<!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>P2249R6: Mixed comparisons for smart pointers</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
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * 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)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * 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
 *
 * Outdated warning for old specs
 *
 * 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)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	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;

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) 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: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		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;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

	/* 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: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		/* 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 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);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) 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);
			color: var(--tocsidebar-heading-text);
		}
		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);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) 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);
			color: var(--tocsidebar-heading-text);
		}

		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;
		color: var(--heading-text);
	}

	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,
	#profile-and-date {
		/* #profile-and-date 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) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		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;
	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 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: inherit;
	}
	dfn var {
		font-style: normal;
	}

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

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

/** 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;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	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;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

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

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

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

	/* 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;
		height: 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,
	.amendment, .correction, .addition {
		margin: 1em auto;
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

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

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

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

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* 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;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* 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;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

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

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

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

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** 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: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    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;
		border-bottom: 1px solid var(--defrow-border);
	}

	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-color: var(--datacell-border);
		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 {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		border-top: 1px solid var(--datacell-border);
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version d5d58a306, updated Fri Jan 26 16:12:28 2024 -0800" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* Boilerplate: style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
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   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2249R6<br>Mixed comparisons for smart pointers</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-02-02">2024-02-02</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:giuseppe.dangelo@kdab.com">Giuseppe D'Angelo</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </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 to enable mixed comparisons for the Standard Library smart
pointer class templates <code class="highlight"><c- n>unique_ptr</c-></code> and <code class="highlight"><c- n>shared_ptr</c-></code>, so that one can compare them against raw pointers.</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="#changelog"><span class="secno">1</span> <span class="content">Changelog</span></a>
    <li><a href="#tonytables"><span class="secno">2</span> <span class="content">Tony Tables</span></a>
    <li>
     <a href="#motivation"><span class="secno">3</span> <span class="content">Motivation and Scope</span></a>
     <ol class="toc">
      <li><a href="#associative"><span class="secno">3.1</span> <span class="content">Associative containers</span></a>
     </ol>
    <li><a href="#impact"><span class="secno">4</span> <span class="content">Impact On The Standard</span></a>
    <li>
     <a href="#designdecisions"><span class="secno">5</span> <span class="content">Design Decisions</span></a>
     <ol class="toc">
      <li>
       <a href="#proposedsignature"><span class="secno">5.1</span> <span class="content">What is the signature of the proposed operators?</span></a>
       <ol class="toc">
        <li><a href="#mixedcomparisons"><span class="secno">5.1.1</span> <span class="content">Allowing mixed comparisons against raw pointers</span></a>
        <li><a href="#array"><span class="secno">5.1.2</span> <span class="content">Handling of array types</span></a>
        <li><a href="#uniqueptrpointer"><span class="secno">5.1.3</span> <span class="content">Custom pointer types for <code class="highlight"><c- n>unique_ptr</c-></code></span></a>
        <li><a href="#generalizedcomparisons"><span class="secno">5.1.4</span> <span class="content">Objects comparable to a smart pointer’s pointer type</span></a>
        <li><a href="#mixedlibrarysmart"><span class="secno">5.1.5</span> <span class="content">Should mixed comparisons between <em>different</em> Standard Library smart pointer classes be allowed?</span></a>
       </ol>
      <li><a href="#totalorder"><span class="secno">5.2</span> <span class="content">Should the comparison operators yield a total order?</span></a>
      <li><a href="#concepts"><span class="secno">5.3</span> <span class="content">Would these operations make the <code class="highlight"><c- n>equality_comparable_with</c-></code> or <code class="highlight"><c- n>three_way_comparable_with</c-></code> concepts satisfied between a smart pointer and a raw pointer?</span></a>
      <li><a href="#uniqueptroperators"><span class="secno">5.4</span> <span class="content">Should <code class="highlight"><c- n>unique_ptr</c-></code> have the full set of ordering operators (<code class="highlight"><c- o>&lt;</c-></code>, <code class="highlight"><c- o>&lt;=</c-></code>, <code class="highlight"><c- o>></c-></code>, <code class="highlight"><c- o>>=</c-></code>), or just <code class="highlight"><c- o>&lt;=></c-></code>?</span></a>
     </ol>
    <li>
     <a href="#technicalspecifications"><span class="secno">6</span> <span class="content">Technical Specifications</span></a>
     <ol class="toc">
      <li><a href="#featuretestingmacro"><span class="secno">6.1</span> <span class="content">Feature testing macro #</span></a>
      <li>
       <a href="#proposedwording"><span class="secno">6.2</span> <span class="content">Proposed wording</span></a>
       <ol class="toc">
        <li><a href="#proposedwordinguniqueptr"><span class="secno">6.2.1</span> <span class="content"><code class="highlight"><c- n>unique_ptr</c-></code></span></a>
        <li><a href="#proposedwordingsharedptr"><span class="secno">6.2.2</span> <span class="content"><code class="highlight"><c- n>shared_ptr</c-></code></span></a>
       </ol>
     </ol>
    <li><a href="#implementation"><span class="secno">7</span> <span class="content">Implementation experience</span></a>
    <li><a href="#acknowledgements"><span class="secno">8</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Changelog</span><a class="self-link" href="#changelog"></a></h2>
   <ul>
    <li data-md>
     <p>R6</p>
     <ul>
      <li data-md>
       <p>Reduce the scope of the proposal: constrain the proposed operators to work only with raw pointers, to avoid clashes with user-defined overloads.</p>
     </ul>
    <li data-md>
     <p>R5</p>
     <ul>
      <li data-md>
       <p>Redesigned the overload set.</p>
      <li data-md>
       <p>Made the <code class="highlight"><c- n>unique_ptr</c-></code> operators <code class="highlight"><c- k>constexpr</c-></code>, following the adoption of <a data-link-type="biblio" href="#biblio-p2273r3" title="Making std::unique_ptr constexpr">[P2273R3]</a>.</p>
     </ul>
    <li data-md>
     <p>R4</p>
     <ul>
      <li data-md>
       <p>Corrected the references to <a data-link-type="biblio" href="#biblio-p2405r0" title="nullopt_t and nullptr_t should both have operator and operator==">[P2405R0]</a> (which had been accidentally referred to as P2403).</p>
     </ul>
    <li data-md>
     <p>R3</p>
     <ul>
      <li data-md>
       <p>Clarified some design decisions after a review on the LEWG mailing list.</p>
      <li data-md>
       <p>Changed the proposed wording for the constraints, in order to exclude types that are derived from <code class="highlight"><c- n>unique_ptr</c-></code> or <code class="highlight"><c- n>shared_ptr</c-></code> specializations.</p>
     </ul>
    <li data-md>
     <p>R2</p>
     <ul>
      <li data-md>
       <p>Fixes to the proposed wording.</p>
     </ul>
    <li data-md>
     <p>R1</p>
     <ul>
      <li data-md>
       <p>Added some clarifications to the design decisions, following a review on the LEWG mailing list.</p>
      <li data-md>
       <p>Made the proposed operators hidden friends.</p>
      <li data-md>
       <p>Rebased on top of the latest draft of the Standard.</p>
      <li data-md>
       <p>Minor fixes.</p>
     </ul>
    <li data-md>
     <p>R0</p>
     <ul>
      <li data-md>
       <p>First submission.</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="tonytables"><span class="secno">2. </span><span class="content">Tony Tables</span><a class="self-link" href="#tonytables"></a></h2>
   <table>
    <thead>
     <tr>
      <th>Before
      <th>After
    <tbody>
     <tr>
      <td>
<pre class="highlight"><c- k>class</c-> <c- nc>Manager</c->
<c- p>{</c->
    <c- c1>// The Manager owns the Objects, therefore it uses smart pointers.</c->
    <c- c1>// The Manager gives out non-owning raw pointers to clients.</c->
    <c- c1>// A client gives the raw pointer back to the Manager, to tell it</c->
    <c- c1>// to act on that Object; the Manager has then to look up the Object</c->
    <c- c1>// in its storage.</c->

