<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P1292R0: Customization Point Functions</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

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

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

  /* Do something nice. */

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

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

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

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

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

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

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

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

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

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

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

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

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

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

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

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		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;
		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;
		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-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
		color: black;
		border-color: #3980B5;
		border-bottom-width: 3px !important;
		margin-bottom: 0px !important;
	}
	.toc a:visited {
		border-color: #054572;
	}
	.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;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* 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 .secno { font-size: 85%; }
	.toc > li li li li li { font-size:   85%;    }
	.toc > li li li li li .secno { font-size: 100%; }

	/* @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 {
			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; }
		}
	/* } */

	@supports (display:grid) {
		/* 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 {
			background: rgba(75%, 75%, 75%, .25);
			border-bottom: 3px solid #054572;
			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 span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
		}
	}

/** 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]) {
		background: #f7f8f9;
	}

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

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

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             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 > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-left: calc(13px + 26.5rem - 50vw);
			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-left: calc(40em - 50vw) !important;
			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-left: 0 !important;
			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 type="text/css">
    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;
    }
  </style>
  <meta content="Bikeshed version e9290f0f6bc01459c8b7bdfa2bb91f6985fc5c67" name="generator">
  <link href="http://wg21.link/p1292r0" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="3d602e6451fd40842eec03ca0282e11018ec3b08" name="document-revision">
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-syntax-highlighting */

.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1292R0<br>Customization Point Functions</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-10-08">2018-10-08</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1292r0">http://wg21.link/p1292r0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:metaprogrammingtheworld@gmail.com">Matt Calabrese</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </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>This paper proposes a new language facility for easily specifying,

    overriding, and invoking single-function customization points that are
    frequently used in generic libraries, such as <code class="highlight"><c- n>swap</c-></code> of the
    standard library, and <code class="highlight"><c- n>AbslHashValue</c-></code> of Abseil[1]. This proposal
    is intended to target a standard after C++20.</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>
     <ol class="toc">
      <li><a href="#Example"><span class="secno">1.1</span> <span class="content">Example</span></a>
      <li><a href="#Motivation"><span class="secno">1.2</span> <span class="content">Motivation</span></a>
     </ol>
    <li>
     <a href="#CPF"><span class="secno">2</span> <span class="content">Customization Point Functions</span></a>
     <ol class="toc">
      <li><a href="#CPF-decl"><span class="secno">2.1</span> <span class="content">Declaring a Customization Point Function</span></a>
      <li>
       <a href="#CPF-override"><span class="secno">2.2</span> <span class="content">Overriding Customization Point Functions</span></a>
       <ol class="toc">
        <li><a href="#CPF-override-semantics"><span class="secno">2.2.1</span> <span class="content">Semantics of Customization Point Functions</span></a>
       </ol>
      <li><a href="#CPF-named-override"><span class="secno">2.3</span> <span class="content">Named Customization Point Function Overrides</span></a>
      <li><a href="#CPF-friend"><span class="secno">2.4</span> <span class="content">Overrides as Friend Functions</span></a>
      <li><a href="#CPF-objects"><span class="secno">2.5</span> <span class="content">Customization Point Functions as Objects</span></a>
      <li><a href="#CPF-pure-virtual"><span class="secno">2.6</span> <span class="content">"Pure Virtual" Customization Point Functions</span></a>
      <li><a href="#CPF-final"><span class="secno">2.7</span> <span class="content">Final Customization Point Functions</span></a>
      <li><a href="#CPF-hierarchical-overrides"><span class="secno">2.8</span> <span class="content">Hierarchical Customization Point Functions</span></a>
     </ol>
    <li><a href="#Polls"><span class="secno">3</span> <span class="content">Suggested Polls</span></a>
    <li><a href="#References"><span class="secno">4</span> <span class="content">References</span></a>
   </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>
   <h3 class="heading settled" data-level="1.1" id="Example"><span class="secno">1.1. </span><span class="content">Example</span><a class="self-link" href="#Example"></a></h3>
   <p>Before going into details, below is a simple example of the core of what is
proposed -- a "virtual", but statically-dispatched, non-member function that
declares a logical entry point for user-customization. Users may explicitly
override such a function for their type from their own namespace, in which
case qualified calls to the customization point will dispatch to the user’s
customized implementation whenever appropriate.</p>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  
<c- c1>// A customization point named "swap", including a default definition.</c->
<c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>T</c-> <c- o>></c->
<c- k>virtual</c-> <c- b>void</c-> <c- n>swap</c-><c- p>(</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>rhs</c-> <c- p>)</c-> <c- p>{</c-> <c- d>/*...*/</c-> <c- p>}</c->
  
