<!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>P0919R3: Heterogeneous lookup for unordered containers</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

  /* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0919R3<br>Heterogeneous lookup for unordered containers</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-11-09">2018-11-09</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0919r3">http://wg21.link/P0919r3</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://www.train-it.eu">Mateusz Pusz</a> (<a class="p-org org" href="http://www.epam.com">Epam Systems</a>) <a class="u-email email" href="mailto:mateusz.pusz@gmail.com">mateusz.pusz@gmail.com</a>
     <dt>Audience:
     <dd>LWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/mpusz/wg21_papers/blob/master/src/0919_heterogeneous_lookup_for_unordered_containers.bs">github.com/mpusz/wg21_papers/blob/master/src/0919_heterogeneous_lookup_for_unordered_containers.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This proposal adds heterogeneous lookup support to the unordered associative containers in the

          C++ Standard Library. As a result, a creation of a temporary key object is not needed when
          different (but compatible) type is provided as a key to the member function. This also makes
          unordered and regular associative container interfaces and functionality more compatible with
          each other.</p>
   <p>With the changes proposed by this paper the following code will work without any additional

          performance hits:</p>
<pre class="language-cpp highlight">          <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>Key</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Value</c-><c- o>></c->
          <c- k>using</c-> <c- n>h_str_umap</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>unordered_map</c-><c- o>&lt;</c-><c- n>Key</c-><c- p>,</c-> <c- n>Value</c-><c- p>,</c-> <c- n>string_hash</c-><c- o>></c-><c- p>;</c->
          <c- n>h_str_umap</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-> <c- n>map</c-> <c- o>=</c-> <c- d>/* ... */</c-><c- p>;</c->
          <c- n>map</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- s>"This does not create a temporary std::string object :-)"</c-><c- n>sv</c-><c- p>);</c->
</pre>
  </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="#motivation-and-scope"><span class="secno">1</span> <span class="content">Motivation and Scope</span></a>
     <ol class="toc">
      <li><a href="#motivation-performance"><span class="secno">1.1</span> <span class="content">Performance related concerns</span></a>
      <li><a href="#motivation-design"><span class="secno">1.2</span> <span class="content">Design related concerns</span></a>
     </ol>
    <li><a href="#prior-work"><span class="secno">2</span> <span class="content">Prior Work</span></a>
    <li><a href="#impact"><span class="secno">3</span> <span class="content">Impact On The Standard</span></a>
    <li>
     <a href="#design-decisions"><span class="secno">4</span> <span class="content">Design Decisions</span></a>
     <ol class="toc">
      <li><a href="#design-heterogeneous-hash"><span class="secno">4.1</span> <span class="content">Heterogeneous hash function object</span></a>
      <li><a href="#design-function-additional-parameters"><span class="secno">4.2</span> <span class="content">Additional parameters in lookup member functions overloads</span></a>
      <li><a href="#design-function-template"><span class="secno">4.3</span> <span class="content">Lookup member functions template overloads</span></a>
     </ol>
    <li><a href="#wording"><span class="secno">5</span> <span class="content">Proposed Wording</span></a>
    <li><a href="#feature-testing"><span class="secno">6</span> <span class="content">Feature Testing</span></a>
    <li><a href="#implementation"><span class="secno">7</span> <span class="content">Implementation Experience</span></a>
    <li><a href="#future-extensions"><span class="secno">8</span> <span class="content">Possible Future Extensions</span></a>
    <li>
     <a href="#revision-history"><span class="secno">9</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r0r3"><span class="secno">9.1</span> <span class="content">r2 ➡ r3 <span>[diff</span>]</span></a>
      <li><a href="#r0r2"><span class="secno">9.2</span> <span class="content">r1 ➡ r2 <span>[diff</span>]</span></a>
      <li><a href="#r0r1"><span class="secno">9.3</span> <span class="content">r0 ➡ r1 <span>[diff</span>]</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">10</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="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <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="motivation-and-scope"><span class="secno">1. </span><span class="content">Motivation and Scope</span><a class="self-link" href="#motivation-and-scope"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-n3657">[N3657]</a> merged into C++14 IS introduced heterogeneous lookup support for ordered associative containers
(<code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>map</c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>set</c-></code>, etc) to the C++ Standard Library. Authors of that document pointed that
the requirement to construct (either implicitly or explicitly) the object of <code class="highlight"><c- n>key_type</c-></code> to do the lookup
may be really expensive.</p>
   <p>Unordered containers still lack support for such functionality and users are often hit by that
performance problem.</p>
   <h3 class="heading settled" data-level="1.1" id="motivation-performance"><span class="secno">1.1. </span><span class="content">Performance related concerns</span><a class="self-link" href="#motivation-performance"></a></h3>
   <p>Consider such use case:</p>
   <div class="example" id="example-1">
    <a class="self-link" href="#example-1"></a> 
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>unordered_map</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-> <c- n>map</c-> <c- o>=</c-> <c- d>/* ... */</c-><c- p>;</c->
<c- k>auto</c-> <c- n>it1</c-> <c- o>=</c-> <c- n>map</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- s>"abc"</c-><c- p>);</c->
<c- k>auto</c-> <c- n>it2</c-> <c- o>=</c-> <c- n>map</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- s>"def"</c-><c- n>sv</c-><c- p>);</c->
</pre>
   </div>
