></code> and <code class="highlight"><c- k="">extern</c-> <c- b="">int</c-> <c- n="">b</c-><c- p="">[];</c-></code> do not alias one another. This is problematic and may be another reason to introduce new distributed array declarations (see <a href="https://vasama.github.io/wg21/P2889R0.html#design_declaration">§ 4.1 Declaring distributed arrays</a>).</p>
   <p>Examples:</p>
<pre class="highlight"><c- k="">extern</c-> <c- b="">int</c-> <c- k="">const</c-> <c- n="">A</c-><c- p="">[];</c->

<c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c->
<c- k="">static</c-> <c- b="">int</c-> <c- k="">const</c-> <c- n="">E1</c-> <c- o="">=</c-> <c- mi="">42</c-><c- p="">;</c->

<c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c->
<c- k="">constexpr</c-> <c- b="">int</c-> <c- n="">E2</c-><c- p="">[]</c-> <c- o="">=</c-> <c- p="">{</c-> <c- mi="">1</c-><c- p="">,</c-> <c- mi="">2</c-> <c- p="">};</c->

<c- c1="">// The template E3 can be used to introduce multiple elements into A.</c->
<c- k="">template</c-><c- o="">&lt;</c-><c- k="">typename</c-> <c- nc="">T</c-><c- o="">&gt;</c->
<c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c->
<c- k="">constexpr</c-> <c- b="">int</c-> <c- n="">E3</c-> <c- o="">=</c-> <c- n="">T</c-><c- o="">::</c-><c- n="">some_value</c-><c- p="">;</c->
</pre>
   <p>In existing usage this pattern is often known as "registration" with types such as <code class="highlight"><c- n="">thing_registry</c-></code> or <code class="highlight"><c- n="">thing_registrar</c-></code> being commonly used alongside macros such as <code class="highlight"><c- n="">REGISTER_THING</c-></code>. For this reason repurposing the now defunct <code class="highlight"><c- k="">register</c-></code> keyword in place of <code class="highlight"><c- n="">__distributed_array</c-></code> might be a good fit.</p>
   <h3 class="heading settled" data-level="4.4" id="design_empty"><span class="secno">4.4. </span><span class="content">Empty distributed arrays</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_empty"></a></h3>
   <p>More so than in the case of regular arrays, it often makes sense to define and use empty distributed arrays. For example, whether the program should perform any initialisation steps provided through a distributed array may depend on its build-time configuration. If empty distributed arrays were not allowed, it would be difficult to detect and handle the case where no initialisation was needed.</p>
   <p>If distributed arrays were given new declaration syntax (see <a href="https://vasama.github.io/wg21/P2889R0.html#design_declaration">§ 4.1 Declaring distributed arrays</a>), then a declaration such as <code class="highlight"><c- n="">T</c-> <c- n="">array</c-><c- p="">[</c-><c- k="">register</c-><c- p="">];</c-></code> (without <code class="highlight"><c- k="">extern</c-></code>) could act as an empty default definition for the distributed array.</p>
   <h3 class="heading settled" data-level="4.5" id="design_size"><span class="secno">4.5. </span><span class="content">Accessing the size of the distributed array</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_size"></a></h3>
   <p>There are a number of alternative approaches for exposing the size of the distributed array, each with its own pros and cons.</p>
   <h4 class="heading settled" data-level="4.5.1" id="design_size_sizeof"><span class="secno">4.5.1. </span><span class="content"><code class="highlight"><c- k="">sizeof</c-></code> operator</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_size_sizeof"></a></h4>
   <p><a data-link-type="biblio" href="https://vasama.github.io/wg21/P2889R0.html#biblio-p2268r0" title="Freestanding Roadmap">[P2268R0]</a> put forth the option of defining <code class="highlight"><c- k="">sizeof</c-></code> on distributed arrays as a non-constant-expression. This represents a clear departure from the existing behaviour of the <code class="highlight"><c- k="">sizeof</c-></code> operator. Additionally the size in bytes of the array is generally less useful than the number of elements in the array. Therefore this proposal does not recommend the use of the <code class="highlight"><c- k="">sizeof</c-></code> operator.</p>
