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  <title>P1950R2: &lt;code>indirect_value&lt;/code>: A Free-Store-Allocated Value Type For C++</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) " ";   }
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 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
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 * Outdated warning for old specs
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 *     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                                    */
/******************************************************************************/

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/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

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	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

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/** Back to Top / ToC Toggle **************************************************/

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		/* See also Overflow section at the bottom */

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/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

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	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#profile-and-date {
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/** Section divider ***********************************************************/

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	:not(.head) > hr::before {
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/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

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	dl dd {
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	.head dd + dd { /* compact for header */
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	/* Put nice boxes around each algorithm. */
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	 border: thin solid var(--algo-border);
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	 margin: .5em calc(-0.5em - 1px);
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	/* Style for switch/case <dl>s */
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	margin-left: -2em;
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	dl.switch {
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	dl.switch > dt {
	text-indent: -1.5em;
	margin-top: 1em;
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	dl.switch > dt + dt {
	margin-top: 0;
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	dl.switch > dt::before {
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	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;
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	dt dfn code, code.idl {
		font-size: inherit;
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	dfn var {
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/** Change Marking ************************************************************/

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
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	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
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	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
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	.amendment del, .correction del, .addition del,
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	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
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/** Miscellaneous improvements to inline formatting ***************************/

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		font-size: 80%
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/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

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	pre {
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		overflow: auto;
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/** Inline Code fragments *****************************************************/

	/* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

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	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
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	/* Backout above styling for W3C logo */
	.head .logo,
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		text-decoration: none;
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/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
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	/* For autogen numbers, add
	  .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

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	.caption, figcaption, caption {
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	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em; }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

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		border-right-style: solid;
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	.amendment > p:first-child,
	.correction > p:first-child,
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	blockquote > :last-child,
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	.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
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		padding-right: 1em;
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	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
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/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
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/** Open issue ****************************************************************/

	.issue {
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		background: #fbe9e9;
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		overflow: auto;
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	.issue::before, .issue > .marker {
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		color: var(--issueheading-text);
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	/* 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;
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		background: #fcfaee;
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	/* 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 {
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		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
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		overflow: auto;
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	.note::before, .note > .marker,
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		color: var(--noteheading-text);
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	/* 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);
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/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
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		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;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom: 3px solid #054572;
			border-bottom: 3px solid var(--toclink-underline);
			margin-bottom: -3px;
		}
		#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 b0de3432c, updated Thu Oct 13 09:16:59 2022 -0700" name="generator">
  <link href="https://wg21.link/P1950" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1950R2<br><code class="highlight"><c- n>indirect_value</c-></code>: A Free-Store-Allocated Value Type For C++</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="2022-10-15">2022-10-15</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P1950">https://wg21.link/P1950</a>
     <dt>Issue Tracking:
     <dd><a href="https://github.com/jbcoe/indirect_value/issues">GitHub</a>
     <dt class="editor">Authors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jonathanbcoe@gmail.com">Jonathan B. Coe</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ant.peacock@gmail.com">Antony Peacock</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/jbcoe/indirect_value/blob/main/documentation/p1950.md">https://github.com/jbcoe/indirect_value/blob/main/documentation/p1950.md</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </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="#abstract"><span class="secno">1</span> <span class="content">Abstract</span></a>
    <li><a href="#change-history"><span class="secno">2</span> <span class="content">Change history</span></a>
    <li>
     <a href="#introduction"><span class="secno">3</span> <span class="content">Introduction</span></a>
     <ol class="toc">
      <li>
       <a href="#motivation"><span class="secno">3.1</span> <span class="content">Motivation</span></a>
       <ol class="toc">
        <li><a href="#node-based-containers"><span class="secno">3.1.1</span> <span class="content">Node-based containers</span></a>
        <li><a href="#hot-cold-splitting"><span class="secno">3.1.2</span> <span class="content">Hot-cold splitting</span></a>
        <li>
         <a href="#pointer-to-implementation---pimpl"><span class="secno">3.1.3</span> <span class="content">Pointer to Implementation - PImpl</span></a>
         <ol class="toc">
          <li><a href="#issues-with-const-propagation"><span class="secno">3.1.3.1</span> <span class="content">Issues with const-propagation</span></a>
          <li><a href="#issues-with-copies"><span class="secno">3.1.3.2</span> <span class="content">Issues with copies</span></a>
          <li><a href="#an-indirect_value-implementation-of-the-pimpl-idiom"><span class="secno">3.1.3.3</span> <span class="content">An <code class="highlight"><c- n>indirect_value</c-></code> implementation of the PImpl Idiom</span></a>
         </ol>
       </ol>
      <li>
       <a href="#design-decisions"><span class="secno">3.2</span> <span class="content">Design decisions</span></a>
       <ol class="toc">
        <li><a href="#should-there-be-be-an-empty-state"><span class="secno">3.2.1</span> <span class="content">Should there be be an empty state?</span></a>
        <li><a href="#how-are-allocators-supported"><span class="secno">3.2.2</span> <span class="content">How are allocators supported?</span></a>
        <li><a href="#is-there-a-small-buffer-object-optimization"><span class="secno">3.2.3</span> <span class="content">Is there a small-buffer object optimization?</span></a>
       </ol>
      <li><a href="#prior-art"><span class="secno">3.3</span> <span class="content">Prior Art</span></a>
      <li><a href="#completeness-of-t"><span class="secno">3.4</span> <span class="content">Completeness of T</span></a>
      <li><a href="#comparison-with-unique_ptrt-and-polymorphic_valuet"><span class="secno">3.5</span> <span class="content">Comparison with <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> and <code class="highlight"><c- n>polymorphic_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code></span></a>
      <li><a href="#impact-on-the-standard"><span class="secno">3.6</span> <span class="content">Impact on the standard</span></a>
     </ol>
    <li>
     <a href="#technical-specifications"><span class="secno">4</span> <span class="content">Technical specifications</span></a>
     <ol class="toc">
      <li><a href="#additions-in-memorysyn-2022"><span class="secno">4.1</span> <span class="content">Additions in [memory.syn] 20.2.2:</span></a>
      <li><a href="#xx-class-template-copier_traits-copiertraits"><span class="secno">4.2</span> <span class="content">X.X Class template <code class="highlight"><c- n>copier_traits</c-></code> [copier.traits]</span></a>
      <li><a href="#xy-class-template-default_copy-defaultcopy"><span class="secno">4.3</span> <span class="content">X.Y Class template <code class="highlight"><c- n>default_copy</c-></code> [default.copy]</span></a>
      <li>
       <a href="#xz-class-template-indirect_value-indirect_value"><span class="secno">4.4</span> <span class="content">X.Z Class template <code class="highlight"><c- n>indirect_value</c-></code> [indirect_value]</span></a>
       <ol class="toc">
        <li><a href="#xz1-class-template-indirect_value-general-indirect_valuegeneral"><span class="secno">4.4.1</span> <span class="content">X.Z.1 Class template <code class="highlight"><c- n>indirect_value</c-></code> general [indirect_value.general]</span></a>
        <li><a href="#xz2-class-template-indirect_value-synopsis-indirect_valuesynopsis"><span class="secno">4.4.2</span> <span class="content">X.Z.2 Class template <code class="highlight"><c- n>indirect_value</c-></code> synopsis [indirect_value.synopsis]</span></a>
        <li><a href="#xz3-class-template-indirect_value-constructors-indirect_valuector"><span class="secno">4.4.3</span> <span class="content">X.Z.3 Class template <code class="highlight"><c- n>indirect_value</c-></code> constructors [indirect_value.ctor]</span></a>
        <li><a href="#xz4-class-template-indirect_value-destructor-indirect_valuedtor"><span class="secno">4.4.4</span> <span class="content">X.Z.4 Class template <code class="highlight"><c- n>indirect_value</c-></code> destructor [indirect_value.dtor]</span></a>
        <li><a href="#xz5-class-template-indirect_value-assignment-indirect_valueassignment"><span class="secno">4.4.5</span> <span class="content">X.Z.5 Class template <code class="highlight"><c- n>indirect_value</c-></code> assignment [indirect_value.assignment]</span></a>
        <li><a href="#xz6-class-template-indirect_value-modifiers-indirect_valuemodifiers"><span class="secno">4.4.6</span> <span class="content">X.Z.6 Class template <code class="highlight"><c- n>indirect_value</c-></code> modifiers [indirect_value.modifiers]</span></a>
        <li><a href="#xz7-class-template-indirect_value-observers-indirect_valueobservers"><span class="secno">4.4.7</span> <span class="content">X.Z.7 Class template <code class="highlight"><c- n>indirect_value</c-></code> observers [indirect_value.observers]</span></a>
        <li><a href="#xz8-class-template-indirect_value-creation-indirect_valuecreation"><span class="secno">4.4.8</span> <span class="content">X.Z.8 Class template <code class="highlight"><c- n>indirect_value</c-></code> creation [indirect_value.creation]</span></a>
       </ol>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">5</span> <span class="content">Acknowledgements</span></a>
    <li><a href="#references"><span class="secno">6</span> <span class="content">References</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="abstract"><span class="secno">1. </span><span class="content">Abstract</span><a class="self-link" href="#abstract"></a></h2>
    Add a class template, <code class="highlight"><c- n>indirect_value</c-></code>, to the C++ Standard Library to support free-store-allocated objects with value-like semantics. 
   <h2 class="heading settled" data-level="2" id="change-history"><span class="secno">2. </span><span class="content">Change history</span><a class="self-link" href="#change-history"></a></h2>
   <p>Changes in P1950r2</p>
   <ul>
    <li data-md>
     <p>Wording fixes.</p>
    <li data-md>
     <p>Add allocator support.</p>
    <li data-md>
     <p><code class="highlight"><c- k>constexpr</c-></code> support.</p>
   </ul>
   <p>Changes in P1950r1</p>
   <ul>
    <li data-md>
     <p>Add design discussion.</p>
    <li data-md>
     <p>Add comparison with <code class="highlight"><c- n>unique_ptr</c-></code> and <code class="highlight"><c- n>polymorphic_value</c-></code>.</p>
    <li data-md>
     <p>Add node-based container and hot-cold splitting as further motivational examples.</p>
    <li data-md>
     <p>Add examples of similar classes already in use.</p>
   </ul>
   <h2 class="heading settled" data-level="3" id="introduction"><span class="secno">3. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
    The class template, <code class="highlight"><c- n>indirect_value</c-></code>, confers value-like semantics on a free-store-allocated object. An <code class="highlight"><c- n>indirect_value</c-></code> may hold an object of a class T, copying the <code class="highlight"><c- n>indirect_value</c-></code> will copy the object <code class="highlight"><c- n>T</c-></code>. When a parent object contains a member of type <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> and
is accessed through a const access path, <code class="highlight"><c- k>const</c-></code>ness will propagate from the parent object to the instance of <code class="highlight"><c- n>T</c-></code> owned by the <code class="highlight"><c- n>indirect_value</c-></code> member. 
   <h3 class="heading settled" data-level="3.1" id="motivation"><span class="secno">3.1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h3>
   <p>It may be desirable for a class member to be an incomplete type or for the storage used by a member object to be separate from the class itself.
In both such cases, the member would traditionally be represented as pointer:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>MyClass</c-> <c- p>{</c->
    <c- n>AType</c-> <c- n>data_member_</c-><c- p>;</c->
    <c- n>AnotherType</c-><c- o>*</c-> <c- n>indirect_data_member_</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>The author of such a class will need to implement special member functions as the compiler-generated special member functions will
not function correctly (indirect data will not be copied, assigned to or deleted, only the pointer).</p>
   <p>Special care will be needed when using the class in multithreaded environments. An instance of <code class="highlight"><c- n>MyClass</c-></code> accessed through a const-access-path
will not propagate the <code class="highlight"><c- k>const</c-></code>-ness to the indirect data (only to the pointer itself) as pointers do not propagate <code class="highlight"><c- k>const</c-></code> to their pointees.</p>
   <p>The class template <code class="highlight"><c- n>indirect_value</c-></code> is a drop-in replacement for pointer members in cases where non-polymorphic data referenced by the
pointer member is logically part of the class. Use of <code class="highlight"><c- n>indirect_value</c-></code> will ensure that the compiler-generated special member functions
behave correctly and that <code class="highlight"><c- k>const</c-></code> is propagated to indirect data.</p>
   <h4 class="heading settled" data-level="3.1.1" id="node-based-containers"><span class="secno">3.1.1. </span><span class="content">Node-based containers</span><a class="self-link" href="#node-based-containers"></a></h4>
   <p>A tree or linked list can be implemented as a node-based container. This gives stable references to data in the nodes
at the cost of memory indirection when accessing data.</p>
   <p>Nodes would typically contain data and pointer(s) to other nodes so that the list or tree can be navigated:</p>
<pre class="language-~cpp highlight"><c- k>class</c-> <c- nc>ForwardListNode</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>data_</c-><c- p>;</c->
    <c- n>ForwardListNode</c-><c- o>*</c-> <c- n>next_</c-><c- p>;</c->
<c- p>};</c->

