<!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>P1035R1: Input range adaptors</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">P1035R1<br>Input range adaptors</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-10-08">2018-10-08</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/p1035">https://wg21.link/p1035</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
     <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>P1035 proposes to introduce seven additional range adaptors in the C++20 timeframe.</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="#acknowledgements"><span class="secno">1</span> <span class="content">Acknowledgements</span></a>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#proposals"><span class="secno">3</span> <span class="content">Proposals</span></a>
     <ol class="toc">
      <li>
       <a href="#take_while_view"><span class="secno">3.1</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-></code></span></a>
       <ol class="toc">
        <li><a href="#motivation①"><span class="secno">3.1.1</span> <span class="content">Motivation</span></a>
        <li><a href="#notes"><span class="secno">3.1.2</span> <span class="content">Notes</span></a>
        <li>
         <a href="#interface-and-specification"><span class="secno">3.1.3</span> <span class="content">Interface and specification</span></a>
         <ol class="toc">
          <li><a href="#take_while_view-constructors"><span class="secno">3.1.3.1</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-></code> constructors</span></a>
          <li><a href="#take_while_view-conversion"><span class="secno">3.1.3.2</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-></code> conversion</span></a>
          <li><a href="#take_while_view-range-begin"><span class="secno">3.1.3.3</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-></code> range begin</span></a>
          <li><a href="#take_while_view-range-end"><span class="secno">3.1.3.4</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-></code> range end</span></a>
         </ol>
        <li><a href="#take_while_view__sentinel"><span class="secno">3.1.4</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code></span></a>
        <li><a href="#take_while_view__sentinel-constructor"><span class="secno">3.1.5</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code> constructor</span></a>
        <li><a href="#take_while_view__sentinel-conversion"><span class="secno">3.1.6</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code> conversion</span></a>
        <li><a href="#take_while_view__sentinel-comparisons"><span class="secno">3.1.7</span> <span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code> comparisons</span></a>
       </ol>
      <li><a href="#viewtake_while"><span class="secno">3.2</span> <span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>take_while</c-></code></span></a>
      <li>
       <a href="#drop_view"><span class="secno">3.3</span> <span class="content"><code class="highlight"><c- n>drop_view</c-></code></span></a>
       <ol class="toc">
        <li><a href="#motivation②"><span class="secno">3.3.1</span> <span class="content">Motivation</span></a>
        <li>
         <a href="#interface"><span class="secno">3.3.2</span> <span class="content">Interface</span></a>
         <ol class="toc">
          <li><a href="#drop_view-constructor"><span class="secno">3.3.2.1</span> <span class="content"><code class="highlight"><c- n>drop_view</c-></code> constructor</span></a>
          <li><a href="#drop_view-conversion"><span class="secno">3.3.2.2</span> <span class="content"><code class="highlight"><c- n>drop_view</c-></code> conversion</span></a>
          <li><a href="#drop_view-range-begin"><span class="secno">3.3.2.3</span> <span class="content"><code class="highlight"><c- n>drop_view</c-></code> range begin</span></a>
          <li><a href="#drop_view-range-end"><span class="secno">3.3.2.4</span> <span class="content"><code class="highlight"><c- n>drop_view</c-></code> range end</span></a>
          <li><a href="#drop_view-size"><span class="secno">3.3.2.5</span> <span class="content"><code class="highlight"><c- n>drop_view</c-></code> size</span></a>
         </ol>
       </ol>
      <li><a href="#viewdrop"><span class="secno">3.4</span> <span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop</c-></code></span></a>
      <li>
       <a href="#drop_while_view"><span class="secno">3.5</span> <span class="content"><code class="highlight"><c- n>drop_while_view</c-></code></span></a>
       <ol class="toc">
        <li><a href="#motivation③"><span class="secno">3.5.1</span> <span class="content">Motivation</span></a>
        <li>
         <a href="#interface①"><span class="secno">3.5.2</span> <span class="content">Interface</span></a>
         <ol class="toc">
          <li><a href="#drop_while_view-constructors"><span class="secno">3.5.2.1</span> <span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> constructors</span></a>
          <li><a href="#drop_while_view-conversion"><span class="secno">3.5.2.2</span> <span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> conversion</span></a>
          <li><a href="#drop_while_view-begin"><span class="secno">3.5.2.3</span> <span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> begin</span></a>
          <li><a href="#drop_while_view-end"><span class="secno">3.5.2.4</span> <span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> end</span></a>
         </ol>
       </ol>
      <li><a href="#viewdrop_while"><span class="secno">3.6</span> <span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop_while</c-></code></span></a>
      <li>
       <a href="#basic_istream_view"><span class="secno">3.7</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code></span></a>
       <ol class="toc">
        <li><a href="#motivation④"><span class="secno">3.7.1</span> <span class="content">Motivation</span></a>
        <li>
         <a href="#interface②"><span class="secno">3.7.2</span> <span class="content">Interface</span></a>
         <ol class="toc">
          <li><a href="#concept-streamextractable"><span class="secno">3.7.2.1</span> <span class="content">Concept <code class="highlight"><c- n>StreamExtractable</c-></code></span></a>
          <li><a href="#concept-streaminsertable"><span class="secno">3.7.2.2</span> <span class="content">Concept <code class="highlight"><c- n>StreamInsertable</c-></code></span></a>
          <li><a href="#basic_istream_view-constructor"><span class="secno">3.7.2.3</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> constructor</span></a>
          <li><a href="#basic_istream_view-begin"><span class="secno">3.7.2.4</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> begin</span></a>
          <li><a href="#basic_istream_view-end"><span class="secno">3.7.2.5</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> end</span></a>
          <li><a href="#basic_istream_view__iterator"><span class="secno">3.7.2.6</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code></span></a>
          <li><a href="#basic_istream_view__iterator-constructor"><span class="secno">3.7.2.7</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> constructor</span></a>
          <li><a href="#basic_istream_view__iterator-next"><span class="secno">3.7.2.8</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> next</span></a>
          <li><a href="#basic_istream_view__iterator-value"><span class="secno">3.7.2.9</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> value</span></a>
          <li><a href="#basic_istream_view__iterator-comparison-functions"><span class="secno">3.7.2.10</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> comparison functions</span></a>
         </ol>
       </ol>
      <li><a href="#basic_istream_view-range-adaptors"><span class="secno">3.8</span> <span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> range adaptors</span></a>
      <li>
       <a href="#zip_with_view"><span class="secno">3.9</span> <span class="content"><code class="highlight"><c- n>zip_with_view</c-></code></span></a>
       <ol class="toc">
        <li><a href="#motivation⑤"><span class="secno">3.9.1</span> <span class="content">Motivation</span></a>
        <li><a href="#__common_tuple"><span class="secno">3.9.2</span> <span class="content"><code class="highlight"><c- n>__common_tuple</c-></code></span></a>
        <li><a href="#__iter_zip_with"><span class="secno">3.9.3</span> <span class="content"><code class="highlight"><c- n>__iter_zip_with</c-></code></span></a>
        <li><a href="#zip_with_view①"><span class="secno">3.9.4</span> <span class="content"><code class="highlight"><c- n>zip_with_view</c-></code></span></a>
       </ol>
      <li><a href="#viewzip_with"><span class="secno">3.10</span> <span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip_with</c-></code></span></a>
      <li><a href="#viewzip"><span class="secno">3.11</span> <span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip</c-></code></span></a>
      <li><a href="#reviewing-iter_value_t-etc-and-introducing-range_value_t-etc"><span class="secno">3.12</span> <span class="content">Reviewing <code class="highlight"><c- n>iter_value_t</c-></code>, etc., and introducing <code class="highlight"><c- n>range_value_t</c-></code>, etc.</span></a>
     </ol>
    <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="acknowledgements"><span class="secno">1. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>I would like to acknowledge the following people for their assistance with this proposal:</p>
   <ol>
    <li data-md>
     <p>Casey Carter, for reviewing all submissions to <a data-link-type="biblio" href="#biblio-cmcstl2">[cmcstl2]</a> and providing feedback that enabled the proposed range adaptors to have high-quality implementations.</p>
    <li data-md>
     <p>Eric Niebler, for providing <a data-link-type="biblio" href="#biblio-range-v3">[range-v3]</a> as a reference implementation.</p>
   </ol>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0789">[P0789]</a> introduces the notion of a range adaptor and twelve pioneering range adaptors that
improve declarative, range-based programming. For example, it is possible to perform an inline,
in-place, lazy reverse like so:</p>
<pre class="language-cpp highlight"><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- k>namespace</c-> <c- n>view</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>view</c-><c- p>;</c->

<c- c1>// Old</c->
<c- k>auto</c-> <c- n>i</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>find</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>rbegin</c-><c- p>(</c-><c- n>employees</c-><c- p>),</c-> <c- n>ranges</c-><c- o>::</c-><c- n>rend</c-><c- p>(</c-><c- n>employees</c-><c- p>),</c-> <c- s>"Lovelace"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>employee</c-><c- o>::</c-><c- n>surname</c-><c- p>);</c->