<c- p>}</c-> <c- c1>// namespace std</c->

<c- c1>// A user’s override (may appear in their own namespace)</c->
<c- b>void</c-> <c- n>swap</c-><c- p>(</c-> <c- n>foo</c-><c- o>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- n>foo</c-><c- o>&amp;</c-> <c- n>rhs</c-> <c- p>)</c-> <c- k>override</c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>swap</c-> <c- p>{</c-> <c- d>/*...*/</c-> <c- p>}</c->
</pre>
   <h3 class="heading settled" data-level="1.2" id="Motivation"><span class="secno">1.2. </span><span class="content">Motivation</span><a class="self-link" href="#Motivation"></a></h3>
   <p>Many C++ libraries introduce simple, named, single-function customization points
through which generic code may interface with user-defined operations. Two
common techniques for implementing these customization points are nested members
of template specializations (such as with traits classes or with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>numeric_limits</c-></code>), and argument dependent lookup
(<code class="highlight"><c- n>swap</c-></code>).</p>
   <p>Both of the aforementioned approaches have advantages and disadvantages. The
traits-based approach allows for groupings of associated types and associated
functions, has a fairly clear declaration, has (relatively) simple rules when
matching template arguments to specializations, but is a bit cumbersome to use
directly and user-specialization often requires jumping into a foreign
namespace.</p>
   <p>The ADL-based approach applies to individual functions as opposed to groupings
of associated properties, so it is a bit narrower in scope, but in that narrow
scope it has some advantages. When a user needs to override such a customization
point, they do not need to enter into a foreign namespace nor do they have to
specialize a type-template just to customize a single function.</p>
   <p>However, the simplicity of ADL-based customization points is in some sense an
illusion. Proper creation and use of ADL customization points is so subtle that
experts in C++ have written extensive articles about them[2] and they are a
frequent source of questions for newcomers and experienced C++ programmers
alike[3]. Ranges even goes so far as to introduce the design pattern of a
"Customization Point Object" at the library level[4], whose primary purpose is
to make it easier to consistently specify and invoke ADL-based customization
points. A prominent programmer in the generic programming community has even
gone so far as to state ADL is the one feature that he would remove from C++ if
he could.[5]</p>
   <p>The following are considered by the author of this proposal to be some core
issues with customization points based on ADL.</p>
   <ol>
    <li data-md>
     <p>They do not have a clear declaration in code.</p>
    <li data-md>
     <p>They are subtle to invoke correctly and doing so incorrectly may not always
produce a compile-time error
(<code class="highlight"><c- k>using</c-> <c- n>std</c-><c- o>::</c-><c- n>swap</c-><c- p>;</c-> <c- n>swap</c-><c- p>(</c-><c- n>lhs</c-><c- p>,</c-> <c- n>rhs</c-><c- p>);</c-></code>).</p>
    <li data-md>
     <p>Associated-namespace rules are rarely understood, cause confusion, and bugs.</p>
    <li data-md>
     <p>Lookup often leads to a large amount of overloads that must be considered.</p>
   </ol>
   <p>This proposal attempts to make function-kind customization points a directly
supported part of the language, with a simpler set of rules than existing
