<!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>P1659R0: starts_with and ends_with</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">P1659R0<br>starts_with and ends_with</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-06-17">2019-06-17</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/p1659">https://wg21.link/p1659</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:cjdb.ns@gmail.com">Christopher Di Bella</a>
     <dt>Audience:
     <dd>LEWG, SG18
     <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 proposal seeks to add std::ranges::starts_with and std::ranges::ends_with, which would work on arbitrary ranges, and also answer questions such as "are the starting elements of `r1` <i>less than</i> the elements of `r2`?" and "are the final elements of `r1` <i>greater than</i> the elements of `r2`?". It also proposes a change to SD-8.</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="#motivation"><span class="secno">1</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#example-usage"><span class="secno">1.1</span> <span class="content">Example usage</span></a>
     </ol>
    <li><a href="#proposed-changes-to-sd-8"><span class="secno">2</span> <span class="content">Proposed changes to SD-8</span></a>
    <li>
     <a href="#proposed-changes-to-c23"><span class="secno">3</span> <span class="content">Proposed changes to C++23</span></a>
     <ol class="toc">
      <li><a href="#25514-starts-with-algstarts_with"><span class="secno">3.1</span> <span class="content">25.5.14 Starts with [alg.starts_with]</span></a>
      <li><a href="#25515-ends-with-algends_with"><span class="secno">3.2</span> <span class="content">25.5.15 Ends with [alg.ends_with]</span></a>
     </ol>
    <li><a href="#reference-implementation"><span class="secno">4</span> <span class="content">Reference implementation</span></a>
    <li><a href="#acknowledgements"><span class="secno">5</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="motivation"><span class="secno">1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>C++20 introduces <code class="highlight"><c- n>basic_string_view</c-><c- o>::</c-><c- n>starts_with</c-></code>, <code class="highlight"><c- n>basic_string_view</c-><c- o>::</c-><c- n>ends_with</c-></code>, <code class="highlight"><c- n>basic_string</c-><c- o>::</c-><c- n>starts_with</c-></code>, and <code class="highlight"><c- n>basic_string</c-><c- o>::</c-><c- n>ends_with</c-></code>. Both <code class="highlight"><c- n>basic_string</c-></code> and <code class="highlight"><c- n>basic_string_view</c-></code> are perculiar container-like types, with many member functions that duplicate
algorithm functionality with minor interface changes (e.g. <code class="highlight"><c- n>compare</c-></code>, <code class="highlight"><c- n>copy</c-></code>, <code class="highlight"><c- n>find</c-></code>, <code class="highlight"><c- n>rfind</c-></code>, <code class="highlight"><c- n>find_first_of</c-></code>, etc.). <a data-link-type="biblio" href="#biblio-p0457">[P0457]</a> §5.1 notes that the decision to add <code class="highlight"><c- n>starts_with</c-></code> and <code class="highlight"><c- n>ends_with</c-></code> as member functions was because (a) it is consistent with the aforementioned member functions, and
(b) that P0457 agrees with <a data-link-type="biblio" href="#biblio-n3609">[N3609]</a> in that <code class="highlight"><c- n>starts_with</c-><c- p>(</c-><c- n>haystack</c-><c- p>,</c-> <c- n>needle</c-><c- p>)</c-></code> is ambiguous with <code class="highlight"><c- n>starts_with</c-><c- p>(</c-><c- n>needle</c-><c- p>,</c-> <c- n>haystack</c-><c- p>)</c-></code>. It should be noted that neither N3609, nor P0457 identified whether
or not they were talking about non-member functions that only operate on string types, or if they
were discussing an algorithm, but the LEWG minutes for P0457 reveal some LWEG interest in making
them algorithms.</p>
   <p>Although there is prior art with respect to <code class="highlight"><c- n>basic_string</c-></code>'s member functions, the author expresses