   <p>In C++17 above code will construct <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> temporary and then will compare it with container’s
elements to find the key. There is no implementation-specific reason to prevent lookup by an arbitrary
key type <code class="highlight"><c- n>T</c-></code>, as long as <code class="highlight"><c- n>hash</c-><c- p>(</c-><c- n>t</c-><c- p>)</c-> <c- o>==</c-> <c- n>hash</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code> for any key <code class="highlight"><c- n>k</c-></code> in the map, if <code class="highlight"><c- n>t</c-> <c- o>==</c-> <c- n>k</c-></code>.</p>
   <h3 class="heading settled" data-level="1.2" id="motivation-design"><span class="secno">1.2. </span><span class="content">Design related concerns</span><a class="self-link" href="#motivation-design"></a></h3>
   <p>Another motivating case is mentioned in <a data-link-type="biblio" href="#biblio-n3573">[N3573]</a>. Consider:</p>
   <div class="example" id="example-2">
    <a class="self-link" href="#example-2"></a> 
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>unordered_set</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- n>set</c-><c- p>;</c->
</pre>
   </div>
   <p>Whilst it’s possible to insert <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> into the set, there are no means to erase or test for
membership, as that would involve constructing two <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> to the same resource.</p>
   <p>In such a case C++ developer is forced to either:</p>
   <ol>
    <li data-md>
     <p>Weaken the design and not use smart pointers for memory ownership management which may result int
stability or security issues.</p>
    <li data-md>
     <p>Provide custom stateful (memory overhead) deleter that only optionally destroys the managed resource as
suggested by <a data-link-type="biblio" href="#biblio-stackoverflow-1">[STACKOVERFLOW-1]</a>:</p>
   </ol>
<pre class="language-cpp highlight"><c- k>class</c-> <c- nc>opt_out_deleter</c-> <c- p>{</c->
  <c- b>bool</c-> <c- n>delete_</c-><c- p>;</c->
<c- k>public</c-><c- o>:</c->
  <c- k>explicit</c-> <c- n>opt_out_deleter</c-><c- p>(</c-><c- b>bool</c-> <c- n>do_delete</c-> <c- o>=</c-> true<c- p>)</c-> <c- o>:</c-> <c- n>delete_</c-><c- p>{</c-><c- n>do_delete</c-><c- p>}</c-> <c- p>{}</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->
  <c- b>void</c-> <c- k>operator</c-><c- p>()(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-><c- p>)</c-> <c- k>const</c->
  <c- p>{</c->
    <c- k>if</c-><c- p>(</c-><c- n>delete_</c-><c- p>)</c-> <c- k>delete</c-> <c- n>p</c-><c- p>;</c->
  <c- p>}</c->
<c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->
<c- k>using</c-> <c- n>set_unique_ptr</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>opt_out_deleter</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->
<c- n>set_unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>make_find_ptr</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>raw</c-><c- p>)</c->
<c- p>{</c->
  <c- k>return</c-> <c- n>set_unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>{</c-><c- n>raw</c-><c- p>,</c-> <c- n>opt_out_deleter</c-><c- p>{</c->false<c- p>}};</c->
<c- p>}</c->

<c- n>set_unique_ptr</c-> <c- n>set</c-> <c- o>=</c-> <c- d>/* ... */</c-><c- p>;</c->
<c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>set</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>make_find_ptr</c-><c- p>(</c-><c- n>raw</c-><c- p>));</c->
</pre>
   <ol start="3">
    <li data-md>
     <p>Use <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unordered_map</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-></code> instead which again results in memory overhead.</p>
   </ol>
   <p>The similar code may also have a different side effect. Let’s consider:</p>
   <div class="example" id="example-3">
    <a class="self-link" href="#example-3"></a> 
<pre class="language-cpp highlight"><c- k>struct</c-> <c- n>my_data</c-> <c- p>{</c->
  <c- b>size_t</c-> <c- n>i</c-><c- p>;</c->
  <c- n>std</c-><c- o>::</c-><c- n>array</c-><c- o>&lt;</c-><c- b>char</c-><c- p>,</c-> <c- mi>256</c-><c- o>></c-> <c- n>data</c-><c- p>;</c->
  <c- k>explicit</c-> <c- nf>my_data</c-><c- p>(</c-><c- b>size_t</c-> <c- n>i_</c-><c- p>)</c-> <c- o>:</c-> <c- n>i</c-><c- p>{</c-><c- n>i_</c-><c- p>}</c->
  <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- n>begin</c-><c- p>(</c-><c- n>data</c-><c- p>),</c-> <c- n>end</c-><c- p>(</c-><c- n>data</c-><c- p>),</c-> <c- mi>0</c-><c- p>);</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- n>my_data_equal</c-> <c- p>{</c->
  <c- b>bool</c-> <c- k>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>my_data</c-><c- o>>&amp;</c-> <c- n>l</c-><c- p>,</c->
                  <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>my_data</c-><c- o>>&amp;</c-> <c- n>r</c-><c- p>)</c-> <c- k>const</c->
  <c- p>{</c-> <c- k>return</c-> <c- n>l</c-><c- o>-></c-><c- n>i</c-> <c- o>==</c-> <c- n>r</c-><c- o>-></c-><c- n>i</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- n>my_data_hash</c-> <c- p>{</c->
  <c- b>size_t</c-> <c- k>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>my_data</c-><c- o>>&amp;</c-> <c- n>v</c-><c- p>)</c-> <c- k>const</c->
  <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- b>size_t</c-><c- o>></c-><c- p>{}(</c-><c- n>v</c-><c- o>-></c-><c- n>i</c-><c- p>);</c-> <c- p>}</c->
<c- p>};</c->

<c- k>using</c-> <c- n>my_set</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>unordered_set</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>my_data</c-><c- o>></c-><c- p>,</c->
                                  <c- n>my_data_hash</c-><c- p>,</c-> <c- n>my_data_equal</c-><c- o>></c-><c- p>;</c->
<c- n>my_set</c-> <c- n>set</c-> <c- o>=</c-> <c- d>/* ... */</c-><c- p>;</c->
<c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>set</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>make_unique</c-><c- o>&lt;</c-><c- n>my_data</c-><c- o>></c-><c- p>(</c-><c- mi>1</c-><c- p>));</c->
</pre>
   </div>
   <p>This case not only introduces a dynamic memory allocation related performance hit on every lookup but also
messes up with nicely defined ownership strategy.</p>
   <h2 class="heading settled" data-level="2" id="prior-work"><span class="secno">2. </span><span class="content">Prior Work</span><a class="self-link" href="#prior-work"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-n3573">[N3573]</a> tried to address this issue. While the motivation described in that paper sounds reasonable