approaches that most programmers would be able to fully understand.</p>
   <h2 class="heading settled" data-level="2" id="CPF"><span class="secno">2. </span><span class="content">Customization Point Functions</span><a class="self-link" href="#CPF"></a></h2>
   <p>Customization point functions are the proposed alternative to the subset of
customization points that developers currently use ADL for.</p>
   <h3 class="heading settled" data-level="2.1" id="CPF-decl"><span class="secno">2.1. </span><span class="content">Declaring a Customization Point Function</span><a class="self-link" href="#CPF-decl"></a></h3>
   <p>A customization point function may be declared at namespace scope just as any
other function template, but with a preceding <code class="highlight"><c- k>virtual</c-></code> keyword.
Customization point functions may be constrained just as any other function
template may be constrained.</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>virtual</c-> <c- k>constexpr</c-> <c- b>void</c-> <c- n>swap</c-><c- p>(</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>rhs</c-> <c- p>)</c->
  <c- k>noexcept</c-><c- p>(</c->    <c- n>std</c-><c- o>::</c-><c- n>is_nothrow_move_constructible_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c->
            <c- o>&amp;&amp;</c-> <c- n>std</c-><c- o>::</c-><c- n>is_nothrow_move_assignable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c->
          <c- p>)</c->
  <c- n>requires</c-> <c- n>MoveConstructible</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>MoveAssignable</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c->
<c- p>{</c->
  <c- n>T</c-> <c- n>temp</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>lhs</c-><c- p>);</c->
  <c- n>lhs</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>rhs</c-><c- p>);</c->
  <c- n>rhs</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>temp</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <p>Note that the usage of the keyword <code class="highlight"><c- k>virtual</c-></code> here is to indicate the
fact that the function may be overridden elsewhere in code in a way that is
dependent on the function’s arguments. However, unlike traditional <code class="highlight"><c- k>virtual</c-></code>, this is not achieved through dynamic dispatch. Whether or
not this particualy syntax is a reasonable choice should be decided by committee
if the overall functionality is considered useful.</p>
   <h3 class="heading settled" data-level="2.2" id="CPF-override"><span class="secno">2.2. </span><span class="content">Overriding Customization Point Functions</span><a class="self-link" href="#CPF-override"></a></h3>
   <p>A customization point function’s primary use is as a non-member function that
may be overridden. Unlike with normal function overloading, an override of a
customization point function must be explicitly declared as an override of that
specific customization point function. There are multiple ways in which a
customization point function’s override may be declared, but the simplest is to
write the function type including the <code class="highlight"><c- k>override</c-></code> specifier, followed
by a <code class="highlight"><c- o>:</c-></code> and the name of the customization point function that is to
be overridden. The override declaration is also a definition if a function body
immediately follows the name of the customization point function.</p>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->