<c- k>class</c-> <c- nc>ForwardList</c-> <c- p>{</c->
    <c- n>ForwardListNode</c-><c- o>*</c-> <c- n>head_</c-><c- p>;</c->
<c- k>public</c-><c- o>:</c->
    <c- c1>// ...</c->
<c- p>};</c->
</pre>
   <p>The special member functions of <code class="highlight"><c- n>ForwardList</c-></code> would need to be user-implemented as the
compiler-generated versions would not copy, move or delete the nodes correctly.</p>
   <p>Care must be taken when implementing <code class="highlight"><c- k>const</c-></code>-qualified member functions as <code class="highlight"><c- k>const</c-></code> will not propagate down
from the <code class="highlight"><c- n>ForwardList</c-></code> to the <code class="highlight"><c- n>ForwardListNode</c-></code>s nor to the data that the nodes contain.</p>
   <p>Implementing the ForwardList and ForwardListNode with <code class="highlight"><c- n>indirect_value</c-></code> corrects these issues:</p>
<pre class="language-~cpp highlight"><c- k>class</c-> <c- nc>ForwardListNode</c-> <c- p>{</c->
    <c- b>int</c-> <c- n>data_</c-><c- p>;</c->
    <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>ForwardListNode</c-><c- o>></c-> <c- n>next_</c-><c- p>;</c->
<c- p>};</c->

<c- k>class</c-> <c- nc>ForwardList</c-> <c- p>{</c->
    <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>ForwardListNode</c-><c- o>></c-> <c- n>head_</c-><c- p>;</c->
<c- k>public</c-><c- o>:</c->
    <c- c1>// ...</c->
<c- p>};</c->
</pre>
   <p>Compiler-generated special member functions will behave correctly and do not need to be written manually. <code class="highlight"><c- k>const</c-></code>-propagation will allow the compiler to ensure that data that is logically part of the container
cannot be modified through a <code class="highlight"><c- k>const</c-></code> access path.</p>
   <h4 class="heading settled" data-level="3.1.2" id="hot-cold-splitting"><span class="secno">3.1.2. </span><span class="content">Hot-cold splitting</span><a class="self-link" href="#hot-cold-splitting"></a></h4>
   <p>When working with collections of data, CPU caches are often under-utilized when algorithms access certain data members
much more frequently than others.  This results in some cache lines becoming busier than others, prefetchers
preemptively caching memory which is later evicted without being used by the CPU or memory bandwidth becoming a
limiting factor in performance.  In such cases segregating data structures in terms of hot and cold data can remove
 pressure on system resources.</p>
<pre class="language-~cpp highlight"><c- k>class</c-> <c- nc>Element</c-> <c- p>{</c->
  <c- n>SmallData</c-> <c- n>frequently_accessed_data</c-><c- p>;</c->
  <c- n>LargeData</c-> <c- n>infrequently_accessed_data</c-><c- p>;</c->
<c- p>};</c->