the proposed solution goes too far and may cause problems. See <a href="#design-decisions">§4 Design Decisions</a> for more details.</p>
   <h2 class="heading settled" data-level="3" id="impact"><span class="secno">3. </span><span class="content">Impact On The Standard</span><a class="self-link" href="#impact"></a></h2>
   <p>This proposal modifies the unordered associative containers in <code class="highlight"><c- o>&lt;</c-><c- n>unordered_map</c-><c- o>></c-></code> and <code class="highlight"><c- o>&lt;</c-><c- n>unordered_set</c-><c- o>></c-></code> by
overloading the lookup member functions with member function templates.</p>
   <p>There are no language changes.</p>
   <p>Almost all existing C++17 code is unaffected because new member functions are disabled from overload
resolution process unless <code class="highlight"><c- n>Hash</c-></code> template parameter has <code class="highlight"><c- n>transparent_key_equal</c-></code> property. That is not the case
for the code created before this proposal.</p>
   <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-heterogeneous-hash"><span class="secno">4.1. </span><span class="content">Heterogeneous hash function object</span><a class="self-link" href="#design-heterogeneous-hash"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-n3573">[N3573]</a> paper suggests adding</p>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-> <c- o>=</c-> <c- b>void</c-><c- o>></c->
  <c- k>struct</c-> <c- n>hash</c-><c- p>;</c->
  
  <c- k>template</c-><c- o>&lt;></c->
  <c- k>struct</c-> <c- n>hash</c-><c- o>&lt;</c-><c- b>void</c-><c- o>></c-> <c- p>{</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->
    <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- k>operator</c-><c- p>()(</c-><c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c->
      <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>std</c-><c- o>::</c-><c- n>decay</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>type</c-><c- o>></c-><c- p>()(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>));</c->
    <c- p>}</c->
  <c- p>};</c->
<c- p>}</c->
</pre>
   <p>While that could be useful and compatible with changes introduced for many operations in <a data-link-type="biblio" href="#biblio-n3421">[N3421]</a>,
there is too big chance of two types being equality-comparable but having incompatible hashes.</p>
   <p>Following issue was pointed out in the <a data-link-type="biblio" href="#biblio-reflector-1">[REFLECTOR-1]</a>.</p>
   <p>For example, under gcc 7.2.0,</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- b>long</c-><c- o>></c-><c- p>{}(</c-><c- o>-</c-><c- mi>1L</c-><c- p>)</c-> <c- o>==</c-> <c- mi>18446744073709551615ULL</c->
<c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-><c- p>{}(</c-><c- o>-</c-><c- mf>1.0</c-><c- p>)</c-> <c- o>==</c-> <c- mi>11078049357879903929ULL</c->
</pre>
   <p>which makes following code fail</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>unordered_set</c-><c- o>&lt;</c-><c- b>double</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;></c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>equal_to</c-><c- o>&lt;>></c-> <c- n>s</c-><c- p>;</c->
<c- n>s</c-><c- p>.</c-><c- n>insert</c-><c- p>(</c-><c- o>-</c-><c- mi>1L</c-><c- p>);</c->                  <c- c1>// Internally converts -1L to -1.0</c->
<c- n>assert</c-><c- p>(</c-><c- n>s</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- o>-</c-><c- mi>1L</c-><c- p>)</c-> <c- o>!=</c-> <c- n>s</c-><c- p>.</c-><c- n>end</c-><c- p>());</c-> <c- c1>// Fails: calls hash&lt;long>(-1L) and gets the wrong bucket</c->
</pre>
   <p>Note that under C++17 rules that code succeeds, because <code class="highlight"><c- n>find</c-><c- p>()</c-></code> also converts its parameter to <code class="highlight"><c- b>double</c-></code> before hashing.</p>
   <p>This proposal intentionally <strong>does not suggest</strong> standardizing heterogeneous hash function object <code class="highlight"><c- k>template</c-><c- o>&lt;></c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- b>void</c-><c- o>></c-></code>. Doing that might be tempting but it requires more investigations and
can be always added via future proposals.</p>
   <h3 class="heading settled" data-level="4.2" id="design-function-additional-parameters"><span class="secno">4.2. </span><span class="content">Additional parameters in lookup member functions overloads</span><a class="self-link" href="#design-function-additional-parameters"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-n3573">[N3573]</a> also proposes adding additional parameters to lookup functions so the users may provide
different hash and equality comparison functor objects for each member function call.</p>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Hash</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;></c-><c- p>,</c-> <c- k>typename</c-> <c- n>Eq</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>equal_to</c-><c- o>&lt;>></c->
<c- n>iterator</c-> <c- n>find</c-><c- p>(</c-><c- n>T</c-> <c- n>t</c-><c- p>,</c-> <c- n>Hash</c-> <c- n>h</c-> <c- o>=</c-> <c- n>Hash</c-><c- p>(),</c-> <c- n>Eq</c-> <c- n>e</c-> <c- o>=</c-> <c- n>Eq</c-><c- p>());</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Hash</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;></c-><c- p>,</c-> <c- k>typename</c-> <c- n>Eq</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>equal_to</c-><c- o>&lt;>></c->
<c- n>const_iterator</c-> <c- n>find</c-><c- p>(</c-><c- n>T</c-> <c- n>t</c-><c- p>,</c-> <c- n>Hash</c-> <c- n>h</c-> <c- o>=</c-> <c- n>Hash</c-><c- p>(),</c-> <c- n>Eq</c-> <c- n>e</c-> <c- o>=</c-> <c- n>Eq</c-><c- p>())</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <p>That is not consistent with the current interface of ordered associative containers and therefore it
is <strong>not proposed</strong> by this paper. If such functionality is considered useful it can be added in the
future by other paper both for ordered and unordered associative containers.</p>
   <h3 class="heading settled" data-level="4.3" id="design-function-template"><span class="secno">4.3. </span><span class="content">Lookup member functions template overloads</span><a class="self-link" href="#design-function-template"></a></h3>
   <p>For consistency reasons this paper proposes heterogeneous lookup for unordered associative containers