    <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>>></c-> <c- n>objects</c-><c- p>;</c->

<c- k>public</c-><c- o>:</c->
    <c- n>Object</c-><c- o>*</c-> <c- n>get_object</c-><c- p>(</c-><c- o>~~~</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c->
        <c- k>return</c-> <c- n>objects</c-><c- p>[</c-><c- o>~~~</c-><c- p>].</c-><c- n>get</c-><c- p>();</c->
    <c- p>}</c->

    <c- b>void</c-> <c- n>drop_object</c-><c- p>(</c-><c- n>Object</c-> <c- o>*</c-><c- n>input</c-><c- p>)</c->
    <c- p>{</c->
        <c- c1>// Must use erase_if and a custom comparison (e.g. a lambda).</c->
        <c- k>auto</c-> <c- n>isEqual</c-> <c- o>=</c-> <c- p>[</c-><c- n>input</c-><c- p>](</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-> <c- o>&amp;</c-><c- n>o</c-><c- p>)</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>o</c-><c- p>.</c-><c- n>get</c-><c- p>()</c-> <c- o>==</c-> <c- n>input</c-><c- p>;</c->
        <c- p>};</c->
        <c- n>erase_if</c-><c- p>(</c-><c- n>objects</c-><c- p>,</c-> <c- n>input</c-><c- p>);</c->
    <c- p>}</c->

    <c- b>ssize_t</c-> <c- n>index_for_object</c-><c- p>(</c-><c- n>Object</c-> <c- o>*</c-><c- n>input</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c->
        <c- c1>// Same story.</c->
        <c- c1>// Code like this (predicates, etc.) may get duplicated all over the place</c->
        <c- c1>// where smart pointers are used in containers/algorithms. Surely,</c->
        <c- c1>// centralizing it is good practice, but there’s always the temptation of</c->
        <c- c1>// just writing the one-liner lambda and "moving on" rather than refactoring...</c->
        <c- k>auto</c-> <c- n>isEqual</c-> <c- o>=</c-> <c- p>[</c-><c- n>input</c-><c- p>](</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-> <c- o>&amp;</c-><c- n>o</c-><c- p>)</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>o</c-><c- p>.</c-><c- n>get</c-><c- p>()</c-> <c- o>==</c-> <c- n>input</c-><c- p>;</c->
        <c- p>};</c->
        <c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>find_if</c-><c- p>(</c-><c- n>objects</c-><c- p>,</c-> <c- n>isEqual</c-><c- p>);</c->
        <c- c1>// etc.</c->
    <c- p>}</c->
<c- p>};</c->
</pre>
      <td>
<pre class="highlight"><c- k>class</c-> <c- nc>Manager</c->
<c- p>{</c->





    <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>>></c-> <c- n>objects</c-><c- p>;</c->

<c- k>public</c-><c- o>:</c->
    <c- n>Object</c-> <c- o>*</c-><c- n>get_object</c-><c- p>(</c-><c- o>~~~</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c->
        <c- k>return</c-> <c- n>objects</c-><c- p>[</c-><c- o>~~~</c-><c- p>].</c-><c- n>get</c-><c- p>();</c->
    <c- p>}</c->

    <c- b>void</c-> <c- n>drop_object</c-><c- p>(</c-><c- n>Object</c-> <c- o>*</c-><c- n>input</c-><c- p>)</c->
    <c- p>{</c->




        <c- c1>// Just use a value-based algorithm, no need for a predicate!</c->
        <ins><c- n>erase</c-><c- p>(</c-><c- n>objects</c-><c- p>,</c-> <c- n>input</c-><c- p>);</c-></ins>
    <c- p>}</c->

    <c- b>ssize_t</c-> <c- n>index_for_object</c-><c- p>(</c-><c- n>Object</c-> <c- o>*</c-><c- n>input</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c->




        <c- c1>// Same, just use a value-based algorithm.</c->
        <c- c1>// Unfortunately, the range version does not work as-is because</c->
        <c- c1>// std::equality_comparable_with&lt;std::unique_ptr&lt;Object>, Object*></c->
        <c- c1>// is not satisfied. See the discussion </c-><a href="#concepts"><c- c1>here</c-></a><c- c1>.</c->
        <ins><c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><del><c- n>ranges</c-><c- o>::</c-></del><c- n>find</c-><c- p>(</c-><c- n>objects</c-><c- p>,</c-> <c- n>input</c-><c- p>);</c-></ins>
        <c- c1>// etc.</c->
    <c- p>}</c->
<c- p>};</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- c1>// Suppose insteat that the Manager needs to use an associative container rather than</c->
<c- c1>// a sequential container (e.g. mapping some data to each object).</c->
<c- c1>// Then, an heterogeneous comparator becomes a necessity -- we can’t possibly</c->
<c- c1>// look up a unique_ptr using another unique_ptr to the same object, especially</c->
<c- c1>// if clients give us non-owning raw pointers to act upon.</c->

<c- c1>// Heterogeneous comparator</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>smart_pointer_comparator</c-> <c- p>{</c->
    <c- k>struct</c-> <c- nc>is_transparent</c-> <c- p>{};</c->

    <c- b>bool</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;</c-><c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;</c-><c- n>rhs</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- n>lhs</c-> <c- o>&lt;</c-> <c- n>rhs</c-><c- p>;</c-> <c- p>}</c->
    <c- b>bool</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;</c-><c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>T</c-> <c- o>*</c-><c- n>rhs</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>()(</c-><c- n>lhs</c-><c- p>.</c-><c- n>get</c-><c- p>(),</c-> <c- n>rhs</c-><c- p>);</c-> <c- p>}</c->
    <c- b>bool</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>T</c-> <c- o>*</c-><c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;</c-><c- n>rhs</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>()(</c-><c- n>lhs</c-><c- p>,</c-> <c- n>rhs</c-><c- p>.</c-><c- n>get</c-><c- p>());</c-> <c- p>}</c->
<c- p>};</c->

<c- c1>// A sorted associative container with some data</c->
<c- n>std</c-><c- o>::</c-><c- n>map</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-><c- p>,</c-> <c- n>Data</c-><c- p>,</c->
    <c- n>smart_pointer_comparator</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>>></c-> <c- n>objects</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- c1>// Heterogeneous lookup using a raw pointer</c->
<c- n>object</c-> <c- o>*</c-><c- n>ptr</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->
<c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>objects</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>ptr</c-><c- p>);</c->
<c- k>if</c-> <c- p>(</c-><c- n>it</c-> <c- o>!=</c-> <c- n>objects</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-> <c- p>{</c-> <c- n>use</c-><c- p>(</c-><c- n>it</c-><c- o>-></c-><c- n>second</c-><c- p>);</c-> <c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- c1>// No need for a custom comparator...</c->

