<c- n>vector</c-><c- o>&lt;</c-><c- n>Element</c-><c- o>></c-> <c- n>elements</c-><c- p>;</c->
<c- k>auto</c-> <c- n>active</c-> <c- o>=</c-> <c- n>find_if</c-><c- p>(</c-><c- n>elements</c-><c- p>.</c-><c- n>begin</c-><c- p>(),</c->
                      <c- n>elements</c-><c- p>.</c-><c- n>end</c-><c- p>(),</c->
                      <c- p>[](</c-><c- k>const</c-> <c- k>auto</c-><c- o>&amp;</c-> <c- n>e</c-><c- p>)</c->
                      <c- p>{</c->
                        <c- k>return</c-> <c- n>e</c-><c- p>.</c-><c- n>frequently_accessed_data</c-><c- p>.</c-><c- n>active</c-><c- p>();</c->
                      <c- p>});</c->
</pre>
   <p>In such cases adding a level of indirection to the larger, less frequently-accessed data can relieve bandwidth
pressure [C. Ericson].  Using <code class="highlight"><c- n>indirect_value</c-></code> in this refactoring does not change guarantees around <code class="highlight"><c- k>const</c-></code>ness. Compiler-generated special member functions will behave correctly and do not need to be manually implemented.</p>
<pre class="language-~cpp highlight"><c- k>class</c-> <c- nc>Element</c-> <c- p>{</c->
  <c- n>SmallData</c-> <c- n>frequently_accessed_data</c-><c- p>;</c->
  <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>LargeData</c-><c- o>></c-> <c- n>infrequently_accessed_data</c-><c- p>;</c->
<c- p>};</c->

<c- n>vector</c-><c- o>&lt;</c-><c- n>Element</c-><c- o>></c-> <c- n>elements</c-><c- p>;</c->
<c- k>auto</c-> <c- n>active</c-> <c- o>=</c-> <c- n>find_if</c-><c- p>(</c-><c- n>elements</c-><c- p>.</c-><c- n>begin</c-><c- p>(),</c->
                      <c- n>elements</c-><c- p>.</c-><c- n>end</c-><c- p>(),</c->
                      <c- p>[](</c-><c- k>const</c-> <c- k>auto</c-><c- o>&amp;</c-> <c- n>e</c-><c- p>)</c->
                      <c- p>{</c->
                        <c- k>return</c-> <c- n>e</c-><c- p>.</c-><c- n>frequently_accessed_data</c-><c- p>.</c-><c- n>active</c-><c- p>();</c->
                      <c- p>});</c->
</pre>
   <h4 class="heading settled" data-level="3.1.3" id="pointer-to-implementation---pimpl"><span class="secno">3.1.3. </span><span class="content">Pointer to Implementation - PImpl</span><a class="self-link" href="#pointer-to-implementation---pimpl"></a></h4>
   <p>In C++, when anything in a class definition changes, dependent classes require recompilation. As early as 1992 the
Handle/Body idiom was suggested to break this dependency [J. Coplien]. From this pattern, the PImpl idiom was
specialised [H. Sutter].  Despite its relative maturity, using the PImpl idiom requires careful thought about copying
and <code class="highlight"><c- k>const</c-></code>-propagation.</p>
<pre class="language-~cpp highlight"><c- c1>// Header file</c->
<c- k>class</c-> <c- nc>widget</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- n>widget</c-><c- p>();</c->
    <c- o>~</c-><c- n>widget</c-><c- p>();</c->
<c- k>private</c-><c- o>:</c->
    <c- k>class</c-> <c- nc>impl</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>impl</c-><c- o>></c-> <c- n>pimpl_</c-><c- p>;</c->
<c- p>};</c->
</pre>
<pre class="language-~cpp highlight"><c- c1>// Implementation file</c->
<c- k>class</c-> <c- nc>widget</c-><c- o>::</c-><c- n>impl</c-> <c- p>{</c->
    <c- c1>// :::</c->
<c- p>};</c->

<c- n>widget</c-><c- o>::</c-><c- n>widget</c-><c- p>()</c-> <c- o>:</c-> <c- n>pimpl_</c-><c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>make_unique</c-><c- o>&lt;</c-><c- n>impl</c-><c- o>></c-><c- p>(</c-> <c- d>/*...*/</c->  <c- p>}</c-> <c- p>{</c-> <c- p>}</c->
<c- c1>// Destructor needs to be defined in a the same TU as &lt;code data-opaque bs-autolink-syntax='`widget::impl`'>widget::impl&lt;/code>.</c->
<c- n>widget</c-><c- o>::~</c-><c- n>widget</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
</pre>
   <p>For convenience, the widget class will be referred to as the “visible class” and impl class the “<code class="highlight"><c- n>PImpl</c-></code> class”. Note, semantically the <code class="highlight"><c- n>PImpl</c-></code> class is the implementation details of the visible class.</p>
   <h5 class="heading settled" data-level="3.1.3.1" id="issues-with-const-propagation"><span class="secno">3.1.3.1. </span><span class="content">Issues with const-propagation</span><a class="self-link" href="#issues-with-const-propagation"></a></h5>
    Using <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code> to store the implementation object introduces an issue - within a <code class="highlight"><c- k>const</c-></code>-qualified
 member function, an instance of the visible class can mutate data inside the implementation object.  This is because <code class="highlight"><c- k>const</c-></code>-qualification applies only to the <code class="highlight"><c- n>unique_ptr</c-></code> value, and not the pointed-to-object. 
   <p>The compiler is unable to make thread-compatibility assumptions for <code class="highlight"><c- k>const</c-></code> objects when <code class="highlight"><c- k>const</c-></code> does not propagate: <code class="highlight"><c- k>const</c-></code> does not mean immutable in the face of pointer-like-member data.</p>
   <p>The desired semantics of a PImpl-owner are value-like, like those of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> which has appropriate <code class="highlight"><c- k>const</c-></code> and non-<code class="highlight"><c- k>const</c-></code>-qualified overloads for <code class="highlight"><c- k>operator</c-><c- o>*</c-></code> and <code class="highlight"><c- k>operator</c-><c- o>-></c-></code>.</p>
   <h5 class="heading settled" data-level="3.1.3.2" id="issues-with-copies"><span class="secno">3.1.3.2. </span><span class="content">Issues with copies</span><a class="self-link" href="#issues-with-copies"></a></h5>
    The copy-constructor and copy-assignment operator of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code> are deleted. Users of a class with a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code> member will be
required to implement copy-construction and copy-assignment [S. Meyers]. 
   <h5 class="heading settled" data-level="3.1.3.3" id="an-indirect_value-implementation-of-the-pimpl-idiom"><span class="secno">3.1.3.3. </span><span class="content">An <code class="highlight"><c- n>indirect_value</c-></code> implementation of the PImpl Idiom</span><a class="self-link" href="#an-indirect_value-implementation-of-the-pimpl-idiom"></a></h5>
    Using <code class="highlight"><c- n>indirect_value</c-></code> to implement the PImpl idiom ensures that the PImpl object is <code class="highlight"><c- k>const</c-></code> when accessed through a <code class="highlight"><c- k>const</c-></code> access path and that compiler-generated special member functions will behave correctly and do not need to be manually implemented. 
<pre class="language-~cpp highlight"><c- c1>// Header file</c->
<c- k>class</c-> <c- nc>widget</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- n>widget</c-><c- p>();</c->
    <c- n>widget</c-><c- p>(</c-><c- n>widget</c-><c- o>&amp;&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- n>widget</c-><c- p>(</c-><c- k>const</c-> <c- n>widget</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>);</c->
    <c- n>widget</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>widget</c-><c- o>&amp;&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- n>widget</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>widget</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>);</c->
    <c- o>~</c-><c- n>widget</c-><c- p>();</c->
<c- k>private</c-><c- o>:</c->
    <c- k>class</c-> <c- nc>impl</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>impl</c-><c- o>>></c-> <c- n>pimpl</c-><c- p>;</c->
<c- p>};</c->
</pre>
<pre class="language-~cpp highlight"><c- c1>// Implementation file</c->
<c- k>class</c-> <c- nc>widget</c-><c- o>::</c-><c- n>impl</c-> <c- p>{</c->
    <c- c1>// :::</c->
<c- p>};</c->