should be provided by the similar means as it is the case for ordered ones. Containers <strong>will only
change their interface when hash functor will define nested tag type called</strong> <code class="highlight"><c- n>transparent_key_equal</c-></code> that specifies transparent equality comparator type to be used by the container <strong>instead of a type
provided (or default type) for</strong> <code class="highlight"><c- n>Pred</c-></code> <strong>template parameter</strong>.</p>
   <p>The container <strong>will fail to compile</strong> (with proper diagnostics applied) when:</p>
   <ul>
    <li data-md>
     <p>equality comparator type provided by the <code class="highlight"><c- n>hasher</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code> is not transparent (does not
provide <code class="highlight"><c- n>is_transparent</c-></code> tag type)</p>
    <li data-md>
     <p><code class="highlight"><c- n>Pred</c-></code> container’s template argument is neither</p>
     <ul>
      <li data-md>
       <p>the default type of that template parameter (namely <code class="highlight"><c- n>equal_to</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-></code>)</p>
      <li data-md>
       <p>the same type as provided by the hasher via <code class="highlight"><c- n>transparent_key_equal</c-></code> tag</p>
     </ul>
   </ul>
   <p><code class="highlight"><c- n>key_equal</c-></code> <strong>member type of the container</strong> will specify either:</p>
   <ul>
    <li data-md>
     <p>the type provided by the <code class="highlight"><c- n>Pred</c-></code> template argument of the container (or its default type) in case the
heterogeneous lookup is disabled</p>
    <li data-md>
     <p>the type provided by the <code class="highlight"><c- n>transparent_key_equal</c-></code> tag of the hash functor object otherwise</p>
   </ul>
   <p class="note" role="note"><span>Note:</span> Changing the specification of the default type in container’s template parameters would cause
      the ABI break, therefore, it is not suggested by that proposal.</p>
   <p>By providing explicit tag <code class="highlight"><c- n>transparent_key_equal</c-></code> in the hash functor object, the user explicitly states that
the intention of this type is to provide coherent and interchangeable hash values for all the types 
supported by the functor’s call operators.</p>
   <p>Concerns raised in <a href="#motivation-and-scope">§1 Motivation and Scope</a> are addressed by this proposal in the following way:</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- n>string_hash</c-> <c- p>{</c->
  <c- k>using</c-> <c- n>transparent_key_equal</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>equal_to</c-><c- o>&lt;></c-><c- p>;</c->  <c- c1>// Pred to use</c->
  <c- k>using</c-> <c- n>hash_type</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-><c- o>></c-><c- p>;</c->  <c- c1>// just a helper local type</c->
  <c- b>size_t</c-> <c- nf>operator</c-><c- p>()(</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>txt</c-><c- p>)</c-> <c- k>const</c->   <c- p>{</c-> <c- k>return</c-> <c- n>hash_type</c-><c- p>{}(</c-><c- n>txt</c-><c- p>);</c-> <c- p>}</c->
  <c- b>size_t</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>string</c-><c- o>&amp;</c-> <c- n>txt</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- n>hash_type</c-><c- p>{}(</c-><c- n>txt</c-><c- p>);</c-> <c- p>}</c->
  <c- b>size_t</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- b>char</c-><c- o>*</c-> <c- n>txt</c-><c- p>)</c-> <c- k>const</c->        <c- p>{</c-> <c- k>return</c-> <c- n>hash_type</c-><c- p>{}(</c-><c- n>txt</c-><c- p>);</c-> <c- p>}</c->
<c- p>};</c->

<c- n>std</c-><c- o>::</c-><c- n>unordered_map</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- p>,</c-> <c- b>int</c-><c- p>,</c-> <c- n>string_hash</c-><c- o>></c-> <c- n>map</c-> <c- o>=</c-> <c- d>/* ... */</c-><c- p>;</c->
<c- n>map</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- s>"abc"</c-><c- p>);</c->
<c- n>map</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- s>"def"</c-><c- n>sv</c-><c- p>);</c->
</pre>
   <p>Note that in the above example the 4th template argument (<code class="highlight"><c- n>Pred</c-></code>) is intentionally skipped and
will be overwritten with the type provided by the <code class="highlight"><c- n>string_hash</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code>.</p>
   <p>In case the user needs to provide custom <code class="highlight"><c- n>Allocator</c-></code> type the <code class="highlight"><c- n>Pred</c-></code> arguments needs to match the type
provided by <code class="highlight"><c- n>hasher</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code>:</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>unordered_map</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- p>,</c-> <c- b>int</c-><c- p>,</c-> <c- n>string_hash</c-><c- p>,</c->
                   <c- n>string_hash</c-><c- o>::</c-><c- n>transparent_key_equal</c-><c- p>,</c->
                   <c- n>std</c-><c- o>::</c-><c- n>allocator</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>pair</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>string</c-><c- p>,</c-> <c- b>int</c-><c- o>>>></c-> <c- n>map</c-> <c- o>=</c-> <c- d>/* ... */</c-><c- p>;</c->
</pre>
   <p>To find more details on how to address all code examples provided in this paper please refer to <a href="#implementation">§7 Implementation Experience</a>.</p>
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">5. </span><span class="content">Proposed Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>The proposed changes are relative to the working draft of the standard as of <a data-link-type="biblio" href="#biblio-n4762">[n4762]</a>.</p>
   <p>Modify <strong>21.2.7 [unord.req]</strong> paragraph 5 as follows:</p>
   <blockquote>
     Two values <code class="highlight"><c- n>k1</c-></code> and <code class="highlight"><c- n>k2</c-></code> 
    <del>of type Key</del>
     are considered equivalent if the container’s key equality predicate 
    <ins><code class="highlight"><c- n>pred</c-><c- p>(</c-><c- n>k1</c-><c- p>,</c-> <c- n>k2</c-><c- p>)</c-></code> is valid and </ins>
     returns <code class="highlight">true</code> when passed those values. If <code class="highlight"><c- n>k1</c-></code> and <code class="highlight"><c- n>k2</c-></code> are equivalent, the
container’s hash function shall return the same value for both. ... 