<c- c1>// ... just use the idiomatic std::less&lt;void></c->
<c- n>std</c-><c- o>::</c-><c- n>map</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-><c- p>,</c-> <c- n>Data</c-><c- p>,</c->
    <ins><c- nl>std</c-><c- p>:</c-><c- n>less</c-><c- o>&lt;></c-></ins><c- o>></c-> <c- n>objects</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- c1>// Heterogeneous lookup</c->
<c- n>Object</c-><c- o>*</c-> <c- n>ptr</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->
<c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>objects</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>ptr</c-><c- p>);</c->
<c- k>if</c-> <c- p>(</c-><c- n>it</c-> <c- o>!=</c-> <c- n>objects</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-> <c- p>{</c-> <c- n>use</c-><c- p>(</c-><c- n>it</c-><c- o>-></c-><c- n>second</c-><c- p>);</c-> <c- p>}</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- c1>// Same, with an unordered associative container.</c->

<c- c1>// Heterogeneous hasher; [util.smartptr.hash] guarantees that both the</c->
<c- c1>// specializations below return the very same value for the same pointer.</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>smart_pointer_hasher</c->
<c- p>{</c->
    <c- k>struct</c-> <c- nc>is_transparent</c-> <c- p>{};</c->
    <c- b>size_t</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- c1>// equal by definition to std::hash&lt;T *>(ptr.get()), that is, (*this)(ptr.get())</c->
        <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>()(</c-><c- n>ptr</c-><c- p>);</c->
    <c- p>}</c->
    <c- b>size_t</c-> <c- nf>operator</c-><c- p>()(</c-><c- n>T</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-><c- p>()(</c-><c- n>ptr</c-><c- p>);</c->
    <c- p>}</c->
<c- p>};</c->

<c- c1>// Heterogeneous equality comparator</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>smart_pointer_equal</c->
<c- p>{</c->
    <c- k>struct</c-> <c- nc>is_transparent</c-> <c- p>{};</c->
    <c- b>bool</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- n>lhs</c-> <c- o>==</c-> <c- n>rhs</c-><c- p>;</c-> <c- p>}</c->
    <c- b>bool</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>T</c-><c- o>*</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- n>lhs</c-><c- p>.</c-><c- n>get</c-><c- p>()</c-> <c- o>==</c-> <c- n>rhs</c-><c- p>;</c-> <c- p>}</c->
    <c- b>bool</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>T</c-><c- o>*</c-> <c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- n>lhs</c-> <c- o>==</c-> <c- n>rhs</c-><c- p>.</c-><c- n>get</c-><c- p>();</c-> <c- p>}</c->
<c- p>};</c->

<c- n>std</c-><c- o>::</c-><c- n>unordered_map</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-><c- p>,</c-> <c- n>Data</c-><c- p>,</c->
    <c- n>smart_pointer_hasher</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-><c- p>,</c->
    <c- n>smart_pointer_equal</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>>></c-> <c- n>objects</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- c1>// Heterogeneous lookup</c->
<c- n>Object</c-><c- o>*</c-> <c- n>ptr</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->
<c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>objects</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>ptr</c-><c- p>);</c->
<c- k>if</c-> <c- p>(</c-><c- n>it</c-> <c- o>!=</c-> <c- n>objects</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-> <c- p>{</c-> <c- n>use</c-><c- p>(</c-><c- n>it</c-><c- o>-></c-><c- n>second</c-><c- p>);</c-> <c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- c1>// </c-><a data-link-type="biblio" href="#biblio-p0919r3" title="Heterogeneous lookup for unordered containers"><c- c1>[P0919R3]</c-></a><c- c1> does not provide a heterogeneous hasher for smart pointers,</c->
<c- c1>// so a custom one is still needed</c->


<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>smart_pointer_hasher</c->
<c- p>{</c->
    <c- k>struct</c-> <c- nc>is_transparent</c-> <c- p>{};</c->
    <c- b>size_t</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- c1>// equal by definition to std::hash&lt;T*>(ptr.get()), that is, (*this)(ptr.get())</c->
        <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>()(</c-><c- n>ptr</c-><c- p>);</c->
    <c- p>}</c->
    <c- b>size_t</c-> <c- nf>operator</c-><c- p>()(</c-><c- n>T</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-><c- p>()(</c-><c- n>ptr</c-><c- p>);</c->
    <c- p>}</c->
<c- p>};</c->

<c- c1>// Custom heterogeneous equality comparator not needed any more</c->











<c- n>std</c-><c- o>::</c-><c- n>unordered_map</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-><c- p>,</c-> <c- n>Data</c-><c- p>,</c->
    <c- n>smart_pointer_hasher</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>></c-><c- p>,</c->
    <ins><c- n>std</c-><c- o>::</c-><c- n>equal_to</c-><c- o>&lt;></c-></ins><c- o>></c-> <c- n>objects</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- c1>// Heterogeneous lookup</c->
<c- n>Object</c-><c- o>*</c-> <c- n>ptr</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->
<c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>objects</c-><c- p>.</c-><c- n>find</c-><c- p>(</c-><c- n>ptr</c-><c- p>);</c->
<c- k>if</c-> <c- p>(</c-><c- n>it</c-> <c- o>!=</c-> <c- n>objects</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-> <c- p>{</c-> <c- n>use</c-><c- p>(</c-><c- n>it</c-><c- o>-></c-><c- n>second</c-><c- p>);</c-> <c- p>}</c->
</pre>
   </table>
   <h2 class="heading settled" data-level="3" id="motivation"><span class="secno">3. </span><span class="content">Motivation and Scope</span><a class="self-link" href="#motivation"></a></h2>
   <p>Smart pointer classes are universally recognized as the idiomatic way to
express ownership of a resource (very incomplete list: <a data-link-type="biblio" href="#biblio-sutter" title="Elements of Modern C++ Style">[Sutter]</a>, <a data-link-type="biblio" href="#biblio-meyers" title="Effective Modern C++, Chapter 4. Smart Pointers">[Meyers]</a>, <a data-link-type="biblio" href="#biblio-r20" title="C++ Core Guidelines, R.20: Use `unique_ptr` or `shared_ptr` to represent ownership">[R.20]</a>). On the other hand, raw pointers (and references) are supposed to be
used as non-owning types to access a resource.</p>
   <p>Both smart pointers and raw pointers, as their name says, share a common
semantic: <b>representing the address of an object</b>.</p>
   <p>This semantic comes with a set of meaningful operations; for instance, asking
if two (smart) pointers represent the address of the same object. <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> is used to express this intent.</p>
   <p>Indeed, with the owning smart pointer class templates available in the Standard
Library (<code class="highlight"><c- n>unique_ptr</c-></code> and <code class="highlight"><c- n>shared_ptr</c-></code>), one can already use <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> between two smart pointer objects (of the same class). However one <em>cannot</em> use
it between a smart pointer and a raw pointer, because the Standard Library is
lacking that set of overloads; instead, one has to manually extract the raw
pointer out of the smart pointer class:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>shared_ptr</c-><c- o>&lt;</c-><c- n>object</c-><c- o>></c-> <c- n>sptr1</c-><c- p>,</c-> <c- n>sptr2</c-><c- p>;</c->
<c- n>object</c-> <c- o>*</c-><c- n>rawptr</c-><c- p>;</c->