concern that our string types have a large set of member functions that either duplicate the
algorithms or are directly incompatible with them, and thus limit the amount of composition that’s
possible. Templates are one of C++'s greatest strengths, and with iterators, we have an extremely
extensible and powerful generic programming model. We should take advantage of this model wherever
possible to ensure that we do not paint ourselves into a corner with a single type.</p>
   <p>At present, it isn’t <i>immediately</i> possible to query whether or not any range -- other than a
standard string type -- is prefixed or suffixed by another range. To do so, one must use <code class="highlight"><c- n>mismatch</c-></code> or <code class="highlight"><c- n>equal</c-></code>, and at least in the case of <code class="highlight"><c- n>ends_with</c-></code>, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>next</c-></code> (C++20).</p>
   <table>
    <tbody>
     <tr align="center">
      <th colspan="2">Before-and-after table
     <tr>
      <th>C++20
      <th>Proposed
     <tr>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>some_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>50</c-><c- p>);</c->
<c- k>auto</c-> <c- n>some_more_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>30</c-><c- p>);</c->
<c- k>if</c-> <c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>mismatch</c-><c- p>(</c-><c- n>some_ints</c-><c- p>,</c-> <c- n>some_more_ints</c-><c- p>).</c-><c- n>in2</c-> <c- o>==</c-> <c- n>end</c-><c- p>(</c-><c- n>some_more_ints</c-><c- p>))</c-> <c- p>{</c->
   <c- c1>// do something</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>some_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>50</c-><c- p>);</c->
<c- k>auto</c-> <c- n>some_more_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>30</c-><c- p>);</c->
<c- k>if</c-> <c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>starts_with</c-><c- p>(</c-><c- n>some_ints</c-><c- p>,</c-> <c- n>some_more_ints</c-><c- p>))</c-> <c- p>{</c->
   <c- c1>// do something</c->
<c- p>}</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>some_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>50</c-><c- p>);</c->
<c- k>auto</c-> <c- n>some_more_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>30</c-><c- p>);</c->
<c- p>{</c->
   <c- k>auto</c-> <c- n>some_ints_tail</c-> <c- o>=</c-> <c- n>subrange</c-><c- p>{</c->
     <c- n>next</c-><c- p>(</c-><c- n>begin</c-><c- p>(</c-><c- n>some_ints</c-><c- p>),</c-> <c- n>distance</c-><c- p>(</c-><c- n>some_more_ints</c-><c- p>),</c-> <c- n>end</c-><c- p>(</c-><c- n>some_ints</c-><c- p>)),</c->
     <c- n>end</c-><c- p>(</c-><c- n>some_ints</c-><c- p>)</c->
   <c- p>};</c->
   <c- k>if</c-> <c- p>(</c-><c- n>not</c-> <c- n>equal</c-><c- p>(</c-><c- n>some_ints_tail</c-><c- p>,</c-> <c- n>some_more_ints</c-><c- p>))</c-> <c- p>{</c->
      <c- c1>// do something</c->
   <c- p>}</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>some_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>50</c-><c- p>);</c->
<c- k>auto</c-> <c- n>some_more_ints</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>30</c-><c- p>);</c->
<c- k>if</c-> <c- p>(</c-><c- n>not</c-> <c- n>ranges</c-><c- o>::</c-><c- n>ends_with</c-><c- p>(</c-><c- n>some_ints</c-><c- p>,</c-> <c- n>some_more_ints</c-><c- p>))</c-> <c- p>{</c->
   <c- c1>// do something</c->
<c- p>}</c->
</pre>
   </table>
   <p>It is interesting to note that, since <code class="highlight"><c- n>starts_with</c-></code> and <code class="highlight"><c- n>ends_with</c-></code> can be implemented using <code class="highlight"><c- n>mismatch</c-></code>, we are able to query more than just "are the first <i>n</i> elements of range one the
same as the entirety of range two?": we’re also able to query "are the first <i>n</i> elements of
range one all greater than the entirety of range two?", where <i>n</i> is the distance of the second
range. See §1.1 for examples. This is something that the string-based member functions are not able
to do, although the author acknowledges that text processing may not require this functionality.</p>