<c- c1>// Special-member functions need to be defined in a the same TU as &lt;code data-opaque bs-autolink-syntax='`widget::impl`'>widget::impl&lt;/code>.</c->
<c- n>widget</c-><c- o>::</c-><c- n>widget</c-><c- p>()</c-> <c- o>:</c-> <c- n>pimpl</c-><c- p>(</c-><c- k>new</c-> <c- n>impl</c-><c- p>)</c-> <c- p>{}</c->
<c- n>widget</c-><c- o>::</c-><c- n>widget</c-><c- p>(</c-><c- n>widget</c-><c- o>&amp;&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
<c- n>widget</c-><c- o>::</c-><c- n>widget</c-><c- p>(</c-><c- k>const</c-> <c- n>widget</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
<c- n>widget</c-><c- o>&amp;</c-> <c- n>widget</c-><c- o>::</c-><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>widget</c-><c- o>&amp;&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
<c- n>widget</c-><c- o>&amp;</c-> <c- n>widget</c-><c- o>::</c-><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>widget</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
<c- n>widget</c-><c- o>::~</c-><c- n>widget</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
</pre>
   <h3 class="heading settled" data-level="3.2" id="design-decisions"><span class="secno">3.2. </span><span class="content">Design decisions</span><a class="self-link" href="#design-decisions"></a></h3>
   <h4 class="heading settled" data-level="3.2.1" id="should-there-be-be-an-empty-state"><span class="secno">3.2.1. </span><span class="content">Should there be be an empty state?</span><a class="self-link" href="#should-there-be-be-an-empty-state"></a></h4>
   <p>There is a design decision to be made about the default constructor of <code class="highlight"><c- n>indirect_value</c-></code>.
It could place <code class="highlight"><c- n>indirect_value</c-></code> in an empty state with no owned object or it could 
default construct an owned object so that <code class="highlight"><c- n>indirect_value</c-></code> is never empty. This would
either add a requirement that <code class="highlight"><c- n>T</c-></code> supports default construction or only support default
construction conditionally when also supported by <code class="highlight"><c- n>T</c-></code>.</p>
   <p>If <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> is never empty then <code class="highlight"><c- n>T</c-></code> must be default constructible for <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> to be a regular type. The default constructor of <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> would then need to allocate memory, which cannot be controlled by the user-supplied
copier or deleter.</p>
   <p>If the default constructed state of <code class="highlight"><c- n>indirect_value</c-></code> is non-empty then the moved from
state should be similarly non-empty. This requires that <code class="highlight"><c- n>T</c-></code> is noexcept move constructible.</p>
   <p>Allowing an empty state for <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> imposes preconditions on operator* and operator->
(similar to those of the raw pointer it may be replacing) but removes requirements on <code class="highlight"><c- n>T</c-></code> and the need
to allocate memory.</p>
   <p>A nullable <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> could be mimicked with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-></code> but this is verbose, does not solve the problem of <code class="highlight"><c- n>T</c-></code> needing to be noexcept 
move-constructible and would require partial template specialization of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> to avoid the overhead that would be incurred by a nullable <code class="highlight"><c- n>indirect_value</c-></code>.  But perhaps
the most unfortunate side effect of this approach is that to access the underlying <code class="highlight"><c- n>T</c-></code> the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> must be dereferenced to get to the <code class="highlight"><c- n>indirect_value</c-></code>, which then requires
a second dereference to get to the underlying <code class="highlight"><c- n>T</c-></code>.  This pattern makes usage unwieldy.</p>
   <p>As designed, <code class="highlight"><c- n>indirect_value</c-></code> has an empty state where it owns no object.
A default constructed <code class="highlight"><c- n>indirect_value</c-></code> is empty and the empty state is the state of a moved-from object.</p>
   <h4 class="heading settled" data-level="3.2.2" id="how-are-allocators-supported"><span class="secno">3.2.2. </span><span class="content">How are allocators supported?</span><a class="self-link" href="#how-are-allocators-supported"></a></h4>
   <p>It may be desirable for users to control the way memory is managed by an instance of a class.
Classes such as <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-></code> allow an allocator to be specified as a template argument
to control memory management. There is existing precedence for allocator support of types in the memory
management library; <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>shared_ptr</c-></code> supports allocators via <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocate_shared</c-></code>, [M. Knejp] suggested
the introduction of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocate_unique</c-></code> to support allocator use with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code>.</p>
   <p>Like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code>, <code class="highlight"><c- n>indirect_value</c-></code> allows a deleter to be provided as a template argument
so that disposal of the owned object can be customized. <code class="highlight"><c- n>indirect_value</c-></code> also allows copying to be
customized by specifying a copier, another template argument.  However, this raises the question of if the 
copier and deleter should be combined types. If not combined then both must hold either a reference to an
allocator (doubling the storage requirements) or, if encapsulating the allocator, then one must own the
allocator while the second holds a reference.  While combining the interfaces into one type would solve this
it would reduce the usage with custom lambdas without specialising for a combined object to be created 
from the two copier and deleter lambda via something akin to the overload pattern. This issue is not 
addressed further, instead opting to keep to the pre-allocator design, but the authors welcome feedback here.</p>
   <p>Memory management for <code class="highlight"><c- n>indirect_value</c-></code> can be fully controlled via the <code class="highlight"><c- n>allocate_indirect_value</c-></code> function
which allows passing in an allocator to control the source of memory.  Custom copier and deleter then use
the allocator for allocations and deallocations.</p>
   <h4 class="heading settled" data-level="3.2.3" id="is-there-a-small-buffer-object-optimization"><span class="secno">3.2.3. </span><span class="content">Is there a small-buffer object optimization?</span><a class="self-link" href="#is-there-a-small-buffer-object-optimization"></a></h4>
   <p>Previous revisions of the paper suggested this was permissible as a quality of implementation improvement.
However, this revision has introduced allocator support, which offers an alternative mechanism for 
supporting the small-buffer optimization.  An allocator, supporting small buffer optimization, can be
provided. This allows achieving the same results while separating the details of the small buffer 
optimization from <code class="highlight"><c- n>indirect_value</c-></code>.</p>
   <h3 class="heading settled" data-level="3.3" id="prior-art"><span class="secno">3.3. </span><span class="content">Prior Art</span><a class="self-link" href="#prior-art"></a></h3>
    There have been previous proposal for deep-copying smart pointers that proposed copy semantics [W. Brown]. <code class="highlight"><c- n>cloned_ptr</c-></code> was proposed in [J. Coe], however under guidance of LEWG this was renamed to <code class="highlight"><c- n>polymorphic_value</c-></code>.  With this change in name came the addition of const propagation. 
