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  <title>P2580R0: Tuple protocol for C-style arrays T[N]</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 {
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	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;			  /* limit line length to 50em for readability   */
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		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
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/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

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

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

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

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

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	.head a:hover > img[src*="logos/W3C"],
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	.head a:active > img[src*="logos/W3C"] {
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		border-color: var(--logo-active-bg);
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	/* see also additional rules in Link Styling section */

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

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

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

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	@media not print {
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			height: 1.33em;
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			box-shadow: 0 0 2px;
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		#toc-nav > #toc-jump {
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			margin-bottom: -1.9em;
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		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
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		#toc-nav:not(:hover) > a:not(:focus) > span + span {
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/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
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			left: 0;
			width: 23.5em;
			max-width: 80%;
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			color: var(--tocsidebar-text);
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			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
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			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
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			padding-left: calc(1em + 26px);
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			color: var(--tocsidebar-text);
			background: inherit;
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			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
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			font-variant: small-caps;
			font-variant: all-small-caps;
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			color: gray;
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		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
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			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
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		body:not(.toc-inline) {
			margin: 0 4em;
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/******************************************************************************/
/*                                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 */
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	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
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	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
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	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 */
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/** Section divider ***********************************************************/

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

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

	p {
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	}

	dd > p:first-child,
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	ul, ol {
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		padding-left: 2em;
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	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 ***********************************************/