<c- c1>// Do both pointers refer to the same object?</c->
<c- k>if</c-> <c- p>(</c-><c- n>sptr1</c-> <c- o>==</c-> <c- n>sptr2</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c->        <c- c1>// WORKS</c->
<c- k>if</c-> <c- p>(</c-><c- n>sptr1</c-> <c- o>==</c-> <c- n>rawptr</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c->       <c- c1>// ERROR, no such operator</c->
<c- k>if</c-> <c- p>(</c-><c- n>sptr1</c-><c- p>.</c-><c- n>get</c-><c- p>()</c-> <c- o>==</c-> <c- n>rawptr</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c-> <c- c1>// WORKS; but why the extra syntax?</c->
</pre>
   <p>This discussion can be easily generalized to the full set of the six relational
operations; these operations have already well-established semantics, and are
indeed already defined between smart pointers objects (of the same class) or
between raw pointers, but they are not supported in mixed scenarios.</p>
   <div class="note" role="note"> We propose to remove this inconsistency by defining the relational operators
between the Standard Library owning smart pointer classes and raw pointers. </div>
   <p>Allowing mixed comparisons isn’t merely a "semantic fixup"; the situation where
one has to compare smart pointers and raw pointers commonly occurs in practice
(the typical use case is outlined in the first example in the <a href="#tonytables">§ 2 Tony Tables</a> above, where a "manager" object gives non-owning raw pointers to clients,
and the clients pass these raw pointers back to the manager, and now the manager
needs to do mixed comparisons).</p>
   <h3 class="heading settled" data-level="3.1" id="associative"><span class="secno">3.1. </span><span class="content">Associative containers</span><a class="self-link" href="#associative"></a></h3>
   <p>Moreover, we believe that allowing mixed comparisons is useful in order to
streamline heterogeneous comparison in associative containers for smart pointer
classes.</p>
   <p>The case of an associative container using a <code class="highlight"><c- n>unique_ptr</c-></code> as its key
type is particularly annoying; one cannot practically <em>ever</em> look up in such a
container using another <code class="highlight"><c- n>unique_ptr</c-></code>, as that would imply having two <code class="highlight"><c- n>unique_ptr</c-></code> objects owning the same object. Instead, the typical lookup is
heterogeneous (by raw pointer); this proposal is one step towards making it
more convenient to use, because it enables the usage of the standard <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>less</c-></code> or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>equal_to</c-></code>.</p>
   <p>We however are not addresssing at all the issue of heterogeneous hashing for
smart pointers. While likely very useful in general, heterogeneous hashing can
be tackled separately by another proposal that builds on top of this one, for
instance, by making the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>hash</c-></code> specializations for Standard smart pointers
1) <em>transparent</em>, and 2) able to hash the smart pointer’s <code class="highlight"><c- n>pointer_type</c-></code> / <code class="highlight"><c- n>element_type</c-><c- o>*</c-></code> as well as the smart pointer object itself. More research
and field experience is needed.</p>
   <h2 class="heading settled" data-level="4" id="impact"><span class="secno">4. </span><span class="content">Impact On The Standard</span><a class="self-link" href="#impact"></a></h2>
   <p>This proposal is a pure library extension. It proposes changes to an existing
header, <code class="highlight"><c- o>&lt;</c-><c- n>memory</c-><c- o>></c-></code>, but it does not require changes to any standard classes or
functions and it does not require changes to any of the standard requirement
tables. The impact is positive: code that was ill-formed before becomes
well-formed.</p>
   <p>This proposal does not depend on any other library extensions.</p>
   <p>This proposal does not require any changes in the core language.</p>
   <p><a data-link-type="biblio" href="#biblio-p0805r2" title="Comparing Containers">[P0805R2]</a> is vaguely related to this proposal. It proposes to add mixed
comparisons between containers of the same type (for instance, to be able to
compare a <code class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> with a <code class="highlight"><c- n>vector</c-><c- o>&lt;</c-><c- b>long</c-><c- o>></c-></code>), without resorting to manual
calls to algorithms; instead, one can use a comparison operator. A quite
verbose call to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>equal</c-><c- p>(</c-><c- n>v1</c-><c- p>.</c-><c- n>cbegin</c-><c- p>(),</c-> <c- n>v1</c-><c- p>.</c-><c- n>cend</c-><c- p>(),</c-> <c- n>v2</c-><c- p>.</c-><c- n>cbegin</c-><c- p>(),</c-> <c- n>v2</c-><c- p>.</c-><c- n>cend</c-><c- p>())</c-></code> can therefore be replaced by a much simpler <code class="highlight"><c- n>v1</c-> <c- o>==</c-> <c- n>v2</c-></code>. In this sense, <a data-link-type="biblio" href="#biblio-p0805r2" title="Comparing Containers">[P0805R2]</a> matches the spirit of the current proposal, although comparing
smart pointers and raw pointer does not require any algorithm, and does not
have such a verbose syntax.</p>
   <h2 class="heading settled" data-level="5" id="designdecisions"><span class="secno">5. </span><span class="content">Design Decisions</span><a class="self-link" href="#designdecisions"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="proposedsignature"><span class="secno">5.1. </span><span class="content">What is the signature of the proposed operators?</span><a class="self-link" href="#proposedsignature"></a></h3>
   <p>The motivating reason of this proposal is that this code should work:</p>
<pre class="highlight"><c- n>smart_pointer</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>smartptr</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->
<c- n>T</c-> <c- o>*</c-><c- n>rawptr</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- k>if</c-> <c- p>(</c-><c- n>smartptr</c-> <c- o>==</c-> <c- n>rawptr</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c-> <c- c1>// OK with this proposal</c->
</pre>
   <p>We could therefore conclude that for instance <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> should have this signature:</p>
<pre class="highlight"><c- c1>// not proposed</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>smart_pointer</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;</c-><c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>T</c-> <c- o>*</c-><c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <p>(The other operators would have a similar one.)</p>
   <p>While this signature "make sense", there is however a number of
subtleties that need to taken into account and are going to be
addressed in the next sections.</p>
   <h4 class="heading settled" data-level="5.1.1" id="mixedcomparisons"><span class="secno">5.1.1. </span><span class="content">Allowing mixed comparisons against raw pointers</span><a class="self-link" href="#mixedcomparisons"></a></h4>
   <p>The rationale of this proposal is that smart pointers should act like
raw pointers when it comes to comparison operators. For instance, raw
pointers allow for mixed comparisons if both pointers are convertible
to their composite pointer type ([expr.type]):</p>
<pre class="highlight"><c- n>Base</c-> <c- o>*</c-><c- n>b</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->
<c- n>Derived</c-> <c- o>*</c-><c- n>d</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- k>if</c-> <c- p>(</c-><c- n>b</c-> <c- o>==</c-> <c- n>d</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c-> <c- c1>// OK</c->
</pre>
   <p>Therefore, we want the following to also work:</p>
<pre class="highlight"><c- n>smart_pointer</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>b</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->
<c- n>Derived</c-> <c- o>*</c-><c- n>d</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- k>if</c-> <c- p>(</c-><c- n>b</c-> <c- o>==</c-> <c- n>d</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c-> <c- c1>// Should also be OK with this proposal</c->
</pre>
   <p>In this case a signature like <code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>smart_pointer</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;</c-><c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>T</c-> <c- o>*</c-><c- n>rhs</c-><c- p>)</c-></code> would not compile, because we cannot
deduce <code class="highlight"><c- n>T</c-></code> in the call above (is it <code class="highlight"><c- n>Base</c-></code> or is it <code class="highlight"><c- n>Derived</c-></code>?).
For this reason, we need to generalize the signature of the comparison
operators, and add constraints to it:</p>
<pre class="highlight"><c- c1>// not proposed</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>U</c-><c- o>></c->
    <c- k>requires</c-> <c- n>equality_comparable</c-><c- o>&lt;</c-><c- n>T</c-> <c- o>*</c-><c- p>,</c-> <c- n>U</c-> <c- o>*></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>smart_pointer</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;</c-><c- n>lhs</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-> <c- o>*</c-><c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <h4 class="heading settled" data-level="5.1.2" id="array"><span class="secno">5.1.2. </span><span class="content">Handling of array types</span><a class="self-link" href="#array"></a></h4>
   <p>The Standard Library smart pointers can be instantiated with array
types. This requires some special handling in the operator signatures:
for a smart pointer <code class="highlight"><c- n>smart_pointer</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, we can’t just check if <code class="highlight"><c- n>T</c-> <c- o>*</c-></code> is
comparable with <code class="highlight"><c- n>U</c-> <c- o>*</c-></code>, because <code class="highlight"><c- n>T</c-></code> may be an array type.</p>
   <p>Therefore, we have to amend the constraint, and use:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>pointer</c-></code> for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code>; and</p>
    <li data-md>
     <p><code class="highlight"><c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>shared_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>element_type</c-> <c- o>*</c-></code> for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>shared_ptr</c-></code>.</p>
   </ul>
   <p>(In other words: the return types for the respective <code class="highlight"><c- n>get</c-><c- p>()</c-></code> functions.)</p>
   <h4 class="heading settled" data-level="5.1.3" id="uniqueptrpointer"><span class="secno">5.1.3. </span><span class="content">Custom pointer types for <code class="highlight"><c- n>unique_ptr</c-></code></span><a class="self-link" href="#uniqueptrpointer"></a></h4>
   <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code> can use a custom pointer type through its deleter.
This pointer type is only required to model <code class="highlight"><c- n>Cpp17NullablePointer</c-></code>,
which means that it’s only guaranteed to be comparable against another
pointer object of the same type, and not in general against raw
pointers.</p>
   <p>In earlier revisions of this proposal we were addressing this issue by
having the comparison operators for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code> work in terms
of its <code class="highlight"><c- n>pointer</c-></code> inner typedef, and not raw pointers. This however
resulted in possible ambiguities: if <code class="highlight"><c- n>pointer</c-></code> is a user-defined type,
the user could have <em>already</em> defined a comparison operator between
that type and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code>. If we add a corresponding one in the
Standard Library, this will risk breaking users' code. The calls to
such an operator could become ambiguous, or, worse, change meaning
(call the operator in the Standard Library rather than the users').</p>
   <p>We do not feel comfortable with this breakage. While in general users
are never supposed to define overloads/customization points/etc. for
Standard Library or language entities, defining an operator overload
that accepts a user-defined type as an input (as well as a
library/language entity) can reasonably be seen as something that the
user has the complete right to do.</p>
   <p>For this reason, **since R6 we are limiting the scope of the mixed
comparison operators for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code> only to language
pointers**. In future, this limitation can be lifted if it is deemed
useful to do so.</p>
   <h4 class="heading settled" data-level="5.1.4" id="generalizedcomparisons"><span class="secno">5.1.4. </span><span class="content">Objects comparable to a smart pointer’s pointer type</span><a class="self-link" href="#generalizedcomparisons"></a></h4>
   <p>Consider a type which is already comparable to a pointer type <code class="highlight"><c- n>T</c-> <c- o>*</c-></code> (such as a user-defined smart pointer type):
should it also become comparable to a Standard Library smart pointer type?</p>
<pre class="highlight"><c- n>T</c-> <c- o>*</c-><c- n>rawPtr</c-><c- p>;</c->
<c- n>my_smart_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>my_ptr</c-><c- p>;</c->     <c- c1>// not in the Standard</c->
<c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>std_ptr</c-><c- p>;</c-> <c- c1>//     in the Standard</c->