   <h4 class="heading settled" data-level="4.5.2" id="design_size_operator"><span class="secno">4.5.2. </span><span class="content">Function-like operator</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_size_operator"></a></h4>
   <p>An operator (strawman syntax <code class="highlight"><c- n="">__distributed_array_size</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c-></code>) provides the greatest implementation flexibility and safety, but the least user flexibility. The implementation is the least limited in its choice of implementation strategy, and programs using the operator improperly can be made ill formed.</p>
   <p>Some alternative syntaxes which avoid the introduction of new keywords include <code class="highlight"><c- k="">sizeof</c-> <c- k="">register</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c-></code> and <code class="highlight"><c- k="">operator</c-> <c- k="">register</c-><c- p="">(</c-><c- n="">A</c-><c- p="">)</c-></code>.</p>
   <h4 class="heading settled" data-level="4.5.3" id="design_size_template"><span class="secno">4.5.3. </span><span class="content">Library template</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_size_template"></a></h4>
   <p>A function template with a const array reference non-type template parameter can likely provide implementation flexibility and safety equal or near equal to that of an operator, but with potentially slightly greater user flexibility. The two may end up being equal in both flexibility and safety however, in which case the library template might be the preferred option.</p>
   <p>Possible implementation:</p>
<pre class="highlight"><c- k="">template</c-><c- o="">&lt;</c-><c- k="">auto</c-> <c- k="">const</c-><c- o="">&amp;</c-> <c- n="">Array</c-><c- o="">&gt;</c->
<c- b="">size_t</c-> <c- n="">distributed_array_size</c-><c- p="">()</c-> <c- k="">noexcept</c-> <c- p="">{</c->
  <c- c1="">// The size might be calculated by subtraction of</c->
  <c- c1="">// pointers to begin and end symbols generated by the linker.</c->
  <c- k="">return</c->
    <c- n="">__builtin_distributed_array_end</c-><c- p="">(</c-><c- n="">Array</c-><c- p="">)</c-> <c- o="">-</c->
    <c- n="">__builtin_distributed_array_begin</c-><c- p="">(</c-><c- n="">Array</c-><c- p="">);</c->
<c- p="">}</c->
</pre>
   <h4 class="heading settled" data-level="4.5.4" id="design_size_function"><span class="secno">4.5.4. </span><span class="content">Library function</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_size_function"></a></h4>
   <p>A function template with a pointer parameter can provide the greatest user flexibility at the cost of lesser implementation flexibility and safety. Calling this function with a pointer pointing to something other than the first element of a distributed array would have to have undefined behaviour.</p>
   <p>Possible implementation:</p>
<pre class="highlight"><c- b="">size_t</c-> <c- nf="">distributed_array_size</c-><c- p="">(</c-><c- k="">auto</c-> <c- k="">const</c-><c- o="">*</c-> <c- k="">const</c-> <c- n="">array</c-><c- p="">)</c-> <c- k="">noexcept</c-> <c- p="">{</c->
  <c- c1="">// The size might be stored before the first element in memory.</c->
  <c- c1="">// Note that this otherwise invalid cast exists only</c->
  <c- c1="">// to illustrate a potential implementation strategy.</c->
  <c- k="">return</c-> <c- k="">reinterpret_cast</c-><c- o="">&lt;</c-><c- b="">size_t</c-> <c- k="">const</c-><c- o="">*&gt;</c-><c- p="">(</c-><c- n="">array</c-><c- p="">)[</c-><c- mi="">-1</c-><c- p="">];</c->
<c- p="">}</c->
</pre>
   <p>If distributed arrays were given distinct types (see <a href="https://vasama.github.io/wg21/P2889R0.html#design_type">§ 4.2 Distributed array type</a>) the signature of this function could be changed to <code class="highlight"><c- b="">size_t</c-> <c- n="">distributed_array_size</c-><c- p="">(</c-><c- k="">auto</c-> <c- k="">const</c-><c- p="">(</c-><c- o="">&amp;</c-><c- n="">array</c-><c- p="">)[</c-><c- k="">register</c-><c- p="">])</c-></code> and incorrect usage could be made ill-formed instead of producing undefined behaviour at runtime.</p>