   </blockquote>
   <p>Modify <strong>21.2.7 [unord.req]</strong> paragraph 11 as follows:</p>
   <blockquote>
     In Table 91: <code class="highlight"><c- n>X</c-></code> denotes an unordered associative container class, <code class="highlight"><c- n>a</c-></code> denotes a value of type <code class="highlight"><c- n>X</c-></code>, <code class="highlight"><c- n>a2</c-></code> denotes a value of a type with nodes compatible with type <code class="highlight"><c- n>X</c-></code> (Table 89), <code class="highlight"><c- n>b</c-></code> denotes a possibly <code class="highlight"><c- k>const</c-></code> value of type <code class="highlight"><c- n>X</c-></code>, <code class="highlight"><c- n>a_uniq</c-></code> denotes a value of type <code class="highlight"><c- n>X</c-></code> when <code class="highlight"><c- n>X</c-></code> supports unique keys, <code class="highlight"><c- n>a_eq</c-></code> denotes a value of type <code class="highlight"><c- n>X</c-></code> when <code class="highlight"><c- n>X</c-></code> supports equivalent keys, 
    <ins><code class="highlight"><c- n>a_tran</c-></code> denotes a possibly <code class="highlight"><c- k>const</c-></code> value of type <code class="highlight"><c- n>X</c-></code> when the <em>qualified-id</em> <code class="highlight"><c- n>X</c-><c- o>::</c-><c- n>hasher</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code> is valid and denotes a type (12.9.2),</ins>
     <code class="highlight"><c- n>i</c-></code> and <code class="highlight"><c- n>j</c-></code> denote input iterators that refer to <code class="highlight"><c- n>value_type</c-></code>, <code class="highlight"><c- p>[</c-><c- n>i</c-><c- p>,</c-> <c- n>j</c-><c- p>)</c-></code> denotes a valid range, <code class="highlight"><c- n>p</c-></code> and <code class="highlight"><c- n>q2</c-></code> denote valid constant iterators to <code class="highlight"><c- n>a</c-></code>, <code class="highlight"><c- n>q</c-></code> and <code class="highlight"><c- n>q1</c-></code> denote valid dereferenceable constant iterators
to <code class="highlight"><c- n>a</c-></code>, <code class="highlight"><c- n>r</c-></code> denotes a valid dereferenceable iterator to <code class="highlight"><c- n>a</c-></code>, <code class="highlight"><c- p>[</c-><c- n>q1</c-><c- p>,</c-> <c- n>q2</c-><c- p>)</c-></code> denotes a valid range in <code class="highlight"><c- n>a</c-></code>, <code class="highlight"><c- n>il</c-></code> denotes a value of type <code class="highlight"><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-></code>, <code class="highlight"><c- n>t</c-></code> denotes a value of type <code class="highlight"><c- n>X</c-><c- o>::</c-><c- n>value_type</c-></code>, <code class="highlight"><c- n>k</c-></code> denotes a value of type <code class="highlight"><c- n>key_type</c-></code>, <code class="highlight"><c- n>hf</c-></code> denotes a possibly <code class="highlight"><c- k>const</c-></code> value of type <code class="highlight"><c- n>hasher</c-></code>, <code class="highlight"><c- n>eq</c-></code> denotes
a possibly <code class="highlight"><c- k>const</c-></code> value of type <code class="highlight"><c- n>key_equal</c-></code>, 
    <ins><code class="highlight"><c- n>ke</c-></code> is a value such that (1) <code class="highlight"><c- n>eq</c-><c- p>(</c-><c- n>r1</c-><c- p>,</c-> <c- n>ke</c-><c- p>)</c-> <c- o>==</c-> <c- n>eq</c-><c- p>(</c-><c- n>ke</c-><c- p>,</c-> <c- n>r1</c-><c- p>)</c-></code> with <code class="highlight"><c- n>r1</c-></code> the key value of <code class="highlight"><c- n>e</c-></code> and <code class="highlight"><c- n>e</c-></code> in <code class="highlight"><c- n>a_tran</c-></code>,
(2) <code class="highlight"><c- n>hf</c-><c- p>(</c-><c- n>r1</c-><c- p>)</c-> <c- o>==</c-> <c- n>hf</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-></code> if <code class="highlight"><c- n>eq</c-><c- p>(</c-><c- n>r1</c-><c- p>,</c-> <c- n>ke</c-><c- p>)</c-></code> is <code class="highlight">true</code>, and (3) <code class="highlight"><c- p>(</c-><c- n>eq</c-><c- p>(</c-><c- n>r1</c-><c- p>,</c-> <c- n>ke</c-><c- p>)</c-> <c- o>&amp;&amp;</c-> <c- n>eq</c-><c- p>(</c-><c- n>r1</c-><c- p>,</c-> <c- n>r2</c-><c- p>))</c-> <c- o>==</c-> <c- n>eq</c-><c- p>(</c-><c- n>r2</c-><c- p>,</c-> <c- n>ke</c-><c- p>)</c-></code> where <code class="highlight"><c- n>r2</c-></code> is
the key of an element in <code class="highlight"><c- n>a_tran</c-></code>,</ins>
     <code class="highlight"><c- n>n</c-></code> denotes a value of type <code class="highlight"><c- n>size_type</c-></code>, <code class="highlight"><c- n>z</c-></code> denotes a value of type <code class="highlight"><c- b>float</c-></code>, and <code class="highlight"><c- n>nh</c-></code> denotes
a non-const rvalue of type <code class="highlight"><c- n>X</c-><c- o>::</c-><c- n>node_type</c-></code>. 
   </blockquote>
   <p>Modify table 72 in section <strong>21.2.7 [unord.req]</strong> as follows:</p>
   <blockquote>
    <table>
     <tbody>
      <tr>
       <th>Expression
       <th>Return type
       <th>Assertion/note pre-/post-condition
       <th>Complexity
      <tr>
       <td>
        <ins>X::key_equal</ins>
       <td>
        <del>Pred</del>
        <ins><code class="highlight"><c- n>Hash</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code> if such a <em>qualified-id</em> is valid and denotes a type (12.9.2);
        otherwise, <code class="highlight"><c- n>Pred</c-></code>.</ins>
       <td>Requires: <code class="highlight"><c- n>key_equal</c-></code> is <code class="highlight"><c- n>CopyConstructible</c-></code>. <code class="highlight"><c- n>key_equal</c-></code> shall be a binary predicate that takes two arguments of type <code class="highlight"><c- n>Key</c-></code>. <code class="highlight"><c- n>key_equal</c-></code> is an equivalence relation.
       <td>compile time
      <tr>
       <td colspan="4">...
      <tr>
       <td><code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code>
       <td><code class="highlight"><c- n>iterator</c-></code>; <code class="highlight"><c- n>const_iterator</c-></code> for const <code class="highlight"><c- n>b</c-></code>.
       <td>Returns an iterator pointing to an element with key equivalent to <code class="highlight"><c- n>k</c-></code>, or <code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>end</c-><c- p>()</c-></code> if no such element exists.
       <td>Average case O(1), worst case O(<code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>).
      <tr>
       <td>
        <ins><code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-></code></ins>
       <td>
        <ins><code class="highlight"><c- n>iterator</c-></code>; <code class="highlight"><c- n>const_iterator</c-></code> for const <code class="highlight"><c- n>a_tran</c-></code>.</ins>
       <td>
        <ins>Returns an iterator pointing to an element with key equivalent to <code class="highlight"><c- n>ke</c-></code>, or <code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>end</c-><c- p>()</c-></code> if no such
    element exists.</ins>
       <td>
        <ins>Average case O(1), worst case O(<code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>).</ins>
      <tr>
       <td><code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>count</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code>
       <td><code class="highlight"><c- n>size_type</c-></code>
       <td>Returns the number of elements with key equivalent to <code class="highlight"><c- n>k</c-></code>.
       <td>Average case O(<code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>count</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code>), worst case O(<code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>).