   <p>Concerns were outlined in both N3609 and P0457 about the ambiguity of whether we are performing <code class="highlight"><c- n>starts_with</c-><c- p>(</c-><c- n>haystack</c-><c- p>,</c-> <c- n>needle</c-><c- p>)</c-></code> or <code class="highlight"><c- n>starts_with</c-><c- p>(</c-><c- n>needle</c-><c- p>,</c-> <c- n>haystack</c-><c- p>)</c-></code>. There is prior art in the
algorithms library that makes the first range the subject of the operation: <code class="highlight"><c- n>mismatch</c-></code>, <code class="highlight"><c- n>equal</c-></code>, <code class="highlight"><c- n>search</c-></code>, <code class="highlight"><c- n>find_first_of</c-></code>, and <code class="highlight"><c- n>lexicographical_compare</c-></code> all take the form <code class="highlight"><c- n>algorithm</c-><c- p>(</c-><c- n>haystack</c-><c- p>,</c-> <c- n>needle</c-><c- p>)</c-></code>, so the author remains unconvinced about the ambiguity.</p>
   <h3 class="heading settled" data-level="1.1" id="example-usage"><span class="secno">1.1. </span><span class="content">Example usage</span><a class="self-link" href="#example-usage"></a></h3>
<pre class="language-cpp highlight"><c- k>auto</c-> <c- n>script</c-> <c- o>=</c-> u8<c- s>"OBI-WAN: Hello, there!</c-><c- se>\n</c-><c- s>"</c->
              u8<c- s>"GENERAL GRIEVOUS: General Kenobi, you are a bold one."</c-><c- n>sv</c-><c- p>;</c->
<c- k>namespace</c-> <c- n>ranges</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- p>;</c->
<c- n>ranges</c-><c- o>::</c-><c- n>starts_with</c-><c- p>(</c-><c- n>script</c-><c- p>,</c-> u8<c- s>"OBI-WAN"</c-><c- n>sv</c-><c- p>);</c->               <c- c1>// returns true</c->
<c- n>ranges</c-><c- o>::</c-><c- n>starts_with</c-><c- p>(</c-><c- n>script</c-><c- p>,</c-> u8<c- s>"ABCDEFG"</c-><c- n>sv</c-><c- p>);</c->               <c- c1>// returns false</c->
<c- n>ranges</c-><c- o>::</c-><c- n>starts_with</c-><c- p>(</c-><c- n>script</c-><c- p>,</c-> u8<c- s>"ABCDEFG"</c-><c- n>sv</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>less</c-><c- p>);</c-> <c- c1>// returns true</c->

<c- k>namespace</c-> <c- n>view</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>view</c-><c- p>;</c->
<c- n>ranges</c-><c- o>::</c-><c- n>ends_with</c-><c- p>(</c-><c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>50</c-><c- p>),</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>30</c-><c- p>)</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>20</c-><c- p>));</c->       <c- c1>// returns true</c->
<c- n>ranges</c-><c- o>::</c-><c- n>ends_with</c-><c- p>(</c-><c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>50</c-><c- p>),</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>30</c-><c- p>)</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>50</c-><c- p>));</c->       <c- c1>// returns false</c->
<c- n>ranges</c-><c- o>::</c-><c- n>ends_with</c-><c- p>(</c-><c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- mi>50</c-><c- p>),</c-> <c- n>view</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- o>-</c-><c- mi>50</c-><c- p>,</c-> <c- o>-</c-><c- mi>40</c-><c- p>),</c-> <c- n>ranges</c-><c- o>::</c-><c- n>greater</c-><c- p>);</c-> <c- c1>// returns true</c->
</pre>
   <h2 class="heading settled" data-level="2" id="proposed-changes-to-sd-8"><span class="secno">2. </span><span class="content">Proposed changes to SD-8</span><a class="self-link" href="#proposed-changes-to-sd-8"></a></h2>
   <p>In response to the concerns outlined in the motivation section of this document, the author would
like to request that LEWG consider discussing adopting in SD-8, a policy of ensuring that options
for algorithms are preferred when a proposal to add a member function to a container-like type is
considered.</p>
   <h2 class="heading settled" data-level="3" id="proposed-changes-to-c23"><span class="secno">3. </span><span class="content">Proposed changes to C++23</span><a class="self-link" href="#proposed-changes-to-c23"></a></h2>
   <p>Add the following text to [algorithm.synopsis]:</p>
<pre class="language-diff highlight">...