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		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;
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	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);
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	/* 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;
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	@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 fb1e763a4, updated Tue Mar 1 13:13:50 2022 -0800" name="generator">
  <link href="https://wg21.link/P2580R0" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="d78676005e6009a88c7364e9f76dacddab02a965" name="document-revision">
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
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  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2580R0<br>Tuple protocol for C-style arrays <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code></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="2022-04-26">2022-04-26</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P2580R0">https://wg21.link/P2580R0</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:p.digiglio91@gmail.com">Paolo Di Giglio</a>
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Audience:
     <dd>Library Evolution Working Group
    </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>I propose C-style array types of known bound, <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code>, should be tuple-like. My aim twofold: improve their usability in contexts where they’re preferable over <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-></code> and provide compile-time index checks through <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code>. The tuple-like protocol implementation I propose for <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code> is designed after <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-></code>’s.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#motivation.automatic_size_deduction"><span class="secno">2.1</span> <span class="content">Automatic size deduction</span></a>
      <li><a href="#motivation.interacting_with_c_apis"><span class="secno">2.2</span> <span class="content">Interacting with C APIs</span></a>
      <li><a href="#motivation.compile-time_bound_check"><span class="secno">2.3</span> <span class="content">Compile-time bound check</span></a>
     </ol>
    <li>
     <a href="#impact_on_the_standard"><span class="secno">3</span> <span class="content">Impact on the standard</span></a>
     <ol class="toc">
      <li><a href="#impact_on_standard.interaction_with_other_papers"><span class="secno">3.1</span> <span class="content">Interaction with other papers</span></a>
     </ol>
    <li>
     <a href="#design_decisions"><span class="secno">4</span> <span class="content">Design decisions</span></a>
     <ol class="toc">
      <li>
       <a href="#design_decisions.proposed_implementation"><span class="secno">4.1</span> <span class="content">Proposed implementation</span></a>
       <ol class="toc">
        <li>
         <a href="#design_decisions.proposed_implementation.tuple_size"><span class="secno">4.1.1</span> <span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code></span></a>
         <ol class="toc">
          <li><a href="#design_decisions.proposed_implementation.tuple_size.const_qualified_specialization"><span class="secno">4.1.1.1</span> <span class="content"><code class="highlight"><c- k>const</c-></code>-qualified specialization</span></a>
          <li><a href="#design_decisions.proposed_implementation.tuple_size.volatile_qualified_specialization"><span class="secno">4.1.1.2</span> <span class="content"><code class="highlight"><c- k>volatile</c-></code>-qualified specializations</span></a>
         </ol>
        <li>
         <a href="#design_decisions.proposed_implementation.tuple_element"><span class="secno">4.1.2</span> <span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code></span></a>
         <ol class="toc">
          <li><a href="#cv-qualified-specializations"><span class="secno">4.1.2.1</span> <span class="content">cv-qualified specializations</span></a>
         </ol>
        <li><a href="#design_decisions.proposed_implementation.get"><span class="secno">4.1.3</span> <span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code></span></a>
       </ol>
      <li><a href="#design_decisions.alternative_implementation"><span class="secno">4.2</span> <span class="content">Alternative implementation</span></a>
     </ol>
    <li><a href="#technical_specifications"><span class="secno">5</span> <span class="content">Technical specifications</span></a>
    <li><a href="#acknowledgements"><span class="secno">6</span> <span class="content">Acknowledgements</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>
   <h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
   <p>The tuple protocol has been introduced in C++11.
If <code class="highlight"><c- n>T</c-></code> is a tuple-like type:</p>
   <ul>
    <li data-md>
     <p>It can be destructured into <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> elements;</p>
    <li data-md>
     <p>Its <code class="highlight"><c- n>I</c-></code>-th element has type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-></code> and can be extracted through the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> function template.</p>
   </ul>
   <p>Since C++17, the tuple protocol interacts with the core language, which allows
structured bindings to expressions of tuple-like types.</p>
   <p>The standard already mandates the following types to be tuple-like:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code> (since C++20),</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple</c-></code> (since C++11),</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pair</c-></code> (since C++11),</p>
    <li data-md>
     <p>and, most relevant to this paper, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-></code> (since C++11).</p>
   </ul>
   <p>In this paper, I propose the standard should make C-style arrays of known
bound, <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code>, tuple-like too.
The implementation of the tuple-like protocol (i.e. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code>) I propose is designed after the existing
one for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-></code>.</p>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>As far as their tuple-like properties are concerned, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-></code> and <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code> are equivalent.
Both have a fixed number of elements, <code class="highlight"><c- n>N</c-></code>, which is known at compile-time; each
element being of type <code class="highlight"><c- n>T</c-></code> and accessible by a compile-time index.</p>
   <p>Implementing the tuple-like protocol for C-style arrays would make them
eligible to be passed as parameters to:</p>
   <ul>
    <li><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>apply</c-></code> (since C++17),
    <li><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>make_from_tuple</c-></code> (since C++17)
    <li>
      and, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_cat</c-></code> (since C++11) 
     <div class="note" role="note"> Prior to <a data-link-type="biblio" href="#biblio-p2165r3">[P2165R3]</a>, the choice whether to support tuple-like
            types besides <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple</c-></code> in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_cat</c-></code> was up to the
            implementers. </div>
   </ul>
   <p>In sections <a href="#motivation.automatic_size_deduction">§ 2.1 Automatic size deduction</a> and <a href="#motivation.interacting_with_c_apis">§ 2.2 Interacting with C APIs</a> I outline some use cases where <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code> may be preferable over <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-></code>.
In such cases, being able to call the above functions without the need for a
temporary <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-></code> would be beneficial.</p>
   <h3 class="heading settled" data-level="2.1" id="motivation.automatic_size_deduction"><span class="secno">2.1. </span><span class="content">Automatic size deduction</span><a class="self-link" href="#motivation.automatic_size_deduction"></a></h3>
   <p>Unlike <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-></code>, compilers are able to deduce the size (only) of a C-style
array from the number of elements in its initializer list:</p>
<pre class="language-cpp highlight"><c- b>int</c-> <c- n>c_arr</c-><c- p>[]</c-> <c- o>=</c-> <c- p>{</c-> <c- mi>0</c-><c- p>,</c-> <c- mi>1</c-><c- p>,</c-> <c- mi>2</c-> <c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- k>sizeof</c-><c- p>(</c-><c- n>c_arr</c-><c- p>)</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>c_arr</c-><c- p>[</c-><c- mi>0</c-><c- p>])</c-> <c- o>==</c-> <c- mi>3</c-><c- p>,</c-> <c- s>""</c-><c- p>);</c->
</pre>
   <p>This limitation of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-></code> was noted by Alisdair Meredith in <a data-link-type="biblio" href="#biblio-n1479">[N1479]</a> itself and lead Zhihao Yuan to float the idea of extending the implementation
of the tuple protocol to C-style arrays in <a data-link-type="biblio" href="#biblio-array-as-a-tuple">[ARRAY-AS-A-TUPLE]</a>.</p>
   <p>CTAD (since C++17) for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-></code> mitigates this problem but doesn’t allow a
user to specify the element type and deduce the size only.
Function template <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>to_array</c-></code> (since C++20) does but poses constraints on
the element type (namely, it has be copy- or move-constructible and non-array).</p>
   <h3 class="heading settled" data-level="2.2" id="motivation.interacting_with_c_apis"><span class="secno">2.2. </span><span class="content">Interacting with C APIs</span><a class="self-link" href="#motivation.interacting_with_c_apis"></a></h3>
   <p>C (or C-like) API may force users to deal with C-style arrays:</p>
<pre class="language-cpp highlight"><c- c1>// File: geometry_c_api.h</c->
<c- cp>#define GEOMETRY_STATUS_OK 0</c->