      <tr>
       <td>
        <ins><code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>count</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-></code></ins>
       <td>
        <ins><code class="highlight"><c- n>size_type</c-></code></ins>
       <td>
        <ins>Returns the number of elements with key equivalent to <code class="highlight"><c- n>ke</c-></code>.</ins>
       <td>
        <ins>Average case O(<code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>count</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-></code>), worst case O(<code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>).</ins>
      <tr>
       <td><code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>contains</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code>
       <td>bool
       <td>Equivalent to <code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-> <c- o>!=</c-> <c- n>b</c-><c- p>.</c-><c- n>end</c-><c- p>()</c-></code>
       <td>Average case O(1), worst case O(<code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>)
      <tr>
       <td>
        <ins><code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>contains</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-></code></ins>
       <td>
        <ins>bool</ins>
       <td>
        <ins>Equivalent to <code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-> <c- o>!=</c-> <c- n>a_tran</c-><c- p>.</c-><c- n>end</c-><c- p>()</c-></code></ins>
       <td>
        <ins>Average case O(1), worst case O(<code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>)</ins>
      <tr>
       <td><code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>equal_range</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code>
       <td><code class="highlight"><c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c-></code>; <code class="highlight"><c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-></code> for const <code class="highlight"><c- n>b</c-></code>.
       <td>Returns a range containing all elements with keys equivalent to <code class="highlight"><c- n>k</c-></code>. Returns <code class="highlight"><c- n>make_pair</c-><c- p>(</c-><c- n>b</c-><c- p>.</c-><c- n>end</c-><c- p>(),</c-> <c- n>b</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-></code> if no such elements exist.
       <td>Average case O(<code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>count</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code>), worst case O(<code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>).
      <tr>
       <td>
        <ins><code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>equal_range</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-></code></ins>
       <td>
        <ins><code class="highlight"><c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c-></code>; <code class="highlight"><c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-></code> for const <code class="highlight"><c- n>a_tran</c-></code>.</ins>
       <td>
        <ins>Returns a range containing all elements with keys equivalent to <code class="highlight"><c- n>ke</c-></code>. Returns <code class="highlight"><c- n>make_pair</c-><c- p>(</c-><c- n>a_tran</c-><c- p>.</c-><c- n>end</c-><c- p>(),</c-> <c- n>a_tran</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-></code> if no such elements exist.</ins>
       <td>
        <ins>Average case O(<code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>count</c-><c- p>(</c-><c- n>ke</c-><c- p>)</c-></code>), worst case O(<code class="highlight"><c- n>a_tran</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>).</ins>
    </table>
   </blockquote>
   <p>Add new paragraphs (18, 19) in <strong>21.2.7 [unord.req]</strong>:</p>
   <blockquote>
    <ins>
      If the <em>qualified-id</em> <code class="highlight"><c- n>Hash</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code> is valid and denotes a type (12.9.2), then the program is ill-formed 
if either: 
     <ul>
      <li data-md>
       <p><em>qualified-id</em> <code class="highlight"><c- n>Hash</c-><c- o>::</c-><c- n>transparent_key_equal</c-><c- o>::</c-><c- n>is_transparent</c-></code> is not valid or does not denote a type, or</p>
      <li data-md>
       <p><code class="highlight"><c- n>Pred</c-></code> is a different type than <code class="highlight"><c- n>equal_to</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-></code> or <code class="highlight"><c- n>Hash</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code>.</p>
     </ul>
    </ins>
   </blockquote>
   <blockquote>
    <ins> The member function templates <code class="highlight"><c- n>find</c-></code>, <code class="highlight"><c- n>count</c-></code>,<code class="highlight"><c- n>equal_range</c-></code>, and <code class="highlight"><c- n>contains</c-></code> shall not participate in overload
resolution unless the <em>qualified-id</em> <code class="highlight"><c- n>Hash</c-><c- o>::</c-><c- n>transparent_key_equal</c-></code> is valid and denotes a type (12.9.2). </ins>
   </blockquote>
   <p>Modify <strong>21.5.4.1 [unord.map.overview]</strong> paragraph 3 as follows:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Key</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>T</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Hash</c-> <c- o>=</c-> <c- n>hash</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Pred</c-> <c- o>=</c-> <c- n>equal_to</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><c- n>pair</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Key</c-><c- p>,</c-> <c- n>T</c-><c- o>>>></c->
  <c- k>class</c-> <c- nc>unordered_map</c-> <c- p>{</c->
  <c- k>public</c-><c- o>:</c->
    <c- c1>// types</c->
    <c- k>using</c-> <c- n>key_type</c->         <c- o>=</c-> <c- n>Key</c-><c- p>;</c->
    <c- k>using</c-> <c- n>mapped_type</c->      <c- o>=</c-> <c- n>T</c-><c- p>;</c->
    <c- k>using</c-> <c- n>value_type</c->       <c- o>=</c-> <c- n>pair</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Key</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-><c- p>;</c->
    <c- k>using</c-> <c- n>hasher</c->           <c- o>=</c-> <c- n>Hash</c-><c- p>;</c->
    <del><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <c- n>Pred</c-><c- p>;</c-></del>
    <ins><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <i><c- n>see</c-> <c- p>[</c-><c- n>unord</c-><c- p>.</c-><c- n>req</c-><c- p>]</c-></i><c- p>;</c-></ins>
    <c- k>using</c-> <c- n>allocator_type</c->   <c- o>=</c-> <c- n>Allocator</c-><c- p>;</c->
</pre>
   </blockquote>
   <blockquote>
<pre class="highlight"><c- c1>// map operations:</c->
