<!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>p0847R0: Deducing this</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">p0847R0<br>Deducing this</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-02-12">12 February 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0847R0">http://wg21.link/P0847R0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Gašper Ažman</span> (<span class="p-org org">gasper dot azman at gmail dot com</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Simon Brand</span> (<span class="p-org org">simon at codeplay dot com</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Ben Deane</span> (<span class="p-org org">ben at elbeno dot com</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Barry Revzin</span> (<span class="p-org org">barry dot revzin at gmail dot com</span>)
     <dt>Audience:
     <dd>EWG
     <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>We propose a new mechanism for specifying or deducing the value category of an instance of a class. In other words, a way to tell from within a member function whether the object it’s invoked on is an lvalue or an rvalue, and whether it is const or volatile.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#motivation"><span class="secno">1</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#proposal"><span class="secno">2</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#name-lookup-candidate-functions"><span class="secno">2.1</span> <span class="content">Name lookup: candidate functions</span></a>
      <li><a href="#type-deduction"><span class="secno">2.2</span> <span class="content">Type deduction</span></a>
      <li><a href="#name-lookup-within-member-functions-with-explicit-object-parameters"><span class="secno">2.3</span> <span class="content">Name lookup: within member functions with explicit object parameters</span></a>
      <li><a href="#by-value-explicit-object-parameters"><span class="secno">2.4</span> <span class="content">By-value explicit object parameters</span></a>
      <li><a href="#writing-the-function-pointer-types-for-such-functions"><span class="secno">2.5</span> <span class="content">Writing the function pointer types for such functions</span></a>
      <li><a href="#teachability-implications"><span class="secno">2.6</span> <span class="content">Teachability Implications</span></a>
      <li><a href="#abi-implications-for-stdfunction-and-related"><span class="secno">2.7</span> <span class="content">ABI implications for <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">function</span></code> and related</span></a>
      <li><a href="#virtual-and-this-as-value"><span class="secno">2.8</span> <span class="content"><code class="highlight"><span class="k">virtual</span></code> and <code class="highlight"><span class="k">this</span></code> as value</span></a>
      <li><a href="#can-static-member-functions-have-a-this-parameter"><span class="secno">2.9</span> <span class="content">Can <code class="highlight"><span class="k">static</span></code> member functions have a <code class="highlight"><span class="k">this</span></code> parameter?</span></a>
      <li><a href="#interplays-with-capturing-this-and-this-in-lambdas"><span class="secno">2.10</span> <span class="content">Interplays with capturing <code class="highlight"><span class="p">[</span><span class="k">this</span><span class="p">]</span></code> and <code class="highlight"><span class="p">[</span><span class="o">*</span><span class="k">this</span><span class="p">]</span></code> in lambdas</span></a>
      <li><a href="#translating-code-to-use-explicit-object-parameters"><span class="secno">2.11</span> <span class="content">Translating code to use explicit object parameters</span></a>
      <li><a href="#alternative-syntax"><span class="secno">2.12</span> <span class="content">Alternative syntax</span></a>
      <li><a href="#unified-function-call-syntax"><span class="secno">2.13</span> <span class="content">Unified Function Call Syntax</span></a>
     </ol>
    <li>
     <a href="#real-world-examples"><span class="secno">3</span> <span class="content">Real-World Examples</span></a>
     <ol class="toc">
      <li><a href="#deduplicating-code"><span class="secno">3.1</span> <span class="content">Deduplicating Code</span></a>
      <li>
       <a href="#recursive-lambdas"><span class="secno">3.2</span> <span class="content">Recursive Lambdas</span></a>
       <ol class="toc">
        <li><a href="#expressions-allowed-for-self-in-lambdas"><span class="secno">3.2.1</span> <span class="content">Expressions allowed for <code class="highlight"><span class="n">self</span></code> in lambdas</span></a>
        <li><a href="#deducing-derived-objects-for-generic-lambdas"><span class="secno">3.2.2</span> <span class="content">Deducing derived objects for generic lambdas</span></a>
       </ol>
      <li><a href="#crtp-without-the-c-r-or-even-t"><span class="secno">3.3</span> <span class="content">CRTP, without the C, R, or even T</span></a>
      <li><a href="#sfinae-friendly-callables"><span class="secno">3.4</span> <span class="content">SFINAE-friendly callables</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">4</span> <span class="content">Acknowledgements</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="motivation"><span class="secno">1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>In C++03, member functions could have <em>cv</em>-qualifications, so it was possible to have scenarios where a particular class would want both a <code class="highlight"><span class="k">const</span></code> and non-<code class="highlight"><span class="k">const</span></code> overload of a particular member (Of course it was possible to also want <code class="highlight"><span class="k">volatile</span></code> overloads, but those are less common). In these cases, both overloads do the same thing - the only difference is in the types accessed and used. This was handled by either simply duplicating the function, adjusting types and qualifications as necessary, or having one delegate to the other. An example of the latter can be found in Scott Meyers' "Effective C++", Item 3:</p>
<pre class="highlight"><span class="k">class</span> <span class="nc">TextBlock</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
  <span class="k">const</span> <span class="kt">char</span><span class="o">&amp;</span> <span class="k">operator</span><span class="p">[](</span><span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">position</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">return</span> <span class="n">text</span><span class="p">[</span><span class="n">position</span><span class="p">];</span>
  <span class="p">}</span>

  <span class="kt">char</span><span class="o">&amp;</span> <span class="k">operator</span><span class="p">[](</span><span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">position</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="k">const_cast</span><span class="o">&lt;</span><span class="kt">char</span><span class="o">&amp;></span><span class="p">(</span>
      <span class="k">static_cast</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">TextBlock</span><span class="o">&amp;></span><span class="p">(</span><span class="k">this</span><span class="p">)[</span><span class="n">position</span><span class="p">]</span>
    <span class="p">);</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   <p>Arguably, neither the duplication or the delegation via <code class="highlight"><span class="k">const_cast</span></code> are great solutions, but they work.</p>
   <p>In C++11, member functions acquired a new axis to specialize on: ref-qualifiers. Now, instead of potentially needing two overloads of a single member function, we might need four: <code class="highlight"><span class="o">&amp;</span></code>, <code class="highlight"><span class="k">const</span><span class="o">&amp;</span></code>, <code class="highlight"><span class="o">&amp;&amp;</span></code>, or <code class="highlight"><span class="k">const</span><span class="o">&amp;&amp;</span></code>. We have three approaches to deal with this: we implement the same member four times, we can have three of the overloads delegate to the fourth, or we can have all four delegate to a helper, private static member function. One example might be the overload set for <code class="highlight"><span class="n">optional</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value</span><span class="p">()</span></code>. The way to implement it would be something like:</p>
   <table>
    <tbody>
     <tr>
      <th>Quadruplication
      <th>Delegation to 4th
      <th>Delegation to helper
     <tr>
      <td>
<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">optional</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">;</span>
        <span class="p">}</span>
        <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">;</span>
        <span class="p">}</span>
        <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
        <span class="p">}</span>
        <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;&amp;</span>
    <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
        <span class="p">}</span>
        <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
    <span class="p">}</span>
    <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
      <td>
<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">optional</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="k">const_cast</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&amp;></span><span class="p">(</span>
            <span class="k">static_cast</span><span class="o">&lt;</span><span class="n">optional</span> <span class="k">const</span><span class="o">&amp;></span><span class="p">(</span>
                <span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">value</span><span class="p">());</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">;</span>
        <span class="p">}</span>
        <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="k">const_cast</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&amp;&amp;></span><span class="p">(</span>
            <span class="k">static_cast</span><span class="o">&lt;</span><span class="n">optional</span> <span class="k">const</span><span class="o">&amp;></span><span class="p">(</span>
                <span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">value</span><span class="p">());</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;&amp;</span>
    <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="k">static_cast</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;&amp;></span><span class="p">(</span>
            <span class="n">value</span><span class="p">());</span>
    <span class="p">}</span>
    <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
      <td>
<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">optional</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">value_impl</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">);</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">value_impl</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">);</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">value_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="o">*</span><span class="k">this</span><span class="p">));</span>
    <span class="p">}</span>

    <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;&amp;</span>
    <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">value_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="o">*</span><span class="k">this</span><span class="p">));</span>
    <span class="p">}</span>

<span class="k">private</span><span class="o">:</span>
    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Opt</span><span class="o">></span>
    <span class="k">static</span> <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span>
    <span class="n">value_impl</span><span class="p">(</span><span class="n">Opt</span><span class="o">&amp;&amp;</span> <span class="n">opt</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">opt</span><span class="p">.</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
        <span class="p">}</span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Opt</span><span class="o">></span><span class="p">(</span><span class="n">opt</span><span class="p">).</span><span class="n">m_value</span><span class="p">;</span>
    <span class="p">}</span>