<c- k>struct</c-> <c- nc>ReferenceFrame</c-><c- p>;</c->
<c- b>int</c-> <c- nf>get_origin</c-><c- p>(</c-><c- k>struct</c-> <c- nc>ReferenceFrame</c-><c- o>*</c-> <c- n>frame</c-><c- p>,</c-> <c- b>double</c-> <c- p>(</c-><c- o>*</c-><c- n>pt</c-><c- p>)[</c-><c- mi>3</c-><c- p>]);</c->
</pre>
   <p>If this paper gets accepted, client code might look like this:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>Point</c->
<c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- k>explicit</c-> <c- n>Point</c-><c- p>(</c-><c- b>double</c-> <c- n>x</c-><c- p>,</c-> <c- b>double</c-> <c- n>y</c-><c- p>,</c-> <c- b>double</c-> <c- n>z</c-><c- p>);</c->
    <c- c1>// ...</c->
<c- p>};</c->

<c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>Point</c-><c- o>></c-> <c- n>get_origin</c-><c- p>(</c-><c- n>ReferenceFrame</c-><c- o>&amp;</c-> <c- n>frame</c-><c- p>)</c->
<c- p>{</c->
    <c- b>double</c-> <c- n>pt</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- p>{</c-> <c- p>};</c->
    <c- k>if</c-> <c- p>(</c-><c- n>get_origin</c-><c- p>(</c-><c- o>&amp;</c-><c- n>frame</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>pt</c-><c- p>)</c-> <c- o>!=</c-> <c- n>GEOMETRY_STATUS_OK</c-><c- p>)</c->
        <c- k>return</c-> <c- p>{</c-> <c- p>};</c->

    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>make_from_tuple</c-><c- o>&lt;</c-><c- n>Point</c-><c- o>></c-><c- p>(</c-><c- n>pt</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <h3 class="heading settled" data-level="2.3" id="motivation.compile-time_bound_check"><span class="secno">2.3. </span><span class="content">Compile-time bound check</span><a class="self-link" href="#motivation.compile-time_bound_check"></a></h3>
   <p>A useful side benefit of the tuple protocol is the bound check performed by
function template <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code>:</p>
<pre class="highlight"><c- b>int</c-> <c- n>c_arr</c-><c- p>[</c-><c- mi>42</c-><c- p>]{};</c->

<c- c1>// This would not not compile because index 42 is out of bounds</c->
<c- c1>//std::get&lt;42>(c_arr) = 42;</c->