<c- c1>// New</c->
<c- k>auto</c-> <c- n>j</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>find</c-><c- p>(</c-><c- n>employees</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>reverse</c-><c- p>,</c-> <c- s>"Lovelace"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>employee</c-><c- o>::</c-><c- n>surname</c-><c- p>);</c->
<c- p>[[</c-><c- nl>assert</c-><c- p>:</c-> <c- n>i</c-> <c- o>==</c-> <c- n>j</c-><c- p>]];</c->
</pre>
   <p>P1035 recognises that P0789 introduces only a few of the widely experimented-with range adaptors in <a data-link-type="biblio" href="#biblio-range-v3">[range-v3]</a>, and would like to add a few more to complete the C++20 phase of range adaptors. To
this end, P1035 discusses range adaptors that are related to those in P0789.</p>
   <h2 class="heading settled" data-level="3" id="proposals"><span class="secno">3. </span><span class="content">Proposals</span><a class="self-link" href="#proposals"></a></h2>
   <p>Unless otherwise requested, each sub-subsection below should be polled individually from other
sub-subsections. Two major questions are to be asked per range adaptor. It is up to LEWG to decide
the exact phrasing, but the author’s intentions are listed below.</p>
   <ol>
    <li data-md>
     <p>Do we want this range adaptor in C++20?</p>
    <li data-md>
     <p>As-is?</p>
    <li data-md>
     <p>With modifications, as suggested by LEWG?</p>
    <li data-md>
     <p>If we do not want this range adaptor in C++20, do we want it in C++23?</p>
    <li data-md>
     <p>As-is?</p>
    <li data-md>
     <p>With modificaitons, as suggested by LEWG?</p>
   </ol>
   <h3 class="heading settled" data-level="3.1" id="take_while_view"><span class="secno">3.1. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-></code></span><a class="self-link" href="#take_while_view"></a></h3>
   <h4 class="heading settled" data-level="3.1.1" id="motivation①"><span class="secno">3.1.1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation①"></a></h4>
   <p>P0789 introduces <code class="highlight"><c- n>take_view</c-></code>, which rangifies the iterator pair <code class="highlight"><c- p>{</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>r</c-><c- p>),</c-> <c- n>ranges</c-><c- o>::</c-><c- n>next</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>r</c-><c- p>),</c-> <c- n>n</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>r</c-><c- p>))}</c-></code>. As an example:</p>
<pre class="language-cpp highlight"><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>{</c-><c- mi>0</c-><c- p>,</c-> <c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-><c- p>,</c-> <c- mi>4</c-><c- p>,</c-> <c- mi>5</c-><c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>distance</c-><c- p>(</c-><c- n>v</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>))</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c-> <c- c1>// prints 3</c->
<c- n>copy</c-><c- p>(</c-><c- n>v</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>),</c-> <c- n>ostream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>cout</c-><c- p>,</c-> <c- s>" "</c-><c- p>));</c-> <c- c1>// prints 0 1 2</c->
<c- n>copy</c-><c- p>(</c-><c- n>v</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- n>distance</c-><c- p>(</c-><c- n>v</c-><c- p>)),</c-> <c- n>ostream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>cout</c-><c- p>,</c-> <c- s>" "</c-><c- p>));</c-> <c- c1>// prints 0 1 2 3 4 5</c->
</pre>
   <p><code class="highlight"><c- n>take_while_view</c-></code> will provide slightly different functionality, akin to having a sentinel that
checks if a certain predicate is satisfied.</p>
   <table>
    <tbody>
     <tr>
      <th>Current
      <th>Proposed
     <tr>
      <td>
<pre class="highlight"><c- k>namespace</c-> <c- n>ranges</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>ranges</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>Integral</c-> <c- n>I</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_odd</c-><c- p>(</c-><c- n>I</c-> <c- k>const</c-> <c- n>i</c-><c- p>)</c-> <c- k>noexcept</c->
<c- p>{</c->
   <c- k>return</c-> <c- n>i</c-> <c- o>%</c-> <c- mi>2</c-><c- p>;</c->
<c- p>}</c->

<c- k>struct</c-> <c- n>is_odd_sentinel</c-> <c- p>{</c->
   <c- k>template</c-> <c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>Iterator</c-> <c- n>I</c-><c- o>></c->
   <c- k>constexpr</c-> <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>I</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>a</c-><c- p>,</c->
      <c- n>is_odd_sentinel</c-><c- p>)</c-> <c- k>noexcept</c->
   <c- p>{</c->
      <c- k>return</c-> <c- n>is_odd</c-><c- p>(</c-><c- o>*</c-><c- n>a</c-><c- p>);</c->
   <c- p>}</c->

   <c- k>template</c-> <c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>Iterator</c-> <c- n>I</c-><c- o>></c->
   <c- k>constexpr</c-> <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>is_odd_sentinel</c-> <c- k>const</c-> <c- n>a</c-><c- p>,</c->
      <c- n>I</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>b</c-><c- p>)</c-> <c- k>noexcept</c->
   <c- p>{</c->
      <c- k>return</c-> <c- n>b</c-> <c- o>==</c-> <c- n>a</c-><c- p>;</c->
   <c- p>}</c->

   <c- k>template</c-> <c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>Iterator</c-> <c- n>I</c-><c- o>></c->
   <c- k>constexpr</c-> <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- n>I</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>a</c-><c- p>,</c->
      <c- n>is_odd_sentinel</c-> <c- k>const</c-> <c- n>b</c-><c- p>)</c-> <c- k>noexcept</c->
   <c- p>{</c->
      <c- k>return</c-> <c- n>not</c-> <c- p>(</c-><c- n>a</c-> <c- o>==</c-> <c- n>b</c-><c- p>);</c->
   <c- p>}</c->

   <c- k>template</c-> <c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>Iterator</c-> <c- n>I</c-><c- o>></c->
   <c- k>constexpr</c-> <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- n>is_odd_sentinel</c-> <c- k>const</c-> <c- n>a</c-><c- p>,</c->
      <c- n>I</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>b</c-><c- p>)</c-> <c- k>noexcept</c->
   <c- p>{</c->
      <c- k>return</c-> <c- n>not</c-> <c- p>(</c-><c- n>b</c-> <c- o>==</c-> <c- n>a</c-><c- p>);</c->
   <c- p>}</c->
<c- p>};</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c->
<c- p>{</c->
   <c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>{</c-><c- mi>0</c-><c- p>,</c-> <c- mi>6</c-><c- p>,</c-> <c- mi>8</c-><c- p>,</c-> <c- mi>9</c-><c- p>,</c-> <c- mi>10</c-><c- p>,</c-> <c- mi>16</c-><c- p>};</c->
   <c- n>ranges</c-><c- o>::</c-><c- n>copy</c-><c- p>(</c->
     <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>v</c-><c- p>),</c->
     <c- n>is_odd_sentinel</c-><c- p>{},</c->
     <c- n>ranges</c-><c- o>::</c-><c- n>ostream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>cout</c-><c- p>,</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-><c- p>));</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- k>namespace</c-> <c- n>ranges</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>ranges</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>Integral</c-> <c- n>I</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_odd</c-><c- p>(</c-><c- n>I</c-> <c- k>const</c-> <c- n>i</c-><c- p>)</c-> <c- k>noexcept</c->
<c- p>{</c->
   <c- k>return</c-> <c- n>i</c-> <c- o>%</c-> <c- mi>2</c-><c- p>;</c->
<c- p>}</c->

<c- k>template</c-> <c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>Integral</c-> <c- n>I</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_even</c-><c- p>(</c-><c- n>I</c-> <c- k>const</c-> <c- n>i</c-><c- p>)</c-> <c- k>noexcept</c->
<c- p>{</c->
   <c- k>return</c-> <c- n>not</c-> <c- n>is_odd</c-><c- p>(</c-><c- n>i</c-><c- p>);</c->
<c- p>}</c->

<c- b>int</c-> <c- n>main</c-><c- p>()</c->
<c- p>{</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- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>{</c-><c- mi>0</c-><c- p>,</c-> <c- mi>6</c-><c- p>,</c-> <c- mi>8</c-><c- p>,</c-> <c- mi>9</c-><c- p>,</c-> <c- mi>10</c-><c- p>,</c-> <c- mi>16</c-><c- p>};</c->
   <c- n>ranges</c-><c- o>::</c-><c- n>copy</c-><c- p>(</c->
      <c- n>v</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>take_while</c-><c- p>(</c-><c- n>is_even</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>),</c->
      <c- n>ranges</c-><c- o>::</c-><c- n>ostream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>cout</c-><c- p>,</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-><c- p>));</c->
<c- p>}</c->
</pre>
     <tr>
      <td><a href="https://wandbox.org/permlink/7UXazMHnbxfOfODQ">Wandbox demo</a>
      <td><a href="https://wandbox.org/permlink/2nS3bzVuQX1DR0yJ">Wandbox demo</a>
   </table>
   <p>The former requires that a user define their own sentinel type: something that while not
expert-friendly, is yet to be established as a widespread idiom in C++, and providing a
range adaptor for this purpose will help programmers determine when a sentinel is _not_
necessary.</p>
   <h4 class="heading settled" data-level="3.1.2" id="notes"><span class="secno">3.1.2. </span><span class="content">Notes</span><a class="self-link" href="#notes"></a></h4>
   <ul>
    <li data-md>
     <p>There is a slight programmer overhead in the naming: the author felt that both <code class="highlight"><c- n>is_odd_sentinel</c-></code> and <code class="highlight"><c- n>is_even_sentinel</c-></code> were applicable names: ultimately, the name <code class="highlight"><c- n>is_odd_sentinel</c-></code> was chosen because it describes the delimiting condition. An equally
valid reason could probably be made for <code class="highlight"><c- n>is_even_sentinel</c-></code>.</p>
    <li data-md>
     <p>A sentinel that takes a lambda may be of interest to LEWG. If there is interest in
this, a proposal could be made in the C++23 timeframe.</p>
   </ul>
   <h4 class="heading settled" data-level="3.1.3" id="interface-and-specification"><span class="secno">3.1.3. </span><span class="content">Interface and specification</span><a class="self-link" href="#interface-and-specification"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- n>View</c-> <c- n>R</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Pred</c-><c- o>></c->
<c- n>requires</c->
   <c- n>InputRange</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c->
   <c- n>std</c-><c- o>::</c-><c- n>is_object_v</c-><c- o>&lt;</c-><c- n>Pred</c-><c- o>></c-> <c- o>&amp;&amp;</c->
   <c- n>IndirectUnaryPredicate</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c->
<c- k>class</c-> <c- nc>take_while_view</c-> <c- o>:</c-> <c- k>public</c-> <c- n>view_interface</c-><c- o>&lt;</c-><c- n>take_while_view</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>Pred</c-><c- o>>></c-> <c- p>{</c->
   <c- k>template</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-> <c- k>class</c-> <c- nc>__sentinel</c-><c- p>;</c-> <c- c1>// exposition-only</c->
<c- k>public</c-><c- o>:</c->
   <c- n>take_while_view</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
   <c- k>constexpr</c-> <c- nf>take_while_view</c-><c- p>(</c-><c- n>R</c-> <c- n>base</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>);</c->