<c- c1>// A customization point function</c->
<c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>T</c-> <c- o>></c->
<c- k>virtual</c-> <c- b>void</c-> <c- n>swap</c-><c- p>(</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>rhs</c-> <c- p>)</c-> <c- p>{</c-> <c- d>/*default definition*/</c-> <c- p>}</c->

<c- p>}</c-> <c- c1>// namespace std</c->

<c- c1>// A user-defined type</c->
<c- k>class</c-> <c- nc>tensor</c-> <c- p>{</c-> <c- d>/*...*/</c-> <c- p>};</c->

<c- c1>// A user-defined override for swap</c->
<c- b>void</c-> <c- p>(</c-> <c- n>tensor</c-><c- o>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- n>tensor</c-><c- o>&amp;</c-> <c- n>rhs</c-> <c- p>)</c-> <c- k>override</c->
  <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>swap</c-> <c- c1>// The customization point function to be overridden</c->
<c- p>{</c->
  <c- d>/*...*/</c->
<c- p>}</c->
</pre>
   <p>Informally, an override of a customization point function must be more
specialized or more constrained than the customization point function that it
overrides, but the return type of an override may differ if the override’s
return type is implicitly convertible to the customization point function’s
return type, or if the customization point’s return type is <code class="highlight"><c- b>void</c-></code>.
Calls made to the customization point function force any such conversions to
take place before returning to the caller.</p>
   <p>Formal details surrounding these requirements and any alterations will be
explored in a future revision if the rest of the proposal is positively
received.</p>
   <h4 class="heading settled" data-level="2.2.1" id="CPF-override-semantics"><span class="secno">2.2.1. </span><span class="content">Semantics of Customization Point Functions</span><a class="self-link" href="#CPF-override-semantics"></a></h4>
   <p>When code attempts to call a customization point function, it is first resolved
as if there were some imaginary function or function template with that
declaration. If a call to such a function would be valid, then it and all of the
direct overrides of that customization point function are treated as though they
were all functions with the same name in some namespace with no other members.
If a qualified call to a function of that name in that namespace would succeed,
then the corresponding function is what is called in the actual program.</p>
   <h3 class="heading settled" data-level="2.3" id="CPF-named-override"><span class="secno">2.3. </span><span class="content">Named Customization Point Function Overrides</span><a class="self-link" href="#CPF-named-override"></a></h3>
   <p>An override of a customization point function may optionally be given a name. To
specify a name for a customization point function override, the declaration is
the same as a normal function declaration, except that it includes the override
specifier and the customization point function that it overrides in the same
manner as for an unnamed override described above. A name given to an overriding
function definition must be a unique name in its namespace (it must be unique
even with respect to other functions in that namespace).</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>It</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Distance</c-> <c- o>></c->
<c- k>virtual</c-> <c- k>constexpr</c-> <c- b>void</c-> <c- n>advance</c-><c- p>(</c-> <c- n>It</c-><c- o>&amp;</c-> <c- n>it</c-><c- p>,</c-> <c- n>Distance</c-> <c- n>n</c-> <c- p>)</c->
  <c- n>requires</c-> <c- n>InputIterator</c-><c- o>&lt;</c-><c- n>It</c-><c- o>></c->
<c- p>{</c->
  <c- k>for</c-><c- p>(</c-> <c- p>;</c-> <c- n>n</c-> <c- o>!=</c-> <c- mi>0</c-><c- p>;</c-> <c- o>--</c-><c- n>n</c-> <c- p>)</c->
    <c- o>++</c-><c- n>it</c-><c- p>;</c->
<c- p>}</c->

