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 <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>p0902r0: Move-only iterators</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,
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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">p0902r0<br>Move-only iterators</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-02-05">5 February 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0902R0">http://wg21.link/P0902R0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ahh@google.com">Andrew Hunter</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: 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>Nothing really requires iterators to be copyable. Can we remove that restriction?</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="#mot"><span class="secno">1</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#whync"><span class="secno">1.1</span> <span class="content">Sidebar: why might an iterator be noncopyable?</span></a>
     </ol>
    <li>
     <a href="#prop"><span class="secno">2</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#output"><span class="secno">2.1</span> <span class="content">What about <code class="highlight"><span class="n">OutputIterator</span></code></span></a>
      <li><a href="#alt"><span class="secno">2.2</span> <span class="content">Alternative: can we retrofit?</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="mot"><span class="secno">1. </span><span class="content">Motivation</span><a class="self-link" href="#mot"></a></h2>
   <p>Modern C++ has left <code class="highlight"><span class="n">InputIterator</span></code> in a curious position. We require
it (through the definition of <code class="highlight"><span class="n">Iterator</span></code>) to be copyable, but precious
little can be done with any copied iterator; only one of a set of
copies can be deferenced.  Why then does it require copyability?</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">InputIt</span><span class="p">,</span> <span class="k">class</span> <span class="nc">OutputIt</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UnaryOperation</span><span class="o">></span>
<span class="n">OutputIt</span> <span class="n">transform</span><span class="p">(</span><span class="n">InputIt</span> <span class="n">first1</span><span class="p">,</span> <span class="n">InputIt</span> <span class="n">last1</span><span class="p">,</span> <span class="n">OutputIt</span> <span class="n">d_first</span><span class="p">,</span>
                   <span class="n">UnaryOperation</span> <span class="n">unary_op</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">return</span> <span class="n">__transform_impl</span><span class="p">(</span><span class="n">first1</span><span class="p">,</span> <span class="n">last1</span><span class="p">,</span> <span class="n">d_first</span><span class="p">,</span> <span class="n">unary_op</span><span class="p">);</span>
<span class="p">}</span>
</pre>
   <p>The above code wants to implement transform by the use of a helper
function, but calling that helper copies the iterator.  There’s an
obvious way to remove that copy:</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">InputIt</span><span class="p">,</span> <span class="k">class</span> <span class="nc">OutputIt</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UnaryOperation</span><span class="o">></span>
<span class="n">OutputIt</span> <span class="n">transform</span><span class="p">(</span><span class="n">InputIt</span> <span class="n">first1</span><span class="p">,</span> <span class="n">InputIt</span> <span class="n">last1</span><span class="p">,</span> <span class="n">OutputIt</span> <span class="n">d_first</span><span class="p">,</span>
                   <span class="n">UnaryOperation</span> <span class="n">unary_op</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">return</span> <span class="n">__transform_impl</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">first1</span><span class="p">),</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">last1</span><span class="p">),</span> <span class="n">d_first</span><span class="p">,</span> <span class="n">unary_op</span><span class="p">);</span>
<span class="p">}</span>
</pre>
   <p>but that feature didn’t <em>exist</em> when <code class="highlight"><span class="n">Iterator</span></code> and <code class="highlight"><span class="n">InputIterator</span></code> were
created. I wasn’t there, but I’ll bet this is why the requirement was specified.</p>
   <p>I’d like to undo that decision (to the extent that is possible.)</p>
   <h3 class="heading settled" data-level="1.1" id="whync"><span class="secno">1.1. </span><span class="content">Sidebar: why might an iterator be noncopyable?</span><a class="self-link" href="#whync"></a></h3>
   <p>Why does this matter? Consider a reader-writer data structure (my
examples all derive from extensions of <a data-link-type="biblio" href="#biblio-cell">[Cell]</a>) where we expose
protected data that can only be safely held while holding a read
lock. <code class="highlight"><span class="n">cell</span></code> and friends ensure this by only allow users to see data
contained in RAII types (greatly
simplified here):</p>
<pre class="highlight"><span class="n">Token</span> <span class="nf">ReadLock</span><span class="p">();</span>
<span class="kt">void</span> <span class="nf">ReadUnlock</span><span class="p">(</span><span class="n">Token</span> <span class="n">t</span><span class="p">);</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span>