template&lt;class ForwardIterator, class Searcher>
  constexpr ForwardIterator
    search(ForwardIterator first, ForwardIterator last, const Searcher&amp; searcher);

+ namespace ranges {
+   // [alg.starts_with], starts_with
+   template&lt;InputIterator I1, Sentinel&lt;I1> S1, InputIterator I2, Sentinel&lt;I2> S2,
+            class Comp = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
+     requires IndirectlyComparable&lt;I1, I2, Comp, Proj1, Proj2>
+   constexpr bool starts_with(I1 first1, S1 last1, I2 first2, S2 last2, Comp comp = {},
+                              Proj1 proj1 = {}, Proj2 proj2 = {});
+   template&lt;InputRange R1, InputRange R2, class Comp = ranges::equal_to, class Proj1 = identity,
+            class Proj2 = identity>
+     requires IndirectlyComparable&lt;iterator_t&lt;R1>, iterator_t&lt;R2>, Comp, Proj1, Proj2>
+   constexpr bool starts_with(R1&amp;&amp; r1, R2&amp;&amp; r2, Comp comp = {}, Proj1 proj1 = {},
+                              Proj2 proj2 = {});
+
+   // [alg.ends_with], ends_with
+   template&lt;ForwardIterator I1, Sentinel&lt;I1> S1, InputIterator I2, Sentinel&lt;I2> S2,
+            class Comp = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
+     requires IndirectlyComparable&lt;I1, I2, Comp, Proj1, Proj2>
+   constexpr bool ends_with(I1 first1, S1 last1, I2 first2, S2 last2, Comp comp = {},
+                            Proj1 proj1 = {}, Proj2 proj2 = {});
+   template&lt;ForwardRange R1, InputRange R2, class Comp = ranges::equal_to, class Proj1 = identity,
+            class Proj2 = identity>
+     requires IndirectlyComparable&lt;iterator_t&lt;R1>, iterator_t&lt;R2>, Comp, Proj1, Proj2>
+   constexpr bool ends_with(R1&amp;&amp; r1, R2&amp;&amp; r2, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
+ }

// [alg.modifying.operations], mutating sequence operations
// [alg.copy], copy
...
</pre>
   <p>Add the following two sections to [alg.nonmodifying]:</p>
   <h3 class="heading settled" data-level="3.1" id="25514-starts-with-algstarts_with"><span class="secno">3.1. </span><span class="content">25.5.14 Starts with [alg.starts_with]</span><a class="self-link" href="#25514-starts-with-algstarts_with"></a></h3>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- n>ranges</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- n>InputIterator</c-> <c- n>I1</c-><c- p>,</c-> <c- n>Sentinel</c-><c- o>&lt;</c-><c- n>I1</c-><c- o>></c-> <c- n>S1</c-><c- p>,</c-> <c- n>InputIterator</c-> <c- n>I2</c-><c- p>,</c-> <c- n>Sentinel</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>></c-> <c- n>S2</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Comp</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>equal_to</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Proj1</c-> <c- o>=</c-> <c- n>identity</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Proj2</c-> <c- o>=</c-> <c- n>identity</c-><c- o>></c->
    <c- n>requires</c-> <c- n>IndirectlyComparable</c-><c- o>&lt;</c-><c- n>I1</c-><c- p>,</c-> <c- n>I2</c-><c- p>,</c-> <c- n>Comp</c-><c- p>,</c-> <c- n>Proj1</c-><c- p>,</c-> <c- n>Proj2</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- n>starts_with</c-><c- p>(</c-><c- n>I1</c-> <c- n>first1</c-><c- p>,</c-> <c- n>S1</c-> <c- n>last1</c-><c- p>,</c-> <c- n>I2</c-> <c- n>first2</c-><c- p>,</c-> <c- n>S2</c-> <c- n>last2</c-><c- p>,</c-> <c- n>Comp</c-> <c- n>comp</c-> <c- o>=</c-> <c- p>{},</c->
                             <c- n>Proj1</c-> <c- n>proj1</c-> <c- o>=</c-> <c- p>{},</c-> <c- n>Proj2</c-> <c- n>proj2</c-> <c- o>=</c-> <c- p>{});</c->
  <c- k>template</c-><c- o>&lt;</c-><c- n>InputRange</c-> <c- n>R1</c-><c- p>,</c-> <c- n>InputRange</c-> <c- n>R2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Comp</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>equal_to</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Proj1</c-> <c- o>=</c-> <c- n>identity</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Proj2</c-> <c- o>=</c-> <c- n>identity</c-><c- o>></c->