<c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>It</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Distance</c-> <c- o>></c->
<c- k>constexpr</c-> <c- b>void</c-> <c- n>advance_bidirectional</c-><c- p>(</c-> <c- n>It</c-><c- o>&amp;</c-> <c- n>it</c-><c- p>,</c-> <c- n>Distance</c-> <c- n>n</c-> <c- p>)</c-> <c- k>override</c->
  <c- n>requires</c-> <c- n>BidirectionalIterator</c-><c- o>&lt;</c-><c- n>It</c-><c- o>></c->
  <c- o>:</c-> <c- n>advance</c->
<c- p>{</c->
  <c- k>if</c-><c- p>(</c-> <c- n>n</c-> <c- o>>=</c-> <c- mi>0</c-> <c- p>)</c->
    <c- k>for</c-><c- p>(</c-> <c- p>;</c-> <c- n>n</c-> <c- o>!=</c-> <c- mi>0</c-><c- p>;</c-> <c- o>--</c-><c- n>n</c-> <c- p>)</c->
      <c- o>++</c-><c- n>it</c-><c- p>;</c->
  <c- k>else</c->
    <c- k>for</c-><c- p>(</c-> <c- p>;</c-> <c- n>n</c-> <c- o>!=</c-> <c- mi>0</c-><c- p>;</c-> <c- o>++</c-><c- n>n</c-> <c- p>)</c->
      <c- o>--</c-><c- n>it</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>One advantage of specification of a name for an override is that it makes it
easier to invoke a specific override, much as it is useful to be able to invoke
a specific <code class="highlight"><c- k>virtual</c-></code> function by qualifying the call with the type
name.</p>
   <p>A named override is itself considered to also be a customization point function.
Implications of this are described later on in this paper.</p>
   <h3 class="heading settled" data-level="2.4" id="CPF-friend"><span class="secno">2.4. </span><span class="content">Overrides as Friend Functions</span><a class="self-link" href="#CPF-friend"></a></h3>
   <p>In order to limit the amount of overrides that are looked up per call, it is
suggested that a customization point function may be able to be overridden in
the body of a class definition, in a similar manner to how an inline <code class="highlight"><c- k>friend</c-></code> function definition behaves with respect to ADL. The details
of this are to be explored in a revision of this paper if the overall idea is
considered useful.</p>
   <h3 class="heading settled" data-level="2.5" id="CPF-objects"><span class="secno">2.5. </span><span class="content">Customization Point Functions as Objects</span><a class="self-link" href="#CPF-objects"></a></h3>
   <p>Despite the name, a customization point function is expected to behave more
similar to a function object than to a function or function template. The
rationale for this is to allow such a function to be able to be easily passed
around to higher-order functions as a single entity. It also more accurately
reflects intended behavior with respect to prohibition of "implicit" overloading
and name lookup. A customization point function *cannot* be overloaded, but
rather, it may only be overridden. Further, a customization point function is to
not be found by ADL, and if when attempting to call a function by name a
customization point function of that name is found by normal name lookup prior
to ADL, then ADL is inhibited. The combination of these rules removes many of
the subtleties of invoking customization points correctly.</p>
   <h3 class="heading settled" data-level="2.6" id="CPF-pure-virtual"><span class="secno">2.6. </span><span class="content">"Pure Virtual" Customization Point Functions</span><a class="self-link" href="#CPF-pure-virtual"></a></h3>
   <p>Similar to <code class="highlight"><c- k>virtual</c-></code> functions, a customization point function may be
declared pure virtual (although in contrast, a pure virtual customization point
function may *not* have a definition). When attempting to call a pure virtual
customization point function, if there is no override that matches the arguments
that are passed, then overload resolution fails.</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>virtual</c-> <c- b>void</c-> <c- n>draw</c-><c- p>(</c-> <c- n>context</c-><c- o>&amp;</c-> <c- n>cont</c-><c- p>,</c-> <c- n>T</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>obj</c-> <c- p>)</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
</pre>
   <h3 class="heading settled" data-level="2.7" id="CPF-final"><span class="secno">2.7. </span><span class="content">Final Customization Point Functions</span><a class="self-link" href="#CPF-final"></a></h3>
   <p>A customization point function may be declared <code class="highlight"><c- k>final</c-></code>, in which case
that function may not be overridden. Programs that attempt to declare an
override for such a function are ill-formed. Though it may seem contradictory in
intent to declare a customization point function as <code class="highlight"><c- k>final</c-></code>, the
primary use of declaring a customization point function as <code class="highlight"><c- k>final</c-></code> is
to take advantage of ADL-inhibition and the ability to safely and easily pass
such functions to higher-order algorithms. This ability is also useful as a
means to counteract the understandable advocation by certain generic library
authors to avoid namespace-scope functions entirely in favor of global function
objects.[6]</p>
   <p>A <code class="highlight"><c- k>final</c-></code> customization point object may include the <code class="highlight"><c- k>virtual</c-></code> specifier, but it is not required to do so.</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>T</c-> <c- o>></c->
<c- n>T</c-> <c- n>square</c-><c- p>(</c-> <c- n>T</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>arg</c-> <c- p>)</c-> <c- k>final</c->
<c- p>{</c->
  <c- k>return</c-> <c- n>arg</c-> <c- o>*</c-> <c- n>arg</c-><c- p>;</c->
<c- p>}</c->