<span class="k">class</span> <span class="nc">snapshot</span> <span class="p">{</span>
 <span class="k">public</span><span class="o">:</span>
  <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">*</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="o">*</span><span class="n">p_</span><span class="p">;</span> <span class="p">}</span>
  <span class="o">~</span><span class="n">snapshot</span><span class="p">()</span> <span class="p">{</span> <span class="n">ReadUnlock</span><span class="p">(</span><span class="n">t_</span><span class="p">);</span> <span class="p">}</span>

  <span class="n">snapshot</span><span class="p">(</span><span class="n">snapshot</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="o">&amp;&amp;</span><span class="n">rhs</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* ... */</span> <span class="p">}</span>
  <span class="n">snapshot</span><span class="p">(</span><span class="k">const</span> <span class="n">snapshot</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="o">&amp;</span><span class="n">rhs</span><span class="p">)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span>
 <span class="k">private</span><span class="o">:</span>
  <span class="n">snapshot</span><span class="p">(</span><span class="n">T</span><span class="o">*</span> <span class="n">p</span><span class="p">,</span> <span class="n">Token</span> <span class="n">t</span><span class="p">)</span> <span class="o">:</span> <span class="n">p_</span><span class="p">(</span><span class="n">p</span><span class="p">),</span> <span class="n">t_</span><span class="p">(</span><span class="n">t</span><span class="p">)</span> <span class="p">{}</span>
  <span class="n">T</span><span class="o">*</span> <span class="n">p_</span><span class="p">;</span>
  <span class="n">Token</span> <span class="n">t_</span><span class="p">;</span>
<span class="p">};</span>
</pre>
   <p>The constructor (only used by <code class="highlight"><span class="n">cell</span></code>'s implementation) ensures that
  a reader lock is held protecting the exposed data; and the <code class="highlight"><span class="n">snapshot</span></code> allows safe access to the locked data: it is not possible
  (without trying quite hard) to accidentally unlock too early, nor
  forget to unlock (so long as you destroy your snapshots.)</p>
   <p>Snapshots are noncopyable first and foremost for safety (there’s no
good reason to copy them) but a more important limit is
correctness. In some major RCU-based implementations (including our
high-performance implementation), just because we <em>hold</em> the read lock
protecting some data does not mean we can <em>take</em> that same read lock
again. (This has to do with our safe disposal mechanism: we have to
guarantee that no <em>new</em> snapshots will be created referencing some
data.)</p>
   <p>So consider a data structure that contains multiple cell-like pieces
of data, like a concurrent <code class="highlight"><span class="n">unordered_map</span></code>: the iterators must contain <code class="highlight"><span class="n">snapshot</span></code>s of the items, but we <em>cannot</em> copy them.  There is a
potential workaround:</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span>
<span class="k">class</span> <span class="nc">Iterator</span> <span class="p">{</span>
 <span class="n">Iterator</span><span class="p">(</span><span class="n">snapshot</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">snap</span><span class="p">)</span> <span class="o">:</span> <span class="n">shared_</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">snap</span><span class="p">))</span> <span class="p">{}</span>
 <span class="k">private</span><span class="o">:</span>
  <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">snapshot</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>></span> <span class="n">shared_</span><span class="p">;</span>
<span class="p">};</span>
</pre>
   <p>But this is absurd--there is no good reason to hold a shared pointer
to what is morally just a special a <code class="highlight"><span class="n">unique_ptr</span></code>, and this requires
allocation on every use of iterators.</p>
   <p>At Google we use these <code class="highlight"><span class="n">cell</span></code>-like map types and often need to iterate
them; we’d very much like for them to have a valid iterator type for
use in range-for constructs; it would also simplify their lookup APIs
if we could match <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">unordered_map</span></code>. But without unreasonable
workarounds, it is not possible for use to create even an <code class="highlight"><span class="n">InputIterator</span></code> for these types as they are fundamentally uncopyable.</p>
   <h2 class="heading settled" data-level="2" id="prop"><span class="secno">2. </span><span class="content">Proposal</span><a class="self-link" href="#prop"></a></h2>
    I propose a fairly simple fix.  Informally (this isn’t intended to be
final wording) we would: 
   <ol>
    <li data-md="">
     <p>strike <code class="highlight"><span class="n">CopyConstructible</span></code> from the requirements in [iterator.iterators]</p>
    <li data-md="">
     <p>Add a concept <code class="highlight"><span class="n">MoveIterator</span></code>, with the current text of [input.iterators]</p>
    <li data-md="">
     <p>Define <code class="highlight"><span class="n">InputIterator</span></code> as a <code class="highlight"><span class="n">CopyConstructible</span></code> <code class="highlight"><span class="n">MoveIterator</span></code>.</p>