    <c- n>requires</c-> <c- n>IndirectlyComparable</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R1</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R2</c-><c- o>></c-><c- p>,</c-> <c- n>Comp</c-><c- p>,</c-> <c- n>Proj1</c-><c- p>,</c-> <c- n>Proj2</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- n>starts_with</c-><c- p>(</c-><c- n>R1</c-><c- o>&amp;&amp;</c-> <c- n>r1</c-><c- p>,</c-> <c- n>R2</c-><c- o>&amp;&amp;</c-> <c- n>r2</c-><c- p>,</c-> <c- n>Comp</c-> <c- n>comp</c-> <c- o>=</c-> <c- p>{},</c-> <c- n>Proj1</c-> <c- n>proj1</c-> <c- o>=</c-> <c- p>{},</c->
                             <c- n>Proj2</c-> <c- n>proj2</c-> <c- o>=</c-> <c- p>{});</c->
<c- p>}</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Equivalent to</i>:</p>
<pre class="language-cpp highlight">    <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>mismatch</c-><c- p>(</c-><c- n>first1</c-><c- p>,</c-> <c- n>last1</c-><c- p>,</c-> <c- n>first2</c-><c- p>,</c-> <c- n>last2</c-><c- p>,</c-> <c- n>comp</c-><c- p>,</c-> <c- n>proj1</c-><c- p>,</c-> <c- n>proj2</c-><c- p>).</c-><c- n>in2</c-> <c- o>==</c-> <c- n>last2</c-><c- p>;</c->
</pre>
    <li data-md>
     <p><i>Complexity</i>: If the types of <code class="highlight"><c- n>first1</c-></code>, <code class="highlight"><c- n>last1</c-></code>, <code class="highlight"><c- n>first2</c-></code>, and <code class="highlight"><c- n>last2</c-></code> pairwise model <code class="highlight"><c- n>SizedSentinel</c-></code> and <code class="highlight"><c- n>last1</c-> <c- o>-</c-> <c- n>first1</c-> <c- o>!=</c-> <c- n>last2</c-> <c- o>-</c-> <c- n>first2</c-></code>, then no applications of the
corresponding predicate and each projection; otherwise, at most <i>min</i>(<code class="highlight"><c- n>last1</c-> <c- o>-</c-> <c- n>first1</c-></code>, <code class="highlight"><c- n>last2</c-> <c- o>-</c-> <c- n>first2</c-><c- p>)</c-></code>) applications of the corresponding predicate and
projections.</p>
   </ol>
   <h3 class="heading settled" data-level="3.2" id="25515-ends-with-algends_with"><span class="secno">3.2. </span><span class="content">25.5.15 Ends with [alg.ends_with]</span><a class="self-link" href="#25515-ends-with-algends_with"></a></h3>
<pre class="language-cpp highlight">  <c- k>template</c-><c- o>&lt;</c-><c- n>ForwardIterator</c-> <c- n>I1</c-><c- p>,</c-> <c- n>Sentinel</c-><c- o>&lt;</c-><c- n>I1</c-><c- o>></c-> <c- n>S1</c-><c- p>,</c-> <c- n>InputIterator</c-> <c- n>I2</c-><c- p>,</c-> <c- n>Sentinel</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>></c-> <c- n>S2</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Comp</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>equal_to</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Proj1</c-> <c- o>=</c-> <c- n>identity</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Proj2</c-> <c- o>=</c-> <c- n>identity</c-><c- o>></c->
    <c- n>requires</c-> <c- n>IndirectlyComparable</c-><c- o>&lt;</c-><c- n>I1</c-><c- p>,</c-> <c- n>I2</c-><c- p>,</c-> <c- n>Comp</c-><c- p>,</c-> <c- n>Proj1</c-><c- p>,</c-> <c- n>Proj2</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- n>ends_with</c-><c- p>(</c-><c- n>I1</c-> <c- n>first1</c-><c- p>,</c-> <c- n>S1</c-> <c- n>last1</c-><c- p>,</c-> <c- n>I2</c-> <c- n>first2</c-><c- p>,</c-> <c- n>S2</c-> <c- n>last2</c-><c- p>,</c-> <c- n>Comp</c-> <c- n>comp</c-> <c- o>=</c-> <c- p>{},</c->
                           <c- n>Proj1</c-> <c- n>proj1</c-> <c- o>=</c-> <c- p>{},</c-> <c- n>Proj2</c-> <c- n>proj2</c-> <c- o>=</c-> <c- p>{});</c->
  <c- k>template</c-><c- o>&lt;</c-><c- n>ForwardRange</c-> <c- n>R1</c-><c- p>,</c-> <c- n>InputRange</c-> <c- n>R2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Comp</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>equal_to</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Proj1</c-> <c- o>=</c-> <c- n>identity</c-><c- p>,</c->
           <c- k>class</c-> <c- nc>Proj2</c-> <c- o>=</c-> <c- n>identity</c-><c- o>></c->