    <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   </table>
   <p>It’s not like this is a complicated function. Far from. But more or less repeating the same code four times, or artificial delegation to avoid doing so, is the kind of thing that begs for a rewrite. Except we can’t really. We <em>have</em> to implement it this way. It seems like we should be able to abstract away the qualifiers. And we can... sort of. As a non-member function, we simply don’t have this problem:</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">optional</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Opt</span><span class="o">></span>
    <span class="k">friend</span> <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span> <span class="n">value</span><span class="p">(</span><span class="n">Opt</span><span class="o">&amp;&amp;</span> <span class="n">o</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="n">o</span><span class="p">.</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Opt</span><span class="o">></span><span class="p">(</span><span class="n">o</span><span class="p">).</span><span class="n">m_value</span><span class="p">;</span>
        <span class="p">}</span>
        <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
    <span class="p">}</span>
    <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   <p>This is great - it’s just one function, that handles all four cases for us. Except it’s a non-member function, not a member function. Different semantics, different syntax, doesn’t help.</p>
   <p>There are many, many cases in code-bases where we need two or four overloads of the same member function for different <code class="highlight"><span class="k">const</span></code>- or ref-qualifiers. More than that, there are likely many cases that a class should have four overloads of a particular member function, but doesn’t simply due to laziness by the developer. We think that there are sufficiently many such cases that they merit a better solution than simply: write it, then write it again, then write it two more times.</p>
   <h2 class="heading settled" data-level="2" id="proposal"><span class="secno">2. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>We propose a new way of declaring a member function that will allow for deducing the type and value category of the class instance parameter, while still being invocable as a member function. We introduce a new kind of parameter that can be provided as the first parameter to any member function: an explicit object parameter. The purpose of this parameter is to bind to the implicit object, allowing it to be deduced.</p>
   <p>An explicit object parameter shall be of the form <code class="highlight"><span class="n">T</span> <span class="p">[</span><span class="k">const</span><span class="p">]</span> <span class="p">[</span><span class="k">volatile</span><span class="p">]</span> <span class="p">[</span><span class="o">&amp;|&amp;&amp;</span><span class="p">]</span> <span class="k">this</span> <span class="o">&lt;</span><span class="n">identifier</span><span class="o">></span></code>, where <code class="highlight"><span class="n">T</span></code> follows the normal rules of type names (e.g. for generic lambdas, it can be <code class="highlight"><span class="k">auto</span></code>), except that it cannot be a pointer type. Member functions with explicit object parameters cannot be <code class="highlight"><span class="k">static</span></code> or have <em>cv</em>- or <em>ref</em>-qualifiers.</p>
   <p>The explicit object parameter can only be the first parameter of a function. It cannot be used in constructors or destructors.</p>
   <p>This is a strict extension to the language; all existing syntax remains valid.</p>
   <p>For the purposes of this proposal, we assume the existence of two new library functions: a metafunction named <code class="highlight"><span class="n">like</span></code> which applies the <em>cv</em> and <em>ref</em> qualifiers from its first type argument onto its second (such that <code class="highlight"><span class="n">like_t</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&amp;</span><span class="p">,</span> <span class="kt">double</span><span class="o">></span></code> is <code class="highlight"><span class="kt">double</span><span class="o">&amp;</span></code>, <code class="highlight"><span class="n">like_t</span><span class="o">&lt;</span><span class="n">X</span> <span class="k">const</span><span class="o">&amp;&amp;</span><span class="p">,</span> <span class="n">Y</span><span class="o">></span></code> is <code class="highlight"><span class="n">Y</span> <span class="k">const</span><span class="o">&amp;&amp;</span></code>, etc.), and a function template named <code class="highlight"><span class="n">forward_like</span></code> which allows you to forward based on the value category of an unrelated type (<code class="highlight"><span class="n">forward_like</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">(</span><span class="n">u</span><span class="p">)</span></code> is short-hand for <code class="highlight"><span class="n">forward</span><span class="o">&lt;</span><span class="n">like_t</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">U</span><span class="o">>></span><span class="p">(</span><span class="n">u</span><span class="p">)</span></code>). Sample implementations of both can be seen <a href="https://github.com/atomgalaxy/isocpp-template-this/blob/master/forward_like.cpp">here</a>.</p>
   <p>With this extension, the example from above can be written like so:</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">optional</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span> <span class="n">value</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">if</span> <span class="p">(</span><span class="n">o</span><span class="p">.</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
            <span class="k">return</span> <span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">m_value</span><span class="p">;</span>
        <span class="p">}</span>
        <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
    <span class="p">}</span>
    <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   <p>We believe that the ability to write <em>cv-ref qualifier</em>-aware member functions without duplication will improve code maintainability, decrease the likelihood of bugs, and allow users to write fast, correct code more easily.</p>
   <p>What follows is a description of how explicit object parameters affect all the important language constructs: name lookup, type deduction, overload resolution, and so forth.</p>
   <h3 class="heading settled" data-level="2.1" id="name-lookup-candidate-functions"><span class="secno">2.1. </span><span class="content">Name lookup: candidate functions</span><a class="self-link" href="#name-lookup-candidate-functions"></a></h3>
   <p>Today, when either invoking a named function or an operator (including the call operator) on an object of class type, name lookup will include both static and non-static member functions found by regular class lookup. Non-static member functions are treated as if there were an implicit object parameter whose type is an lvalue or rvalue reference to <em>cv</em> <code class="highlight"><span class="n">X</span></code> (where the reference and <em>cv</em> qualifiers are determined based on the function’s qualifiers) which binds to the object on which the function was invoked. For static member functions, the implicit object parameter is effectively discarded, so they will not be mentioned further.</p>
   <p>For non-static member functions with the new <strong>explicit</strong> object parameter, lookup will work the same way as other member functions today, except rather than implicitly determining the type of the object parameter based on the <em>cv</em>- and <em>ref</em>-qualifiers of the member function, these are set by the parameter itself. The following examples illustrate this concept. Note that the explicit object parameter does not need to be named:</p>
   <table>
    <tbody>
     <tr>
      <th>C++17
      <th>With Explicit Object
     <tr>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">X</span> <span class="p">{</span>
    <span class="c1">// implicit object has type X&amp;</span>
<span class="c1"></span>    <span class="kt">void</span> <span class="n">foo</span><span class="p">();</span>

    <span class="c1">// implicit object has type X const&amp;</span>
<span class="c1"></span>    <span class="kt">void</span> <span class="nf">foo</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>

    <span class="c1">// implicit object has type X&amp;&amp;</span>
<span class="c1"></span>    <span class="kt">void</span> <span class="nf">bar</span><span class="p">()</span> <span class="o">&amp;&amp;</span><span class="p">;</span>

    <span class="cm">/* ex_baz has no C++17 equivalent */</span>
<span class="p">};</span>
</pre>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">X</span> <span class="p">{</span>
    <span class="c1">// explicit object, named self, has type X&amp;</span>
<span class="c1"></span>    <span class="kt">void</span> <span class="n">ex_foo</span><span class="p">(</span><span class="n">X</span><span class="o">&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">);</span>

    <span class="c1">// explicit object, named self, has type X const&amp;</span>
<span class="c1"></span>    <span class="kt">void</span> <span class="nf">ex_foo</span><span class="p">(</span><span class="n">X</span> <span class="k">const</span><span class="o">&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">);</span>

    <span class="c1">// explicit object, unnamed, has type X&amp;&amp;</span>
<span class="c1"></span>    <span class="kt">void</span> <span class="nf">ex_bar</span><span class="p">(</span><span class="n">X</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">);</span>

    <span class="c1">// explicit object, unnamed, has type X</span>
<span class="c1"></span>    <span class="c1">// copied or moved from original object</span>
<span class="c1"></span>    <span class="kt">void</span> <span class="nf">ex_baz</span><span class="p">(</span><span class="n">X</span> <span class="k">this</span><span class="p">);</span>
<span class="p">};</span>
</pre>
   </table>
   <p>The overload resolution rules for this new set of candidate functions remains unchanged - we’re simply being explicit rather than implicit about the object parameter. Given a call to <code class="highlight"><span class="n">x</span><span class="p">.</span><span class="n">ex_foo</span><span class="p">()</span></code>, overload resolution would select the first <code class="highlight"><span class="n">ex_foo</span><span class="p">()</span></code> overload if <code class="highlight"><span class="n">x</span></code> isn’t <code class="highlight"><span class="k">const</span></code> and the second if it is.</p>
   <p>Since the first parameter in member functions that have an explicit object parameter binds to the class object on which the member function is being invoked, the first provided argument is used to initialize the second function parameter, if any:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">C</span> <span class="p">{</span>
    <span class="kt">int</span> <span class="n">get</span><span class="p">(</span><span class="n">C</span> <span class="k">const</span><span class="o">&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">,</span> <span class="kt">int</span> <span class="n">i</span><span class="p">);</span>
<span class="p">};</span>

<span class="n">C</span> <span class="n">c</span><span class="p">;</span>
<span class="n">c</span><span class="p">.</span><span class="n">get</span><span class="p">(</span><span class="mi">4</span><span class="p">);</span> <span class="c1">// self is a C const&amp;, initialized to c</span>
<span class="c1"></span>          <span class="c1">// i is initialized with 4</span>
</pre>
   <h3 class="heading settled" data-level="2.2" id="type-deduction"><span class="secno">2.2. </span><span class="content">Type deduction</span><a class="self-link" href="#type-deduction"></a></h3>
   <p>One of the main motivations of this proposal is to deduce the <em>cv</em>-qualifiers and value category of the class object, so the explicit object parameter needs to be deducible. We do not propose any change in the template deduction rules for member functions with explicit object parameters - it will just be deduced from the class object the same way as any other function parameter:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">X</span> <span class="p">{</span>
    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="kt">void</span> <span class="n">foo</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">,</span> <span class="kt">int</span> <span class="n">i</span><span class="p">);</span>
<span class="p">};</span>

<span class="n">X</span> <span class="n">x</span><span class="p">;</span>
<span class="n">x</span><span class="p">.</span><span class="n">foo</span><span class="p">(</span><span class="mi">4</span><span class="p">);</span> <span class="c1">// Self deduces as X&amp;</span>
<span class="c1"></span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">x</span><span class="p">).</span><span class="n">foo</span><span class="p">(</span><span class="mi">2</span><span class="p">);</span> <span class="c1">// Self deduces as X</span>
</pre>
   <p>Since deduction rules do not change, and the explicit object parameter is deduced from the object the function is called on, this has the interesting effect of possibly deducing derived types, which can best be illustrated by the following example:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">B</span> <span class="p">{</span>
    <span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">get</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span>
        <span class="c1">// NB: specifically self.i, not just i or this->i</span>
<span class="c1"></span>        <span class="k">return</span> <span class="n">self</span><span class="p">.</span><span class="n">i</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">};</span>

<span class="k">struct</span> <span class="nl">D</span> <span class="p">:</span> <span class="n">B</span> <span class="p">{</span>
    <span class="c1">// shadows B::i</span>
<span class="c1"></span>    <span class="kt">double</span> <span class="n">i</span> <span class="o">=</span> <span class="mf">3.14</span><span class="p">;</span>
<span class="p">};</span>

<span class="n">B</span> <span class="n">b</span><span class="p">{};</span>
<span class="n">B</span> <span class="k">const</span> <span class="n">cb</span><span class="p">{};</span>
<span class="n">D</span> <span class="n">d</span><span class="p">{};</span>