<c- n>std</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-> <c- n>range1</c-><c- p>,</c-> <c- n>out_it</c-><c- p>,</c-> <c- n>square</c-> <c- p>);</c->
</pre>
   <h3 class="heading settled" data-level="2.8" id="CPF-hierarchical-overrides"><span class="secno">2.8. </span><span class="content">Hierarchical Customization Point Functions</span><a class="self-link" href="#CPF-hierarchical-overrides"></a></h3>
   <p>As mentioned earlier, a named customization point function override is itself
considered to be its own customization point function. This means that such an
override may be explicitly overridden by name (it may also be pure and it may be <code class="highlight"><c- k>final</c-></code>). This allows customization point functions to naturally form
hierarchies of overrides, as opposed to a strictly flat set of overloads, in a
way that may more clearly and more efficiently represent concept-based
overloads.</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>It</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Distance</c-> <c- o>></c->
<c- k>virtual</c-> <c- k>constexpr</c-> <c- b>void</c-> <c- n>advance</c-><c- p>(</c-> <c- n>It</c-><c- o>&amp;</c-> <c- n>it</c-><c- p>,</c-> <c- n>Distance</c-> <c- n>n</c-> <c- p>)</c->
  <c- n>requires</c-> <c- n>InputIterator</c-><c- o>&lt;</c-><c- n>It</c-><c- o>></c->
<c- p>{</c->
  <c- k>for</c-><c- p>(</c-> <c- p>;</c-> <c- n>n</c-> <c- o>!=</c-> <c- mi>0</c-><c- p>;</c-> <c- o>--</c-><c- n>n</c-> <c- p>)</c->
    <c- o>++</c-><c- n>it</c-><c- p>;</c->
<c- p>}</c->

<c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>It</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Distance</c-> <c- o>></c->
<c- k>constexpr</c-> <c- b>void</c-> <c- n>advance_bidirectional</c-><c- p>(</c-> <c- n>It</c-><c- o>&amp;</c-> <c- n>it</c-><c- p>,</c-> <c- n>Distance</c-> <c- n>n</c-> <c- p>)</c-> <c- k>override</c->
  <c- n>requires</c-> <c- n>BidirectionalIterator</c-><c- o>&lt;</c-><c- n>It</c-><c- o>></c->
  <c- o>:</c-> <c- n>advance</c->
<c- p>{</c->
  <c- k>if</c-><c- p>(</c-> <c- n>n</c-> <c- o>>=</c-> <c- mi>0</c-> <c- p>)</c->
    <c- k>for</c-><c- p>(</c-> <c- p>;</c-> <c- n>n</c-> <c- o>!=</c-> <c- mi>0</c-><c- p>;</c-> <c- o>--</c-><c- n>n</c-> <c- p>)</c->
      <c- o>++</c-><c- n>it</c-><c- p>;</c->
  <c- k>else</c->
    <c- k>for</c-><c- p>(</c-> <c- p>;</c-> <c- n>n</c-> <c- o>!=</c-> <c- mi>0</c-><c- p>;</c-> <c- o>++</c-><c- n>n</c-> <c- p>)</c->
      <c- o>--</c-><c- n>it</c-><c- p>;</c->
<c- p>}</c->