<c- n>iterator</c->       <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>const_iterator</c-> <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>iterator</c->       <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>const_iterator</c-> <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>size_type</c-> <c- nf>count</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>size_type</c-> <c- n>count</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- b>bool</c-> <c- nf>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- b>bool</c-> <c- n>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <p>In <strong>21.5.5.1 [unord.multimap.overview]</strong> add:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Key</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>T</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Hash</c-> <c- o>=</c-> <c- n>hash</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Pred</c-> <c- o>=</c-> <c- n>equal_to</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><c- n>pair</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Key</c-><c- p>,</c-> <c- n>T</c-><c- o>>>></c->
  <c- k>class</c-> <c- nc>unordered_multimap</c-> <c- p>{</c->
  <c- k>public</c-><c- o>:</c->
    <c- c1>// types</c->
    <c- k>using</c-> <c- n>key_type</c->         <c- o>=</c-> <c- n>Key</c-><c- p>;</c->
    <c- k>using</c-> <c- n>mapped_type</c->      <c- o>=</c-> <c- n>T</c-><c- p>;</c->
    <c- k>using</c-> <c- n>value_type</c->       <c- o>=</c-> <c- n>pair</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Key</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-><c- p>;</c->
    <c- k>using</c-> <c- n>hasher</c->           <c- o>=</c-> <c- n>Hash</c-><c- p>;</c->
    <del><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <c- n>Pred</c-><c- p>;</c-></del>
    <ins><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <i><c- n>see</c-> <c- p>[</c-><c- n>unord</c-><c- p>.</c-><c- n>req</c-><c- p>]</c-></i><c- p>;</c-></ins>
    <c- k>using</c-> <c- n>allocator_type</c->   <c- o>=</c-> <c- n>Allocator</c-><c- p>;</c->
</pre>
   </blockquote>
   <blockquote>
<pre class="highlight"><c- c1>// map operations:</c->
<c- n>iterator</c->       <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>const_iterator</c-> <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>iterator</c->       <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>const_iterator</c-> <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>size_type</c-> <c- nf>count</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>size_type</c-> <c- n>count</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- b>bool</c-> <c- nf>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- b>bool</c-> <c- n>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <p>In <strong>21.5.6.1 [unord.set.overview]</strong> add:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Key</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Hash</c-> <c- o>=</c-> <c- n>hash</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Pred</c-> <c- o>=</c-> <c- n>equal_to</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><c- n>pair</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Key</c-><c- p>,</c-> <c- n>T</c-><c- o>>>></c->
  <c- k>class</c-> <c- nc>unordered_set</c-> <c- p>{</c->
  <c- k>public</c-><c- o>:</c->
    <c- c1>// types</c->
    <c- k>using</c-> <c- n>key_type</c->         <c- o>=</c-> <c- n>Key</c-><c- p>;</c->
    <c- k>using</c-> <c- n>value_type</c->       <c- o>=</c-> <c- n>Key</c-><c- p>;</c->
    <c- k>using</c-> <c- n>hasher</c->           <c- o>=</c-> <c- n>Hash</c-><c- p>;</c->
    <del><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <c- n>Pred</c-><c- p>;</c-></del>
    <ins><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <i><c- n>see</c-> <c- p>[</c-><c- n>unord</c-><c- p>.</c-><c- n>req</c-><c- p>]</c-></i><c- p>;</c-></ins>
    <c- k>using</c-> <c- n>allocator_type</c->   <c- o>=</c-> <c- n>Allocator</c-><c- p>;</c->
</pre>
   </blockquote>
   <blockquote>
<pre class="highlight"><c- c1>// set operations:</c->
<c- n>iterator</c->       <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>const_iterator</c-> <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>iterator</c->       <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>const_iterator</c-> <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>size_type</c-> <c- nf>count</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>size_type</c-> <c- n>count</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- b>bool</c-> <c- nf>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- b>bool</c-> <c- n>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <p>In <strong>21.5.7.1 [unord.multiset.overview]</strong> add:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Key</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Hash</c-> <c- o>=</c-> <c- n>hash</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Pred</c-> <c- o>=</c-> <c- n>equal_to</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><c- n>pair</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Key</c-><c- p>,</c-> <c- n>T</c-><c- o>>>></c->
  <c- k>class</c-> <c- nc>unordered_multiset</c-> <c- p>{</c->
  <c- k>public</c-><c- o>:</c->
    <c- c1>// types</c->
    <c- k>using</c-> <c- n>key_type</c->         <c- o>=</c-> <c- n>Key</c-><c- p>;</c->
    <c- k>using</c-> <c- n>value_type</c->       <c- o>=</c-> <c- n>Key</c-><c- p>;</c->
    <c- k>using</c-> <c- n>hasher</c->           <c- o>=</c-> <c- n>Hash</c-><c- p>;</c->
    <del><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <c- n>Pred</c-><c- p>;</c-></del>
    <ins><c- k>using</c-> <c- n>key_equal</c->        <c- o>=</c-> <i><c- n>see</c-> <c- p>[</c-><c- n>unord</c-><c- p>.</c-><c- n>req</c-><c- p>]</c-></i><c- p>;</c-></ins>
    <c- k>using</c-> <c- n>allocator_type</c->   <c- o>=</c-> <c- n>Allocator</c-><c- p>;</c->
</pre>
   </blockquote>
   <blockquote>
<pre class="highlight"><c- c1>// set operations:</c->
<c- n>iterator</c->       <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>const_iterator</c-> <c- nf>find</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>iterator</c->       <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>const_iterator</c-> <c- n>find</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>size_type</c-> <c- nf>count</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>size_type</c-> <c- n>count</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- b>bool</c-> <c- nf>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- b>bool</c-> <c- n>contains</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
<c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>key_type</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>iterator</c-><c- p>,</c-> <c- n>iterator</c-><c- o>></c->             <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>K</c-><c- o>></c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>const_iterator</c-><c- p>,</c-> <c- n>const_iterator</c-><c- o>></c-> <c- n>equal_range</c-><c- p>(</c-><c- k>const</c-> <c- n>K</c-><c- o>&amp;</c-> <c- n>k</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <h2 class="heading settled" data-level="6" id="feature-testing"><span class="secno">6. </span><span class="content">Feature Testing</span><a class="self-link" href="#feature-testing"></a></h2>
   <p>Add the following row to a <strong>Table 35</strong> in <strong>16.3.1 [support.limits.general]</strong> paragraph 3:</p>
   <table>
    <tbody>
     <tr>
      <th>Macro name
      <th>Value
      <th>Header(s)
     <tr>
      <td colspan="3">...