<span class="n">b</span><span class="p">.</span><span class="n">foo</span><span class="p">();</span>            <span class="c1">// #1</span>
<span class="c1"></span><span class="n">cb</span><span class="p">.</span><span class="n">foo</span><span class="p">();</span>           <span class="c1">// #2</span>
<span class="c1"></span><span class="n">d</span><span class="p">.</span><span class="n">foo</span><span class="p">();</span>            <span class="c1">// #3</span>
<span class="c1"></span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">d</span><span class="p">).</span><span class="n">foo</span><span class="p">();</span> <span class="c1">// #4</span>
</pre>
   <p>The proposed behavior of these calls is:</p>
   <ol>
    <li data-md="">
     <p><code class="highlight"><span class="n">Self</span></code> is deduced as <code class="highlight"><span class="n">B</span><span class="o">&amp;</span></code>, this call returns an <code class="highlight"><span class="kt">int</span><span class="o">&amp;</span></code> to <code class="highlight"><span class="n">B</span><span class="o">::</span><span class="n">i</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span class="n">Self</span></code> is deduced as <code class="highlight"><span class="n">B</span> <span class="k">const</span><span class="o">&amp;</span></code>, this calls returns an <code class="highlight"><span class="kt">int</span> <span class="k">const</span><span class="o">&amp;</span></code> to <code class="highlight"><span class="n">B</span><span class="o">::</span><span class="n">i</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span class="n">Self</span></code> is deduced as <code class="highlight"><span class="n">D</span><span class="o">&amp;</span></code>, this call returns a <code class="highlight"><span class="kt">double</span><span class="o">&amp;</span></code> to <code class="highlight"><span class="n">D</span><span class="o">::</span><span class="n">i</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span class="n">Self</span></code> is deduced as <code class="highlight"><span class="n">D</span></code>, this call returns a <code class="highlight"><span class="kt">double</span><span class="o">&amp;&amp;</span></code> to <code class="highlight"><span class="n">D</span><span class="o">::</span><span class="n">i</span></code></p>
   </ol>
   <p>When we deduce the object parameter, we don’t just deduce the <em>cv</em>- and <em>ref</em>-qualifiers. We may also get a derived type. This follows from the normal template deduction rules. In <code class="highlight"><span class="cp">#3</span></code>, for instance, the object parameter is an lvalue of type <code class="highlight"><span class="n">D</span></code>, so <code class="highlight"><span class="n">Self</span></code> deduces as <code class="highlight"><span class="n">D</span><span class="o">&amp;</span></code>.</p>
   <h3 class="heading settled" data-level="2.3" id="name-lookup-within-member-functions-with-explicit-object-parameters"><span class="secno">2.3. </span><span class="content">Name lookup: within member functions with explicit object parameters</span><a class="self-link" href="#name-lookup-within-member-functions-with-explicit-object-parameters"></a></h3>
   <p>So far, we’ve only considered how member functions with explicit object parameters get found with name lookup and how they deduce that parameter. Now let’s move on to how the bodies of these functions actually behave. Consider a slightly expanded version of the previous example:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">B</span> <span class="p">{</span>
    <span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">f1</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">i</span><span class="p">;</span> <span class="p">}</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">f2</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">i</span><span class="p">;</span> <span class="p">}</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">f3</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">i</span><span class="p">;</span> <span class="p">}</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">f4</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="n">self</span><span class="p">).</span><span class="n">i</span><span class="p">;</span> <span class="p">}</span>
<span class="p">};</span>

<span class="k">struct</span> <span class="nl">D</span> <span class="p">:</span> <span class="n">B</span> <span class="p">{</span>
    <span class="kt">double</span> <span class="n">i</span> <span class="o">=</span> <span class="mf">3.14</span><span class="p">;</span>
<span class="p">};</span>
</pre>
   <p>Consider invoking each of these functions with an lvalue of type <code class="highlight"><span class="n">D</span></code>. We have no interest in changing template deduction rules, which means that <code class="highlight"><span class="n">Self</span></code> in each case will be <code class="highlight"><span class="n">D</span><span class="o">&amp;</span></code>. The design space consists of the following alternatives:</p>
   <ol>
    <li data-md="">
     <p><code class="highlight"><span class="k">this</span></code> is a <code class="highlight"><span class="n">B</span><span class="o">*</span></code> (non-<code class="highlight"><span class="k">const</span></code> because the explicit object parameter is non-<code class="highlight"><span class="k">const</span></code>), unqualified lookup looks in <code class="highlight"><span class="n">B</span></code>'s scope first. This means that <code class="highlight"><span class="n">f1</span></code> returns an <code class="highlight"><span class="kt">int</span><span class="o">&amp;</span></code> (to <code class="highlight"><span class="n">B</span><span class="o">::</span><span class="n">i</span></code>) and <code class="highlight"><span class="n">f2</span></code> returns an <code class="highlight"><span class="kt">int</span><span class="o">&amp;</span></code>. <code class="highlight"><span class="n">f3</span></code> is ill-formed, because <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">D</span><span class="o">&amp;></span></code> takes a <code class="highlight"><span class="n">D</span><span class="o">&amp;</span></code> and we’re passing in a <code class="highlight"><span class="n">B</span><span class="o">&amp;</span></code>, which is not a conversion that can be done implicitly. <code class="highlight"><span class="n">f4</span></code> returns a <code class="highlight"><span class="kt">double</span><span class="o">&amp;</span></code> to <code class="highlight"><span class="n">D</span><span class="o">::</span><span class="n">i</span></code>, because while <code class="highlight"><span class="o">*</span><span class="k">this</span></code> is a <code class="highlight"><span class="n">B</span></code>, <code class="highlight"><span class="n">self</span></code> actually is the object parameter, hence is a <code class="highlight"><span class="n">D</span></code>.</p>
    <li data-md="">
     <p><code class="highlight"><span class="k">this</span></code> is a <code class="highlight"><span class="n">D</span><span class="o">*</span></code> (non-<code class="highlight"><span class="k">const</span></code>), but unqualified lookup looks in <code class="highlight"><span class="n">B</span></code>'s scope. This means that <code class="highlight"><span class="n">f1</span></code> returns an <code class="highlight"><span class="kt">int</span><span class="o">&amp;</span></code>, but <code class="highlight"><span class="n">f2</span></code> returns a <code class="highlight"><span class="kt">double</span><span class="o">&amp;</span></code>. <code class="highlight"><span class="n">f3</span></code> and <code class="highlight"><span class="n">f4</span></code> both return <code class="highlight"><span class="kt">double</span><span class="o">&amp;</span></code>.</p>
    <li data-md="">
     <p><code class="highlight"><span class="k">this</span></code> is a <code class="highlight"><span class="n">D</span><span class="o">*</span></code> and unqualified lookup looks in <code class="highlight"><span class="n">D</span></code>'s scope too. All four functions return a <code class="highlight"><span class="kt">double</span><span class="o">&amp;</span></code>.</p>
    <li data-md="">
     <p>While the previous three options are variations on these functions behaving as non-static member functions, we could instead also consider these functions to behave as either static member functions or non-member <code class="highlight"><span class="k">friend</span></code> functions. In this case, any member access would require direct access from the explicit object parameter, so <code class="highlight"><span class="n">f1</span></code>, <code class="highlight"><span class="n">f2</span></code>, and <code class="highlight"><span class="n">f3</span></code> are all ill-formed. <code class="highlight"><span class="n">f4</span></code> returns a <code class="highlight"><span class="kt">double</span><span class="o">&amp;</span></code>.</p>
   </ol>
   <p>In our view, options 2 and 3 are too likely to be lead to programmer errors, even by experts. It would be very surprising if <code class="highlight"><span class="n">f1</span></code> didn’t return <code class="highlight"><span class="n">B</span><span class="o">::</span><span class="n">i</span></code>, or if <code class="highlight"><span class="n">f1</span></code> and <code class="highlight"><span class="n">f2</span></code> behaved differently. Option 4 is interesting, but would be more verbose without clear benefit.</p>
   <p>We favor option 1, since <code class="highlight"><span class="n">f1</span></code>, <code class="highlight"><span class="n">f2</span></code> and <code class="highlight"><span class="n">f4</span></code> all end up having behavior that is reasonable given the other rules for name lookup we already have. <code class="highlight"><span class="n">f2</span></code> and <code class="highlight"><span class="n">f4</span></code> returning different things might appear strange at first glance, but <code class="highlight"><span class="n">f2</span></code> references <code class="highlight"><span class="k">this</span><span class="o">-></span><span class="n">i</span></code> and <code class="highlight"><span class="n">f4</span></code> references <code class="highlight"><span class="n">self</span><span class="p">.</span><span class="n">i</span></code> - programmers would have to be aware that these may be different. Where this option could go wrong is <code class="highlight"><span class="n">f3</span></code>, which looks like it yields an lvalue or rvalue reference to <code class="highlight"><span class="n">B</span><span class="o">::</span><span class="n">i</span></code> but is actually casting <code class="highlight"><span class="o">*</span><span class="k">this</span></code> to a derived object. But since this is ill-formed, this is a <em>compile</em> error rather than a runtime bug. Hence, not only do we get typically expected behavior, but we’re also protected from a potential source of error by the compiler.</p>
   <p>The proper way to implement forwarding is to use one of the two hypothetical library functions mentioned earlier:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">B</span> <span class="p">{</span>
    <span class="k">template</span> <span class="o">&lt;</span> <span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">f3</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">i</span><span class="p">;</span>            <span class="c1">// ok if Self and *this are the same type</span>
<span class="c1"></span>                                                       <span class="c1">// compile other if Self is a derived type</span>
<span class="c1"></span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">like_t</span><span class="o">&lt;</span><span class="n">Self</span><span class="p">,</span> <span class="n">B</span><span class="o">>></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">i</span><span class="p">;</span> <span class="c1">// always ok</span>
<span class="c1"></span>        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">i</span><span class="p">;</span>       <span class="c1">// always ok</span>
<span class="c1"></span>    <span class="p">}</span>
<span class="p">};</span>
</pre>
   <p>The rule that we’re establishing is that <code class="highlight"><span class="k">this</span></code> remains a pointer to the class type of the member function it is used in, and unqualified lookup looks in class scope as usual. Additionally, <code class="highlight"><span class="k">this</span></code> is either a pointer to the explicit object parameter or, in the case of deducing a derived type, one of its base class subobjects.</p>
   <h3 class="heading settled" data-level="2.4" id="by-value-explicit-object-parameters"><span class="secno">2.4. </span><span class="content">By-value explicit object parameters</span><a class="self-link" href="#by-value-explicit-object-parameters"></a></h3>
   <p>We think they are a logical extension of the mechanism, and would go a long way towards making member functions as powerful as inline friend functions, with the only difference being the call syntax. One implication of this is that the explicit object parameter would be move-constructed in cases where the object is an rvalue (or constructed in place for prvalues), allowing you to treat chained builder member functions that return a new object uniformly without having to resort to templates.</p>
   <p>We continue to follow the rule established in the previous section: <code class="highlight"><span class="k">this</span></code> is a pointer to the explicit object parameter or one of its base class subobjects. In the example below, <code class="highlight"><span class="k">this</span> <span class="o">==</span> <span class="o">&amp;</span><span class="n">self</span></code> and all accesses to <code class="highlight"><span class="n">s</span></code> could also be rewritten as <code class="highlight"><span class="k">this</span><span class="o">-></span><span class="n">s</span></code> or <code class="highlight"><span class="n">self</span><span class="p">.</span><span class="n">s</span></code>:</p>
<pre class="highlight"><span class="k">class</span> <span class="nc">string_builder</span> <span class="p">{</span>
  <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">s</span><span class="p">;</span>