   <h3 class="heading settled" data-level="4.6" id="design_ordering"><span class="secno">4.6. </span><span class="content">Ordering of distributed array elements</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_ordering"></a></h3>
   <ul>
    <li data-md="">
     <p>Non-inline elements of a distributed array defined within a single translation unit are ordered according to the order of their definitions.</p>
    </li><li data-md="">
     <p>Inline elements of a distributed array are ordered relative to other elements according to the order of their definitions, if that order is the same in all translation units. Otherwise the order is unspecified.</p>
    </li><li data-md="">
     <p>The relative ordering of sets of elements from different translation units is unspecified, but is the same for all distributed arrays. In particular this facilitates the definition of parallel distributed arrays with the same order of elements.</p>
   </li></ul>
   <h3 class="heading settled" data-level="4.7" id="design_trivial"><span class="secno">4.7. </span><span class="content">Non-trivial element types</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_trivial"></a></h3>
   <p>Accessing a distributed array forces initialisation of variables of static storage duration from all translation units contributing to the array. Order of initialisation of distributed array elements may be left entirely unspecified, except where relative ordering of the elements is specified. However specifying the order of initialisation to match the ordering of the elements should not impose any additional difficulty in implementation of the feature either, as the relative ordering of elements from different translation units is itself unspecified. The initialisation of distributed array elements may be interleaved with initialisation of other non-trivial objects with static storage duration.</p>
   <h3 class="heading settled" data-level="4.8" id="design_thread"><span class="secno">4.8. </span><span class="content">Thread storage duration</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#design_thread"></a></h3>
   <p>Distributed arrays with thread storage duration may have some interesting use cases, but they are not currently being proposed. Any distributed array definition declared <code class="highlight"><c- k="">thread_local</c-></code> is therefore ill-formed.</p>
   <h3 class="heading settled" data-level="4.9" id="implementation_dynamic"><span class="secno">4.9. </span><span class="content">Dynamic linking</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#implementation_dynamic"></a></h3>
   <p>Any attempt to dynamically link together multiple distributed array definitions into a single array is obviously problematic in the presence of dynamic loading and unloading of libraries. On Windows where each DLL (Dynamic Link Library) is practically its own program, this is not a problem.</p>
   <p>On Linux and similar systems the best solution might be for the dynamic linker to consider distributed array definitions for the same array declaration from two shared libraries to be in conflict with one another. In other words, the set of distributed array definitions present when statically linking the shared library would produce a single regular array definition. The dynamic linker would never have to deal with any distributed array definitions. This matches the existing behaviour of linkers for those arrays already being used to implement other features (see <a href="https://vasama.github.io/wg21/P2889R0.html#implementation">§ 5 Implementation experience</a>).</p>
   <p>With dynamic linking being out of scope for the C++ standard, this issue should not have a major effect on the proposal.</p>
   <h2 class="heading settled" data-level="5" id="implementation"><span class="secno">5. </span><span class="content">Implementation experience</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#implementation"></a></h2>