<c- c1>// Suppose that this works because `my_smart_ptr` has an overloaded operator==</c->
<c- k>if</c-> <c- p>(</c-><c- n>rawPtr</c-> <c- o>==</c-> <c- n>my_ptr</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c->

<c- c1>// Then should this also work?</c->
<c- k>if</c-> <c- p>(</c-><c- n>std_ptr</c-> <c- o>==</c-> <c- n>my_ptr</c-><c- p>)</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c->
</pre>
   <p>In principle one could conclude that the last line should also work, as
the comparison semantics for smart pointers follow the identical model
of comparisons for a raw pointer. However, once more, it is perfectly
allowed for a user-defined type to have already defined an overload of <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> between itself and a Standard Library smart pointer class
template; redefining the same operator again can easily create source
incompatibilities.</p>
   <p>For this reason, we are <strong>not proposing this extension</strong>, and we are
limiting the scope of the proposed operators to comparisons between
smart pointer classes and raw pointers.</p>
   <h4 class="heading settled" data-level="5.1.5" id="mixedlibrarysmart"><span class="secno">5.1.5. </span><span class="content">Should mixed comparisons between <em>different</em> Standard Library smart pointer classes be allowed?</span><a class="self-link" href="#mixedlibrarysmart"></a></h4>
   <p>In principle, we could define comparisons between different Standard
Library smart pointer classes. Although <a data-link-type="biblio" href="#biblio-sd-8" title="SD-8: Standard Library Compatibility">[SD-8]</a> isn’t crystal clear on
this, it is reasonable to expect that users are not allowed to define
operators between two types that they do not control, so there is no
danger at breaking user code here.</p>
   <p>There are some considerations that in our opinion apply to this case.</p>
   <p>The first is whether such an operation makes sense. From the abstract
point of view of "comparing the addresses of two objects", the
operation is meaningful, even if the addresses are represented by
instances of different smart pointer classes. As mentioned before, the
currently existing comparison operators of the smart pointer classes
implement these semantics.</p>
   <p>On the other hand: the Standard Library smart pointer classes that this
proposal deals with are all <em>owning</em> classes. One could therefore
reason that there is little utility at allowing a mixed comparison,
because it’s likely to be a mistake by the user. In a "ordinary"
program, such comparisons would inevitably yield false, because the
same object cannot be owned by two smart pointers of different classes
(if it is, there is a bug in the program).</p>
   <p>There is some semantic leeway here, represented by the fact that the
smart pointer classes can hold custom deleters (incl. empty/no-op
deleters), aliased pointers (in the case of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>shared_ptr</c-></code>), as well
as as fancy pointer types (in the case of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code>). In
principle, one can write a perfectly legal example where the same
object is owned by smart pointers of different classes, using custom
deleters and/or custom fancy pointer types, and then wants to compare
the smart pointers (compare the addresses of the objects owned by
them).</p>
   <p>In some ways, this is hardly a realistic use case for allowing
comparisons between different smart pointer classes. The danger of
misuse of such comparisons, again in "ordinary" code, seems be stricly
greater than their usefulness, given the unlikelihood of valid use
cases -- in the majority of ordinary usages, the comparisons would be
meaningless.</p>
   <p>We have therefore a tension between the abstract/ideal domain of the
operations, and the practical usage. The problem is that <strong>trying to
solve it via the type system alone isn’t possible</strong>. For instance,
type-erased deleters (in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>shared_ptr</c-></code>) make it impossible to know
if, given a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>X</c-><c- p>,</c-> <c- n>D</c-><c- o>></c-></code> and a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>shared_ptr</c-><c- o>&lt;</c-><c- n>X</c-><c- o>></c-></code> object,
a comparison between them is meaningless or instead they have somehow
been designed to "work together".</p>
   <p>In R6 we are going proposing the more conservative solution: that is, <strong>we are not going to propose mixed comparisons between different
smart pointer classes</strong>. They can always be added at a later time if so
desired.</p>
   <h3 class="heading settled" data-level="5.2" id="totalorder"><span class="secno">5.2. </span><span class="content">Should the comparison operators yield a total order?</span><a class="self-link" href="#totalorder"></a></h3>
   <p>The current semantics of comparisons between smart pointer classes <em>extend</em> the ones defined by the language operators when used on the
respective raw pointer types: smart pointer classes use <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>less</c-></code> or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>compare_three_way</c-></code> and therefore always yield a strict weak
ordering, even when the built-in comparison operator for pointers would
not guarantee any specific ordering.</p>
   <p>For instance:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>a</c-><c- p>(</c-><c- k>new</c-> <c- b>int</c-><c- p>(</c-><c- mi>123</c-><c- p>));</c->
<c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>b</c-><c- p>(</c-><c- k>new</c-> <c- b>int</c-><c- p>(</c-><c- mi>456</c-><c- p>));</c->