  <span class="k">operator</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="p">(</span><span class="n">string_builder</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">s</span><span class="p">);</span>
  <span class="p">}</span>

  <span class="n">string_builder</span> <span class="k">operator</span><span class="o">*</span><span class="p">(</span><span class="n">string_builder</span> <span class="k">this</span> <span class="n">self</span><span class="p">,</span> <span class="kt">int</span> <span class="n">n</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">assert</span><span class="p">(</span><span class="n">n</span> <span class="o">></span> <span class="mi">0</span><span class="p">);</span>

    <span class="n">s</span><span class="p">.</span><span class="n">reserve</span><span class="p">(</span><span class="n">s</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">*</span> <span class="n">n</span><span class="p">);</span>
    <span class="k">auto</span> <span class="k">const</span> <span class="n">size</span> <span class="o">=</span> <span class="n">s</span><span class="p">.</span><span class="n">size</span><span class="p">();</span>
    <span class="k">for</span> <span class="p">(</span><span class="k">auto</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="n">n</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span> <span class="p">{</span>
      <span class="n">s</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="n">size</span><span class="p">);</span>
    <span class="p">}</span>
    <span class="k">return</span> <span class="n">self</span><span class="p">;</span>
  <span class="p">}</span>

  <span class="n">string_builder</span> <span class="n">bop</span><span class="p">(</span><span class="n">string_builder</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">s</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"bop"</span><span class="p">);</span>
    <span class="k">return</span> <span class="n">self</span><span class="p">;</span>
  <span class="p">}</span>
<span class="p">};</span>

<span class="c1">// this is optimally efficient as far as allocations go</span>
<span class="c1"></span><span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="k">const</span> <span class="n">x</span> <span class="o">=</span> <span class="p">(</span><span class="n">string_builder</span><span class="p">{</span><span class="s">"asdf"</span><span class="p">}</span> <span class="o">*</span> <span class="mi">5</span><span class="p">).</span><span class="n">bop</span><span class="p">().</span><span class="n">bop</span><span class="p">();</span>
</pre>
   <p>In this example, <code class="highlight"><span class="n">x</span></code> would hold the value <code class="highlight"><span class="s">"asdfasdfasdfasdfasdfbopbop"</span></code>.</p>
   <p>Of course, implementing this example with templated explicit object parameter member functions would have been slightly more efficient due to also saving on move constructions, but the by-value usage makes for simpler code.</p>
   <h3 class="heading settled" data-level="2.5" id="writing-the-function-pointer-types-for-such-functions"><span class="secno">2.5. </span><span class="content">Writing the function pointer types for such functions</span><a class="self-link" href="#writing-the-function-pointer-types-for-such-functions"></a></h3>
   <p>Currently, we write member function pointers like so:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">Y</span> <span class="p">{</span>
  <span class="kt">int</span> <span class="n">f</span><span class="p">(</span><span class="kt">int</span> <span class="n">a</span><span class="p">,</span> <span class="kt">int</span> <span class="n">b</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="o">&amp;</span><span class="n">Y</span><span class="o">::</span><span class="n">f</span><span class="p">),</span> <span class="kt">int</span> <span class="p">(</span><span class="n">Y</span><span class="o">::*</span><span class="p">)(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;></span><span class="p">);</span>
</pre>
   <p>All the member functions that take references already have a function pointer syntax - they are just alternate ways of writing functions we can already write.</p>
   <p>The only one that does not have such a syntax is the pass-by-value method, all others have pre-existing signatures that do just fine.</p>
   <p>We are asking for suggestions for syntax for these function pointers. We give our first pass here:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">Z</span> <span class="p">{</span>
  <span class="c1">// same as int f(int a, int b) const&amp;;</span>
<span class="c1"></span>  <span class="kt">int</span> <span class="n">f</span><span class="p">(</span><span class="n">Z</span> <span class="k">const</span><span class="o">&amp;</span> <span class="k">this</span><span class="p">,</span> <span class="kt">int</span> <span class="n">a</span><span class="p">,</span> <span class="kt">int</span> <span class="n">b</span><span class="p">);</span>

  <span class="kt">int</span> <span class="nf">g</span><span class="p">(</span><span class="n">Z</span> <span class="k">this</span><span class="p">,</span> <span class="kt">int</span> <span class="n">a</span><span class="p">,</span> <span class="kt">int</span> <span class="n">b</span><span class="p">);</span>
<span class="p">};</span>

<span class="c1">// f is still the same as Y::f</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="o">&amp;</span><span class="n">Z</span><span class="o">::</span><span class="n">f</span><span class="p">),</span> <span class="kt">int</span> <span class="p">(</span><span class="n">Z</span><span class="o">::*</span><span class="p">)(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;></span><span class="p">);</span>
<span class="c1">// but would this alternate syntax make any sense?</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="o">&amp;</span><span class="n">Z</span><span class="o">::</span><span class="n">f</span><span class="p">),</span> <span class="kt">int</span> <span class="p">(</span><span class="o">*</span><span class="p">)(</span><span class="n">Z</span><span class="o">::</span><span class="k">const</span><span class="o">&amp;</span><span class="p">,</span> <span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="p">)</span><span class="o">></span><span class="p">);</span>
<span class="c1">// It allows us to specify the syntax for Z as a pass-by-value member function</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="o">&amp;</span><span class="n">Z</span><span class="o">::</span><span class="n">g</span><span class="p">),</span> <span class="kt">int</span> <span class="p">(</span><span class="o">*</span><span class="p">)(</span><span class="n">Z</span><span class="o">::</span><span class="p">,</span> <span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="p">)</span><span class="o">></span><span class="p">);</span>
</pre>
   <p>Such an approach unifies, to a degree, the member functions and the rest of the function type spaces, since it communicates not only that the first parameter is special, but also its type and calling convention.</p>
   <h3 class="heading settled" data-level="2.6" id="teachability-implications"><span class="secno">2.6. </span><span class="content">Teachability Implications</span><a class="self-link" href="#teachability-implications"></a></h3>
   <p>Using <code class="highlight"><span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span></code> follows existing patterns for dealing with forwarding references.</p>
   <p>Explicitly naming the object as the <code class="highlight"><span class="k">this</span></code>-designated first parameter fits with many programmers' mental model of the <code class="highlight"><span class="k">this</span></code> pointer being the first parameter to member functions "under the hood" and is comparable to usage in other languages, e.g. Python and Rust.</p>
   <p>This also makes the definition of "<code class="highlight"><span class="k">const</span></code> member function" more obvious, meaning it can more easily be taught to students.</p>
   <p>It also works as a more obvious way to teach how <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">bind</span></code> and <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">function</span></code> work with a member function pointer by making the pointer explicit.</p>
   <h3 class="heading settled" data-level="2.7" id="abi-implications-for-stdfunction-and-related"><span class="secno">2.7. </span><span class="content">ABI implications for <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">function</span></code> and related</span><a class="self-link" href="#abi-implications-for-stdfunction-and-related"></a></h3>
   <p>If references and pointers do not have the same representation for member functions, this effectively says "for the purposes of the <code class="highlight"><span class="k">this</span></code>-designated first parameter, they do."</p>
   <p>This matters because code written in the "<code class="highlight"><span class="k">this</span></code> is a pointer" syntax with the <code class="highlight"><span class="k">this</span><span class="o">-></span></code> notation needs to be assembly-identical to code written with the <code class="highlight"><span class="n">self</span><span class="p">.</span></code> notation; the two are just different ways to implement a function with the same signature.</p>
   <h3 class="heading settled" data-level="2.8" id="virtual-and-this-as-value"><span class="secno">2.8. </span><span class="content"><code class="highlight"><span class="k">virtual</span></code> and <code class="highlight"><span class="k">this</span></code> as value</span><a class="self-link" href="#virtual-and-this-as-value"></a></h3>
   <p>Virtual member functions are always dispatched based on the type of the object the dot -- or arrow, in case of pointer -- operator is being used on. Once the member function is located, the parameter <code class="highlight"><span class="k">this</span></code> is constructed with the appropriate move or copy constructor and passed as the <code class="highlight"><span class="k">this</span></code> parameter, which might incur slicing.</p>
   <p>Effectively, there is no change from current behavior -- only a slight addition of a new overload that behaves the way a user would expect.</p>
   <p>Virtual member function templates are not allowed in the language today, and this paper does not propose a change from the current behavior for member functions with explicit object parameters either.</p>
   <h3 class="heading settled" data-level="2.9" id="can-static-member-functions-have-a-this-parameter"><span class="secno">2.9. </span><span class="content">Can <code class="highlight"><span class="k">static</span></code> member functions have a <code class="highlight"><span class="k">this</span></code> parameter?</span><a class="self-link" href="#can-static-member-functions-have-a-this-parameter"></a></h3>
   <p>No. Static member functions currently do not have an implicit <code class="highlight"><span class="k">this</span></code> parameter, and therefore have no reason to have an explicit one.</p>
   <h3 class="heading settled" data-level="2.10" id="interplays-with-capturing-this-and-this-in-lambdas"><span class="secno">2.10. </span><span class="content">Interplays with capturing <code class="highlight"><span class="p">[</span><span class="k">this</span><span class="p">]</span></code> and <code class="highlight"><span class="p">[</span><span class="o">*</span><span class="k">this</span><span class="p">]</span></code> in lambdas</span><a class="self-link" href="#interplays-with-capturing-this-and-this-in-lambdas"></a></h3>
   <p>In the function parameter list, <code class="highlight"><span class="k">this</span></code> just designates the explicit object parameter. It does not, in any way, change the meaning of <code class="highlight"><span class="k">this</span></code> in the body of the lambda.</p>
   <table>
    <tbody>
     <tr>
      <th>C++17
      <th>Proposed
     <tr>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">X</span> <span class="p">{</span>
    <span class="kt">int</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">;</span>