<c- c1>// Not OK: this is UB and compiles  </c->
<c- n>c_arr</c-><c- p>[</c-><c- mi>42</c-><c- p>]</c-> <c- o>=</c-> <c- mi>42</c-><c- p>;</c->
</pre>
   <p>Making <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code> tuple-like would help users prevent the above class of bugs
without any need for static analysis tools or sanitizers.</p>
   <h2 class="heading settled" data-level="3" id="impact_on_the_standard"><span class="secno">3. </span><span class="content">Impact on the standard</span><a class="self-link" href="#impact_on_the_standard"></a></h2>
   <p>This proposal is a pure library extension.
It proposes changes to an existing header, <code class="highlight"><c- o>&lt;</c-><c- n>tuple</c-><c- o>></c-></code>, but it does not require
changes to any standard classes or functions.</p>
   <p>This proposal does not require changes in the core language.
It does not produce changes in the core language either.
Even though the tuple protocol interferes with the core language, which
provides structured-binding support for tuple-like types, the standard already
defines special rules for structured bindings to C-style arrays.</p>
   <p>This proposal does not depend on any other library extension.
In section <a href="#design_decisions.proposed_implementation">§ 4.1 Proposed implementation</a>, I propose an
implementation in standard C++11.</p>
   <h3 class="heading settled" data-level="3.1" id="impact_on_standard.interaction_with_other_papers"><span class="secno">3.1. </span><span class="content">Interaction with other papers</span><a class="self-link" href="#impact_on_standard.interaction_with_other_papers"></a></h3>
   <p>With this proposal, <code class="highlight"><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code> would satisfy exposition-only concept <i><code class="highlight"><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></code></i> introduced by Corentin Jabot with <a data-link-type="biblio" href="#biblio-p2165r3">[P2165R3]</a>.
So, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple</c-></code>s and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pair</c-></code>s would be constructible from and comparable
with C-style arrays:</p>
<pre class="language-cpp highlight"><c- b>int</c-> <c- n>c_arr</c-><c- p>[]</c-> <c- o>=</c-> <c- p>{</c-> <c- mi>0</c-><c- p>,</c-> <c- mi>1</c-> <c- p>};</c->
<c- c1>//std::tuple&lt;int, int, int> t = c_arr; // Error: different tuple size</c->
<c- n>std</c-><c- o>::</c-><c- n>pair</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-> <c- n>p</c-> <c- o>=</c-> <c- n>c_arr</c-><c- p>;</c->
<c- n>p</c-> <c- o>==</c-> <c- n>c_arr</c-><c- p>;</c-> <c- c1>// Ok: evaluates to true</c->
<c- n>p</c-> <c- o>&lt;</c-> <c- n>c_arr</c-><c- p>;</c->  <c- c1>// Ok: evaluates to false</c->
</pre>
<pre class="language-cpp highlight"><c- b>int</c-> <c- n>c_arr</c-><c- p>[]</c-> <c- o>=</c-> <c- p>{</c-> <c- mi>0</c-><c- p>,</c-> <c- mi>1</c-><c- p>,</c-> <c- mi>2</c-> <c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-> <c- n>t</c-> <c- o>=</c-> <c- n>c_arr</c-><c- p>;</c->
<c- c1>//std::pair&lt;int, int> p = c_arr; // Error: different tuple size</c->
<c- n>t</c-> <c- o>==</c-> <c- n>c_arr</c-><c- p>;</c-> <c- c1>// Ok: evaluates to true</c->
<c- n>t</c-> <c- o>&lt;</c-> <c- n>c_arr</c-><c- p>;</c->  <c- c1>// Ok: evaluates to false</c->
</pre>
   <h2 class="heading settled" data-level="4" id="design_decisions"><span class="secno">4. </span><span class="content">Design decisions</span><a class="self-link" href="#design_decisions"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="design_decisions.proposed_implementation"><span class="secno">4.1. </span><span class="content">Proposed implementation</span><a class="self-link" href="#design_decisions.proposed_implementation"></a></h3>
   <p>In the following subsections, I outline my proposed implementation of the tuple
protocol for C-style arrays of known bound in standard C++11.
My implementation is designed after <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-></code>'s.</p>
   <h4 class="heading settled" data-level="4.1.1" id="design_decisions.proposed_implementation.tuple_size"><span class="secno">4.1.1. </span><span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code></span><a class="self-link" href="#design_decisions.proposed_implementation.tuple_size"></a></h4>
   <p>For the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> class template, I propose the following specializations:</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- c1>// (ts)</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->

<c- c1>// (ts.c)</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-> <c- k>const</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->

<c- p>}</c->
</pre>
   <h5 class="heading settled" data-level="4.1.1.1" id="design_decisions.proposed_implementation.tuple_size.const_qualified_specialization"><span class="secno">4.1.1.1. </span><span class="content"><code class="highlight"><c- k>const</c-></code>-qualified specialization</span><a class="self-link" href="#design_decisions.proposed_implementation.tuple_size.const_qualified_specialization"></a></h5>
   <p>Specialization <code class="highlight"><c- n>ts</c-><c- p>.</c-><c- n>c</c-></code> is required because:</p>
   <ul>
    <li data-md>
     <p>The standard already defines a <code class="highlight"><c- k>const</c-></code>-qualified specialization for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> (and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code>);</p>
    <li data-md>
     <p>Applying cv-qualifiers to an array type applies the qualifiers to the element type and any array type whose elements are of cv-qualified type is considered to have the same cv-qualification <a data-link-type="biblio" href="#biblio-cpp-ref-array">[CPP-REF-ARRAY]</a>.</p>
   </ul>
   <p>So, by not defining <code class="highlight"><c- n>ts</c-><c- p>.</c-><c- n>c</c-></code>, the following code</p>
<pre class="highlight"><c- k>using</c-> <c- n>const_array_t</c-> <c- o>=</c-> <c- b>int</c-> <c- k>const</c-> <c- p>[</c-><c- mi>42</c-><c- p>];</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-><c- o>&lt;</c-><c- n>array_t</c-><c- o>>::</c-><c- n>value</c-> <c- o>==</c-> <c- mi>42</c-><c- p>,</c-> <c- s>"Size OK"</c-><c- p>);</c->
</pre>
   <p>would fail to compile because the template instanciation for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> is ambiguous.
In fact both the following specializations would be viable candidates:</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- c1>// Already in the standard</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>T</c-><c- o>></c->
    <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->
<c- c1>// With T = int[42]</c->