<c- k>if</c-> <c- p>(</c-><c- n>a</c-> <c- o>&lt;</c-> <c- n>b</c-><c- p>)</c->             <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c-> <c- c1>// well-defined behavior</c->
<c- k>if</c-> <c- p>(</c-><c- n>a</c-><c- p>.</c-><c- n>get</c-><c- p>()</c-> <c- o>&lt;</c-> <c- n>b</c-><c- p>.</c-><c- n>get</c-><c- p>())</c-> <c- p>{</c-> <c- o>~~~</c-> <c- p>}</c-> <c- c1>// unspecified behavior [expr.rel/4.3]</c->
</pre>
   <p>The operations that we are proposing would similarly always be
well-defined and yield a total order, by using the very same function
objects.</p>
   <h3 class="heading settled" data-level="5.3" id="concepts"><span class="secno">5.3. </span><span class="content">Would these operations make the <code class="highlight"><c- n>equality_comparable_with</c-></code> or <code class="highlight"><c- n>three_way_comparable_with</c-></code> concepts satisfied between a smart pointer and a raw pointer?</span><a class="self-link" href="#concepts"></a></h3>
   <p><strong>No.</strong> Adding the operations would indeed bring the Standard Library’s
smart pointer classes "one step closer" to satisfy those concepts, but
they would still be unsatisfied because there is no <code class="highlight"><c- n>common_reference_t</c-></code> between a smart pointer and a raw pointer.</p>
   <p>Changing that is out of scope for the present proposal (and orthogonal
to it).</p>
   <div class="note" role="note">
     In general, the current situation of comparison concepts and Standard
Library smart pointers is "suboptimal". For instance: 
<pre class="highlight"><c- k>static_assert</c-><c- p>(</c-><c- n>equality_comparable_with</c-><c- o>&lt;</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>nullptr_t</c-><c- o>></c-><c- p>);</c->  <c- c1>// (1) OK since C++23 (ERROR in C++20)</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>equality_comparable_with</c-><c- o>&lt;</c-><c- n>shared_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>nullptr_t</c-><c- o>></c-><c- p>);</c->  <c- c1>// (2) OK</c->

<c- k>static_assert</c-><c- p>(</c-><c- n>three_way_comparable_with</c-><c- o>&lt;</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>nullptr_t</c-><c- o>></c-><c- p>);</c-> <c- c1>// (3) ERROR</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>three_way_comparable_with</c-><c- o>&lt;</c-><c- n>shared_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>nullptr_t</c-><c- o>></c-><c- p>);</c-> <c- c1>// (4) ERROR</c->
</pre>
    <p>... despite the existence of the related operators between smart
pointers and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>nullptr_t</c-></code>.</p>
    <ul>
     <li data-md>
      <p>(1) and (3) used to fail in C++20 because the concepts required <code class="highlight"><c- n>unique_ptr</c-></code> to be copiable.</p>
     <li data-md>
      <p>(3) and (4) fail because they require <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>nullptr_t</c-></code> to be three-way comparable, which it isn’t (<code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>nullptr_t</c-></code> lacks relational operations).</p>
    </ul>
    <p>An analysis and discussion is available in <a data-link-type="biblio" href="#biblio-so_unique_ptr_comparable_thread" title="Why is unique_ptr not equality_comparable_with nullptr_t in C++20?">this thread on StackOverflow</a>.</p>
    <p><a data-link-type="biblio" href="#biblio-p2404r3" title="Move-only types for equality_comparable_with, totally_ordered_with, and three_way_comparable_with">[P2404R3]</a> (merged in C++23) and <a data-link-type="biblio" href="#biblio-p2405r0" title="nullopt_t and nullptr_t should both have operator and operator==">[P2405R0]</a> (unactive) aim at closing these
semantics gaps (for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>nullptr_t</c-></code>, not for raw pointers in general).</p>
   </div>
   <p>An unfortunate consequence of this is that, for the moment being, we will yet
not be able to use many range-based algorithms using heterogeneous comparisons:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Object</c-><c- o>>></c-> <c- n>objects</c-><c- p>;</c->

<c- n>Object</c-> <c- o>*</c-><c- n>ptr</c-> <c- o>=</c-> <c- o>~~~</c-><c- p>;</c->