    <span class="k">auto</span> <span class="nf">getter</span><span class="p">()</span> <span class="k">const</span>
    <span class="p">{</span>
        <span class="k">return</span> <span class="p">[</span><span class="o">*</span><span class="k">this</span><span class="p">](){</span>
            <span class="k">return</span> <span class="n">x</span>       <span class="c1">// refers to X::x</span>
<span class="c1"></span>                <span class="o">+</span> <span class="k">this</span><span class="o">-></span><span class="n">y</span><span class="p">;</span> <span class="c1">// refers to X::y</span>
<span class="c1"></span>        <span class="p">};</span>
    <span class="p">}</span>
<span class="p">};</span>
</pre>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">X</span> <span class="p">{</span>
    <span class="kt">int</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">;</span>

    <span class="k">auto</span> <span class="nf">getter</span><span class="p">()</span> <span class="k">const</span>
    <span class="p">{</span>
        <span class="k">return</span> <span class="p">[</span><span class="o">*</span><span class="k">this</span><span class="p">](</span><span class="k">auto</span> <span class="k">const</span><span class="o">&amp;</span> <span class="k">this</span> <span class="cm">/* self */</span><span class="p">){</span>
            <span class="k">return</span> <span class="n">x</span>       <span class="c1">// still refers to X::x</span>
<span class="c1"></span>                <span class="o">+</span> <span class="k">this</span><span class="o">-></span><span class="n">y</span><span class="p">;</span> <span class="c1">// still refers to X::y</span>
<span class="c1"></span>        <span class="p">};</span>
    <span class="p">}</span>
<span class="p">};</span>
</pre>
   </table>
   <p>If other language features play with what <code class="highlight"><span class="k">this</span></code> means, they are completely orthogonal and do not have interplays with this proposal. However, it should be obvious that developers have great potential for introducing hard-to-read code if they are at all changing the meaning of <code class="highlight"><span class="k">this</span></code> in function bodies, especially in conjunction with this proposal.</p>
   <h3 class="heading settled" data-level="2.11" id="translating-code-to-use-explicit-object-parameters"><span class="secno">2.11. </span><span class="content">Translating code to use explicit object parameters</span><a class="self-link" href="#translating-code-to-use-explicit-object-parameters"></a></h3>
   <p>The most common qualifier overload sets for member functions are:</p>
   <ol>
    <li data-md="">
     <p><code class="highlight"><span class="k">const</span></code> and non-<code class="highlight"><span class="k">const</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span class="o">&amp;</span></code>, <code class="highlight"><span class="k">const</span><span class="o">&amp;</span></code>, <code class="highlight"><span class="o">&amp;&amp;</span></code>, and <code class="highlight"><span class="k">const</span><span class="o">&amp;&amp;</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span class="k">const</span><span class="o">&amp;</span></code> and <code class="highlight"><span class="o">&amp;&amp;</span></code></p>
   </ol>
   <p>Some examples:</p>
   <table>
    <tbody>
     <tr>
      <th>1
      <th>2
      <th>3
     <tr>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">foo</span> <span class="p">{</span>
  <span class="kt">void</span> <span class="n">bar</span><span class="p">();</span>
  <span class="kt">void</span> <span class="nf">bar</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>
<span class="p">};</span>
</pre>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">foo</span> <span class="p">{</span>
  <span class="kt">void</span> <span class="n">bar</span><span class="p">()</span> <span class="o">&amp;</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">bar</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">bar</span><span class="p">()</span> <span class="o">&amp;&amp;</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">bar</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;&amp;</span><span class="p">;</span>
<span class="p">};</span>
</pre>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">foo</span> <span class="p">{</span>
  <span class="kt">void</span> <span class="n">bar</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">bar</span><span class="p">()</span> <span class="o">&amp;&amp;</span><span class="p">;</span>
<span class="p">};</span>
</pre>
   </table>
   <p>All three of these can be handled by a single perfect-forwarding overload, like this:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">foo</span> <span class="p">{</span>
    <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">Self</span><span class="o">></span>
    <span class="kt">void</span> <span class="n">bar</span> <span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">);</span>
<span class="p">};</span>
</pre>
   <p>This overload is callable for all <code class="highlight"><span class="k">const</span></code>- and ref-qualified object parameters, just like the above examples. It is also callable for <code class="highlight"><span class="k">volatile</span></code>-qualified objects, so the code is not entirely equivalent; however, the <code class="highlight"><span class="k">volatile</span></code> versions are unlikely to be valid and more likely to be simply left out for the sake of brevity. The only major difference is in the third case, where non-<code class="highlight"><span class="k">const</span></code> lvalue arguments would be non-<code class="highlight"><span class="k">const</span></code> inside the function body, and <code class="highlight"><span class="k">const</span></code> rvalue arguments would be <code class="highlight"><span class="k">const</span><span class="o">&amp;&amp;</span></code> instead of <code class="highlight"><span class="k">const</span><span class="o">&amp;</span></code>. Again, this is unlikely to cause correctness issues unless <code class="highlight"><span class="k">this</span></code> has other member functions called on it which do semantically different things depending on the cv-qualification of <code class="highlight"><span class="k">this</span></code>.</p>
   <h3 class="heading settled" data-level="2.12" id="alternative-syntax"><span class="secno">2.12. </span><span class="content">Alternative syntax</span><a class="self-link" href="#alternative-syntax"></a></h3>
   <p>Instead of qualifying the parameter with <code class="highlight"><span class="k">this</span></code>, to mark it as the parameter for deducing <code class="highlight"><span class="k">this</span></code>, we could let the first parameter be <em>named</em> <code class="highlight"><span class="k">this</span></code> instead:</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">struct</span> <span class="n">X</span> <span class="p">{</span>
    <span class="n">T</span> <span class="n">value</span><span class="p">;</span>

    <span class="c1">// as proposed</span>
<span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span> <span class="n">foo</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="n">self</span><span class="p">).</span><span class="n">value</span><span class="p">;</span>
    <span class="p">}</span>

    <span class="c1">// alternative</span>
<span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">This</span><span class="o">></span>
    <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span> <span class="n">foo</span><span class="p">(</span><span class="n">This</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">This</span><span class="o">></span><span class="p">(</span><span class="k">this</span><span class="p">).</span><span class="n">value</span><span class="p">;</span>
    <span class="p">}</span>
</pre>
   <p>In the example above, <code class="highlight"><span class="k">this</span></code> is no longer a pointer. Since <code class="highlight"><span class="k">this</span></code> can never be a null pointer in valid code in the first place, this is an advantage, but may add confusion to the language. It also leaves us at a loss as to how one can address the derived object and the base object, as outlined in the previous section.</p>
   <p>Rather than naming the first parameter <code class="highlight"><span class="k">this</span></code>, we can also consider introducing a dummy template parameter where the qualifications normally reside. This syntax is also ill-formed today, and is purely a language extension:</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">struct</span> <span class="n">X</span> <span class="p">{</span>
    <span class="n">T</span> <span class="n">value</span><span class="p">;</span>

    <span class="c1">// as proposed</span>
<span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span> <span class="n">foo</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="n">self</span><span class="p">).</span><span class="n">value</span><span class="p">;</span>
    <span class="p">}</span>

    <span class="c1">// alternative</span>
<span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">This</span><span class="o">></span>
    <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span> <span class="n">foo</span><span class="p">()</span> <span class="n">This</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">This</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">value</span><span class="p">;</span>
    <span class="p">}</span>