<c- k>template</c-><c- o>&lt;</c-> <c- k>class</c-> <c- nc>It</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Distance</c-> <c- o>></c->
<c- k>constexpr</c-> <c- b>void</c-> <c- n>advance_random_access</c-><c- p>(</c-> <c- n>It</c-><c- o>&amp;</c-> <c- n>it</c-><c- p>,</c-> <c- n>Distance</c-> <c- n>n</c-> <c- p>)</c-> <c- k>override</c->
  <c- n>requires</c-> <c- n>RandomAccessIterator</c-><c- o>&lt;</c-><c- n>It</c-><c- o>></c->
  <c- o>:</c-> <c- n>advance_bidirectional</c->
<c- p>{</c->
  <c- n>it</c-> <c- o>+=</c-> <c- n>n</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>Note that the above code example has some advantages over both traditional
concept-based overloading and of branching via <code class="highlight"><c- k>if</c-> <c- k>constexpr</c-></code>. First,
because the overrides are hierarchical, each level of override functions only
needs to undergo substitution if the previous level succeeds substitution. In
naturally hierarchical cases, this hypothetically may lead to better
compile-time performance and/or simpler error messages when compared to normal
concept-based overloads, though there is no implementation experience to verify
this. As well, unlike with a nested <code class="highlight"><c- k>if</c-> <c- k>constexpr</c-></code> chain, the
hierarchical overrides naturally form an open set of branches that is exposed to
the user, and for which they may further customize.</p>
   <p>A named customization point function override that is intended to be <code class="highlight"><c- k>final</c-></code> must use the keyword <code class="highlight"><c- k>final</c-></code> instead of <code class="highlight"><c- k>override</c-></code> in its declaration.</p>
   <h2 class="heading settled" data-level="3" id="Polls"><span class="secno">3. </span><span class="content">Suggested Polls</span><a class="self-link" href="#Polls"></a></h2>
   <p>Should this proposal be elaborated on in a future revision?</p>
   <p>Are the motivating problems worth solving at the language level at all?</p>
   <p>Is the functionality of <code class="highlight"><c- k>final</c-></code> non-member functions worth proposing
separately?</p>
   <h2 class="heading settled" data-level="4" id="References"><span class="secno">4. </span><span class="content">References</span><a class="self-link" href="#References"></a></h2>
   <p>[1]: Matt Kulukundis: "Tip of the Week #152: AbslHashValue and You" <a href="https://abseil.io/tips/152">https://abseil.io/tips/152</a></p>
   <p>[2]: Eric Niebler: "Customization Point Design in C++11 and Beyond" <a href="http://ericniebler.com/2014/10/21/customization-point-design-in-c11-and-beyond/">http://ericniebler.com/2014/10/21/customization-point-design-in-c11-and-beyond/</a></p>
   <p>[3]: Stack Overflow Search for ADL <a href="https://stackoverflow.com/search?q=ADL">https://stackoverflow.com/search?q=ADL</a></p>
   <p>[4]: N4762 "Working Draft, Standard for Programming Language C++" [customization.point.object]: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4762.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4762.pdf</a></p>
   <p>[5]: Jens Weller "An Interview with Sean Parent" <a href="https://www.meetingcpp.com/blog/items/interview-with-sean-parent.html">https://www.meetingcpp.com/blog/items/interview-with-sean-parent.html</a></p>
   <p>[6]: Eric Niebler: "C++11 Library Design" <a href="https://github.com/boostcon/cppnow_presentations_2014/blob/master/files/cxx11-library-design.pdf">https://github.com/boostcon/cppnow_presentations_2014/blob/master/files/cxx11-library-design.pdf</a></p>
  </main>
<script>
(function() {
  "use strict";
  var collapseSidebarText = '<span aria-hidden="true">←</span> '
                          + '<span>Collapse Sidebar</span>';
  var expandSidebarText   = '<span aria-hidden="true">→</span> '
                          + '<span>Pop Out Sidebar</span>';
  var tocJumpText         = '<span aria-hidden="true">↑</span> '
                          + '<span>Jump to Table of Contents</span>';

  var sidebarMedia = window.matchMedia('screen and (min-width: 78em)');
  var autoToggle   = function(e){ toggleSidebar(e.matches) };
  if(sidebarMedia.addListener) {
    sidebarMedia.addListener(autoToggle);
  }

  function toggleSidebar(on) {
    if (on == undefined) {
      on = !document.body.classList.contains('toc-sidebar');
    }

    /* Don’t scroll to compensate for the ToC if we’re above it already. */
    var headY = 0;
    var head = document.querySelector('.head');
    if (head) {
      // terrible approx of "top of ToC"
      headY += head.offsetTop + head.offsetHeight;
    }
    var skipScroll = window.scrollY < headY;

    var toggle = document.getElementById('toc-toggle');
    var tocNav = document.getElementById('toc');
    if (on) {
      var tocHeight = tocNav.offsetHeight;
      document.body.classList.add('toc-sidebar');
      document.body.classList.remove('toc-inline');
      toggle.innerHTML = collapseSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, 0 - tocHeight);
      }
      tocNav.focus();
      sidebarMedia.addListener(autoToggle); // auto-collapse when out of room
    }
    else {
      document.body.classList.add('toc-inline');
      document.body.classList.remove('toc-sidebar');
      toggle.innerHTML = expandSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, tocNav.offsetHeight);
      }
      if (toggle.matches(':hover')) {
        /* Unfocus button when not using keyboard navigation,
           because I don’t know where else to send the focus. */
        toggle.blur();
      }
    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn’t exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn’t standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don’t have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we’re at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can’t find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>