<c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>find</c-><c- p>(</c-><c- n>objects</c-><c- p>,</c-> <c- n>ptr</c-><c- p>);</c-> <c- c1>// ERROR; must use find_if and a predicate :-(</c->
</pre>
   <p>On the other hand, non-range algorithms would work just fine:</p>
<pre class="highlight"><c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>find</c-><c- p>(</c-><c- n>objects</c-><c- p>.</c-><c- n>begin</c-><c- p>(),</c-> <c- n>objects</c-><c- p>.</c-><c- n>end</c-><c- p>(),</c-> <c- n>ptr</c-><c- p>);</c-> <c- c1>// OK</c->
<c- n>erase</c-><c- p>(</c-><c- n>objects</c-><c- p>,</c-> <c- n>ptr</c-><c- p>);</c-> <c- c1>// OK</c->
</pre>
   <p>Future evolutions can close this gap by providing the missing building
blocks.</p>
   <h3 class="heading settled" data-level="5.4" id="uniqueptroperators"><span class="secno">5.4. </span><span class="content">Should <code class="highlight"><c- n>unique_ptr</c-></code> have the full set of ordering operators (<code class="highlight"><c- o>&lt;</c-></code>, <code class="highlight"><c- o>&lt;=</c-></code>, <code class="highlight"><c- o>></c-></code>, <code class="highlight"><c- o>>=</c-></code>), or just <code class="highlight"><c- o>&lt;=></c-></code>?</span><a class="self-link" href="#uniqueptroperators"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p1614r2" title="The Mothership Has Landed: Adding <=> to the Library">[P1614R2]</a> added support for <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code> across the Standard Library.
Notably, it <em>added</em> <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code> for <code class="highlight"><c- n>unique_ptr</c-></code>, leaving the other
four ordering operators (<code class="highlight"><c- o>&lt;</c-></code>, <code class="highlight"><c- o>&lt;=</c-></code>, <code class="highlight"><c- o>></c-></code>, <code class="highlight"><c- o>>=</c-></code>) untouched. On the
other hand, when looking at <code class="highlight"><c- n>shared_ptr</c-></code>, the same paper <em>replaced</em> these four
operators with <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code>.</p>
   <p>We believe that this was done in order to preserve the semantics for the
existing operators, which are defined in terms of customization points
(notably, <code class="highlight"><c- n>common_type</c-></code>; <code class="highlight"><c- n>unique_ptr</c-></code> can work in terms of a custom "fancy"
pointer type).</p>
   <p>We are not bound by any pre-existing semantics, and the operators we
are adding only act between <code class="highlight"><c- n>unique_ptr</c-></code> and a raw pointer type, so we
are just proposing <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code> for <code class="highlight"><c- n>unique_ptr</c-></code>.</p>
   <h2 class="heading settled" data-level="6" id="technicalspecifications"><span class="secno">6. </span><span class="content">Technical Specifications</span><a class="self-link" href="#technicalspecifications"></a></h2>
   <p>All the proposed changes are relative to <a data-link-type="biblio" href="#biblio-n4971" title="Working Draft, Programming Languages — C++">[N4971]</a>.</p>
   <h3 class="heading settled" data-level="6.1" id="featuretestingmacro"><span class="secno">6.1. </span><span class="content">Feature testing macro #</span><a class="self-link" href="#featuretestingmacro"></a></h3>
   <p>Add to the list in [version.syn]:</p>
<pre class="highlight"><c- cp>#define __cpp_lib_mixed_smart_pointer_comparisons YYYYMML  </c-><c- c1>// also in &lt;memory></c->
</pre>
   <p>with the value specified as usual (year and month of adoption).</p>
   <h3 class="heading settled" data-level="6.2" id="proposedwording"><span class="secno">6.2. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposedwording"></a></h3>
   <h4 class="heading settled" data-level="6.2.1" id="proposedwordinguniqueptr"><span class="secno">6.2.1. </span><span class="content"><code class="highlight"><c- n>unique_ptr</c-></code></span><a class="self-link" href="#proposedwordinguniqueptr"></a></h4>
   <p>Modify [unique.ptr.single.general] as shown:</p>
   <blockquote>
<pre class="highlight"><ins>
    <c- c1>// [unique.ptr.single.mixed.cmp], mixed comparisons</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>D2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
      <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c->
        <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>D2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
      <c- k>friend</c-> <c- k>constexpr</c-> <c- n>compare_three_way_result_t</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>::</c-><c- n>pointer</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*></c->
        <c- k>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
    <c- c1>// disable copy from lvalue</c->
    <c- n>unique_ptr</c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
    <c- n>unique_ptr</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
<c- p>};</c->
</pre>
   </blockquote>
   <p>Add a new section at the end of the [unique.ptr.single] chapter:</p>
   <blockquote>
    <ins><strong>?.?.?.?.? Mixed comparison operators</strong> [unique.ptr.single.mixed.cmp]</ins>
<pre class="highlight"><ins>
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>D2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c->
    <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
</pre>
    <ins>1 <em>Constraints</em>:</ins>
    <br> 
    <ul>
     <li>
      <ins><code class="highlight"><c- n>is_pointer_v</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>unique_ptr</c-><c- o>&lt;</c-><c- n>U2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>::</c-><c- n>pointer</c-><c- o>></c-></code> is <code class="highlight">true</code>;</ins>
     <li>
      <ins><code class="highlight"><c- n>equality_comparable_with</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>unique_ptr</c-><c- o>&lt;</c-><c- n>U2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>::</c-><c- n>pointer</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*></c-></code> is <code class="highlight">true</code>.</ins>
    </ul>
    <ins>2 <em>Returns</em>: <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>get</c-><c- p>()</c-> <c- o>==</c-> <c- n>y</c-></code>.</ins>
    <br> 
<pre class="highlight"><ins>
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>D2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>constexpr</c-> <c- n>compare_three_way_result_t</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>::</c-><c- n>pointer</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*></c->
    <c- k>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
</pre>
    <ins>3 <em>Constraints</em>:</ins>
    <br> 
    <ul>
     <li>
      <ins><code class="highlight"><c- n>is_pointer_v</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>unique_ptr</c-><c- o>&lt;</c-><c- n>U2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>::</c-><c- n>pointer</c-><c- o>></c-></code> is <code class="highlight">true</code>;</ins>
     <li>
      <ins><code class="highlight"><c- n>three_way_comparable_with</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>unique_ptr</c-><c- o>&lt;</c-><c- n>U2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>::</c-><c- n>pointer</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*></c-></code> is <code class="highlight">true</code>.</ins>
    </ul>
    <ins>4 <em>Returns</em>: <code class="highlight"><c- n>compare_three_way</c-><c- p>()(</c-><c- n>x</c-><c- p>.</c-><c- n>get</c-><c- p>(),</c-> <c- n>y</c-><c- p>)</c-></code>.</ins>
   </blockquote>
   <p>Modify [unique.ptr.runtime.general] as shown:</p>
   <blockquote>
<pre class="highlight"><ins>
    <c- c1>// mixed comparisons</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>D2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
      <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c->
        <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>D2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
      <c- k>friend</c-> <c- k>constexpr</c-> <c- n>compare_three_way_result_t</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>::</c-><c- n>pointer</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*></c->
        <c- k>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- n>D2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
    <c- c1>// disable copy from lvalue</c->
    <c- n>unique_ptr</c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
    <c- n>unique_ptr</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>unique_ptr</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
<c- p>};</c->
</pre>
   </blockquote>
   <h4 class="heading settled" data-level="6.2.2" id="proposedwordingsharedptr"><span class="secno">6.2.2. </span><span class="content"><code class="highlight"><c- n>shared_ptr</c-></code></span><a class="self-link" href="#proposedwordingsharedptr"></a></h4>
   <p>Modify [util.smartptr.shared.general] as shown:</p>
   <blockquote>
<pre class="highlight">    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
      <c- b>bool</c-> <c- n>owner_before</c-><c- p>(</c-><c- k>const</c-> <c- n>weak_ptr</c-><c- o>&lt;</c-><c- n>U</c-><c- o>>&amp;</c-> <c- n>b</c-><c- p>)</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->