   </ol>
   <p>(With some obvious changes elsewhere adding <code class="highlight"><span class="n">MoveIterator</span></code> to the
various lists of kinds of iterator.)</p>
   <p>We edit <code class="highlight"><span class="n">InputIterator</span></code> to ensure all current code that creates and
uses iterators maintains precisely the same requirements (but see <a href="#alt">§2.2 Alternative: can we retrofit?</a> for another possibility.)  The new concept is free for anyone
to create algorithms with, and can be applied to data structures for
both their access APIs and use of range-for iteration.</p>
   <p>A much larger extension would be retrofitting <code class="highlight"><span class="n">MoveIterator</span></code> across
the standard library: the vast majority of iterator algorithms can
obviously accept a <code class="highlight"><span class="n">MoveIterator</span></code> with no serious implementation
difficulty, but it’s not clear which <em>implementations</em>, if any, have
innocent but unnecessary copies within (say) <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">copy</span></code>. In this
paper I propose we simply punt on this and do not change the required
iterator tags on any algorithm. The ability to have honest-to-goodness <code class="highlight"><span class="n">Iterator</span></code> concepts on our data structures and use range-for syntax
gets us quite far already.  If this proves successful and there is
demand for <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">copy</span></code> or <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">transform</span></code> or anything else we can add
it then.</p>
   <p>Another question is iterator tags.  The natural thing would be to redefine:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">move_iterator_tag</span> <span class="p">{</span> <span class="p">};</span>
<span class="k">struct</span> <span class="nl">input_iterator_tag</span> <span class="p">:</span> <span class="k">public</span> <span class="n">move_iterator_tag</span> <span class="p">{</span> <span class="p">};</span>
</pre>
   <p>The biggest problem here is that algorithms that are currently
guaranteed to work (and have explicit overloads for) any C++17 iterator
category will no longer work for any iterator, until they are updated
to have <code class="highlight"><span class="n">move_iterator_tag</span></code> overloads.  However, it is important to 
emphasize that this alone will not break any user code. User code will 
continue to work with any concrete iterator type that it worked with 
before, and will not break unless some part of the program is changed 
to produce a MoveIterator instead of an InputIterator. (This becomes 
more of an issue if we retroactively downclassify standard library
iterators to <code class="highlight"><span class="n">MoveIterator</span></code>s, but as we make clear above, we don’t
propose doing so unless and until pressing need arises.)</p>
   <h3 class="heading settled" data-level="2.1" id="output"><span class="secno">2.1. </span><span class="content">What about <code class="highlight"><span class="n">OutputIterator</span></code></span><a class="self-link" href="#output"></a></h3>
   <p>Much of the same argument above applies to <code class="highlight"><span class="n">OutputIterator</span></code>: it’s
requireed to be copyable, but there’s not much that can be done with
it.  It’s very possible we should apply the same transformation.</p>
   <p>As our use cases all revolve around read access, I’ve left out any
attempt to do this just to keep things shorter. I have no strong
opinion for or against.</p>
   <h3 class="heading settled" data-level="2.2" id="alt"><span class="secno">2.2. </span><span class="content">Alternative: can we retrofit?</span><a class="self-link" href="#alt"></a></h3>
   <p>There’s a more ambitious choice we could make here: strike <code class="highlight"><span class="n">CopyConstructible</span></code> from <code class="highlight"><span class="n">Iterator</span></code>, and add it to <code class="highlight"><span class="n">ForwardIterator</span></code> (which clearly needs it!) instead of <code class="highlight"><span class="n">InputIterator</span></code>, without adding any new class of iterators
whatsoever.  This is tempting!  My entire argument is that any
reasonable use of <code class="highlight"><span class="n">InputIterator</span></code> doesn’t need to copy, and in
principle this is backwards-compatibile: all extant <code class="highlight"><span class="n">InputIterators</span></code> are still perfectly good, as is all extant code that uses them.</p>
   <p>The problem is the same retrofitting workload I want to avoid that
explains why I don’t think we should change the standard library’s
iterator requirements yet: code that used the new less restrictive
definition of <code class="highlight"><span class="n">InputIterator</span></code> would want to use extant algorithms on
those iterators, and there’s no guarantee at all that any
implementation would support that. What’s worse, that applies to both
standard library algorithms <em>and</em> any user-created algorithm on <code class="highlight"><span class="n">InputIterator</span></code>s that contains copies.</p>
   <p>I think this is worth considering, but probably not practical.</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-cell">[Cell]
   <dd>Andrew Hunter; Geoffrey Romer. <a href="http://wg21.link/P0561R3">An RAII Interface for Deferred Reclamation</a>. November 11, 2017. URL: <a href="http://wg21.link/P0561R3">http://wg21.link/P0561R3</a>
  </dl>