   <p>This paper is not unique in these ideas. GitHub code search finds 602k lines of code referencing "PImpl" and 99 C
++ repositories claiming to provide generic implementations of Pimpl.  Google Carbon Language, an experimental successor to C++ includes <code class="highlight"><c- n>IndirectValue</c-></code> as a vocabulary type. Additionally other authors have addressed this
 topic [A. Upadyshev].  Some generic implementations of note in the wild are:</p>
   <table>
    <thead>
     <tr>
      <th>Project
      <th>Link
      <th>Deep Copying
      <th>Const Propagation
    <tbody>
     <tr>
      <td>Carbon Language
      <td>https://github.com/carbon-language/carbon-lang/blob/trunk/common/indirect_value.h
      <td>Yes
      <td>Yes
     <tr>
      <td>Boost Pimpl
      <td>https://github.com/sean-/boost-pimpl
      <td>Yes
      <td>Yes
     <tr>
      <td>pimpl
      <td>https://github.com/JonChesterfield/pimpl
      <td>Yes
      <td>No
     <tr>
      <td>impl_ptr
      <td>https://github.com/sth/impl_ptr
      <td>Yes
      <td>Yes
     <tr>
      <td>Simpl
      <td>https://github.com/oliora/samples/blob/master/spimpl.h
      <td>Yes
      <td>Yes
     <tr>
      <td>smart_pimpl
      <td>https://github.com/SBreezyx/smart_pimpl
      <td>No
      <td>No
     <tr>
      <td>pimpl_on_stack
      <td>https://github.com/kuznetsss/pimpl_on_stack/
      <td>Yes
      <td>Yes
     <tr>
      <td>deep_const_ptr
      <td>https://github.com/torbjoernk/deep_const_ptr
      <td>No
      <td>Yes
   </table>
   <p>Divergence on issues such as deep copying and const propagation along with subtleties in implementation
mean that a single standardized solution would be of broad benefit to the community of C++ users.</p>
   <h3 class="heading settled" data-level="3.4" id="completeness-of-t"><span class="secno">3.4. </span><span class="content">Completeness of T</span><a class="self-link" href="#completeness-of-t"></a></h3>
    Smart pointer types in the Standard Library expect that some of the members can be instantiated with incomplete types
[H.Hinnant]. Similarly, this is the case for <code class="highlight"><c- n>indirect_value</c-></code>, the table outlines the expected behaviour for incomplete
pointer types: 
   <table>
    <thead>
     <tr>
      <th>Function
      <th>Description
      <th>Incomplete/Complete
    <tbody>
     <tr>
      <td><code class="highlight"><c- n>indirect_value</c-><c- p>()</c-></code>
      <td>Default constructor
      <td>Incomplete
     <tr>
      <td><code class="highlight"><c- n>indirect_value</c-><c- p>(</c-><c- k>const</c-> <c- n>indirect_value</c-><c- o>&amp;</c-><c- p>)</c-></code>
      <td>Copy-constructor
      <td>Complete
     <tr>
      <td><code class="highlight"><c- n>indirect_value</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&amp;&amp;</c-><c- p>)</c-></code>
      <td>Move-constructor
      <td>Incomplete
     <tr>
      <td><code class="highlight"><c- o>~</c-><c- n>indirect_value</c-><c- p>()</c-></code>
      <td>Destructor
      <td>Complete
     <tr>
      <td><code class="highlight"><c- n>indirect_value</c-><c- o>&amp;</c-> <c- n>indirect_value</c-><c- o>::</c-><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>indirect_value</c-><c- o>&amp;</c-><c- p>)</c-></code>
      <td>Copy-assignment
      <td>Complete
     <tr>
      <td><code class="highlight"><c- n>indirect_value</c-><c- o>&amp;</c-> <c- n>indirect_value</c-><c- o>::</c-><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&amp;&amp;</c-><c- p>)</c-></code>
      <td>Move-assignment
      <td>Complete
     <tr>
      <td><code class="highlight"><c- n>T</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-></code>
      <td>Indirection-operator
      <td>Incomplete 
     <tr>
      <td><code class="highlight"><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-></code>
      <td>Indirection-operator
      <td>Incomplete 
     <tr>
      <td><code class="highlight"><c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>noexcept</c-></code>
      <td>Member-of-pointer-operator 
      <td>Incomplete 
     <tr>
      <td><code class="highlight"><c- k>const</c-> <c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-></code>
      <td>Member-of-pointer-operator 
      <td>Incomplete 
     <tr>
      <td><code class="highlight"><c- k>explicit</c-> <c- k>operator</c-> <c- b>bool</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-></code>
      <td>Bool-operator
      <td>Incomplete
     <tr>
      <td><code class="highlight"><c- b>void</c-> <c- n>swap</c-><c- p>(</c-><c- n>indirect_value</c-><t><c- o>&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-></t></code>
      <td>Swap
      <td>Incomplete
   </table>
   <h3 class="heading settled" data-level="3.5" id="comparison-with-unique_ptrt-and-polymorphic_valuet"><span class="secno">3.5. </span><span class="content">Comparison with <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> and <code class="highlight"><c- n>polymorphic_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code></span><a class="self-link" href="#comparison-with-unique_ptrt-and-polymorphic_valuet"></a></h3>
   <p>The class template <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> is in many respects similar to <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> and the proposed <code class="highlight"><c- n>polymorphic_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> [J. Coe]. The table below highlights the similarities and differences:</p>
   <table>
    <thead>
     <tr>
      <th>Behaviour
      <th><code class="highlight"><c- n>indirect_value</c-></code>
      <th><code class="highlight"><c- n>polymorphic_value</c-></code>
      <th><code class="highlight"><c- n>unique_ptr</c-></code>
    <tbody>
     <tr>
      <td>Default constructed state
      <td>Empty
      <td>Empty
      <td>Empty
     <tr>
      <td>Copyable
      <td>Yes
      <td>Yes
      <td>No
     <tr>
      <td>Const-propagating
      <td>Yes
      <td>Yes
      <td>No
     <tr>
      <td>Polymorphic
      <td>No
      <td>Yes
      <td>Yes
     <tr>
      <td>Customizable memory access
      <td>Yes
      <td>No
      <td>Yes
   </table>
   <h3 class="heading settled" data-level="3.6" id="impact-on-the-standard"><span class="secno">3.6. </span><span class="content">Impact on the standard</span><a class="self-link" href="#impact-on-the-standard"></a></h3>
    This proposal is a pure library extension. It requires additions to be made to the standard library header <code class="highlight"><c- o>&lt;</c-><c- n>memory</c-><c- o>></c-></code>. 
   <h2 class="heading settled" data-level="4" id="technical-specifications"><span class="secno">4. </span><span class="content">Technical specifications</span><a class="self-link" href="#technical-specifications"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="additions-in-memorysyn-2022"><span class="secno">4.1. </span><span class="content">Additions in [memory.syn] 20.2.2:</span><a class="self-link" href="#additions-in-memorysyn-2022"></a></h3>
<pre class="highlight"><c- c1>// [indirect.value], class template indirect_value</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>default_copy</c-><c- p>;</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>copier_traits</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>C</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>default_copy</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>class</c-> <c- nc>D</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>copier_traits</c-><c- o>&lt;</c-><c- n>C</c-><c- o>>::</c-><c- n>deleter_type</c-><c- o>></c-> <c- k>class</c-> <c- nc>indirect_value</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- p>...</c-><c- n>Ts</c-><c- o>></c-> 
  <c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>make_indirect_value</c-><c- p>(</c-><c- n>Ts</c-><c- o>&amp;&amp;</c-> <c- p>...</c-><c- n>ts</c-><c- p>);</c-> 