   <c- k>template</c-> <c- o>&lt;</c-><c- n>ViewableRange</c-> <c- n>O</c-><c- o>></c->
   <c- n>requires</c-> <c- n>constructible</c-><c- o>-</c-><c- n>from</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>O</c-><c- o>></c->
   <c- k>constexpr</c-> <c- n>take_while_view</c-><c- p>(</c-><c- n>O</c-><c- o>&amp;&amp;</c-> <c- n>o</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>);</c->

   <c- k>constexpr</c-> <c- n>R</c-> <c- nf>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
   <c- k>constexpr</c-> <c- k>const</c-> <c- n>Pred</c-><c- o>&amp;</c-> <c- n>pred</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>;</c->
   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->

   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>;</c->
   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
<c- k>private</c-><c- o>:</c->
   <c- n>R</c-> <c- n>base_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
   <c- n>semiregular</c-><c- o>&lt;</c-><c- n>Pred</c-><c- o>></c-> <c- n>pred_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
<c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Pred</c-><c- o>></c->
<c- n>take_while_view</c-><c- p>(</c-><c- n>R</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- n>Pred</c-><c- p>)</c-> <c- o>-></c-> <c- n>take_while_view</c-><c- o>&lt;</c-><c- n>all_view</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> <c- n>Pred</c-><c- o>></c-><c- p>;</c->
</pre>
   <h5 class="heading settled" data-level="3.1.3.1" id="take_while_view-constructors"><span class="secno">3.1.3.1. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-></code> constructors</span><a class="self-link" href="#take_while_view-constructors"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- nf>take_while_view</c-><c- p>(</c-><c- n>R</c-> <c- n>base</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>);</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>base_</c-></code> with <code class="highlight"><c- n>base</c-></code> and initialises <code class="highlight"><c- n>pred_</c-></code> with <code class="highlight"><c- n>pred</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- n>ViewableRange</c-> <c- n>O</c-><c- o>></c->
<c- n>requires</c-> <c- n>constructible</c-><c- o>-</c-><c- n>from</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>O</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>take_while_view</c-><c- p>(</c-><c- n>O</c-><c- o>&amp;&amp;</c-> <c- n>o</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>);</c->
</pre>
   <ol start="2">
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>base_</c-></code> with <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>all</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>O</c-><c- o>></c-><c- p>(</c-><c- n>o</c-><c- p>))</c-></code> and initialises <code class="highlight"><c- n>pred_</c-></code> with <code class="highlight"><c- n>pred</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.1.3.2" id="take_while_view-conversion"><span class="secno">3.1.3.2. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-></code> conversion</span><a class="self-link" href="#take_while_view-conversion"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>R</c-> <c- nf>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- n>base_</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>const</c-> <c- n>Pred</c-><c- o>&amp;</c-> <c- n>pred</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ol start="2">
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- n>pred_</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.1.3.3" id="take_while_view-range-begin"><span class="secno">3.1.3.3. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-></code> range begin</span><a class="self-link" href="#take_while_view-range-begin"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>;</c->
<c- k>constexpr</c-> <c- k>auto</c-> <c- n>begin</c-><c- p>()</c-> <c- k>const</c-> <c- n>requries</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c->
</pre>
   <ol>
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.1.3.4" id="take_while_view-range-end"><span class="secno">3.1.3.4. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-></code> range end</span><a class="self-link" href="#take_while_view-range-end"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>;</c->
<c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>__sentinel</c-><c- o>&lt;</c-><c- n>is_const_v</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-><c- o>>></c-><c- p>(</c-><c- o>&amp;</c-><c- n>pred</c-><c- p>());</c-></code>.</p>
   </ol>
   <h4 class="heading settled" data-level="3.1.4" id="take_while_view__sentinel"><span class="secno">3.1.4. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code></span><a class="self-link" href="#take_while_view__sentinel"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Pred</c-><c- o>></c->
<c- k>template</c-><c- o>&lt;</c-><c- b>bool</c-> <c- n>Const</c-><c- o>></c->
<c- k>class</c-> <c- nc>take_while_view</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>Pred</c-><c- o>>::</c-><c- n>__sentinel</c-> <c- p>{</c->
   <c- k>using</c-> <c- n>Parent</c-> <c- o>=</c-> <c- n>conditional_t</c-><c- o>&lt;</c-><c- n>Const</c-><c- p>,</c-> <c- k>const</c-> <c- n>take_while_view</c-><c- p>,</c-> <c- n>take_wile_view</c-><c- o>></c-><c- p>;</c->
   <c- k>using</c-> <c- n>Base</c-> <c- o>=</c-> <c- n>conditional_t</c-><c- o>&lt;</c-><c- n>Const</c-><c- p>,</c-> <c- k>const</c-> <c- n>R</c-><c- p>,</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
   <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>end_</c-><c- p>{};</c-> <c- c1>// exposition-only</c->
   <c- k>const</c-> <c- n>Pred</c-><c- o>*</c-> <c- n>pred_</c-><c- p>{};</c-> <c- c1>// pred</c->
<c- k>public</c-><c- o>:</c->
   <c- n>__sentinel</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
   <c- k>constexpr</c-> <c- k>explicit</c-> <c- nf>__sentinel</c-><c- p>(</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>end</c-><c- p>,</c-> <c- k>const</c-> <c- n>Pred</c-><c- o>*</c-> <c- n>pred</c-><c- p>);</c->
   <c- k>constexpr</c-> <c- nf>__sentinel</c-><c- p>(</c-><c- n>__sentinel</c-><c- o>&lt;!</c-><c- n>Const</c-><c- o>></c-> <c- n>s</c-><c- p>)</c->
     <c- n>requires</c-> <c- n>Const</c-> <c- o>&amp;&amp;</c-> <c- n>ConvertibleTo</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>></c->

   <c- k>constexpr</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>base</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- n>end_</c-><c- p>;</c-> <c- p>}</c->

   <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
   <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>);</c->
   <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
   <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>);</c->
<c- p>};</c->
</pre>
   <h4 class="heading settled" data-level="3.1.5" id="take_while_view__sentinel-constructor"><span class="secno">3.1.5. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code> constructor</span><a class="self-link" href="#take_while_view__sentinel-constructor"></a></h4>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>explicit</c-> <c- nf>__sentinel</c-><c- p>(</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>end</c-><c- p>,</c-> <c- k>const</c-> <c- n>Pred</c-><c- o>*</c-> <c- n>pred</c-><c- p>);</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>end_</c-></code> with <code class="highlight"><c- n>end</c-></code>, and <code class="highlight"><c- n>pred_</c-></code> with <code class="highlight"><c- n>pred</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- nf>__sentinel</c-><c- p>(</c-><c- n>__sentinel</c-><c- o>&lt;!</c-><c- n>Const</c-><c- o>></c-> <c- n>s</c-><c- p>)</c->
   <c- n>requires</c-> <c- n>Const</c-> <c- o>&amp;&amp;</c-> <c- n>ConvertibleTo</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>></c-><c- p>;</c->
</pre>
   <ol start="2">
    <li data-md>
     <p><i>Effects</i> Initialises <code class="highlight"><c- n>end_</c-></code> with <code class="highlight"><c- n>s</c-><c- p>.</c-><c- n>end_</c-></code> and <code class="highlight"><c- n>pred_</c-></code> with <code class="highlight"><c- n>s</c-><c- p>.</c-><c- n>pred_</c-></code>.</p>
   </ol>
   <h4 class="heading settled" data-level="3.1.6" id="take_while_view__sentinel-conversion"><span class="secno">3.1.6. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code> conversion</span><a class="self-link" href="#take_while_view__sentinel-conversion"></a></h4>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ol start="3">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>end_</c-><c- p>;</c-></code></p>
   </ol>
   <h4 class="heading settled" data-level="3.1.7" id="take_while_view__sentinel-comparisons"><span class="secno">3.1.7. </span><span class="content"><code class="highlight"><c- n>take_while_view</c-><c- o>::</c-><c- n>__sentinel</c-></code> comparisons</span><a class="self-link" href="#take_while_view__sentinel-comparisons"></a></h4>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>)</c->
</pre>
   <ol start="4">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>x</c-><c- p>.</c-><c- n>end_</c-> <c- o>!=</c-> <c- n>y</c-> <c- o>&amp;&amp;</c-> <c- o>!</c-><c- p>(</c-><c- o>*</c-><c- n>x</c-><c- p>.</c-><c- n>pred_</c-><c- p>)(</c-><c- o>*</c-><c- n>y</c-><c- p>);</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <ol start="5">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>y</c-> <c- o>==</c-> <c- n>x</c-><c- p>;</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <ol start="6">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- o>!</c-><c- p>(</c-><c- n>x</c-> <c- o>==</c-> <c- n>y</c-><c- p>);</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>__sentinel</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <ol start="7">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- o>!</c-><c- p>(</c-><c- n>y</c-> <c- o>==</c-> <c- n>x</c-><c- p>);</c-></code>.</p>
   </ol>
   <h3 class="heading settled" data-level="3.2" id="viewtake_while"><span class="secno">3.2. </span><span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>take_while</c-></code></span><a class="self-link" href="#viewtake_while"></a></h3>
   <p>The name <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>take_while</c-></code> denotes a range adaptor object. Let <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> be expressions such that type <code class="highlight"><c- n>T</c-></code> is <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code>. Then, the expression <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>take_while</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code>is expression-equivalent to:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>take_while_view</c-><c- p>{</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>InputRange</c-></code> and if <code class="highlight"><c- n>F</c-></code> is an object, and
models <code class="highlight"><c- n>IndirectUnaryPredicate</c-></code>.</p>
    <li data-md>
     <p>Otherwise <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>view</c-><c- o>::</c-><c- n>take_while</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is ill-formed.</p>
   </ol>
   <h3 class="heading settled" data-level="3.3" id="drop_view"><span class="secno">3.3. </span><span class="content"><code class="highlight"><c- n>drop_view</c-></code></span><a class="self-link" href="#drop_view"></a></h3>
   <h4 class="heading settled" data-level="3.3.1" id="motivation②"><span class="secno">3.3.1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation②"></a></h4>
   <p><code class="highlight"><c- n>drop_view</c-></code> is the complement to <code class="highlight"><c- n>take_view</c-></code>: instead of providing the user with the
first _n_ elements, it provides the user with all _but_ the first _n_ elements.</p>
   <table>
    <tbody>
     <tr>
      <th>Current (C++17)
      <th>Proposed (C++20)
     <tr>
      <td>
<pre class="highlight">    <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- k>auto</c-> <c- n>i</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>next</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>employees</c-><c- p>),</c-> <c- mi>5</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>employees</c-><c- p>));</c->
    <c- k>auto</c-> <c- n>j</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>find</c-><c- p>(</c-><c- n>i</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>employees</c-><c- p>),</c-> <c- s>"Lovelace"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>employees</c-><c- o>::</c-><c- n>surname</c-><c- p>);</c->
      