<c- c1>// Proposed in this paper</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->
<c- c1>// With T = int const, N = 42</c->

<c- p>}</c->
</pre>
   <h5 class="heading settled" data-level="4.1.1.2" id="design_decisions.proposed_implementation.tuple_size.volatile_qualified_specialization"><span class="secno">4.1.1.2. </span><span class="content"><code class="highlight"><c- k>volatile</c-></code>-qualified specializations</span><a class="self-link" href="#design_decisions.proposed_implementation.tuple_size.volatile_qualified_specialization"></a></h5>
   <p>The existing specializations of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> (and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code>)
for <code class="highlight"><c- k>volatile</c-></code>-qualified types were deprecated in C++20 and will be removed in
C++23, as per <a data-link-type="biblio" href="#biblio-p1831r1">[P1831R1]</a>.
So, there is no need to provide specializations of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> (nor <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code>) for:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>T</c-> <c- k>volatile</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>T</c-> <c- k>volatile</c-> <c- k>const</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-></code>.</p>
   </ul>
   <h4 class="heading settled" data-level="4.1.2" id="design_decisions.proposed_implementation.tuple_element"><span class="secno">4.1.2. </span><span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code></span><a class="self-link" href="#design_decisions.proposed_implementation.tuple_element"></a></h4>
   <p>For the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code> class template, I propose the following specializations:</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- c1>// (te)</c->
<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>Idx</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>Idx</c-><c- p>,</c-> <c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c->
<c- p>{</c->
    <c- k>static_assert</c-><c- p>(</c-><c- n>Idx</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>,</c-> <c- s>"Index out of bounds"</c-><c- p>);</c->
    <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->
<c- p>};</c->

<c- c1>// (te.c)</c->
<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>Idx</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>Idx</c-><c- p>,</c-> <c- n>T</c-> <c- k>const</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c->
<c- p>{</c->
    <c- k>static_assert</c-><c- p>(</c-><c- n>Idx</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>,</c-> <c- s>"Index out of bounds"</c-><c- p>);</c->
    <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>T</c-> <c- k>const</c-><c- p>;</c->
<c- p>};</c->

<c- p>}</c->
</pre>
   <h5 class="heading settled" data-level="4.1.2.1" id="cv-qualified-specializations"><span class="secno">4.1.2.1. </span><span class="content">cv-qualified specializations</span><a class="self-link" href="#cv-qualified-specializations"></a></h5>
   <p>The reasoning behind the introduction of specialization <code class="highlight"><c- n>ts</c-><c- p>.</c-><c- n>c</c-></code> in <a href="#design_decisions.proposed_implementation.tuple_size">§ 4.1.1 std::tuple_size</a> also motivates the
introduction of specialization <code class="highlight"><c- n>te</c-><c- p>.</c-><c- n>c</c-></code> here.</p>
   <p>As for the <code class="highlight"><c- k>volatile</c-></code>-qualified specializations of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code>, I
explain why there’s no need for those in section <a href="#design_decisions.proposed_implementation.tuple_size.volatile_qualified_specialization">§ 4.1.1.2 volatile-qualified specializations</a>.
Please note however that, by my proposal, the following code would compile:</p>
<pre class="highlight"><c- k>using</c-> <c- n>v_array_t</c-> <c- o>=</c-> <c- b>int</c-> <c- k>volatile</c-><c- p>[</c-><c- mi>42</c-><c- p>];</c->
<c- k>using</c-> <c- n>cv_array_t</c-> <c- o>=</c-> <c- b>int</c-> <c- k>volatile</c-> <c- k>const</c-><c- p>[</c-><c- mi>42</c-><c- p>];</c->

<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- mi>0</c-><c- p>,</c-> <c- n>v_array_t</c-><c- o>></c-><c- p>,</c-> <c- b>int</c-> <c- k>volatile</c-><c- o>></c-><c- p>,</c-> <c- s>""</c-><c- p>);</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>v_array_t</c-><c- o>></c-> <c- o>==</c-> <c- mi>42</c-><c- p>,</c-> <c- s>""</c-><c- p>);</c->