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>A</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>allocator</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>allocate_indirect_value</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>allocator_arg_t</c-><c- p>,</c-> <c- n>A</c-><c- o>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- n>Ts</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>ts</c-><c- p>);</c-> 

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>void</c-> <c- n>swap</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>p</c-><c- p>,</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>u</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <h3 class="heading settled" data-level="4.2" id="xx-class-template-copier_traits-copiertraits"><span class="secno">4.2. </span><span class="content">X.X Class template <code class="highlight"><c- n>copier_traits</c-></code> [copier.traits]</span><a class="self-link" href="#xx-class-template-copier_traits-copiertraits"></a></h3>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</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>copier_traits</c-> <c- p>{</c->
        <c- k>using</c-> <c- n>deleter_type</c-> <c- o>=</c-> <c- o>*</c-><c- n>see</c-> <c- n>below</c-><c- o>*</c-><c- p>;</c->
    <c- p>};</c->
<c- p>}</c->
</pre>
   <code class="highlight"><c- k>using</c-> <c- n>deleter_type</c-> <c- o>=</c-> <em><c- n>see</c-> <c- n>below</c-></em><c- p>;</c-></code> 
   <ul>
    <li data-md>
     <p>Type: <code class="highlight"><c- n>T</c-><c- o>::</c-><c- n>deleter_type</c-></code> if the <em>qualified-id</em> <code class="highlight"><c- n>T</c-><c- o>::</c-><c- n>deleter_type</c-></code> is valid and denotes a type;
otherwise, <code class="highlight"><c- b>void</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>U</c-><c- o>*</c-><c- p>)</c-></code> if <code class="highlight"><c- n>T</c-></code> is of the form <code class="highlight"><c- n>U</c-><c- o>*</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>V</c-><c- p>)</c-></code> for types <code class="highlight"><c- n>U</c-></code> and <code class="highlight"><c- n>V</c-></code>;
otherwise, there is no member <code class="highlight"><c- n>deleter_type</c-></code>.</p>
   </ul>
   <h3 class="heading settled" data-level="4.3" id="xy-class-template-default_copy-defaultcopy"><span class="secno">4.3. </span><span class="content">X.Y Class template <code class="highlight"><c- n>default_copy</c-></code> [default.copy]</span><a class="self-link" href="#xy-class-template-default_copy-defaultcopy"></a></h3>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</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>default_copy</c-> <c- p>{</c->
        <c- k>using</c-> <c- n>deleter_type</c-> <c- o>=</c-> <c- n>default_delete</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c->
        <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
    <c- p>};</c->
<c- p>}</c-> <c- c1>// namespace std</c->
</pre>
   The class template default_copy [J. Coe] serves as the default copier for the class template <code class="highlight"><c- n>indirect_value</c-></code>.
The template parameter <code class="highlight"><c- n>T</c-></code> of <code class="highlight"><c- n>default_copy</c-></code> may be an incomplete type. 
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- nf>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></code></p>
   <ul>
    <li data-md>
     <p>Returns: <code class="highlight"><c- k>new</c-> <c- n>T</c-><c- p>(</c-><c- n>t</c-><c- p>);</c-></code></p>
   </ul>
   <h3 class="heading settled" data-level="4.4" id="xz-class-template-indirect_value-indirect_value"><span class="secno">4.4. </span><span class="content">X.Z Class template <code class="highlight"><c- n>indirect_value</c-></code> [indirect_value]</span><a class="self-link" href="#xz-class-template-indirect_value-indirect_value"></a></h3>
   <h4 class="heading settled" data-level="4.4.1" id="xz1-class-template-indirect_value-general-indirect_valuegeneral"><span class="secno">4.4.1. </span><span class="content">X.Z.1 Class template <code class="highlight"><c- n>indirect_value</c-></code> general [indirect_value.general]</span><a class="self-link" href="#xz1-class-template-indirect_value-general-indirect_valuegeneral"></a></h4>
    An <code class="highlight"><c- n>indirect_value</c-></code> is an object that owns another object and manages that other object through a pointer. More precisely, an indirect value is an object <code class="highlight"><c- n>v</c-></code> that stores a pointer to a second object <code class="highlight"><c- n>p</c-></code> and will dispose of <code class="highlight"><c- n>p</c-></code> when <code class="highlight"><c- n>v</c-></code> is itself destroyed (e.g., when leaving block scope (9.7)). In this context, <code class="highlight"><c- n>v</c-></code> is said to own <code class="highlight"><c- n>p</c-></code>. 
   <p>An <code class="highlight"><c- n>indirect_value</c-></code> object is empty if it does not own a pointer.</p>
   <p>Copying a non-empty <code class="highlight"><c- n>indirect_value</c-></code> will copy the owned object so that the copied <code class="highlight"><c- n>indirect_value</c-></code> will have its own unique copy of the owned object.</p>
   <p>Copying from an empty <code class="highlight"><c- n>indirect_value</c-></code> produces another empty <code class="highlight"><c- n>indirect_value</c-></code>.</p>
   <p>Copying and disposal of the owned object can be customised by supplying a copier and deleter.</p>
   <p>The template parameter <code class="highlight"><c- n>T</c-></code> of <code class="highlight"><c- n>indirect_value</c-></code> must be a non-union class type.</p>
   <p>The template parameter <code class="highlight"><c- n>T</c-></code> of <code class="highlight"><c- n>indirect_value</c-></code> may be an incomplete type.</p>
   <p>A copier and deleter are said to be present if a <code class="highlight"><c- n>indirect_value</c-></code> object is constructed from a non-null pointer, or from a <code class="highlight"><c- n>indirect_value</c-></code> object where a copier and deleter are present.</p>
   <h4 class="heading settled" data-level="4.4.2" id="xz2-class-template-indirect_value-synopsis-indirect_valuesynopsis"><span class="secno">4.4.2. </span><span class="content">X.Z.2 Class template <code class="highlight"><c- n>indirect_value</c-></code> synopsis [indirect_value.synopsis]</span><a class="self-link" href="#xz2-class-template-indirect_value-synopsis-indirect_valuesynopsis"></a></h4>
<pre class="highlight">    <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>C</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>default_copy</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>class</c-> <c- nc>D</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>copier_traits</c-><c- o>&lt;</c-><c- n>C</c-><c- o>>::</c-><c- n>deleter_type</c-><c- o>></c->
    <c- k>class</c-> <c- nc>indirect_value</c-> <c- p>{</c->
    <c- k>public</c-><c- o>:</c->
        <c- k>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->

        <c- c1>// Constructors</c->
        <c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- k>explicit</c-> <c- n>indirect_value</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-><c- p>,</c-> <c- n>C</c-> <c- n>c</c-><c- o>=</c-><c- n>C</c-><c- p>{},</c-> <c- n>D</c-> <c- n>d</c-><c- o>=</c-><c- n>D</c-><c- p>{});</c->

        <c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>(</c-><c- k>const</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- n>p</c-><c- p>);</c->
        <c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

        <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- p>...</c-><c- n>Ts</c-><c- o>></c->
        <c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>in_place_t</c-><c- p>,</c-> <c- n>Ts</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>ts</c-><c- p>);</c->  <c- c1>// See below</c->

        <c- c1>// Destructor</c->
        <c- k>constexpr</c-> <c- o>~</c-><c- n>indirect_value</c-><c- p>();</c->

        <c- c1>// Assignment</c->
        <c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- n>p</c-><c- p>);</c->
        <c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

        <c- c1>// Modifiers</c->
        <c- k>constexpr</c-> <c- b>void</c-> <c- nf>swap</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

        <c- c1>// Observers</c->
        <c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>();</c->
        <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>();</c->
        <c- k>constexpr</c-> <c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- k>const</c-> <c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- k>explicit</c-> <c- k>operator</c-> <c- b>bool</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- p>};</c->

    <c- c1>// indirect_value creation</c->
    <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- p>...</c-><c- n>Ts</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>make_indirect_value</c-><c- p>(</c-><c- n>Ts</c-><c- o>&amp;&amp;</c-> <c- p>...</c-><c- n>ts</c-><c- p>);</c->  <c- c1>// See below</c->

    <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>A</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>allocator</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>allocate_indirect_value</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>allocator_arg_t</c-><c- p>,</c-> <c- n>A</c-><c- o>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- n>Ts</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>ts</c-><c- p>);</c-> 