</pre>
      <td>
<pre class="highlight">    <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- k>auto</c-> <c- n>j</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>find</c-><c- p>(</c-><c- n>employees</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>5</c-><c- p>),</c-> <c- s>"Lovelace"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>employees</c-><c- o>::</c-><c- n>surname</c-><c- p>);</c->
      
</pre>
   </table>
   <h4 class="heading settled" data-level="3.3.2" id="interface"><span class="secno">3.3.2. </span><span class="content">Interface</span><a class="self-link" href="#interface"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>View</c-> <c- n>R</c-><c- o>></c->
<c- k>class</c-> <c- nc>drop_view</c-> <c- o>:</c-> <c- k>public</c-> <c- n>view_interface</c-><c- o>&lt;</c-><c- n>drop_view</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-> <c- p>{</c->
   <c- k>using</c-> <c- n>D</c-> <c- o>=</c-> <c- n>iter_distance_t</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-><c- p>;</c-> <c- c1>// exposition-only</c->
<c- k>public</c-><c- o>:</c->
   <c- n>drop_view</c-><c- p>();</c->
   <c- k>constexpr</c-> <c- nf>drop_view</c-><c- p>(</c-><c- n>R</c-> <c- n>base</c-><c- p>,</c-> <c- n>D</c-> <c- n>count</c-><c- p>);</c->

   <c- k>template</c-><c- o>&lt;</c-><c- n>ViewableRange</c-> <c- n>O</c-><c- o>></c->
   <c- n>requires</c-> <c- n>constructible</c-><c- o>-</c-><c- n>from</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>O</c-><c- o>></c->
   <c- k>constexpr</c-> <c- n>drop_view</c-><c- p>(</c-><c- n>O</c-><c- o>&amp;&amp;</c-> <c- n>o</c-><c- p>,</c-> <c- n>D</c-> <c- n>count</c-><c- p>);</c->

   <c- k>constexpr</c-> <c- n>R</c-> <c- nf>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- p>(</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>RandomAccessRange</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>);</c->
   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>RandomAccessRange</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- p>(</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>);</c->
   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->

   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>size</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>SizedRange</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>;</c->
   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>size</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>SizedRange</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
<c- k>private</c-><c- o>:</c->
   <c- n>R</c-> <c- n>base_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
   <c- n>D</c-> <c- n>count_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
<c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- n>Range</c-> <c- n>R</c-><c- o>></c->
<c- n>drop_view</c-><c- p>(</c-><c- n>R</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-><c- p>)</c-> <c- o>-></c-> <c- n>drop_view</c-><c- o>&lt;</c-><c- n>all_view</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-><c- p>;</c->
</pre>
   <h5 class="heading settled" data-level="3.3.2.1" id="drop_view-constructor"><span class="secno">3.3.2.1. </span><span class="content"><code class="highlight"><c- n>drop_view</c-></code> constructor</span><a class="self-link" href="#drop_view-constructor"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- nf>drop_view</c-><c- p>(</c-><c- n>R</c-> <c- n>base</c-><c- p>,</c-> <c- n>D</c-> <c- n>count</c-><c- p>);</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>base_</c-></code> with <code class="highlight"><c- n>base</c-></code> and <code class="highlight"><c- n>count_</c-></code> with <code class="highlight"><c- n>count</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>ViewableRange</c-> <c- n>O</c-><c- o>></c->
<c- n>requires</c-> <c- n>constructible</c-><c- o>-</c-><c- n>from</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>O</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>drop_view</c-><c- p>(</c-><c- n>O</c-><c- o>&amp;&amp;</c-> <c- n>o</c-><c- p>,</c-> <c- n>D</c-> <c- n>count</c-><c- p>);</c->
</pre>
   <ol start="2">
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>base_</c-></code> with <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>all</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>O</c-><c- o>></c-><c- p>(</c-><c- n>o</c-><c- p>))</c-></code> and <code class="highlight"><c- n>count_</c-></code> with <code class="highlight"><c- n>count</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.3.2.2" id="drop_view-conversion"><span class="secno">3.3.2.2. </span><span class="content"><code class="highlight"><c- n>drop_view</c-></code> conversion</span><a class="self-link" href="#drop_view-conversion"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>R</c-> <c- nf>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ol start="3">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>base_</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.3.2.3" id="drop_view-range-begin"><span class="secno">3.3.2.3. </span><span class="content"><code class="highlight"><c- n>drop_view</c-></code> range begin</span><a class="self-link" href="#drop_view-range-begin"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- p>(</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>RandomAccessRange</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>);</c->
<c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>RandomAccessRange</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
</pre>
   <ol start="4">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>next</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>),</c-> <c- n>count_</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>base_</c-><c- p>));</c-></code>.</p>
    <li data-md>
     <p>_Remarks_: In order to provide the amortized constant time complexity required by the
Range concept, this function caches the result within the <code class="highlight"><c- n>drop_view</c-></code> for use on
subsequent calls.</p>
   </ol>
   <h5 class="heading settled" data-level="3.3.2.4" id="drop_view-range-end"><span class="secno">3.3.2.4. </span><span class="content"><code class="highlight"><c- n>drop_view</c-></code> range end</span><a class="self-link" href="#drop_view-range-end"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- p>(</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>);</c->
<c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
</pre>
   <ol start="6">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.3.2.5" id="drop_view-size"><span class="secno">3.3.2.5. </span><span class="content"><code class="highlight"><c- n>drop_view</c-></code> size</span><a class="self-link" href="#drop_view-size"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>auto</c-> <c- nf>size</c-><c- p>()</c-> <c- n>requires</c-> <c- o>!</c-><c- n>SimpleView</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>SizedRange</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>;</c->
<c- k>constexpr</c-> <c- k>auto</c-> <c- nf>size</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>SizedRange</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>R</c-><c- o>></c-><c- p>;</c->
</pre>
   <ol start="7">
    <li data-md>
     <p>Equivalent to:</p>
   </ol>
<pre class="language-cpp highlight"><c- k>auto</c-> <c- k>const</c-> <c- n>size</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>size</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c->
<c- k>auto</c-> <c- k>const</c-> <c- n>count</c-> <c- o>=</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><c- n>size</c-><c- p>)</c-><c- o>></c-><c- p>(</c-><c- n>count_</c-><c- p>);</c->
<c- k>return</c-> <c- n>size</c-> <c- o>&lt;</c-> <c- n>count</c-> <c- o>?</c-> <c- mi>0</c-> <c- o>:</c-> <c- n>size</c-> <c- o>-</c-> <c- n>count</c-><c- p>;</c->
</pre>
   <h3 class="heading settled" data-level="3.4" id="viewdrop"><span class="secno">3.4. </span><span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop</c-></code></span><a class="self-link" href="#viewdrop"></a></h3>
   <p>The name <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop</c-></code> denotes a range adaptor object. Let <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> be expressions such that type <code class="highlight"><c- n>T</c-></code> is <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code>. Then, the expression <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code>is expression-equivalent to:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>drop_view</c-><c- p>{</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>InputRange</c-></code> and <code class="highlight"><c- n>F</c-></code> is implicitly convertible to <code class="highlight"><c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-></code>.</p>
    <li data-md>
     <p>Otherwise <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is ill-formed.</p>
   </ol>
   <h3 class="heading settled" data-level="3.5" id="drop_while_view"><span class="secno">3.5. </span><span class="content"><code class="highlight"><c- n>drop_while_view</c-></code></span><a class="self-link" href="#drop_while_view"></a></h3>
   <h4 class="heading settled" data-level="3.5.1" id="motivation③"><span class="secno">3.5.1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation③"></a></h4>
   <p>The motivation for <code class="highlight"><c- n>drop_while_view</c-></code> is the union of <code class="highlight"><c- n>drop_view</c-></code> and <code class="highlight"><c- n>take_while_view</c-></code>.
Unlike the others, there are two demonstrations of <code class="highlight"><c- n>drop_while_view</c-></code> below.</p>
   <table>
    <tbody>
     <tr>
      <th>Current (C++17)
      <th>Proposed (C++20) v1
     <tr>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>begin</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>find_if_not</c-><c- p>(</c-><c- n>employees</c-><c- p>,</c-> <c- p>[](</c-><c- k>auto</c-> <c- k>const</c-> <c- n>holidays</c-><c- p>){</c-> <c- k>return</c-> <c- n>holidays</c-> <c- o>&lt;</c-> <c- mi>20</c-><c- p>;</c-> <c- p>},</c-> <c- o>&amp;</c-><c- n>employee</c-><c- o>::</c-><c- n>holidays</c-><c- p>);</c->
<c- n>ranges</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- n>begin</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>employees</c-><c- p>),</c-> <c- n>ranges</c-><c- o>::</c-><c- n>ostream_iterator</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>cout</c-><c- p>,</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-><c- p>),</c-> <c- p>[](</c-><c- k>auto</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>e</c-><c- p>)</c-> <c- p>{</c->
   <c- k>return</c-> <c- n>e</c-><c- p>.</c-><c- n>given_name</c-><c- p>()</c-> <c- o>+</c-> <c- n>e</c-><c- p>.</c-><c- n>surname</c-><c- p>();</c-> <c- p>});</c->
</pre>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>too_many_holidays</c-> <c- o>=</c-> <c- n>employees</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>drop_while</c-><c- p>([](</c-><c- k>auto</c-> <c- k>const</c-> <c- n>e</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>e</c-><c- p>.</c-><c- n>holidays</c-><c- p>()</c-> <c- o>&lt;</c-> <c- mi>20</c-><c- p>;</c-> <c- p>});</c->
<c- n>ranges</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- n>too_many_holidays</c-><c- p>,</c->
   <c- n>ranges</c-><c- o>::</c-><c- n>ostream_iterator</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>cout</c-><c- p>,</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-><c- p>),</c->
   <c- p>[](</c-><c- k>auto</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>e</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>e</c-><c- p>.</c-><c- n>given_name</c-><c- p>()</c-> <c- o>+</c-> <c- n>e</c-><c- p>.</c-><c- n>surname</c-><c- p>();</c-> <c- p>});</c->
</pre>
   </table>
   <h4 class="heading settled" data-level="3.5.2" id="interface①"><span class="secno">3.5.2. </span><span class="content">Interface</span><a class="self-link" href="#interface①"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>View</c-> <c- n>R</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Pred</c-><c- o>></c->
<c- n>requires</c->
  <c- n>InputRange</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c->
  <c- n>std</c-><c- o>::</c-><c- n>is_object_v</c-><c- o>&lt;</c-><c- n>Pred</c-><c- o>></c-> <c- o>&amp;&amp;</c->
  <c- n>IndirectPredicate</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c->
<c- k>class</c-> <c- nc>drop_while_view</c-> <c- o>:</c-> <c- k>public</c-> <c- n>view_interface</c-><c- o>&lt;</c-><c- n>drop_while_view</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>Pred</c-><c- o>>></c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
   <c- n>drop_while_view</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