<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- mi>0</c-><c- p>,</c-> <c- n>cv_array_t</c-><c- o>></c-><c- p>,</c-> <c- b>int</c-> <c- k>volatile</c-> <c- k>const</c-><c- o>></c-><c- p>,</c-> <c- s>""</c-><c- p>);</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>cv_array_t</c-><c- o>></c-> <c- o>==</c-> <c- mi>42</c-><c- p>,</c-> <c- s>""</c-><c- p>);</c->
</pre>
   <p>I believe this to be correct.
While none of the above <code class="highlight"><c- k>static_assert</c-></code>s would compile after <a data-link-type="biblio" href="#biblio-p1831r1">[P1831R1]</a> with:</p>
<pre class="highlight"><c- k>using</c-> <c- n>v_array_t</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- mi>42</c-><c- o>></c-> <c- k>volatile</c-><c- p>;</c->
<c- k>using</c-> <c- n>cv_array_t</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- mi>42</c-><c- o>></c-> <c- k>volatile</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <p>They would both compile with:</p>
<pre class="highlight"><c- k>using</c-> <c- n>v_array_t</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- b>int</c-> <c- k>volatile</c-><c- p>,</c-> <c- mi>42</c-><c- o>></c-><c- p>;</c->
<c- k>using</c-> <c- n>cv_array_t</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- b>int</c-> <c- k>volatile</c-> <c- k>const</c-><c- p>,</c-> <c- mi>42</c-><c- o>></c-><c- p>;</c->
</pre>
   <h4 class="heading settled" data-level="4.1.3" id="design_decisions.proposed_implementation.get"><span class="secno">4.1.3. </span><span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code></span><a class="self-link" href="#design_decisions.proposed_implementation.get"></a></h4>
   <p>For the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> function templates, I propose:</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>Idx</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-> <c- p>{</c->
    <c- k>static_assert</c-><c- p>(</c-><c- n>Idx</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>,</c-> <c- s>"Index out of bounds"</c-><c- p>);</c->
    <c- k>return</c-> <c- n>arr</c-><c- p>[</c-><c- n>Idx</c-><c- p>];</c->
<c- p>}</c->

<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>Idx</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-> <c- p>{</c->
    <c- k>static_assert</c-><c- p>(</c-><c- n>Idx</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>,</c-> <c- s>"Index out of bounds"</c-><c- p>);</c->
    <c- k>return</c-> <c- n>move</c-><c- p>(</c-><c- n>arr</c-><c- p>[</c-><c- n>Idx</c-><c- p>]);</c->
<c- p>}</c->

<c- p>}</c->
</pre>
   <h3 class="heading settled" data-level="4.2" id="design_decisions.alternative_implementation"><span class="secno">4.2. </span><span class="content">Alternative implementation</span><a class="self-link" href="#design_decisions.alternative_implementation"></a></h3>
   <p>Another possible implementation for the C-style array specializations of class
templates <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code> in C++20 is the following
(courtesy of Arthur O’Dwyer):</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>requires</c-><c- p>(</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>remove_const_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>)</c->
<c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-> <c- p>{};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>Idx</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>requires</c-><c- p>(</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>remove_const_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>)</c->
<c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>Idx</c-><c- p>,</c-> <c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- p>{</c->
    <c- k>static_assert</c-><c- p>(</c-><c- n>Idx</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>,</c-> <c- s>"Index out of bounds"</c-><c- p>);</c->
    <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->
<c- p>};</c->

<c- p>}</c->
</pre>
   <p>The <code class="highlight"><c- k>requires</c-></code> clause SFINAEs out the specializations of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code> for <code class="highlight"><c- k>const</c-></code>-qualified array types and prevents the
ambiguous-template-instantiation compilation error described in section <a href="#design_decisions.proposed_implementation.tuple_size">§ 4.1.1 std::tuple_size</a>.</p>
   <p>As for the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> function templates, implementing them by means of the <code class="highlight"><c- k>requires</c-></code> clause is not feasible.
In fact, the following:</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>Idx</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>requires</c-><c- p>(</c-><c- n>Idx</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>)</c->
<c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>arr</c-><c- p>[</c-><c- n>Idx</c-><c- p>];</c->
<c- p>}</c->