    <c- c1>// indirect_value specialized algorithms</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
    <c- k>constexpr</c-> <c- b>void</c-> <c- n>swap</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>p</c-><c- p>,</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>u</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- p>}</c-> <c- c1>// end namespace std</c->
</pre>
   <h4 class="heading settled" data-level="4.4.3" id="xz3-class-template-indirect_value-constructors-indirect_valuector"><span class="secno">4.4.3. </span><span class="content">X.Z.3 Class template <code class="highlight"><c- n>indirect_value</c-></code> constructors [indirect_value.ctor]</span><a class="self-link" href="#xz3-class-template-indirect_value-constructors-indirect_valuector"></a></h4>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c-></code></p>
   <ul>
    <li data-md>
     <p>Effects: Constructs an empty <code class="highlight"><c- n>indirect_value</c-></code>.</p>
    <li data-md>
     <p>Postconditions: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> false</code>.</p>
   </ul>
   <p><code class="highlight"><c- k>constexpr</c-> <c- k>explicit</c-> <c- n>indirect_value</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-><c- p>,</c-> <c- n>C</c-> <c- n>c</c-><c- o>=</c-><c- n>C</c-><c- p>{},</c-> <c- n>D</c-> <c- n>d</c-><c- o>=</c-><c- n>D</c-><c- p>{});</c-></code></p>
   <ul>
    <li data-md>
     <p>Effects: Constructs a <code class="highlight"><c- n>indirect_value</c-></code> which owns p, initializing the stored pointer with p. The copier and deleter of the <code class="highlight"><c- n>indirect_value</c-></code> constructed are moved from <code class="highlight"><c- n>c</c-></code> and <code class="highlight"><c- n>d</c-></code>. If <code class="highlight"><c- n>p</c-></code> is null, creates an empty object.</p>
    <li data-md>
     <p>Constraints: <code class="highlight"><c- n>is_nothrow_move_assignable_v</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-></code> is true. <code class="highlight"><c- n>is_nothrow_move_assignable_v</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-></code> is true.</p>
    <li data-md>
     <p>Preconditions: <code class="highlight"><c- n>C</c-></code> and <code class="highlight"><c- n>D</c-></code> meet the <i>Cpp17CopyConstructible</i> requirements. If the arguments <code class="highlight"><c- n>c</c-></code> and/or <code class="highlight"><c- n>d</c-></code> are not supplied, then <code class="highlight"><c- n>C</c-></code> and/or <code class="highlight"><c- n>D</c-></code> respectively are default constructible types that are not pointer types. If <code class="highlight"><c- n>p</c-></code> is non-null then the expression <code class="highlight"><c- n>c</c-><c- p>(</c-><c- o>*</c-><c- n>p</c-><c- p>)</c-></code> returns a non-null <code class="highlight"><c- n>T</c-><c- o>*</c-></code> is as if copy constructed from *p. The expression <code class="highlight"><c- n>d</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-></code> has well-defined behaviour, and does not throw exceptions.</p>
    <li data-md>
     <p>Postconditions: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> <c- b>bool</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-></code>.</p>
    <li data-md>
     <p>Mandates: The expression <code class="highlight"><c- n>d</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-></code> is well-formed.</p>
   </ul>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>(</c-><c- k>const</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- n>p</c-><c- p>);</c-></code></p>
   <ul>
    <li data-md>
     <p>Constraints: <code class="highlight"><c- n>is_copy_constructible_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> is true.</p>
    <li data-md>
     <p>Effects: Creates a <code class="highlight"><c- n>indirect_value</c-></code> object that owns a copy of the object managed by <code class="highlight"><c- n>p</c-></code>. The copy is created by the copier in <code class="highlight"><c- n>p</c-></code>. If <code class="highlight"><c- n>p</c-></code> has a custom copier and deleter then the custom copier and deleter of the <code class="highlight"><c- n>indirect_value</c-></code> constructed are copied from those in <code class="highlight"><c- n>p</c-></code>.</p>
    <li data-md>
     <p>Remarks: The constructor must work with pointer to an complete type <code class="highlight"><c- n>T</c-></code>.</p>
    <li data-md>
     <p>Throws: Any exception thrown by the copier or <code class="highlight"><c- n>bad_alloc</c-></code> if required storage cannot be obtained.</p>
    <li data-md>
     <p>Postconditions: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> <c- b>bool</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-></code>.</p>
   </ul>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c-></code></p>
   <ul>
    <li data-md>
     <p>Effects: Move-constructs an <code class="highlight"><c- n>indirect_value</c-></code> instance from <code class="highlight"><c- n>p</c-></code>. If <code class="highlight"><c- n>p</c-></code> has a custom copier and deleter then the copier and deleter of the <code class="highlight"><c- n>indirect_value</c-></code> constructed are the same as those in <code class="highlight"><c- n>p</c-></code>.</p>
    <li data-md>
     <p>Postconditions: <code class="highlight"><c- o>*</c-><c- k>this</c-></code> contains the old value of <code class="highlight"><c- n>p</c-></code>. <code class="highlight"><c- n>p</c-></code> is empty.</p>
   </ul>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>indirect_value</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>in_place_t</c-><c- p>,</c-> <c- n>Ts</c-><c- o>&amp;&amp;</c-> <c- p>...</c-><c- n>ts</c-><c- p>);</c-></code></p>
   <ul>
    <li data-md>
     <p>Effects: Constructs an <code class="highlight"><c- n>indirect_value</c-></code> which owns an object of type <code class="highlight"><c- n>T</c-></code> direct-non-list-initialized with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>></c-><c- p>(</c-><c- n>ts</c-><c- p>)...</c-></code></p>
    <li data-md>
     <p>Throws: Any exception thrown by the selected constructor of <code class="highlight"><c- n>T</c-></code> or <code class="highlight"><c- n>bad_alloc</c-></code> if required storage cannot be obtained.</p>
    <li data-md>
     <p>Preconditions: <code class="highlight"><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>C</c-><c- p>,</c-> <c- n>default_copy</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>is_same_v</c-><c- o>&lt;</c-><c- n>D</c-><c- p>,</c-> <c- n>default_delete</c-><c- o>></c-></code>.</p>
   </ul>
   <h4 class="heading settled" data-level="4.4.4" id="xz4-class-template-indirect_value-destructor-indirect_valuedtor"><span class="secno">4.4.4. </span><span class="content">X.Z.4 Class template <code class="highlight"><c- n>indirect_value</c-></code> destructor [indirect_value.dtor]</span><a class="self-link" href="#xz4-class-template-indirect_value-destructor-indirect_valuedtor"></a></h4>
    <code class="highlight"><c- k>constexpr</c-> <c- o>~</c-><c- n>indirect_value</c-><c- p>();</c-></code> 
   <ul>
    <li data-md>
     <p>Effects: If <code class="highlight"><c- o>*</c-><c- k>this</c-></code> there are no effects. If a custom deleter <code class="highlight"><c- n>d</c-></code> is present then <code class="highlight"><c- n>d</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-></code> is called and the copier and deleter are destroyed. Otherwise the destructor of the managed object is called.</p>
    <li data-md>
     <p>Remarks: The constructor must work with pointer to an complete type <code class="highlight"><c- n>T</c-></code>.</p>
   </ul>
   <h4 class="heading settled" data-level="4.4.5" id="xz5-class-template-indirect_value-assignment-indirect_valueassignment"><span class="secno">4.4.5. </span><span class="content">X.Z.5 Class template <code class="highlight"><c- n>indirect_value</c-></code> assignment [indirect_value.assignment]</span><a class="self-link" href="#xz5-class-template-indirect_value-assignment-indirect_valueassignment"></a></h4>
    <code class="highlight"><c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- n>p</c-><c- p>);</c-></code> 
   <ul>
    <li data-md>
     <p>Constraints: <code class="highlight"><c- n>is_copy_assignable_v</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-></code> is true. <code class="highlight"><c- n>is_copy_assignable_v</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-></code> is true.</p>
    <li data-md>
     <p>Effects: If <code class="highlight"><c- n>p</c-></code> is empty, assigns an empty object. Otherwise creates an object that owns a copy of the object managed by <code class="highlight"><c- n>p</c-></code>. The copy is created by the copier in <code class="highlight"><c- n>p</c-></code>, and the copier and deleter of <code class="highlight"><c- o>*</c-><c- k>this</c-></code> are copied from those in <code class="highlight"><c- n>p</c-></code>.</p>
    <li data-md>
     <p>Remarks: The function must work with pointer to an complete type <code class="highlight"><c- n>T</c-></code>.</p>
    <li data-md>
     <p>Throws: Any exception thrown by the copier or <code class="highlight"><c- n>bad_alloc</c-></code> if required storage cannot be obtained.</p>
    <li data-md>
     <p>Returns: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
    <li data-md>