   <c- k>constexpr</c-> <c- nf>drop_while_view</c-><c- p>(</c-><c- n>R</c-> <c- n>base</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>);</c->

   <c- k>template</c-><c- o>&lt;</c-><c- n>ViewableRange</c-> <c- n>O</c-><c- o>></c->
   <c- n>requires</c-> <c- n>constructible</c-><c- o>-</c-><c- n>from</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>O</c-><c- o>></c->
   <c- k>constexpr</c-> <c- n>drop_while_view</c-><c- p>(</c-><c- n>O</c-><c- o>&amp;&amp;</c-> <c- n>o</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>);</c->

   <c- k>constexpr</c-> <c- n>R</c-> <c- nf>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
   <c- k>constexpr</c-> <c- n>Pred</c-> <c- nf>pred</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>();</c->
   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>();</c->
<c- k>private</c-><c- o>:</c->
   <c- n>R</c-> <c- n>base_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
   <c- n>semiregular</c-><c- o>&lt;</c-><c- n>Pred</c-><c- o>></c-> <c- n>pred_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
<c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Pred</c-><c- o>></c->
<c- n>drop_while_view</c-><c- p>(</c-><c- n>R</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- n>Pred</c-><c- p>)</c-> <c- o>-></c-> <c- n>drop_while_view</c-><c- o>&lt;</c-><c- n>all_view</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> <c- n>Pred</c-><c- o>></c-><c- p>;</c->
</pre>
   <h5 class="heading settled" data-level="3.5.2.1" id="drop_while_view-constructors"><span class="secno">3.5.2.1. </span><span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> constructors</span><a class="self-link" href="#drop_while_view-constructors"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- nf>drop_while_view</c-><c- p>(</c-><c- n>R</c-> <c- n>base</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>);</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>base_</c-></code> with <code class="highlight"><c- n>base</c-></code> and initialises <code class="highlight"><c- n>pred_</c-></code> with <code class="highlight"><c- n>pred</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>ViewableRange</c-> <c- n>O</c-><c- o>></c->
<c- n>requires</c-> <c- n>constructible</c-><c- o>-</c-><c- n>from</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>O</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>drop_while_view</c-><c- p>(</c-><c- n>O</c-><c- o>&amp;&amp;</c-> <c- n>o</c-><c- p>,</c-> <c- n>Pred</c-> <c- n>pred</c-><c- p>)</c->
</pre>
   <ol start="2">
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>base_</c-></code> with <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>all</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>O</c-><c- o>></c-><c- p>(</c-><c- n>o</c-><c- p>))</c-></code>, and intialises <code class="highlight"><c- n>pred_</c-></code> with <code class="highlight"><c- n>pred</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.5.2.2" id="drop_while_view-conversion"><span class="secno">3.5.2.2. </span><span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> conversion</span><a class="self-link" href="#drop_while_view-conversion"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>R</c-> <c- nf>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ol start="3">
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- n>base_</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>Pred</c-> <c- nf>pred</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ol start="4">
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- n>pred_</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.5.2.3" id="drop_while_view-begin"><span class="secno">3.5.2.3. </span><span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> begin</span><a class="self-link" href="#drop_while_view-begin"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>();</c->
</pre>
   <ol start="5">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>find_if_not</c-><c- p>(</c-><c- n>base_</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>ref</c-><c- p>(</c-><c- n>pred_</c-><c- p>));</c-></code>.</p>
    <li data-md>
     <p>_Remarks_: In order to provide the amortized constant time complexity required by the <code class="highlight"><c- n>Range</c-></code> concept, this function caches the result within the <code class="highlight"><c- n>drop_while_view</c-></code> for use on subsequent
calls.</p>
   </ol>
   <h5 class="heading settled" data-level="3.5.2.4" id="drop_while_view-end"><span class="secno">3.5.2.4. </span><span class="content"><code class="highlight"><c- n>drop_while_view</c-></code> end</span><a class="self-link" href="#drop_while_view-end"></a></h5>
   <ol start="7">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c-></code>.</p>
   </ol>
   <h3 class="heading settled" data-level="3.6" id="viewdrop_while"><span class="secno">3.6. </span><span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop_while</c-></code></span><a class="self-link" href="#viewdrop_while"></a></h3>
   <p>The name <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop_while</c-></code> denotes a range adaptor object. Let <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> be expressions such that
type <code class="highlight"><c- n>T</c-></code> is <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code>. Then, the expression <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop_while</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code>is expression-equivalent
to:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>drop_while_view</c-><c- p>{</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>InputRange</c-></code>, and <code class="highlight"><c- n>F</c-></code> is both an object and models <code class="highlight"><c- n>IndirectUnaryPredicate</c-></code>.</p>
    <li data-md>
     <p>Otherwise <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is ill-formed.</p>
   </ol>
   <h3 class="heading settled" data-level="3.7" id="basic_istream_view"><span class="secno">3.7. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code></span><a class="self-link" href="#basic_istream_view"></a></h3>
   <h4 class="heading settled" data-level="3.7.1" id="motivation④"><span class="secno">3.7.1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation④"></a></h4>
   <p><code class="highlight"><c- n>istream_iterator</c-></code> is an abstraction over a <code class="highlight"><c- n>basic_istream</c-></code> object, so that it may be used as though
it were an input iterator. It is a great way to populate a container from the get-go, or fill a
container later in its lifetime. This is great, as copy is a standard algorithm that cleanly
communicates that we’re copying something from one range into another. There aren’t any Hidden
Surprises<sup>TM</sup>. This is also good when writing generic code, because the generic library
author does not need to care how things are inserted at the end of <code class="highlight"><c- n>v</c-></code>, only that they are.</p>
   <table>
    <tbody>
     <tr>
      <th>Without <code class="highlight"><c- n>istream_iterator</c-></code>
      <th>With <code class="highlight"><c- n>istream_iterator</c-></code>
     <tr>
      <td>
<pre class="highlight">    <c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- p>[]{</c->
       <c- k>auto</c-> <c- n>result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{};</c->
       <c- k>for</c-> <c- p>(</c-><c- k>auto</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>std</c-><c- o>::</c-><c- n>cin</c-> <c- o>>></c-> <c- n>i</c-><c- p>;)</c-> <c- p>{</c->
          <c- n>result</c-><c- p>.</c-><c- n>push_back</c-><c- p>(</c-><c- n>i</c-><c- p>);</c->
       <c- p>}</c->
    <c- p>}();</c->
    <c- c1>// ...</c->
    <c- k>for</c-> <c- p>(</c-><c- k>auto</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>std</c-><c- o>::</c-><c- n>cin</c-> <c- o>>></c-> <c- n>i</c-><c- p>;)</c-> <c- p>{</c->
       <c- n>result</c-><c- p>.</c-><c- n>push_back</c-><c- p>(</c-><c- n>i</c-><c- p>);</c->
    <c- p>}</c->
  
</pre>
      <td>
<pre class="highlight"><c- o>&lt;</c-><c- n>p</c-> <c- n>line</c-><c- o>-</c-><c- n>number</c-><c- o>=</c-><c- mi>616</c-><c- o>></c-><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>(</c-><c- n>istream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>cin</c-><c- p>},</c-> <c- n>istream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{});</c->
    <c- c1>// ...</c->
    <c- n>copy</c-><c- p>(</c-><c- n>istream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>cin</c-><c- p>},</c-> <c- n>istream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{},</c-> <c- n>back_inserter</c-><c- p>(</c-><c- n>v</c-><c- p>));</c-><c- o>&lt;/</c-><c- n>p</c-><c- o>></c->