    <c- n>requires</c-> <c- n>IndirectlyComparable</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R1</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R2</c-><c- o>></c-><c- p>,</c-> <c- n>Comp</c-><c- p>,</c-> <c- n>Proj1</c-><c- p>,</c-> <c- n>Proj2</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- n>ends_with</c-><c- p>(</c-><c- n>R1</c-><c- o>&amp;&amp;</c-> <c- n>r1</c-><c- p>,</c-> <c- n>R2</c-><c- o>&amp;&amp;</c-> <c- n>r2</c-><c- p>,</c-> <c- n>Comp</c-> <c- n>comp</c-> <c- o>=</c-> <c- p>{},</c-> <c- n>Proj1</c-> <c- n>proj1</c-> <c- o>=</c-> <c- p>{},</c-> <c- n>Proj2</c-> <c- n>proj2</c-> <c- o>=</c-> <c- p>{});</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Equivalent to</i>:</p>
<pre class="language-cpp highlight">    <c- k>const</c-> <c- k>auto</c-> <c- n>first</c-> <c- o>=</c-> <c- n>subrange</c-><c- p>{</c-><c- n>first1</c-><c- p>,</c-> <c- n>last1</c-><c- p>};</c->
    <c- k>const</c-> <c- k>auto</c-> <c- n>second</c-> <c- o>=</c-> <c- n>subrange</c-><c- p>{</c-><c- n>first2</c-><c- p>,</c-> <c- n>last2</c-><c- p>};</c->
    <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>equal</c-><c- p>(</c-><c- n>first</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>distance</c-><c- p>(</c-><c- n>second</c-><c- p>)),</c-> <c- n>second</c-><c- p>,</c-> <c- n>comp</c-><c- p>,</c-> <c- n>proj1</c-><c- p>,</c-> <c- n>proj2</c-><c- p>);</c->
</pre>
    <li data-md>
     <p><i>Complexity</i>: If the types of <code class="highlight"><c- n>first1</c-></code>, <code class="highlight"><c- n>last1</c-></code>, <code class="highlight"><c- n>first2</c-></code>, and <code class="highlight"><c- n>last2</c-></code> pairwise model <code class="highlight"><c- n>SizedSentinel</c-></code> and <code class="highlight"><c- n>last1</c-> <c- o>-</c-> <c- n>first1</c-> <c- o>!=</c-> <c- n>last2</c-> <c- o>-</c-> <c- n>first2</c-></code>, then no applications of the
corresponding predicate and each projection; otherwise, at most <i>min</i>(<code class="highlight"><c- n>last1</c-> <c- o>-</c-> <c- n>first1</c-></code>, <code class="highlight"><c- n>last2</c-> <c- o>-</c-> <c- n>first2</c-></code>) applications of the corresponding predicate and
projections.</p>
   </ol>
   <h2 class="heading settled" data-level="4" id="reference-implementation"><span class="secno">4. </span><span class="content">Reference implementation</span><a class="self-link" href="#reference-implementation"></a></h2>
   <p>Both <code class="highlight"><a data-link-type="biblio" href="#biblio-starts_with"><c- p>[</c-><c- n>starts_with</c-><c- p>]</c-></a></code> and <code class="highlight"><a data-link-type="biblio" href="#biblio-ends_with"><c- p>[</c-><c- n>ends_with</c-><c- p>]</c-></a></code> have been implemented in range-v3.</p>
   <h2 class="heading settled" data-level="5" id="acknowledgements"><span class="secno">5. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>The author would like to thank Arien Judge for reviewing the proposal, Johel Ernesto Guerrero Peña
for providing an implementation for <code class="highlight"><c- n>ends_with</c-></code>, and Eric Niebler for merging the respective pull
requests to range-v3.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-ends_with">[ENDS_WITH]
   <dd>Johel Ernesto Guerrero and Eric Niebler. <a href="https://git.io/fjzq0">ranges::ends_with</a>. URL: <a href="https://git.io/fjzq0">https://git.io/fjzq0</a>
   <dt id="biblio-n3609">[N3609]
   <dd>Jeffrey Yasskin. <a href="https://wg21.link/n3609">string_view: a non-owning reference to a string, revision 3</a>. URL: <a href="https://wg21.link/n3609">https://wg21.link/n3609</a>
   <dt id="biblio-p0457">[P0457]
   <dd>Mikhail Maltsev. <a href="https://wg21.link/p0457">String Prefix and Suffix Checking</a>. URL: <a href="https://wg21.link/p0457">https://wg21.link/p0457</a>
   <dt id="biblio-starts_with">[STARTS_WITH]
   <dd>Christopher Di Bella and Eric Niebler. <a href="https://git.io/fjzqR">ranges::starts_with</a>. URL: <a href="https://git.io/fjzqR">https://git.io/fjzqR</a>
  </dl>