    <span class="c1">// another alternative</span>
<span class="c1"></span>    <span class="k">decltype</span><span class="p">(</span><span class="k">auto</span><span class="p">)</span> <span class="n">foo</span><span class="p">()</span> <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
        <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">value</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">};</span>
</pre>
   <p>This has the benefit of not muddying the parameter list, and it keeps the qualifier deduction off to the side where qualifiers usually live.</p>
   <h3 class="heading settled" data-level="2.13" id="unified-function-call-syntax"><span class="secno">2.13. </span><span class="content">Unified Function Call Syntax</span><a class="self-link" href="#unified-function-call-syntax"></a></h3>
   <p>The proposed use of <code class="highlight"><span class="k">this</span></code> as the first parameter seems as if these members functions really had better be written as non-members to begin with, and might suggest that the solution to this problem is really unified function call syntax (UFCS).</p>
   <p>However, several of the motivating examples presented in this proposal are implementations of operators that cannot be implemented as non-members (<code class="highlight"><span class="p">()</span></code>, <code class="highlight"><span class="p">[]</span></code>, <code class="highlight"><span class="o">-></span></code>, and unary <code class="highlight"><span class="o">*</span></code>). UFCS alone would be insufficient. Additionally, the member function overload sets in all of these examples exist as <em>member</em> functions, not free functions, today. Having to take a member function overload set and rewrite it as a non-member (likely <code class="highlight"><span class="k">friend</span></code>ed) free function template is very much changing the intended design of a class to overcome a language hurdle. This proposal contends that it would be more in keeping with programmer intent to allow member functions to stay member functions.</p>
   <h2 class="heading settled" data-level="3" id="real-world-examples"><span class="secno">3. </span><span class="content">Real-World Examples</span><a class="self-link" href="#real-world-examples"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="deduplicating-code"><span class="secno">3.1. </span><span class="content">Deduplicating Code</span><a class="self-link" href="#deduplicating-code"></a></h3>
   <p>This proposal can de-duplicate and de-quadruplicate a large amount of code. In each case, the single function is only slightly more complex than the initial two or four, which makes for a huge win. What follows are a few examples of how repeated code can be reduced.</p>
   <p>The particular implementation of optional is Simon’s, and can be viewed on <a href="https://github.com/TartanLlama/optional">GitHub</a>, and this example includes some functions that are proposed in <a href="https://wg21.link/p0798">P0798</a>, with minor changes to better suit this format:</p>
   <table>
    <tbody>
     <tr>
      <th>C++17
      <th>This proposal
     <tr>
      <td>
<pre class="highlight"><span class="k">class</span> <span class="nc">TextBlock</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
  <span class="k">const</span> <span class="kt">char</span><span class="o">&amp;</span>
  <span class="k">operator</span><span class="p">[](</span><span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">position</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">return</span> <span class="n">text</span><span class="p">[</span><span class="n">position</span><span class="p">];</span>
  <span class="p">}</span>

  <span class="kt">char</span><span class="o">&amp;</span> <span class="k">operator</span><span class="p">[](</span><span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">position</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="k">const_cast</span><span class="o">&lt;</span><span class="kt">char</span><span class="o">&amp;></span><span class="p">(</span>
      <span class="k">static_cast</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">TextBlock</span><span class="o">&amp;></span>
        <span class="p">(</span><span class="k">this</span><span class="p">)[</span><span class="n">position</span><span class="p">]</span>
    <span class="p">);</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
      <td>
<pre class="highlight"><span class="k">class</span> <span class="nc">TextBlock</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
  <span class="k">auto</span><span class="o">&amp;</span> <span class="k">operator</span><span class="p">[](</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">,</span>
                   <span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">position</span><span class="p">)</span> <span class="p">{</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">return</span> <span class="n">text</span><span class="p">[</span><span class="n">position</span><span class="p">];</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
     <tr>
      <td>
<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">optional</span> <span class="p">{</span>
  <span class="c1">// ...</span>
<span class="c1"></span>  <span class="k">constexpr</span> <span class="n">T</span><span class="o">*</span> <span class="k">operator</span><span class="o">-></span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">addressof</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
  <span class="p">}</span>

  <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">*</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="n">std</span><span class="o">::</span><span class="n">addressof</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
      <td>
<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">optional</span> <span class="p">{</span>
  <span class="c1">// ...</span>
<span class="c1"></span>  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="k">auto</span> <span class="k">operator</span><span class="o">-></span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">addressof</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
     <tr>
      <td>
<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">optional</span> <span class="p">{</span>
  <span class="c1">// ...</span>
<span class="c1"></span>  <span class="k">constexpr</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="o">&amp;</span> <span class="p">{</span>
    <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">;</span>
  <span class="p">}</span>

  <span class="k">constexpr</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="o">&amp;</span> <span class="p">{</span>
    <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">;</span>
  <span class="p">}</span>

  <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;&amp;</span> <span class="k">operator</span><span class="o">*</span><span class="p">()</span> <span class="o">&amp;&amp;</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
  <span class="p">}</span>

  <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;&amp;</span>
  <span class="k">operator</span><span class="o">*</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
  <span class="p">}</span>

  <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
      <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">;</span>
    <span class="p">}</span>
    <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
  <span class="p">}</span>

  <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
      <span class="k">return</span> <span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">;</span>
    <span class="p">}</span>
    <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
  <span class="p">}</span>

  <span class="k">constexpr</span> <span class="n">T</span><span class="o">&amp;&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="o">&amp;&amp;</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
      <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
    <span class="p">}</span>
    <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
  <span class="p">}</span>

  <span class="k">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;&amp;</span> <span class="n">value</span><span class="p">()</span> <span class="k">const</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
      <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="k">this</span><span class="o">-></span><span class="n">m_value</span><span class="p">);</span>
    <span class="p">}</span>
    <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
      <td>
<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">optional</span> <span class="p">{</span>
  <span class="c1">// ...</span>
<span class="c1"></span>  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">like_t</span><span class="o">&lt;</span><span class="n">Self</span><span class="p">,</span> <span class="n">T</span><span class="o">>&amp;&amp;</span> <span class="k">operator</span><span class="o">*</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">m_value</span><span class="p">;</span>
  <span class="p">}</span>

  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">like_t</span><span class="o">&lt;</span><span class="n">Self</span><span class="p">,</span> <span class="n">T</span><span class="o">>&amp;&amp;</span> <span class="n">value</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">has_value</span><span class="p">())</span> <span class="p">{</span>
      <span class="k">return</span> <span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">m_value</span><span class="p">;</span>
    <span class="p">}</span>
    <span class="k">throw</span> <span class="n">bad_optional_access</span><span class="p">();</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
     <tr>
      <td>
<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">optional</span> <span class="p">{</span>
  <span class="c1">// ...</span>
<span class="c1"></span>  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="k">auto</span> <span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span> <span class="n">f</span><span class="p">)</span> <span class="o">&amp;</span> <span class="p">{</span>
    <span class="k">using</span> <span class="n">result</span> <span class="o">=</span>
      <span class="n">invoke_result_t</span><span class="o">&lt;</span><span class="n">F</span><span class="p">,</span> <span class="n">T</span><span class="o">&amp;></span><span class="p">;</span>
    <span class="k">static_assert</span><span class="p">(</span>
      <span class="n">is_optional</span><span class="o">&lt;</span><span class="n">result</span><span class="o">>::</span><span class="n">value</span><span class="p">,</span>
      <span class="s">"F must return an optional"</span><span class="p">);</span>

    <span class="k">return</span> <span class="nf">has_value</span><span class="p">()</span>
        <span class="o">?</span> <span class="n">invoke</span><span class="p">(</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">),</span> <span class="o">**</span><span class="k">this</span><span class="p">)</span>
        <span class="o">:</span> <span class="n">nullopt</span><span class="p">;</span>
  <span class="p">}</span>

  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="k">auto</span> <span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span> <span class="n">f</span><span class="p">)</span> <span class="o">&amp;&amp;</span> <span class="p">{</span>
    <span class="k">using</span> <span class="n">result</span> <span class="o">=</span>
      <span class="n">invoke_result_t</span><span class="o">&lt;</span><span class="n">F</span><span class="p">,</span> <span class="n">T</span><span class="o">&amp;&amp;></span><span class="p">;</span>
    <span class="k">static_assert</span><span class="p">(</span>
      <span class="n">is_optional</span><span class="o">&lt;</span><span class="n">result</span><span class="o">>::</span><span class="n">value</span><span class="p">,</span>
      <span class="s">"F must return an optional"</span><span class="p">);</span>

    <span class="k">return</span> <span class="nf">has_value</span><span class="p">()</span>
        <span class="o">?</span> <span class="n">invoke</span><span class="p">(</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</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="o">**</span><span class="k">this</span><span class="p">))</span>
        <span class="o">:</span> <span class="n">nullopt</span><span class="p">;</span>
  <span class="p">}</span>

  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="k">auto</span> <span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span> <span class="n">f</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span> <span class="p">{</span>
    <span class="k">using</span> <span class="n">result</span> <span class="o">=</span>
      <span class="n">invoke_result_t</span><span class="o">&lt;</span><span class="n">F</span><span class="p">,</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;></span><span class="p">;</span>
    <span class="k">static_assert</span><span class="p">(</span>
      <span class="n">is_optional</span><span class="o">&lt;</span><span class="n">result</span><span class="o">>::</span><span class="n">value</span><span class="p">,</span>
      <span class="s">"F must return an optional"</span><span class="p">);</span>

    <span class="k">return</span> <span class="nf">has_value</span><span class="p">()</span>
        <span class="o">?</span> <span class="n">invoke</span><span class="p">(</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">),</span> <span class="o">**</span><span class="k">this</span><span class="p">)</span>
        <span class="o">:</span> <span class="n">nullopt</span><span class="p">;</span>
  <span class="p">}</span>

  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="k">auto</span> <span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span> <span class="n">f</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;&amp;</span> <span class="p">{</span>
    <span class="k">using</span> <span class="n">result</span> <span class="o">=</span>
      <span class="n">invoke_result_t</span><span class="o">&lt;</span><span class="n">F</span><span class="p">,</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;&amp;></span><span class="p">;</span>
    <span class="k">static_assert</span><span class="p">(</span>
      <span class="n">is_optional</span><span class="o">&lt;</span><span class="n">result</span><span class="o">>::</span><span class="n">value</span><span class="p">,</span>
      <span class="s">"F must return an optional"</span><span class="p">);</span>

    <span class="k">return</span> <span class="nf">has_value</span><span class="p">()</span>
        <span class="o">?</span> <span class="n">invoke</span><span class="p">(</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</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="o">**</span><span class="k">this</span><span class="p">))</span>
        <span class="o">:</span> <span class="n">nullopt</span><span class="p">;</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
      <td>
<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">optional</span> <span class="p">{</span>
  <span class="c1">// ...</span>
<span class="c1"></span>  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="p">,</span> <span class="k">typename</span> <span class="n">F</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="k">auto</span>
  <span class="n">and_then</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">,</span> <span class="n">F</span><span class="o">&amp;&amp;</span> <span class="n">f</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">using</span> <span class="n">val</span> <span class="o">=</span> <span class="k">decltype</span><span class="p">((</span>
        <span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">).</span><span class="n">m_value</span><span class="p">));</span>
    <span class="k">using</span> <span class="n">result</span> <span class="o">=</span> <span class="n">invoke_result_t</span><span class="o">&lt;</span><span class="n">F</span><span class="p">,</span> <span class="n">val</span><span class="o">></span><span class="p">;</span>