  
</pre>
   </table>
   <p>The problem with <code class="highlight"><c- n>istream_iterator</c-></code> is that we need to provide a bogus <code class="highlight"><c- n>istream_iterator</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>{}</c-></code> (or <code class="highlight"><c- n>default_sentinel</c-><c- p>{}</c-></code>) every time we want to use it; this acts as the sentinel for <code class="highlight"><c- n>istream_iterator</c-></code>. It is bogus, because the code is equivalent to saying "from the first element of
the istream range until the last element of the istream range", but an <code class="highlight"><c- n>istream</c-></code> range does not have
a last element. Instead, we denote the end of an istream range to be when the <code class="highlight"><c- n>istream</c-></code>'s failbit is
set. This is otherwise known as the _end-of-stream_ iterator value, but it doesn’t denote a
'past-the-last element' in the same way that call to <code class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>end</c-></code> does. Because it is the same
for every range, the _end-of-stream_ object may as well be dropped, so that we can write code that
resembles the code below.</p>
<pre class="language-cpp highlight"><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>istream_view</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>cin</c-><c- p>});</c->
<c- c1>// ...</c->
<c- n>copy</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>istream_view</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>cin</c-><c- p>},</c-> <c- n>back_inserter</c-><c- p>(</c-><c- n>v</c-><c- p>));</c->
</pre>
   <p>This code is cleaner: we are implicitly saying "until our <code class="highlight"><c- n>basic_istream</c-></code> object fails, insert our
input into the back of <code class="highlight"><c- n>v</c-></code>". There is less to read (and type!), and the code is simpler for it.</p>
   <h4 class="heading settled" data-level="3.7.2" id="interface②"><span class="secno">3.7.2. </span><span class="content">Interface</span><a class="self-link" href="#interface②"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>CharT</c-> <c- o>=</c-> <c- b>char</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-> <c- o>=</c-> <c- n>char_traits</c-><c- o>&lt;</c-><c- n>CharT</c-><c- o>>></c->
<c- n>concept</c-> <c- b>bool</c-> <c- n>StreamExtractable</c-> <c- o>=</c-> <c- n>see</c-><c- o>-</c-><c- n>below</c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>charT</c-> <c- o>=</c-> <c- b>char</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>char_traits</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>>></c->
<c- n>concept</c-> <c- b>bool</c-> <c- n>StreamInsertable</c-> <c- o>=</c-> <c- n>see</c-><c- o>-</c-><c- n>below</c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- n>Semiregular</c-> <c- n>Val</c-><c- p>,</c-> <c- k>class</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-> <c- o>=</c-> <c- n>char_traits</c-><c- o>&lt;</c-><c- n>CharT</c-><c- o>>></c->
<c- n>requires</c-> <c- n>StreamExtractable</c-><c- o>&lt;</c-><c- n>Val</c-><c- p>,</c-> <c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>></c->
<c- k>class</c-> <c- nc>basic_istream_view</c-> <c- o>:</c-> <c- k>public</c-> <c- n>view_interface</c-><c- o>&lt;</c-><c- n>basic_istream_view</c-><c- o>&lt;</c-><c- n>Val</c-><c- p>,</c-> <c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>></c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
   <c- n>basic_istream_view</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

   <c- k>explicit</c-> <c- k>constexpr</c-> <c- nf>basic_istream_view</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>basic_istream</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>);</c->

   <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>();</c->
   <c- k>constexpr</c-> <c- n>default_sentinel</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>private</c-><c- o>:</c->
   <c- k>struct</c-> <c- n>__iterator</c-><c- p>;</c-> <c- c1>// exposition-only</c->
   <c- n>std</c-><c- o>::</c-><c- n>basic_istream</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>*</c-> <c- n>stream_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
   <c- n>Val</c-> <c- n>object_</c-><c- p>;</c-> <c- c1>// exposition-only</c->
<c- p>};</c->
</pre>
   <h5 class="heading settled" data-level="3.7.2.1" id="concept-streamextractable"><span class="secno">3.7.2.1. </span><span class="content">Concept <code class="highlight"><c- n>StreamExtractable</c-></code></span><a class="self-link" href="#concept-streamextractable"></a></h5>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>CharT</c-> <c- o>=</c-> <c- b>char</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-> <c- o>=</c-> <c- n>char_traits</c-><c- o>&lt;</c-><c- n>CharT</c-><c- o>>></c->
<c- n>concept</c-> <c- b>bool</c-> <c- n>StreamExtractable</c-> <c- o>=</c->
   <c- n>requires</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>basic_istream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>is</c-><c- p>,</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c->
      <c- p>{</c-> <c- n>is</c-> <c- o>>></c-> <c- n>t</c-> <c- p>}</c-> <c- o>-></c-> <c- n>Same</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>basic_istream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>>&amp;</c-><c- p>;</c->
   <c- p>};</c->
</pre>
   <ol>
    <li data-md>
     <p><i>Remarks</i>: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>is</c-><c- p>)</c-> <c- o>==</c-> <c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>is</c-> <c- o>&lt;&lt;</c-> <c- n>t</c-><c- p>)</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.7.2.2" id="concept-streaminsertable"><span class="secno">3.7.2.2. </span><span class="content">Concept <code class="highlight"><c- n>StreamInsertable</c-></code></span><a class="self-link" href="#concept-streaminsertable"></a></h5>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>charT</c-> <c- o>=</c-> <c- b>char</c-><c- p>,</c-> <c- k>class</c-> <c- nc>traits</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>char_traits</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>>></c->
<c- n>concept</c-> <c- b>bool</c-> <c- n>StreamInsertable</c-> <c- o>=</c->
   <c- n>requires</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>basic_ostream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>&amp;</c-> <c- n>os</c-><c- p>,</c-> <c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c->
      <c- p>{</c-> <c- n>os</c-> <c- o>&lt;&lt;</c-> <c- n>t</c-> <c- p>}</c-> <c- o>-></c-> <c- n>Same</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>basic_ostream</c-><c- o>&lt;</c-><c- n>charT</c-><c- p>,</c-> <c- n>traits</c-><c- o>>>&amp;</c-><c- p>;</c->
   <c- p>};</c->
</pre>
   <ol start="2">
    <li data-md>
     <p><i>Remarks</i>: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>os</c-><c- p>)</c-> <c- o>==</c-> <c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>os</c-> <c- o>>></c-> <c- n>t</c-><c- p>)</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.7.2.3" id="basic_istream_view-constructor"><span class="secno">3.7.2.3. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> constructor</span><a class="self-link" href="#basic_istream_view-constructor"></a></h5>
<pre class="language-cpp highlight"><c- k>explicit</c-> <c- k>constexpr</c-> <c- nf>basic_istream_view</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>basic_istream</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>&amp;</c-> <c- n>stream</c-><c- p>);</c->
</pre>
   <ol start="3">
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>stream_</c-></code> to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>stream</c-><c- p>)</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.7.2.4" id="basic_istream_view-begin"><span class="secno">3.7.2.4. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> begin</span><a class="self-link" href="#basic_istream_view-begin"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- k>auto</c-> <c- nf>begin</c-><c- p>();</c->
</pre>
   <ol start="4">
    <li data-md>
     <p><i>Effects</i>: Equivalent to</p>
   </ol>
<pre class="language-cpp highlight"><c- o>*</c-><c- n>stream_</c-> <c- o>>></c-> <c- n>object_</c-><c- p>;</c->
<c- k>return</c-> <c- n>__iterator</c-><c- p>{</c-><c- o>*</c-><c- k>this</c-><c- p>};</c->
</pre>
   <h5 class="heading settled" data-level="3.7.2.5" id="basic_istream_view-end"><span class="secno">3.7.2.5. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> end</span><a class="self-link" href="#basic_istream_view-end"></a></h5>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>default_sentinel</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <ol start="5">
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- n>default_sentinel</c-><c- p>{}</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.7.2.6" id="basic_istream_view__iterator"><span class="secno">3.7.2.6. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code></span><a class="self-link" href="#basic_istream_view__iterator"></a></h5>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Val</c-><c- p>,</c-> <c- k>class</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Traits</c-><c- o>></c->
<c- k>class</c-> <c- nc>basic_istream_view</c-><c- o>&lt;</c-><c- n>Val</c-><c- p>,</c-> <c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>::</c-><c- n>__iterator</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
   <c- k>using</c-> <c- n>iterator_category</c-> <c- o>=</c-> <c- n>input_iterator_tag</c-><c- p>;</c->
   <c- k>using</c-> <c- n>difference_type</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- b>ptrdiff_t</c-><c- p>;</c->
   <c- k>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>Val</c-><c- p>;</c->

   <c- n>__iterator</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

   <c- k>explicit</c-> <c- k>constexpr</c-> <c- nf>__iterator</c-><c- p>(</c-><c- n>istream_view</c-><c- o>&lt;</c-><c- n>Val</c-><c- o>>&amp;</c-> <c- n>parent</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

   <c- n>__iterator</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>++</c-><c- p>();</c->
   <c- b>void</c-> <c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>);</c->