<ins>
    <c- c1>// [util.smartptr.shared.mixed.cmp], mixed comparisons</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
        <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>b</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
      <c- k>friend</c-> <c- n>compare_three_way_result_t</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>::</c-><c- n>element_type</c-><c- o>*</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*&lt;</c->
        <c- k>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>b</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
<c- p>};</c->
</pre>
   </blockquote>
   <p>Insert a new section after [util.smartptr.shared.cmp]:</p>
   <blockquote>
    <ins><strong>?.?.?.? Mixed comparison operators</strong> [util.smartptr.shared.mixed.cmp]</ins>
<pre class="highlight"><ins>
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
    <c- k>friend</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>b</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
</pre>
    <ins>1 <em>Constraints</em>: <code class="highlight"><c- n>equality_comparable_with</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>::</c-><c- n>element_type</c-><c- o>*</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*></c-></code> is <code class="highlight">true</code>.</ins>
    <br> 
    <ins>2 <em>Returns</em>: <code class="highlight"><c- n>a</c-><c- p>.</c-><c- n>get</c-><c- p>()</c-> <c- o>==</c-> <c- n>b</c-></code>.</ins>
<pre class="highlight"><ins>
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T2</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- n>compare_three_way_result_t</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>::</c-><c- n>element_type</c-><c- o>*</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*&lt;</c->
    <c- k>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*</c-> <c- n>b</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
</pre>
    <ins>3 <em>Constraints</em>: <code class="highlight"><c- n>three_way_comparable_with</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>shared_ptr</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>>::</c-><c- n>element_type</c-><c- o>*</c-><c- p>,</c-> <c- k>const</c-> <c- n>U</c-><c- o>*></c-></code> is <code class="highlight">true</code>.</ins>
    <br> 
    <ins>4 <em>Returns</em>: <code class="highlight"><c- n>compare_three_way</c-><c- p>()(</c-><c- n>a</c-><c- p>.</c-><c- n>get</c-><c- p>(),</c-> <c- n>b</c-><c- p>)</c-></code>.</ins>
   </blockquote>
   <h2 class="heading settled" data-level="7" id="implementation"><span class="secno">7. </span><span class="content">Implementation experience</span><a class="self-link" href="#implementation"></a></h2>
   <p>A working prototype of the changes proposed by this paper,
done on top of GCC 13, is available in this <a data-link-type="biblio" href="#biblio-p2249-gcc" title="P2249 prototype implementation">GCC branch</a> on
GitHub.</p>
   <h2 class="heading settled" data-level="8" id="acknowledgements"><span class="secno">8. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Credits for this idea go to Marc Mutz, who raised the question on the <a data-link-type="biblio" href="#biblio-marcmutzreflector" title="unique_ptr<T> @ T* relational operators / comparison">LEWG reflector</a>, receiving a positive feedback.</p>
   <p>Thanks to the reviewers of early drafts of this paper on the <a data-link-type="biblio" href="#biblio-std-proposals" title="P2249 discussion on the std-proposals mailing list">std-proposals</a> mailing list.</p>
   <p>Thanks to KDAB for supporting this work.</p>
   <p>All remaining errors are ours and ours only.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-marcmutzreflector">[MarcMutzReflector]
   <dd>Marc Mutz. <a href="https://lists.isocpp.org/lib-ext/2020/07/15873.php"><cite>unique_ptr&lt;T> @ T* relational operators / comparison</cite></a>. URL: <a href="https://lists.isocpp.org/lib-ext/2020/07/15873.php">https://lists.isocpp.org/lib-ext/2020/07/15873.php</a>
   <dt id="biblio-meyers">[Meyers]
   <dd>Scott Meyers. <a href="https://www.oreilly.com/library/view/effective-modern-c/9781491908419/ch04.html"><cite>Effective Modern C++, Chapter 4. Smart Pointers</cite></a>. URL: <a href="https://www.oreilly.com/library/view/effective-modern-c/9781491908419/ch04.html">https://www.oreilly.com/library/view/effective-modern-c/9781491908419/ch04.html</a>
   <dt id="biblio-n4971">[N4971]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4971"><cite>Working Draft, Programming Languages — C++</cite></a>. 18 December 2023. URL: <a href="https://wg21.link/n4971">https://wg21.link/n4971</a>
   <dt id="biblio-p0805r2">[P0805R2]
   <dd>Marshall Clow. <a href="https://wg21.link/p0805r2"><cite>Comparing Containers</cite></a>. 22 June 2018. URL: <a href="https://wg21.link/p0805r2">https://wg21.link/p0805r2</a>
   <dt id="biblio-p0919r3">[P0919R3]
   <dd>Mateusz Pusz. <a href="https://wg21.link/p0919r3"><cite>Heterogeneous lookup for unordered containers</cite></a>. 9 November 2018. URL: <a href="https://wg21.link/p0919r3">https://wg21.link/p0919r3</a>
   <dt id="biblio-p1614r2">[P1614R2]
   <dd>Barry Revzin. <a href="https://wg21.link/p1614r2"><cite>The Mothership Has Landed: Adding &lt;=> to the Library</cite></a>. 28 July 2019. URL: <a href="https://wg21.link/p1614r2">https://wg21.link/p1614r2</a>
   <dt id="biblio-p2249-gcc">[P2249-GCC]
   <dd>Giuseppe D'Angelo. <a href="https://github.com/dangelog/gcc/tree/P2249_mixed_smart_pointer_comparisons"><cite>P2249 prototype implementation</cite></a>. URL: <a href="https://github.com/dangelog/gcc/tree/P2249_mixed_smart_pointer_comparisons">https://github.com/dangelog/gcc/tree/P2249_mixed_smart_pointer_comparisons</a>
   <dt id="biblio-p2273r3">[P2273R3]
   <dd>Andreas Fertig. <a href="https://wg21.link/p2273r3"><cite>Making std::unique_ptr constexpr</cite></a>. 9 November 2021. URL: <a href="https://wg21.link/p2273r3">https://wg21.link/p2273r3</a>
   <dt id="biblio-p2404r3">[P2404R3]
   <dd>Justin Bassett. <a href="https://wg21.link/p2404r3"><cite>Move-only types for equality_comparable_with, totally_ordered_with, and three_way_comparable_with</cite></a>. 8 July 2022. URL: <a href="https://wg21.link/p2404r3">https://wg21.link/p2404r3</a>
   <dt id="biblio-p2405r0">[P2405R0]
   <dd>Justin Bassett. <a href="https://wg21.link/p2405r0"><cite>nullopt_t and nullptr_t should both have operator and operator==</cite></a>. 15 July 2021. URL: <a href="https://wg21.link/p2405r0">https://wg21.link/p2405r0</a>
   <dt id="biblio-r20">[R.20]
   <dd>Bjarne Stroustrup; Herb Sutter. <a href="https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#r20-use-unique_ptr-or-shared_ptr-to-represent-ownership"><cite>C++ Core Guidelines, R.20: Use `unique_ptr` or `shared_ptr` to represent ownership</cite></a>. URL: <a href="https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#r20-use-unique_ptr-or-shared_ptr-to-represent-ownership">https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#r20-use-unique_ptr-or-shared_ptr-to-represent-ownership</a>
   <dt id="biblio-sd-8">[SD-8]
   <dd>Bryce Lelbach. <a href="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility"><cite>SD-8: Standard Library Compatibility</cite></a>. URL: <a href="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility">https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility</a>
   <dt id="biblio-so_unique_ptr_comparable_thread">[SO_unique_ptr_comparable_thread]
   <dd><a href="https://stackoverflow.com/questions/66937947/why-is-unique-ptr-not-equality-comparable-with-nullptr-t-in-c20"><cite>Why is unique_ptr not equality_comparable_with nullptr_t in C++20?</cite></a>. URL: <a href="https://stackoverflow.com/questions/66937947/why-is-unique-ptr-not-equality-comparable-with-nullptr-t-in-c20">https://stackoverflow.com/questions/66937947/why-is-unique-ptr-not-equality-comparable-with-nullptr-t-in-c20</a>
   <dt id="biblio-std-proposals">[Std-proposals]
   <dd><a href="https://lists.isocpp.org/std-proposals/2021/01/2308.php"><cite>P2249 discussion on the std-proposals mailing list</cite></a>. URL: <a href="https://lists.isocpp.org/std-proposals/2021/01/2308.php">https://lists.isocpp.org/std-proposals/2021/01/2308.php</a>
   <dt id="biblio-sutter">[Sutter]
   <dd>Herb Sutter. <a href="https://herbsutter.com/elements-of-modern-c-style/"><cite>Elements of Modern C++ Style</cite></a>. URL: <a href="https://herbsutter.com/elements-of-modern-c-style/">https://herbsutter.com/elements-of-modern-c-style/</a>
  </dl>