<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>Idx</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>requires</c-><c- p>(</c-><c- n>Idx</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>)</c->
<c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>move</c-><c- p>(</c-><c- n>arr</c-><c- p>[</c-><c- n>Idx</c-><c- p>]);</c->
<c- p>}</c->

<c- p>}</c->
</pre>
   <p>may lead to an inconsistent behavior with the existing implementation for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-></code> in unevaluated contexts:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- mi>42</c-><c- o>></c-> <c- n>cpp_arr</c-><c- p>{};</c->
<c- k>using</c-> <c- n>cpp_elem_ptr_t</c-> <c- o>=</c-> <c- k>decltype</c-><c- p>(</c-><c- o>&amp;</c-><c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>42</c-><c- o>></c-><c- p>(</c-><c- n>cpp_arr</c-><c- p>));</c->
<c- c1>// cpp_elem_ptr_t is int*</c->

<c- c1>//int c_arr[42]{};</c->
<c- c1>//using c_elem_ptr_t = decltype(&amp;std::get&lt;42>(c_arr));</c->
<c- c1>// error: no matching function for call to 'get&lt;42>(int [42])'</c->
</pre>
   <h2 class="heading settled" data-level="5" id="technical_specifications"><span class="secno">5. </span><span class="content">Technical specifications</span><a class="self-link" href="#technical_specifications"></a></h2>
   <p>In this section, I present the changes I propose to the standard.
The wording is based on <a data-link-type="biblio" href="#biblio-n4910">[N4910]</a>.</p>
   <p><strong>Modify section</strong> "Header <code class="highlight"><c- o>&lt;</c-><c- n>tuple</c-><c- o>></c-></code> synopsis <code class="highlight"><c- p>[</c-><c- n>tuple</c-><c- p>.</c-><c- n>syn</c-><c- p>]</c-></code>":</p>
<pre class="highlight"><c- c1>// 22.4.6, tuple helper classes</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_size</c-><c- p>;</c-> <c- c1>// not defined</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>T</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Types</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>...</c-><c- o>>></c-><c- p>;</c->
</pre>
   <div style="background-color: #00ff0010">
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-> <c- k>const</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-><c- p>;</c->
</pre>
   </div>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_element</c-><c- p>;</c-> <c- c1>// not defined</c->
<c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- k>const</c-> <c- n>T</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Types</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>...</c-><c- o>>></c-><c- p>;</c->
</pre>
   <div style="background-color: #00ff0010">
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
 <c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>T</c-> <c- k>const</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-><c- p>;</c->
</pre>
   </div>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>using</c-> <c- n>tuple_element_t</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>T</c-><c- o>>::</c-><c- n>type</c-><c- p>;</c->