   <c- n>Val</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

   <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>__iterator</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel</c-><c- p>);</c->
   <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>default_sentinel</c-> <c- n>y</c-><c- p>,</c-> <c- n>__iterator</c-> <c- n>x</c-><c- p>);</c->
   <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- n>__iterator</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel</c-> <c- n>y</c-><c- p>);</c->
   <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- n>default_sentinel</c-> <c- n>y</c-><c- p>,</c-> <c- n>__iterator</c-> <c- n>x</c-><c- p>);</c->
<c- k>private</c-><c- o>:</c->
   <c- n>basic_istream_view</c-><c- o>&lt;</c-><c- n>Val</c-><c- p>,</c-> <c- n>CharT</c-><c- p>,</c-> <c- n>Traits</c-><c- o>>*</c-> <c- n>parent_</c-> <c- o>=</c-> <c- k>nullptr</c-><c- p>;</c-> <c- c1>// exposition-only</c->
<c- p>};</c->
</pre>
   <h5 class="heading settled" data-level="3.7.2.7" id="basic_istream_view__iterator-constructor"><span class="secno">3.7.2.7. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> constructor</span><a class="self-link" href="#basic_istream_view__iterator-constructor"></a></h5>
<pre class="language-cpp highlight"><c- k>explicit</c-> <c- k>constexpr</c-> <c- nf>__iterator</c-><c- p>(</c-><c- n>istream_view</c-><c- o>&lt;</c-><c- n>Val</c-><c- o>>&amp;</c-> <c- n>parent</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <ol start="6">
    <li data-md>
     <p><i>Effects</i>: Initialises <code class="highlight"><c- n>parent_</c-></code> with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>addressof</c-><c- p>(</c-><c- n>parent_</c-><c- p>)</c-></code>.</p>
   </ol>
   <h5 class="heading settled" data-level="3.7.2.8" id="basic_istream_view__iterator-next"><span class="secno">3.7.2.8. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> next</span><a class="self-link" href="#basic_istream_view__iterator-next"></a></h5>
<pre class="language-cpp highlight"><c- n>__iterator</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>++</c-><c- p>();</c->
<c- b>void</c-> <c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>);</c->
</pre>
   <ol start="7">
    <li data-md>
     <p><i>Effects</i>: Equivalent to</p>
   </ol>
<pre class="language-cpp highlight"><c- o>*</c-><c- n>parent_</c-><c- o>-></c-><c- n>stream_</c-> <c- o>>></c-> <c- n>parent_</c-><c- o>-></c-><c- n>object_</c-><c- p>;</c->
</pre>
   <h5 class="heading settled" data-level="3.7.2.9" id="basic_istream_view__iterator-value"><span class="secno">3.7.2.9. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> value</span><a class="self-link" href="#basic_istream_view__iterator-value"></a></h5>
<pre class="language-cpp highlight"><c- n>Val</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ol start="8">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- n>parent_</c-><c- o>-></c-><c- n>value_</c-><c- p>;</c-></code></p>
   </ol>
   <h5 class="heading settled" data-level="3.7.2.10" id="basic_istream_view__iterator-comparison-functions"><span class="secno">3.7.2.10. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-><c- o>::</c-><c- n>__iterator</c-></code> comparison functions</span><a class="self-link" href="#basic_istream_view__iterator-comparison-functions"></a></h5>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>__iterator</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel</c-><c- p>);</c->
</pre>
   <ol start="9">
    <li data-md>
     <p><i>Effects</i>: Equivalent to <code class="highlight"><c- k>return</c-> <c- o>!*</c-><c- n>x</c-><c- p>.</c-><c- n>parent_</c-><c- o>-></c-><c- n>stream_</c-><c- p>;</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>default_sentinel</c-> <c- n>y</c-><c- p>,</c-> <c- n>__iterator</c-> <c- n>x</c-><c- p>);</c->
</pre>
   <ol start="10">
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- n>x</c-> <c- o>==</c-> <c- n>y</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- n>__iterator</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <ol start="11">
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- o>!</c-><c- p>(</c-><c- n>x</c-> <c- o>==</c-> <c- n>y</c-><c- p>)</c-></code>.</p>
   </ol>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- n>default_sentinel</c-> <c- n>y</c-><c- p>,</c-> <c- n>__iterator</c-> <c- n>x</c-><c- p>);</c->
</pre>
   <ol start="12">
    <li data-md>
     <p><i>Returns</i>: <code class="highlight"><c- o>!</c-><c- p>(</c-><c- n>x</c-> <c- o>==</c-> <c- n>y</c-><c- p>)</c-></code>.</p>
   </ol>
   <h3 class="heading settled" data-level="3.8" id="basic_istream_view-range-adaptors"><span class="secno">3.8. </span><span class="content"><code class="highlight"><c- n>basic_istream_view</c-></code> range adaptors</span><a class="self-link" href="#basic_istream_view-range-adaptors"></a></h3>
   <p>The names <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>istream</c-></code>, <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>wistream</c-></code>, <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>u16istream</c-></code>, and <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>u32istream</c-></code> denote range
adaptors. Let <code class="highlight"><c- n>E</c-></code> be an expression and <code class="highlight"><c- n>T</c-></code> be a type distinct from <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code>.</p>
   <ol>
    <li data-md>
     <p>Then, the expression <code class="highlight"><c- n>istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is expression-equivalent to:</p>
    <li data-md>
     <p><code class="highlight"><c- n>basic_istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- b>char</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> if <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code> models <code class="highlight"><c- n>DerivedFrom</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>istream</c-><c- o>></c-></code> and <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>StreamExtractable</c-></code>.</p>
    <li data-md>
     <p>Otherwise <code class="highlight"><c- n>istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is ill-formed.</p>
    <li data-md>
     <p>Then, the expression <code class="highlight"><c- n>wistream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is expression-equivalent to:</p>
    <li data-md>
     <p><code class="highlight"><c- n>basic_istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- b>wchar_t</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> if <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code> models <code class="highlight"><c- n>DerivedFrom</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>wistream</c-><c- o>></c-></code> and <code class="highlight"><c- n>T</c-></code> modes <code class="highlight"><c- n>StreamExtractable</c-></code>.</p>
    <li data-md>
     <p>Otherwise <code class="highlight"><c- n>wistream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is ill-formed.</p>
    <li data-md>
     <p>Then, the expression <code class="highlight"><c- n>u16istream_view</c-></code> is expression-equivalent to:</p>
    <li data-md>
     <p><code class="highlight"><c- n>basic_istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- b>char16_t</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> if <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code> models <code class="highlight"><c- n>DerivedFrom</c-><c- o>&lt;</c-><c- n>basic_istream</c-><c- o>&lt;</c-><c- b>char16_t</c-><c- o>>></c-></code> and <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>StreamExtractable</c-></code>.</p>
    <li data-md>
     <p>Otherwise <code class="highlight"><c- n>u16istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is ill-formed.</p>
    <li data-md>
     <p>Then, the expression <code class="highlight"><c- n>u32istream_view</c-></code> is expression-equivalent to:</p>
    <li data-md>
     <p><code class="highlight"><c- n>basic_istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- b>char32_t</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> if <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-></code> models <code class="highlight"><c- n>DerivedFrom</c-><c- o>&lt;</c-><c- n>basic_istream</c-><c- o>&lt;</c-><c- b>char32_t</c-><c- o>>></c-></code> and <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>StreamExtractable</c-></code>.</p>
    <li data-md>
     <p>Otherwise <code class="highlight"><c- n>u32istream_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is ill-formed.</p>
   </ol>
   <h3 class="heading settled" data-level="3.9" id="zip_with_view"><span class="secno">3.9. </span><span class="content"><code class="highlight"><c- n>zip_with_view</c-></code></span><a class="self-link" href="#zip_with_view"></a></h3>
   <h4 class="heading settled" data-level="3.9.1" id="motivation⑤"><span class="secno">3.9.1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation⑤"></a></h4>
   <p>A zip, also known as a <a data-link-type="biblio" href="#biblio-convolution">[convolution]</a> operation, performs a transformation on multiple input
ranges. The typical zip operation transforms several input ranges into a single input range
containing a tuple with the <i>i</i>th element from each range, and terminates when the smallest
finite range is delimited. The following example has been adapted from <a data-link-type="biblio" href="#biblio-p0836">[P0836]</a> §2.1.</p>
   <table>
    <tbody>
     <tr>
      <th>Current (C++17)
      <th>Proposed (C++20)
     <tr>
      <td>
<pre class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-> <c- k>const</c-> <c- n>x</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- n>vector</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-> <c- k>const</c-> <c- n>y</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- b>float</c-> <c- n>a</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- c1>// ...</c->
<c- k>auto</c-> <c- n>result</c-> <c- o>=</c-> <c- n>vector</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-><c- p>(</c-><c- n>size</c-><c- p>(</c-><c- n>x</c-><c- p>));</c->
<c- n>transform</c-><c- p>(</c-><c- n>x</c-><c- p>,</c-> <c- n>y</c-><c- p>,</c-> <c- n>begin</c-><c- p>(</c-><c- n>result</c-><c- p>),</c-> <c- p>[</c-><c- n>a</c-><c- p>](</c-><c- k>auto</c-> <c- k>const</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- k>const</c-> <c- n>y</c-><c- p>)</c-> <c- p>{</c->
   <c- k>return</c-> <c- n>a</c-> <c- o>*</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c-> <c- p>});</c->
</pre>
      <td>
<pre class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-> <c- k>const</c-> <c- n>x</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- n>vector</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-> <c- k>const</c-> <c- n>y</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- b>float</c-> <c- k>const</c-> <c- n>a</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- c1>// ...</c->
<c- k>auto</c-> <c- k>const</c-> <c- n>pair_mult</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>ax</c-><c- p>){</c-> <c- k>auto</c-> <c- p>[</c-><c- n>a</c-><c- p>,</c-> <c- n>x</c-><c- p>]</c-> <c- o>=</c-> <c- n>ax</c-><c- p>;</c-> <c- k>return</c-> <c- n>a</c-> <c- o>*</c-> <c- n>x</c-><c- p>;</c-> <c- p>};</c->