    <span class="k">static_assert</span><span class="p">(</span>
      <span class="n">is_optional</span><span class="o">&lt;</span><span class="n">result</span><span class="o">>::</span><span class="n">value</span><span class="p">,</span>
      <span class="s">"F must return an optional"</span><span class="p">);</span>

    <span class="k">return</span> <span class="nf">has_value</span><span class="p">()</span>
        <span class="o">?</span> <span class="n">invoke</span><span class="p">(</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">),</span>
                 <span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span>
                   <span class="p">(</span><span class="n">self</span><span class="p">).</span><span class="n">m_value</span><span class="p">)</span>
        <span class="o">:</span> <span class="n">nullopt</span><span class="p">;</span>
  <span class="p">}</span>
  <span class="c1">// ...</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   </table>
   <p>Keep in mind that there are a few more functions in P0798 that have this lead to this explosion of overloads, so the code difference and clarity is dramatic.</p>
   <p>For those that dislike returning auto in these cases, it is very easy to write a metafunction that matches the appropriate qualifiers from a type. Certainly simpler than copying and pasting code and hoping that the minor changes were made correctly in every case.</p>
   <h3 class="heading settled" data-level="3.2" id="recursive-lambdas"><span class="secno">3.2. </span><span class="content">Recursive Lambdas</span><a class="self-link" href="#recursive-lambdas"></a></h3>
   <p>This proposal also allows for an alternative solution to implementing a recursive lambda, since now we open up the possibility of allowing a lambda to reference itself:</p>
<pre class="highlight"><span class="c1">// as proposed in [P0839]</span>
<span class="c1"></span><span class="k">auto</span> <span class="n">fib</span> <span class="o">=</span> <span class="p">[]</span> <span class="n">self</span> <span class="p">(</span><span class="kt">int</span> <span class="n">n</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">n</span> <span class="o">&lt;</span> <span class="mi">2</span><span class="p">)</span> <span class="k">return</span> <span class="n">n</span><span class="p">;</span>
    <span class="k">return</span> <span class="nf">self</span><span class="p">(</span><span class="n">n</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span> <span class="o">+</span> <span class="n">self</span><span class="p">(</span><span class="n">n</span><span class="o">-</span><span class="mi">2</span><span class="p">);</span>
<span class="p">};</span>

<span class="c1">// this proposal</span>
<span class="c1"></span><span class="k">auto</span> <span class="n">fib</span> <span class="o">=</span> <span class="p">[](</span><span class="k">auto</span><span class="o">&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">,</span> <span class="kt">int</span> <span class="n">n</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">n</span> <span class="o">&lt;</span> <span class="mi">2</span><span class="p">)</span> <span class="k">return</span> <span class="n">n</span><span class="p">;</span>
    <span class="k">return</span> <span class="nf">self</span><span class="p">(</span><span class="n">n</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span> <span class="o">+</span> <span class="n">self</span><span class="p">(</span><span class="n">n</span><span class="o">-</span><span class="mi">2</span><span class="p">);</span>
<span class="p">};</span>
</pre>
   <p>This simply works following the established rules. The call operator of the closure object can have an explicit object parameter too, so <code class="highlight"><span class="n">self</span></code> in this example <em>is</em> the closure object.</p>
   <p>If the lambda would otherwise decay to a function pointer, <code class="highlight"><span class="o">&amp;</span><span class="n">self</span></code> shall have the value of that function pointer.</p>
   <h4 class="heading settled" data-level="3.2.1" id="expressions-allowed-for-self-in-lambdas"><span class="secno">3.2.1. </span><span class="content">Expressions allowed for <code class="highlight"><span class="n">self</span></code> in lambdas</span><a class="self-link" href="#expressions-allowed-for-self-in-lambdas"></a></h4>
<pre class="highlight">  <span class="n">self</span><span class="p">(...);</span>      <span class="c1">// call with appropriate signature</span>
<span class="c1"></span>  <span class="k">decltype</span><span class="p">(</span><span class="n">self</span><span class="p">);</span> <span class="c1">// evaluates to the type of the lambda with the appropriate</span>
<span class="c1"></span>                  <span class="c1">// cv-ref qualifiers</span>
<span class="c1"></span>  <span class="o">&amp;</span><span class="n">self</span><span class="p">;</span>          <span class="c1">// the address of either the closure object or function pointer</span>
<span class="c1"></span>  <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">self</span><span class="p">)</span> <span class="c1">// you’re allowed to move yourself into an algorithm...</span>
<span class="c1"></span>  <span class="cm">/* ... and all other things you’re allowed to do with the lambda itself. */</span>
</pre>
   <p>Within lambda expressions, the <code class="highlight"><span class="k">this</span></code> parameter still does not allow one to refer to the members of the closure object, which has no defined storage or layout, nor do its members have names. Instead it allows one to deduce the value category of the lambda and access its members -- including various call operators -- in the way appropriate for the value category.</p>
   <h4 class="heading settled" data-level="3.2.2" id="deducing-derived-objects-for-generic-lambdas"><span class="secno">3.2.2. </span><span class="content">Deducing derived objects for generic lambdas</span><a class="self-link" href="#deducing-derived-objects-for-generic-lambdas"></a></h4>
   <p>When we deduce the object parameter, it doesn’t have to be the same type as the class - it could be a derived type. This isn’t typically relevant for lambdas, which would just be used as is, but it does provide an interesting opportunity when it comes to visitation. One tool to make it easier to invoke <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">visit</span><span class="p">()</span></code> on a <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">variant</span></code> is to write an overload utility which ends up creating a new type that inherits from all of the provided types. Consider a scenario where we have a <code class="highlight"><span class="n">Tree</span></code> type which is a <code class="highlight"><span class="n">variant</span><span class="o">&lt;</span><span class="n">Leaf</span><span class="p">,</span> <span class="n">Node</span><span class="o">></span></code>, where a <code class="highlight"><span class="n">Node</span></code> contains two sub-<code class="highlight"><span class="n">Tree</span></code>s. We could count the number of leaves in a <code class="highlight"><span class="n">Tree</span></code> as follows:</p>
<pre class="highlight"><span class="kt">int</span> <span class="nf">num_leaves</span><span class="p">(</span><span class="n">Tree</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">tree</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">visit</span><span class="p">(</span><span class="n">overload</span><span class="p">(</span>
        <span class="p">[](</span><span class="n">Leaf</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span> <span class="p">},</span>
        <span class="p">[](</span><span class="k">auto</span> <span class="k">const</span><span class="o">&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">,</span> <span class="n">Node</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">n</span><span class="p">)</span> <span class="p">{</span>
            <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">visit</span><span class="p">(</span><span class="n">self</span><span class="p">,</span> <span class="n">n</span><span class="p">.</span><span class="n">left</span><span class="p">)</span> <span class="o">+</span> <span class="n">std</span><span class="o">::</span><span class="n">visit</span><span class="p">(</span><span class="n">self</span><span class="p">,</span> <span class="n">n</span><span class="p">.</span><span class="n">right</span><span class="p">);</span>
        <span class="p">}</span>
    <span class="p">),</span> <span class="n">tree</span><span class="p">);</span>
<span class="p">}</span>
</pre>
   <p>In this example, <code class="highlight"><span class="n">self</span></code> doesn’t deduce as that inner lambda, it deduces at the overload object, so this works.</p>
   <h3 class="heading settled" data-level="3.3" id="crtp-without-the-c-r-or-even-t"><span class="secno">3.3. </span><span class="content">CRTP, without the C, R, or even T</span><a class="self-link" href="#crtp-without-the-c-r-or-even-t"></a></h3>
   <p>Today, a common design pattern is the Curiously Recurring Template Pattern. This implies passing the derived type as a template parameter to a base class template, as a way of achieving static polymorphism. If we wanted to just outsource implementing postfix incrementing to a base, we could use CRTP for that. But with explicit object parameters that deduce to the derived objects already, we don’t need any curious recurrence. We can just use standard inheritance and let deduction just do its thing. The base class doesn’t even need to be a template:</p>
   <table>
    <tbody>
     <tr>
      <th>C++17
      <th>Proposed
     <tr>
      <td>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Derived</span><span class="o">></span>
<span class="k">struct</span> <span class="n">add_postfix_increment</span> <span class="p">{</span>
    <span class="n">Derived</span> <span class="k">operator</span><span class="o">++</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">auto</span><span class="o">&amp;</span> <span class="n">self</span> <span class="o">=</span> <span class="k">static_cast</span><span class="o">&lt;</span><span class="n">Derived</span><span class="o">&amp;></span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">);</span>

        <span class="n">Derived</span> <span class="nf">tmp</span><span class="p">(</span><span class="n">self</span><span class="p">);</span>
        <span class="o">++</span><span class="n">self</span><span class="p">;</span>
        <span class="k">return</span> <span class="n">tmp</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">};</span>

<span class="k">struct</span> <span class="nl">some_type</span> <span class="p">:</span> <span class="n">add_postfix_increment</span><span class="o">&lt;</span><span class="n">some_type</span><span class="o">></span> <span class="p">{</span>
    <span class="n">some_type</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">++</span><span class="p">()</span> <span class="p">{</span> <span class="p">...</span> <span class="p">}</span>
<span class="p">};</span>
</pre>
      <td>
<pre class="highlight"><span class="k">struct</span> <span class="n">add_postfix_increment</span> <span class="p">{</span>
    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="o">></span>
    <span class="n">Self</span> <span class="k">operator</span><span class="o">++</span><span class="p">(</span><span class="n">Self</span><span class="o">&amp;</span> <span class="k">this</span> <span class="n">self</span><span class="p">,</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
        <span class="n">Self</span> <span class="n">tmp</span><span class="p">(</span><span class="n">self</span><span class="p">);</span>
        <span class="o">++</span><span class="n">self</span><span class="p">;</span>
        <span class="k">return</span> <span class="n">tmp</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">};</span>