     <tr>
      <td>__cpp_lib_generic_associative_lookup
      <td>201304L
      <td>&lt;map> &lt;set>
     <tr>
      <td>
       <ins>__cpp_lib_generic_unordered_lookup</ins>
      <td>
      <td>
       <ins>&lt;unordered_map> &lt;unordered_set></ins>
   </table>
   <h2 class="heading settled" data-level="7" id="implementation"><span class="secno">7. </span><span class="content">Implementation Experience</span><a class="self-link" href="#implementation"></a></h2>
   <p>Changes related to this proposal as well as answers to all of the code examples provided in this paper
are partially implemented in <a href="https://github.com/mpusz/unordered_v2">GitHub repo</a> against <a href="https://libcxx.llvm.org">libc++ 5.0.0</a>.</p>
   <p>Simple performance tests provided there proved more than:</p>
   <ul>
    <li data-md>
     <p>20% performance gain for short text (SSO used in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> temporary) in <a href="#example-1">EXAMPLE 1</a></p>
    <li data-md>
     <p>35% performance gain for long text (dynamic memory allocation in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> temporary) in <a href="#example-1">EXAMPLE 1</a></p>
    <li data-md>
     <p>85% performance gain in <a href="#example-3">EXAMPLE 3</a></p>
   </ul>
   <h2 class="heading settled" data-level="8" id="future-extensions"><span class="secno">8. </span><span class="content">Possible Future Extensions</span><a class="self-link" href="#future-extensions"></a></h2>
   <p><a href="#design-heterogeneous-hash">§4.1 Heterogeneous hash function object</a> and <a href="#design-function-additional-parameters">§4.2 Additional parameters in lookup member functions overloads</a> are not proposed by
this paper but can be explored and proposed in the future.</p>
   <h2 class="heading settled" data-level="9" id="revision-history"><span class="secno">9. </span><span class="content">Revision History</span><a class="self-link" href="#revision-history"></a></h2>
   <h3 class="heading settled" data-level="9.1" id="r0r3"><span class="secno">9.1. </span><span class="content">r2 ➡ r3 <a href="https://github.com/mpusz/wg21-papers/commit/69181bbfb95858c26c675aeee25070c56039659b">[diff</a>]</span><a class="self-link" href="#r0r3"></a></h3>
   <ul>
    <li data-md>
     <p>Rebased to <a data-link-type="biblio" href="#biblio-n4762">[n4762]</a></p>
    <li data-md>
     <p>Wording updated according to the LWG feedback</p>
    <li data-md>
     <p>Feature test macro name changed</p>
   </ul>
   <h3 class="heading settled" data-level="9.2" id="r0r2"><span class="secno">9.2. </span><span class="content">r1 ➡ r2 <a href="https://github.com/mpusz/wg21_papers/commit/b08f5a6c6ab4385a8e3aac4473bafcd0906b3d94">[diff</a>]</span><a class="self-link" href="#r0r2"></a></h3>
   <ul>
    <li data-md>
     <p>Added support for <code class="highlight"><c- n>contains</c-><c- p>()</c-></code> member functions introduced by <a data-link-type="biblio" href="#biblio-p0458r2">[P0458r2]</a></p>
   </ul>
   <h3 class="heading settled" data-level="9.3" id="r0r1"><span class="secno">9.3. </span><span class="content">r0 ➡ r1 <a href="https://github.com/mpusz/wg21_papers/commit/8296c704573a8fb7f3a1ecc37f9eef9449dbc737">[diff</a>]</span><a class="self-link" href="#r0r1"></a></h3>
   <p>The paper was reviewed by LEWG at the 2018 Jacksonville meeting and resulted with the following straw polls</p>
   <ul>
    <li data-md>
     <p>Do we want encourage to do more work in that subject? Unanimous consent.</p>
    <li data-md>
     <p>Put the comparator in the hash (and forbid specifying it as an explicit 4th template parameter).</p>
   </ul>
<pre class="highlight"><c- o>|</c-> <c- n>SF</c-> <c- o>|</c-> <c- n>F</c-> <c- o>|</c-> <c- n>N</c-> <c- o>|</c-> <c- n>A</c-> <c- o>|</c-> <c- n>SA</c-> <c- o>|</c->
<c- o>|</c-> <c- mi>6</c->  <c- o>|</c-> <c- mi>8</c-> <c- o>|</c-> <c- mi>2</c-> <c- o>|</c-> <c- mi>0</c-> <c- o>|</c-> <c- mi>2</c->  <c- o>|</c->
</pre>
   <p><em>r1</em> changes the way the transparent equality comparator is provided to the class template.
Instead of depending on the user to do the right thing in providing both hasher and comparator that
are transitive and compatible with each other, now the design forces hasher to provide compatible
comparator.</p>
   <h2 class="heading settled" data-level="10" id="acknowledgements"><span class="secno">10. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to Casey Carter for initial review of this proposal and help with wording issues.</p>
   <p>Special thanks and recognition goes to <a href="http://www.epam.com">Epam Systems</a> for supporting my
membership in the ISO C++ Committee and the production of this proposal.</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="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-n3657">[N3657]
   <dd>J. Wakely, S. Lavavej, J. Muñoz. <a href="https://wg21.link/n3657">Adding heterogeneous comparison lookup to associative containers (rev 4)</a>. 19 March 2013. URL: <a href="https://wg21.link/n3657">https://wg21.link/n3657</a>
   <dt id="biblio-n4762">[N4762]
   <dd>Richard Smith. <a href="https://wg21.link/n4762">Working Draft, Standard for Programming Language C+</a>. 7 July 2018. URL: <a href="https://wg21.link/n4762">https://wg21.link/n4762</a>
   <dt id="biblio-p0458r2">[P0458r2]
   <dd>Mikhail Maltsev. <a href="https://wg21.link/p0458r2">Checking for Existence of an Element in Associative Containers</a>. URL: <a href="https://wg21.link/p0458r2">https://wg21.link/p0458r2</a>
  </dl>
  <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-n3421">[N3421]
   <dd>Stephan T. Lavavej. <a href="https://wg21.link/n3421">Making Operator Functors greater&lt;></a>. 20 September 2012. URL: <a href="https://wg21.link/n3421">https://wg21.link/n3421</a>
   <dt id="biblio-n3573">[N3573]
   <dd>Mark Boyall. <a href="https://wg21.link/n3573">Heterogenous extensions to unordered containers</a>. 10 March 2013. URL: <a href="https://wg21.link/n3573">https://wg21.link/n3573</a>
   <dt id="biblio-reflector-1">[REFLECTOR-1]
   <dd>Joe Gottman. <a href="https://groups.google.com/a/isocpp.org/d/msg/std-proposals/mRu7rIrDAEw/bYMyojZRaiEJ">N3573: Why heterogenous extensions to unordered containers will not work</a>. URL: <a href="https://groups.google.com/a/isocpp.org/d/msg/std-proposals/mRu7rIrDAEw/bYMyojZRaiEJ">https://groups.google.com/a/isocpp.org/d/msg/std-proposals/mRu7rIrDAEw/bYMyojZRaiEJ</a>
   <dt id="biblio-stackoverflow-1">[STACKOVERFLOW-1]
   <dd>Xeo. <a href="https://stackoverflow.com/a/17853770">Using a std::unordered_set of std::unique_ptr</a>. URL: <a href="https://stackoverflow.com/a/17853770">https://stackoverflow.com/a/17853770</a>
  </dl>