<c- c1>// 22.4.7, element access</c->
<c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Types</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>...</c-><c- o>>>&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>tuple</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>...</c-><c- o>>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>...</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Types</c-><c- o>></c->
  <c- k>constexpr</c-> <c- k>const</c-> <c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>get</c-><c- p>(</c-><c- k>const</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>...</c-><c- o>>&amp;&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <div style="background-color: #00ff0010">
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   </div>
   <p><strong>Modify section</strong> "Tuple helper classes <code class="highlight"><c- p>[</c-><c- n>tuple</c-><c- p>.</c-><c- n>helper</c-><c- p>]</c-></code>":</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>tuple_size</c-><c- p>;</c->
</pre>
   <p><sup>1</sup> All specializations of <code class="highlight"><c- n>tuple_size</c-></code> meet the <i>Cpp17UnaryTypeTrait</i> requirements (21.3.2) with a base
characteristic of <code class="highlight"><c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-></code> for some <code class="highlight"><c- n>N</c-></code>.</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Types</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>...</c-><c- o>>></c-> <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>...(</c-><c- n>Types</c-><c- p>)</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->
</pre>
   <div style="background-color: #00ff0010">
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-> <c- k>const</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- o>:</c-> <c- k>public</c-> <c- n>integral_constant</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->
</pre>
   </div>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Types</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>...</c-><c- o>>></c-> <c- p>{</c->
    <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>TI</c-><c- p>;</c->
  <c- p>};</c->
</pre>
   <sup>2</sup> <i>Mandates:</i> <code class="highlight"><c- n>I</c-> <c- o>&lt;</c-> <c- k>sizeof</c-><c- p>...(</c-><c- n>Types</c-><c- p>)</c-></code>.<br> <sup>3</sup> <i>Type:</i> <code class="highlight"><c- n>TI</c-></code> is the type of the <code class="highlight"><c- n>I</c-></code>-th element of <code class="highlight"><c- n>Types</c-></code>, where indexing is zero-based. 
   <div style="background-color: #00ff0010">
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>T</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- p>{</c->
      <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->
  <c- p>};</c->
</pre>
    <sup>4</sup> <i>Mandates:</i> <code class="highlight"><c- n>I</c-> <c- o>&lt;</c-> <c- n>N</c-></code>. 
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- n>Idx</c-><c- p>,</c-> <c- n>T</c-> <c- k>const</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-><c- o>></c-> <c- p>{</c->
      <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>T</c-> <c- k>const</c-><c- p>;</c->
  <c- p>};</c->
</pre>
    <sup>5</sup> <i>Mandates:</i> <code class="highlight"><c- n>I</c-> <c- o>&lt;</c-> <c- n>N</c-></code>. 
   </div>
   <p><strong>Append to section</strong> "Element access <code class="highlight"><c- p>[</c-><c- n>tuple</c-><c- p>.</c-><c- n>elem</c-><c- p>]</c-></code>":</p>
   <div style="background-color: #00ff0010">
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>get</c-><c- p>(</c-><c- n>T</c-> <c- p>(</c-><c- o>&amp;&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <sup>9</sup> <i>Mandates:</i> <code class="highlight"><c- n>I</c-> <c- o>&lt;</c-> <c- n>N</c-></code>.<br> <sup>10</sup> <i>Returns:</i> A reference to the <code class="highlight"><c- n>I</c-></code>-th element of <code class="highlight"><c- n>arr</c-></code>, where indexing is zero-based. 
   </div>
   <h2 class="heading settled" data-level="6" id="acknowledgements"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>I’d like to thank (sorted by <code class="highlight"><c- p>[](</c-><c- n>Dev</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- n>Dev</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>lhs</c-><c- p>.</c-><c- n>name</c-> <c- o>&lt;</c-> <c- n>rhs</c-><c- p>.</c-><c- n>name</c-><c- p>;</c-> <c- p>}</c-></code>)</p>
   <ul>
    <li data-md>
     <p>Arthur O’Dwyer,</p>
    <li data-md>
     <p>Barry Revzin,</p>
    <li data-md>
     <p>Giuseppe D’Angelo,</p>
    <li data-md>
     <p>Jason McKesson,</p>
    <li data-md>
     <p>Jens Maurer,</p>
    <li data-md>
     <p>Lénárd Szolnoki,</p>
    <li data-md>
     <p>Nikolay Mihaylov,</p>
    <li data-md>
     <p>Zhihao Yuan</p>
   </ul>
   <p>for their valuable feedbacks which made this paper possible.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-array-as-a-tuple">[ARRAY-AS-A-TUPLE]
   <dd>Zhihao Yuan. <a href="https://blog.miator.net/post/39362111475/use-array-as-a-tuple"><cite>Use array as a tuple</cite></a>. 2012-12-31. URL: <a href="https://blog.miator.net/post/39362111475/use-array-as-a-tuple">https://blog.miator.net/post/39362111475/use-array-as-a-tuple</a>
   <dt id="biblio-cpp-ref-array">[CPP-REF-ARRAY]
   <dd><a href="https://en.cppreference.com/w/cpp/language/array"><cite>Array declaration on cpp.reference.com</cite></a>. URL: <a href="https://en.cppreference.com/w/cpp/language/array">https://en.cppreference.com/w/cpp/language/array</a>
   <dt id="biblio-n1479">[N1479]
   <dd>Alisdair Meredith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1479.html"><cite>A Proposal to Add a Fixed Size Array Wrapper to the Standard Library Technical Report</cite></a>. 2003-04-23. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1479.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1479.html</a>
   <dt id="biblio-n4910">[N4910]
   <dd>Thomas Köppe. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf"><cite>Working Draft, Standard for Programming Language C++</cite></a>. 2022-03-17. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf</a>
   <dt id="biblio-p1831r1">[P1831R1]
   <dd>JF Bastien. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1831r1.html"><cite>Deprecating volatile: library</cite></a>. 2020-02-12. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1831r1.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1831r1.html</a>
   <dt id="biblio-p2165r3">[P2165R3]
   <dd>Corentin Jabot. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2165r3.pdf"><cite>Compatibility between tuple, pair and tuple-like objects</cite></a>. 2022-01-19. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2165r3.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2165r3.pdf</a>
  </dl>