   <p>Existing linkers already support merging symbols into arrays from multiple translation units in order to implement static initialisers, thread local storage and exception handling tables. More work is needed to provide a prototype implementation or testimonials from implementers regarding the feasibility of implementing of this feature however.</p>
   <h2 class="heading settled" data-level="6" id="examples"><span class="secno">6. </span><span class="content">Examples</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#examples"></a></h2>
   <p>These examples use the library function syntax to access the size of the distributed array. See <a href="https://vasama.github.io/wg21/P2889R0.html#design_size_function">§ 4.5.4 Library function</a>.</p>
   <h3 class="heading settled" data-level="6.1" id="examples_test"><span class="secno">6.1. </span><span class="content">Unit test framework</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#examples_test"></a></h3>
   <p>The example code is simplified for readability. A real unit test framework would likely use macros to hide any boilerplate code.</p>
   <h4 class="heading settled" data-level="6.1.1" id="examples_test_header"><span class="secno">6.1.1. </span><span class="content">Framework header</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#examples_test_header"></a></h4>
<pre class="highlight"><c- k="">using</c-> <c- n="">test_case_callback</c-> <c- o="">=</c-> <c- b="">void</c-><c- p="">();</c->

<c- c1="">// Array of test case callbacks.</c->
<c- k="">extern</c-> <c- n="">test_case_callback</c-><c- o="">*</c-> <c- k="">const</c-> <c- n="">test_cases</c-><c- p="">[];</c->
</pre>
   <h4 class="heading settled" data-level="6.1.2" id="examples_test_user"><span class="secno">6.1.2. </span><span class="content">User test code</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#examples_test_user"></a></h4>
<pre class="highlight"><c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">test_cases</c-><c- p="">)</c->
<c- n="">test_case_callback</c-><c- o="">*</c-> <c- k="">const</c-> <c- n="">my_test_case</c-> <c- o="">=</c-> <c- n="">my_test_case_function_1</c-><c- p="">;</c->

<c- n="">__distributed_array</c-><c- p="">(</c-><c- n="">test_cases</c-><c- p="">)</c->
<c- n="">test_case_callback</c-><c- o="">*</c-> <c- k="">const</c-> <c- n="">my_test_case_array</c-><c- p="">[]</c-> <c- o="">=</c-> <c- p="">{</c->
  <c- n="">my_test_case_function_2</c-><c- p="">,</c->
  <c- n="">my_test_case_function_3</c-><c- p="">,</c->
<c- p="">};</c->
</pre>
   <h4 class="heading settled" data-level="6.1.3" id="examples_test_source"><span class="secno">6.1.3. </span><span class="content">Framework source</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#examples_test_source"></a></h4>
<pre class="highlight"><c- b="">void</c-> <c- nf="">run_unit_tests</c-><c- p="">()</c-> <c- p="">{</c->
  <c- n="">test_case_callback</c-><c- o="">*</c-> <c- k="">const</c-><c- o="">*</c-> <c- k="">const</c-> <c- n="">test_data</c-> <c- o="">=</c-> <c- o="">::</c-><c- n="">test_cases</c-><c- p="">;</c->
  <c- b="">size_t</c-> <c- k="">const</c-> <c- n="">test_count</c-> <c- o="">=</c-> <c- n="">std</c-><c- o="">::</c-><c- n="">distributed_array_size</c-><c- p="">(</c-><c- n="">test_data</c-><c- p="">);</c->
  <c- n="">std</c-><c- o="">::</c-><c- n="">span</c-> <c- k="">const</c-> <c- n="">tests</c-> <c- o="">=</c-> <c- n="">std</c-><c- o="">::</c-><c- n="">span</c-><c- p="">(</c-><c- n="">test_data</c-><c- p="">,</c-> <c- n="">test_count</c-><c- p="">);</c->

  <c- k="">for</c-> <c- p="">(</c-><c- n="">test_case_callback</c-><c- o="">*</c-> <c- k="">const</c-> <c- n="">test</c-> <c- o="">:</c-> <c- n="">tests</c-><c- p="">)</c-> <c- p="">{</c->
    <c- n="">test</c-><c- p="">();</c->
  <c- p="">}</c->
<c- p="">}</c->
</pre>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="https://vasama.github.io/wg21/P2889R0.html#informative"></a></h3>
  <dl>
   <dt id="biblio-p2268r0">[P2268R0]
   </dt><dd>Ben Craig. <a href="https://wg21.link/p2268r0"><cite>Freestanding Roadmap</cite></a>. 10 December 2020. URL: <a href="https://wg21.link/p2268r0">https://wg21.link/p2268r0</a>
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