<c- k>auto</c-> <c- k>const</c-> <c- n>pair_plus</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>axpy</c-><c- p>){</c-> <c- k>auto</c-> <c- p>[</c-><c- n>ax</c-><c- p>,</c-> <c- n>y</c-><c- p>]</c-> <c- o>=</c-> <c- n>axpy</c-><c- p>;</c-> <c- k>return</c-> <c- n>ax</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>ax</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>zip</c-><c- p>(</c-><c- n>view</c-><c- o>::</c-><c- n>repeat</c-><c- p>(</c-><c- n>a</c-><c- p>),</c-> <c- n>x</c-><c- p>)</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- n>pair_mult</c-><c- p>);</c->
<c- k>auto</c-> <c- n>saxpy</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>zip</c-><c- p>(</c-><c- n>ax</c-><c- p>,</c-> <c- n>y</c-><c- p>)</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- n>pair_plus</c-><c- p>);</c->
<c- k>auto</c-> <c- k>const</c-> <c- n>result</c-> <c- o>=</c-> <c- n>vector</c-><c- p>(</c-><c- n>begin</c-><c- p>(</c-><c- n>saxpy</c-><c- p>),</c-> <c- n>end</c-><c- p>(</c-><c- n>saxpy</c-><c- p>));</c->
</pre>
   </table>
   <p>The benefits of this proposed approach include:</p>
   <ul>
    <li data-md>
     <p>More <i>declared</i> operations, leading to more declarative -- rather than imperative -- style
programming.</p>
    <li data-md>
     <p>Eliminates state.</p>
    <li data-md>
     <p><code class="highlight"><c- n>result</c-></code> can be declared <code class="highlight"><c- k>const</c-></code> without needing to rely on IILE. (Note the lack of IILE and the
non-<code class="highlight"><c- k>const</c-></code>ness in the former).</p>
    <li data-md>
     <p>Temporary storage is eliminated, which P0836 §2.1 articulates as beneficial for heterogeneous
programming.</p>
   </ul>
   <p><code class="highlight"><c- n>zip_with</c-></code> is a generalisation of <code class="highlight"><c- n>zip</c-></code>, such that we can apply an <i>n</i>-ary function in place of
the <code class="highlight"><c- n>transform</c-></code>s above. The following is an example of how <code class="highlight"><c- n>zip_with</c-></code> can refine <code class="highlight"><c- n>zip</c-></code> when tuples
are not necessary.</p>
<pre class="language-cpp highlight"><c- n>vector</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-> <c- k>const</c-> <c- n>x</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- n>vector</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-> <c- k>const</c-> <c- n>y</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- b>float</c-> <c- k>const</c-> <c- n>a</c-> <c- o>=</c-> <c- c1>// ...</c->
<c- c1>// ...</c->
<c- k>auto</c-> <c- n>ax</c-> <c- o>=</c-> <c- n>x</c-> <c- o>|</c-> <c- n>view</c-><c- o>::</c-><c- n>transform</c-><c- p>([</c-><c- n>a</c-><c- p>](</c-><c- k>auto</c-> <c- k>const</c-> <c- n>x</c-><c- p>)</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-> <c- o>*</c-> <c- n>x</c-><c- p>;</c-> <c- p>});</c->
<c- k>auto</c-> <c- n>saxpy</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>zip_with</c-><c- p>(</c-><c- n>plus</c-><c- o>&lt;></c-><c- p>{},</c-> <c- n>ax</c-><c- p>,</c-> <c- n>y</c-><c- p>);</c->
<c- k>auto</c-> <c- k>const</c-> <c- n>result</c-> <c- o>=</c-> <c- n>vector</c-><c- p>(</c-><c- n>begin</c-><c- p>(</c-><c- n>saxpy</c-><c- p>),</c-> <c- n>end</c-><c- p>(</c-><c- n>saxpy</c-><c- p>));</c->
</pre>
   <p>Another motivating example for using <code class="highlight"><c- n>zip</c-></code> involves replacing raw loops with algorithms and range
adaptors, but still being able to index the operation in question.</p>
   <table>
    <tbody>
     <tr>
      <th>Current (C++17)
      <th>Proposed (C++20)
     <tr>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>vector</c-><c- p>(</c-><c- n>istream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>cin</c-><c- p>),</c-> <c- n>istream_iterator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>());</c->
<c- k>auto</c-> <c- n>weighted_sum</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
<c- k>for</c-> <c- p>(</c-><c- k>auto</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- b>unsigned</c-> <c- b>int</c-><c- o>></c-><c- n>i</c-> <c- o>&lt;</c-> <c- n>size</c-><c- p>(</c-><c- n>v</c-><c- p>);</c-> <c- o>++</c-><c- n>i</c-><c- p>)</c-> <c- p>{</c->
   <c- n>weighted_sum</c-> <c- o>+=</c-> <c- n>i</c-> <c- o>*</c-> <c- n>v</c-><c- p>[</c-><c- n>i</c-><c- p>];</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>in</c-> <c- o>=</c-> <c- n>view</c-><c- o>::</c-><c- n>zip</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- n>istream_range</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>cin</c-><c- p>});</c->
<c- k>auto</c-> <c- n>weighted_sum</c-> <c- o>=</c-> <c- n>accumulate</c-><c- p>(</c-><c- n>begin</c-><c- p>(</c-><c- n>in</c-><c- p>),</c-> <c- n>end</c-><c- p>(</c-><c- n>in</c-><c- p>),</c-> <c- p>[](</c-><c- k>auto</c-> <c- k>const</c-> <c- n>a</c-><c- p>,</c-> <c- k>auto</c-> <c- k>const</c-> <c- n>b</c-><c- p>){</c->
   <c- k>auto</c-> <c- k>const</c-> <c- p>[</c-><c- n>index</c-><c- p>,</c-> <c- n>in</c-><c- p>]</c-> <c- o>=</c-> <c- n>b</c-><c- p>;</c->
   <c- k>return</c-> <c- n>a</c-> <c- o>+</c-> <c- p>(</c-><c- n>index</c-> <c- o>*</c-> <c- n>in</c-><c- p>);</c->
<c- p>});</c->
</pre>
   </table>
   <p>In the following text, <code class="highlight"><c- n>zip</c-></code> is implemented in terms of <code class="highlight"><c- n>zip_with</c-></code>.</p>
   <h4 class="heading settled" data-level="3.9.2" id="__common_tuple"><span class="secno">3.9.2. </span><span class="content"><code class="highlight"><c- n>__common_tuple</c-></code></span><a class="self-link" href="#__common_tuple"></a></h4>
   <p>TODO (released before San Diego).</p>
   <h4 class="heading settled" data-level="3.9.3" id="__iter_zip_with"><span class="secno">3.9.3. </span><span class="content"><code class="highlight"><c- n>__iter_zip_with</c-></code></span><a class="self-link" href="#__iter_zip_with"></a></h4>
   <p>TODO (released before San Diego).</p>
   <h4 class="heading settled" data-level="3.9.4" id="zip_with_view①"><span class="secno">3.9.4. </span><span class="content"><code class="highlight"><c- n>zip_with_view</c-></code></span><a class="self-link" href="#zip_with_view①"></a></h4>
   <p>TODO (released before San Diego).</p>
   <h3 class="heading settled" data-level="3.10" id="viewzip_with"><span class="secno">3.10. </span><span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip_with</c-></code></span><a class="self-link" href="#viewzip_with"></a></h3>
   <p>The name <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip_with</c-></code> denotes a range adaptor object. Let <code class="highlight"><c- n>Args</c-><c- o>&amp;&amp;</c-><c- p>...</c-></code> be a parameter pack, with <code class="highlight"><c- n>args</c-><c- p>...</c-></code> as its pack expansion, and let <code class="highlight"><c- n>F</c-></code> be an expression. Then, the expression <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip_with</c-><c- p>(</c-><c- n>F</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...)</c-></code> is expression-equivalent to:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>zip_with_view</c-><c- p>{</c-><c- n>F</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...}</c-></code>, if all parameters in the parameter pack <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>...</c-></code> model <code class="highlight"><c- n>InputRange</c-></code>, and if <code class="highlight"><c- n>F</c-></code> models <code class="highlight"><c- n>RegularInvocable</c-><c- o>&lt;</c-><c- n>iter_reference_t</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>>>></c-><c- p>...</c-><c- o>></c-></code>.</p>
    <li data-md>
     <p>Otherwise, <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip_with</c-><c- p>(</c-><c- n>F</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...)</c-></code> is ill-formed.</p>
   </ol>
   <h3 class="heading settled" data-level="3.11" id="viewzip"><span class="secno">3.11. </span><span class="content"><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip</c-></code></span><a class="self-link" href="#viewzip"></a></h3>
   <p>The name <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip</c-></code> denotes a range adptor object. Let <code class="highlight"><c- n>Args</c-><c- o>&amp;&amp;</c-><c- p>...</c-></code> be a parameter pack, with <code class="highlight"><c- n>args</c-><c- p>...</c-></code> as its pack expansion. Then, the expression <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...)</c-></code> is
expression-equivalent to:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip_with</c-><c- p>{</c-><c- n>make_pair</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...}</c-></code>, if <code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>Args</c-><c- p>)</c-> <c- o>==</c-> <c- mi>2</c-></code>, and both
parameters in the parameter pack <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>...</c-></code> model <code class="highlight"><c- n>InputRange</c-></code>.</p>
    <li data-md>
     <p><code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip_with</c-><c- p>{</c-><c- n>make_tuple</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...}</c-></code>, if <code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>Args</c-><c- p>)</c-> <c- o>!=</c-> <c- mi>2</c-></code>, and all
parameters in the parameter pack <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>...</c-></code> model <code class="highlight"><c- n>InputRange</c-></code>.</p>
    <li data-md>
     <p>Otherwise, <code class="highlight"><c- n>view</c-><c- o>::</c-><c- n>zip</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...)</c-></code> is ill-formed.</p>
   </ol>
   <h3 class="heading settled" data-level="3.12" id="reviewing-iter_value_t-etc-and-introducing-range_value_t-etc"><span class="secno">3.12. </span><span class="content">Reviewing <code class="highlight"><c- n>iter_value_t</c-></code>, etc., and introducing <code class="highlight"><c- n>range_value_t</c-></code>, etc.</span><a class="self-link" href="#reviewing-iter_value_t-etc-and-introducing-range_value_t-etc"></a></h3>
   <p>P1035 also proposes to review <code class="highlight"><c- n>iter_value_t</c-></code> on ranges, and to introduce a new <code class="highlight"><c- n>range_value_t</c-></code>, if
it is deemed appropriate.</p>
   <p>TODO (released before San Diego).</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-cmcstl2">[CMCSTL2]
   <dd>Casey Carter &amp; others. <a href=" https://github.com/CaseyCarter/cmcstl2">cmcstl2 C++ library</a>. URL: <a href=" https://github.com/CaseyCarter/cmcstl2"> https://github.com/CaseyCarter/cmcstl2</a>
   <dt id="biblio-convolution">[CONVOLUTION]
   <dd>Wikipedia. <a href="https://en.wikipedia.org/wiki/Convolution_(computer_science)">Convolution</a>. URL: <a href="https://en.wikipedia.org/wiki/Convolution_(computer_science)">https://en.wikipedia.org/wiki/Convolution_(computer_science)</a>
   <dt id="biblio-p0789">[P0789]
   <dd>Eric Niebler. <a href="https://wg21.link/p0789">Range Adaptors and Utilities</a>. URL: <a href="https://wg21.link/p0789">https://wg21.link/p0789</a>
   <dt id="biblio-p0836">[P0836]
   <dd>Gordon Brown; et al. <a href="https://wg21.link/p0836">P0836 Introduce Parallelism to the Ranges TS</a>. URL: <a href="https://wg21.link/p0836">https://wg21.link/p0836</a>
   <dt id="biblio-range-v3">[RANGE-V3]
   <dd>Eric Niebler &amp; others. <a href="https://github.com/ericniebler/range-v3">ranges-v3 C++ library</a>. URL: <a href="https://github.com/ericniebler/range-v3">https://github.com/ericniebler/range-v3</a>
  </dl>