<span class="k">struct</span> <span class="nl">some_type</span> <span class="p">:</span> <span class="n">add_postfix_increment</span> <span class="p">{</span>
    <span class="n">some_type</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">++</span><span class="p">()</span> <span class="p">{</span> <span class="p">...</span> <span class="p">}</span>
<span class="p">};</span>
</pre>
   </table>
   <p>The example at right isn’t much shorter, but it is certainly simpler.</p>
   <h3 class="heading settled" data-level="3.4" id="sfinae-friendly-callables"><span class="secno">3.4. </span><span class="content">SFINAE-friendly callables</span><a class="self-link" href="#sfinae-friendly-callables"></a></h3>
   <p>A seemingly unrelated problem to the question of code quadruplication is that of writing these numerous overloads for function wrappers, as demonstrated in <a href="https://wg21.link/p0826">P0826</a>. Consider what happens if we implement <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">not_fn</span><span class="p">()</span></code>, as currently specified:</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
<span class="k">class</span> <span class="nc">call_wrapper</span> <span class="p">{</span>
    <span class="n">F</span> <span class="n">f</span><span class="p">;</span>
<span class="k">public</span><span class="o">:</span>
    <span class="c1">// ...</span>
<span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span><span class="p">...</span> <span class="n">Args</span><span class="o">></span>
    <span class="k">auto</span> <span class="k">operator</span><span class="p">()(</span><span class="n">Args</span><span class="o">&amp;&amp;</span><span class="p">...</span> <span class="p">)</span> <span class="o">&amp;</span>
        <span class="o">-></span> <span class="k">decltype</span><span class="p">(</span><span class="o">!</span><span class="n">declval</span><span class="o">&lt;</span><span class="n">invoke_result_t</span><span class="o">&lt;</span><span class="n">F</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">Args</span><span class="p">...</span><span class="o">>></span><span class="p">());</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span><span class="p">...</span> <span class="n">Args</span><span class="o">></span>
    <span class="k">auto</span> <span class="k">operator</span><span class="p">()(</span><span class="n">Args</span><span class="o">&amp;&amp;</span><span class="p">...</span> <span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span>
        <span class="o">-></span> <span class="k">decltype</span><span class="p">(</span><span class="o">!</span><span class="n">declval</span><span class="o">&lt;</span><span class="n">invoke_result_t</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">F</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">Args</span><span class="p">...</span><span class="o">>></span><span class="p">());</span>

    <span class="c1">// ... same for &amp;&amp; and const &amp;&amp; ...</span>
<span class="c1"></span><span class="p">};</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
<span class="k">auto</span> <span class="n">not_fn</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span> <span class="n">f</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">call_wrapper</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">F</span><span class="o">>></span><span class="p">{</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">)};</span>
<span class="p">}</span>
</pre>
   <p>As described in the paper, this implementation has two pathological cases: one in which the callable is SFINAE-unfriendly (which would cause a call to be ill-formed, when it could otherwise work), and one in which overload is deleted (which would cause a call to fallback to a different overload, when it should fail):</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">unfriendly</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">auto</span> <span class="k">operator</span><span class="p">()(</span><span class="n">T</span> <span class="n">v</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="kt">int</span><span class="o">></span><span class="p">);</span>
        <span class="k">return</span> <span class="n">v</span><span class="p">;</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">auto</span> <span class="k">operator</span><span class="p">()(</span><span class="n">T</span> <span class="n">v</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span>
        <span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_same_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="kt">double</span><span class="o">></span><span class="p">);</span>
        <span class="k">return</span> <span class="n">v</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">};</span>

<span class="k">struct</span> <span class="n">fun</span> <span class="p">{</span>
    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span><span class="p">...</span> <span class="n">Args</span><span class="o">></span>
    <span class="kt">void</span> <span class="k">operator</span><span class="p">()(</span><span class="n">Args</span><span class="o">&amp;&amp;</span><span class="p">...)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span><span class="p">...</span> <span class="n">Args</span><span class="o">></span>
    <span class="kt">bool</span> <span class="k">operator</span><span class="p">()(</span><span class="n">Args</span><span class="o">&amp;&amp;</span><span class="p">...)</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> true<span class="p">;</span> <span class="p">}</span>
<span class="p">};</span>

<span class="n">std</span><span class="o">::</span><span class="n">not_fn</span><span class="p">(</span><span class="n">unfriendly</span><span class="p">{})(</span><span class="mi">1</span><span class="p">);</span> <span class="c1">// static assert!</span>
<span class="c1"></span>                              <span class="c1">// even though the non-const overload is viable and would be the best</span>
<span class="c1"></span>                              <span class="c1">// match, during overload resolution, both overloads of unfriendly have</span>
<span class="c1"></span>                              <span class="c1">// to be instantiated - and the second one is a hard compile error.</span>
<span class="c1"></span>
<span class="n">std</span><span class="o">::</span><span class="n">not_fn</span><span class="p">(</span><span class="n">fun</span><span class="p">{})();</span>         <span class="c1">// ok!? Returns false</span>
<span class="c1"></span>                              <span class="c1">// even though we want the non-const overload to be deleted, the const</span>
<span class="c1"></span>                              <span class="c1">// overload of the call_wrapper ends up being viable - and the only viable</span>
<span class="c1"></span>                              <span class="c1">// candidate.</span>
</pre>
   <p>Gracefully handling SFINAE-unfriendly callables is <strong>not solvable</strong> in C++ today. Preventing fallback can be solved by the addition of yet another four overloads, so that each of the four <em>cv</em>/ref-qualifiers leads to a pair of overloads: one enabled and one <code class="highlight"><span class="n">deleted</span></code>.</p>
   <p>This proposal solves both problems by simply allowing <code class="highlight"><span class="k">this</span></code> to be deduced. The following is a complete implementation of <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">not_fn</span></code>:</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
<span class="k">struct</span> <span class="n">call_wrapper</span> <span class="p">{</span>
    <span class="n">F</span> <span class="n">f</span><span class="p">;</span>

    <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Self</span><span class="p">,</span> <span class="k">typename</span><span class="p">...</span> <span class="n">Args</span><span class="o">></span>
    <span class="k">auto</span> <span class="k">operator</span><span class="p">()(</span><span class="n">Self</span><span class="o">&amp;&amp;</span> <span class="k">this</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;&amp;</span><span class="p">...</span> <span class="n">args</span><span class="p">)</span>
        <span class="o">-></span> <span class="k">decltype</span><span class="p">(</span><span class="o">!</span><span class="n">invoke</span><span class="p">(</span><span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">),</span> <span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)...))</span>
    <span class="p">{</span>
        <span class="k">return</span> <span class="o">!</span><span class="n">invoke</span><span class="p">(</span><span class="n">forward_like</span><span class="o">&lt;</span><span class="n">Self</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">),</span> <span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)...);</span>
    <span class="p">}</span>
<span class="p">};</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">F</span><span class="o">></span>
<span class="k">auto</span> <span class="n">not_fn</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span> <span class="n">f</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">call_wrapper</span><span class="o">&lt;</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">F</span><span class="o">>></span><span class="p">{</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">)};</span>
<span class="p">}</span>

<span class="n">not_fn</span><span class="p">(</span><span class="n">unfriendly</span><span class="p">{})(</span><span class="mi">1</span><span class="p">);</span> <span class="c1">// ok</span>
<span class="c1"></span><span class="n">not_fn</span><span class="p">(</span><span class="n">fun</span><span class="p">{})();</span>         <span class="c1">// error</span>
</pre>
   <p>Here, there is only one overload with everything deduced together. The first example now works correctly. <code class="highlight"><span class="n">Self</span></code> gets deduced as <code class="highlight"><span class="n">call_wrapper</span><span class="o">&lt;</span><span class="n">unfriendly</span><span class="o">></span></code>, and the one <code class="highlight"><span class="k">operator</span><span class="p">()</span></code> will only consider <code class="highlight"><span class="n">unfriendly</span></code>'s non-<code class="highlight"><span class="k">const</span></code> call operator. The <code class="highlight"><span class="k">const</span></code> one is simply never considered, so does not have an opportunity to cause problems. The call works.</p>
   <p>The second example now fails correctly. Previously, we had four candidates: the two non-<code class="highlight"><span class="k">const</span></code> ones were removed from the overload set due to <code class="highlight"><span class="n">fun</span></code>'s non-<code class="highlight"><span class="k">const</span></code> call operator being <code class="highlight"><span class="k">delete</span></code>d, and the two <code class="highlight"><span class="k">const</span></code> ones which were viable. But now, we only have one candidate. <code class="highlight"><span class="n">Self</span></code> gets deduced as <code class="highlight"><span class="n">call_wrapper</span><span class="o">&lt;</span><span class="n">fun</span><span class="o">></span></code>, which requires <code class="highlight"><span class="n">fun</span></code>'s non-<code class="highlight"><span class="k">const</span></code> call operator to be well-formed. Since it is not, the call is an error. There is no opportunity for fallback since there is only one overload ever considered.</p>
   <p>As a result, this singular overload then has precisely the desired behavior: working, for <code class="highlight"><span class="n">unfriendly</span></code>, and not working, for <code class="highlight"><span class="n">fun</span></code>.</p>
   <h2 class="heading settled" data-level="4" id="acknowledgements"><span class="secno">4. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>The authors would like to thank:</p>
   <ul>
    <li data-md="">
     <p>Jonathan Wakely, for bringing us all together by pointing out we were writing the same paper, twice</p>
    <li data-md="">
     <p>Graham Heynes, Andrew Bennieston, Jeff Snyder for early feedback regarding the meaning of <code class="highlight"><span class="k">this</span></code> inside function bodies</p>
    <li data-md="">
     <p>Chandler Carruth, Amy Worthington, Jackie Chen, Vittorio Romeo, Tristan Brindle, Agustín Bergé, Louis Dionne, and Michael Park for early feedback</p>
    <li data-md="">
     <p>Guilherme Hartmann for his guidance with the implementation</p>
   </ul>
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