     <p>Postconditions: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> <c- b>bool</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-></code>.</p>
   </ul>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>indirect_value</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c-></code></p>
   <ul>
    <li data-md>
     <p>Constraints: <code class="highlight"><c- n>is_nothrow_move_assignable_v</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-></code> is true. <code class="highlight"><c- n>is_nothrow_move_assignable_v</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-></code> is true.</p>
    <li data-md>
     <p>Effects: Ownership of the resource managed by <code class="highlight"><c- n>p</c-></code> is transferred to this. If <code class="highlight"><c- n>p</c-></code> has a custom copier and deleter then the copier and deleter of <code class="highlight"><c- o>*</c-><c- k>this</c-></code> is the same as those in <code class="highlight"><c- n>p</c-></code>.</p>
    <li data-md>
     <p>Remarks: The function must work with pointer to an complete type <code class="highlight"><c- n>T</c-></code>.</p>
    <li data-md>
     <p>Returns: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
    <li data-md>
     <p>Postconditions: <code class="highlight"><c- o>*</c-><c- k>this</c-></code> contains the old value of <code class="highlight"><c- n>p</c-></code>. <code class="highlight"><c- n>p</c-></code> is empty.</p>
   </ul>
   <h4 class="heading settled" data-level="4.4.6" id="xz6-class-template-indirect_value-modifiers-indirect_valuemodifiers"><span class="secno">4.4.6. </span><span class="content">X.Z.6 Class template <code class="highlight"><c- n>indirect_value</c-></code> modifiers [indirect_value.modifiers]</span><a class="self-link" href="#xz6-class-template-indirect_value-modifiers-indirect_valuemodifiers"></a></h4>
    <code class="highlight"><c- k>constexpr</c-> <c- b>void</c-> <c- nf>swap</c-><c- p>(</c-><c- n>indirect_value</c-><t><c- o>&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c-></t></code> 
   <ul>
    <li data-md>
     <p>Effects: Exchanges the contents of <code class="highlight"><c- n>p</c-></code> and <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
   </ul>
   <h4 class="heading settled" data-level="4.4.7" id="xz7-class-template-indirect_value-observers-indirect_valueobservers"><span class="secno">4.4.7. </span><span class="content">X.Z.7 Class template <code class="highlight"><c- n>indirect_value</c-></code> observers [indirect_value.observers]</span><a class="self-link" href="#xz7-class-template-indirect_value-observers-indirect_valueobservers"></a></h4>
<pre class="highlight"><c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>();</c->
<c- k>constexpr</c-> <c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <ul>
    <li data-md>
     <p>Preconditions: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-></code> is true.</p>
    <li data-md>
     <p>Returns: A reference to the owned object.</p>
   </ul>
<pre class="highlight"><c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>constexpr</c-> <c- k>const</c-> <c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <ul>
    <li data-md>
     <p>Preconditions: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-></code> is true.</p>
    <li data-md>
     <p>Returns: A pointer to the owned object.</p>
   </ul>
   <p><code class="highlight"><c- k>constexpr</c-> <c- k>explicit</c-> <c- k>operator</c-> <c- b>bool</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c-></code></p>
   <ul>
    <li data-md>
     <p>Returns: false if the <code class="highlight"><c- n>indirect_value</c-></code> is empty, otherwise true.</p>
   </ul>
   <h4 class="heading settled" data-level="4.4.8" id="xz8-class-template-indirect_value-creation-indirect_valuecreation"><span class="secno">4.4.8. </span><span class="content">X.Z.8 Class template <code class="highlight"><c- n>indirect_value</c-></code> creation [indirect_value.creation]</span><a class="self-link" href="#xz8-class-template-indirect_value-creation-indirect_valuecreation"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-><c- o>=</c-><c- n>T</c-><c- p>,</c-> <c- k>class</c-> <c- p>...</c-><c- n>Ts</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>make_indirect_value</c-><c- p>(</c-><c- n>Ts</c-><c- o>&amp;&amp;</c-> <c- p>...</c-><c- n>ts</c-><c- p>);</c->
</pre>
   <ul>
    <li data-md>
     <p>Constraints: <code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>U</c-><c- p>,</c-> <c- n>Ts</c-><c- p>...</c-><c- o>></c-></code> is true.</p>
    <li data-md>
     <p>Preconditions: <code class="highlight"><c- n>U</c-></code> meets the <code class="highlight"><c- n>Cpp17CopyConstructible</c-></code> requirements.</p>
    <li data-md>
     <p>Returns: A <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> owning an object of type direct-non-list-initialized with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>></c-><c- p>(</c-><c- n>ts</c-><c- p>)...</c-></code>.</p>
    <li data-md>
     <p>Preconditions: <code class="highlight"><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>C</c-><c- p>,</c-> <c- n>default_copy</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>is_same_v</c-><c- o>&lt;</c-><c- n>D</c-><c- p>,</c-> <c- n>default_delete</c-><c- o>></c-></code>.</p>
   </ul>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U</c-> <c- o>=</c-> <c- n>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>A</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>allocator</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>allocate_indirect_value</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>allocator_arg_t</c-><c- p>,</c-> <c- n>A</c-><c- o>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- n>Ts</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>ts</c-><c- p>);</c-> 
</pre>
   <ul>
    <li data-md>
     <p>Constraints: <code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>U</c-><c- p>,</c-> <c- n>Ts</c-><c- p>...</c-><c- o>></c-></code> is true.</p>
    <li data-md>
     <p>Preconditions: <code class="highlight"><c- n>U</c-></code> meets the <code class="highlight"><c- n>Cpp17CopyConstructible</c-></code> requirements.</p>
    <li data-md>
     <p>Returns: A <code class="highlight"><c- n>indirect_value</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> owning an object of type direct-non-list-initialized with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>></c-><c- p>(</c-><c- n>ts</c-><c- p>)...</c-></code>.</p>
    <li data-md>
     <p>Preconditions <code class="highlight"><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>C</c-><c- p>,</c-> <c- n>allocator_copy</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>is_same_v</c-><c- o>&lt;</c-><c- n>D</c-><c- p>,</c-> <c- n>allocator_delete</c-><c- o>></c-></code>.</p>
   </ul>
   <h2 class="heading settled" data-level="5" id="acknowledgements"><span class="secno">5. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
    Contributions to the open source reference implementation have forwarded the design, including but not limited to contribution from Kilian Henneberger, Ed Catmur, Stephen Kelly &amp; Malcolm Parsons.
The authors would like to thank Thomas Russell for the original suggestions of allocator support, Bengt Gustafsson for useful discussion on SBO and nullstate representation via std::optional specialisation, Jonathan Wakely &amp; Daniel Krügler for document reviews, Andrew Bennieston for useful discussions on the topic and the BSI panel for on-going support. 
   <h2 class="heading settled" data-level="6" id="references"><span class="secno">6. </span><span class="content">References</span><a class="self-link" href="#references"></a></h2>
   <p>[W. Brown] <a href="open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3339.pdf">n3339: A Preliminary Proposal for a Deep-Copying Smart Pointer, Walter E. Brown, 2012</a></p>
   <p>[J. Coe] <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0201r5.html">p0201r5: A polymorphic value-type for C++</a></p>
   <p>[J. Coplien] Advanced C++ Programming Styles and Idioms (Addison-Wesley), James O. Coplien, 1992</p>
   <p>[C. Ericson] <a href="http://realtimecollisiondetection.net/pubs/GDC03_Ericson_Memory_Optimization.ppt">Memory Optimization, Christer Ericson, Games Developers Conference, 2003</a></p>
   <p>[R. Grimm] <a href="https://www.modernescpp.com/index.php/visiting-a-std-variant-with-the-overload-pattern">Visiting a std::variant with the Overload Pattern</a></p>
   <p>[H. Hinnant] <a href="http://howardhinnant.github.io/incomplete.html">“Incomplete types and shared_ptr / unique_ptr”, Howard Hinnant, 2011</a></p>
   <p>[M. Knejp] <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0316r0.html">P0316R0: allocate_unique and allocator_delete</a></p>
   <p>[H. Sutter] <a href="http://www.gotw.ca/publications/mill04.htm">"Pimpls - Beauty Marks You Can Depend On", Herb Sutter, 1998</a></p>
   <p>[Impl] <a href="https://github.com/jbcoe/indirect_value">Reference implementation: indirect_value, J.B.Coe</a></p>
   <p>[S. Meyers] Effective Modern C++, Item 22: When using the Pimpl Idiom, define special member functions in the implementation file, Scott Meyers, 2014</p>
   <p>[A. Upadyshev] <a href="http://oliora.github.io/2015/12/29/pimpl-and-rule-of-zero.html">PIMPL, Rule of Zero and Scott Meyers, Andrey Upadyshev, 2015</a></p>
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