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  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P0901R7: Size feedback in operator new</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
		color: black;
		color: var(--text);
		background-color: white;
		background-color: var(--bg);
	}

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		background: transparent top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

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

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

	p.copyright,
	p.copyright small { font-size: small }

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

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

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

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

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

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

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

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

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

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

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	 border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd;
	  border: thin solid var(--algo-border);
	  border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

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

	del {
		color: red;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #080;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

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

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

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

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

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: none;
		border-bottom: 1px solid #707070;
		border-bottom: 1px solid var(--a-normal-underline);
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		border-bottom-color: #bbb;
		border-bottom-color: var(--a-visited-underline);
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		border-color: #c00;
		border-color: var(--a-active-underline);
	}

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

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

	img {
		border-style: none;
	}

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

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

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
		height: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

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

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

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}


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

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

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

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

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

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #e0cb52;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52e052;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

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

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

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

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

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

/** 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-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* 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: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;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* 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 .secno { font-size: 85%; }
	.toc > li li li li li { font-size:   85%;    }
	.toc > li li li li li .secno { font-size: 100%; }

	/* @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 {
			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 span:not(.dfn-paneled) {
			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;
	}

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

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@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 table positioning:
		   "content column" is 50ems wide at max; less on smaller screens.
		   Extra space (after ToC + content) is empty on the right.

		   1. When table < content column, centers table in column.
		   2. When content < table < available, left-aligns.
		   3. When table > 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 type="text/css">
    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 89ebb6ab, updated Fri Oct 9 15:32:07 2020 -0700" name="generator">
  <link href="http://wg21.link/P0901R7" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="c46fe2926e8f768ed7b829913c9c9eac27dcd923" name="document-revision">
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
<style>/* style-colors */

/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
html {
    color: black;
    color: var(--text);
    background-color: white;
    background-color: var(--bg);
}
:root {
    color-scheme: light dark;

    --text: black;
    --bg: white;

    --logo-bg: #1a5e9a;
    --logo-active-bg: #c00;
    --logo-text: white;

    --tocnav-normal-text: #707070;
    --tocnav-normal-bg: var(--bg);
    --tocnav-hover-text: var(--tocnav-normal-text);
    --tocnav-hover-bg: #f8f8f8;
    --tocnav-active-text: #c00;
    --tocnav-active-bg: var(--tocnav-normal-bg);

    --tocsidebar-text: var(--text);
    --tocsidebar-bg: #f7f8f9;
    --tocsidebar-shadow: rgba(0,0,0,.1);
    --tocsidebar-heading-text: hsla(203,20%,40%,.7);

    --toclink-text: var(--text);
    --toclink-underline: #3980b5;
    --toclink-visited-text: var(--toclink-text);
    --toclink-visited-underline: #054572;

    --heading-text: #005a9c;

    --hr-text: var(--text);

    --algo-border: #def;

    --del-text: red;
    --del-bg: transparent;
    --ins-text: #080;
    --ins-bg: transparent;

    --a-normal-text: #034575;
    --a-normal-underline: #707070;
    --a-visited-text: var(--a-normal-text);
    --a-visited-underline: #bbb;
    --a-hover-bg: rgba(75%, 75%, 75%, .25);
    --a-active-text: #c00;
    --a-active-underline: #c00;

    --blockquote-border: silver;
    --blockquote-bg: transparent;
    --blockquote-text: currentcolor;

    --issue-border: #e05252;
    --issue-bg: #fbe9e9;
    --issue-text: var(--text);
    --issueheading-text: #831616;

    --example-border: #e0cb52;
    --example-bg: #fcfaee;
    --example-text: var(--text);
    --exampleheading-text: #574b0f;

    --note-border: #52e052;
    --note-bg: #e9fbe9;
    --note-text: var(--text);
    --noteheading-text: hsl(120, 70%, 30%);
    --notesummary-underline: silver;

    --assertion-border: #aaa;
    --assertion-bg: #eee;
    --assertion-text: black;

    --advisement-border: orange;
    --advisement-bg: #fec;
    --advisement-text: var(--text);
    --advisementheading-text: #b35f00;

    --warning-border: red;
    --warning-bg: hsla(40,100%,50%,0.95);
    --warning-text: var(--text);

    --def-border: #8ccbf2;
    --def-bg: #def;
    --def-text: var(--text);
    --defrow-border: #bbd7e9;

    --datacell-border: silver;

    --indexinfo-text: #707070;

    --indextable-hover-text: black;
    --indextable-hover-bg: #f7f8f9;

    --outdatedspec-bg: rgba(0, 0, 0, .5);
    --outdatedspec-text: black;
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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">P0901R7<br>Size feedback in operator new</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2020-11-13">2020-11-13</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0901R7">http://wg21.link/P0901R7</a>
     <dt class="editor">Authors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:andrewhhunter@gmail.com">Andrew Hunter</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ckennelly@google.com">Chris Kennelly</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>CWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Provide access to actual malloc buffer sizes for users.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#mot"><span class="secno">1</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li>
       <a href="#nallocx"><span class="secno">1.1</span> <span class="content">nallocx: not as awesome as it looks</span></a>
       <ol class="toc">
        <li><a href="#whatval"><span class="secno">1.1.1</span> <span class="content">nallocx must give a conservative answer</span></a>
        <li><a href="#speed"><span class="secno">1.1.2</span> <span class="content">nallocx duplicates work</span></a>
        <li><a href="#feedback"><span class="secno">1.1.3</span> <span class="content">nallocx hides information from malloc</span></a>
       </ol>
      <li><a href="#afteralloc"><span class="secno">1.2</span> <span class="content">after allocation is too late</span></a>
      <li><a href="#realloc"><span class="secno">1.3</span> <span class="content">realloc’s day has passed</span></a>
     </ol>
    <li><a href="#prop"><span class="secno">2</span> <span class="content">Proposal</span></a>
    <li>
     <a href="#alternatives"><span class="secno">3</span> <span class="content">Alternative Designs Considered</span></a>
     <ol class="toc">
      <li><a href="#splode"><span class="secno">3.1</span> <span class="content"><em>How</em> many <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>'s?</span></a>
      <li><a href="#trivial"><span class="secno">3.2</span> <span class="content">Implementation difficulty</span></a>
      <li><a href="#sizeddelete"><span class="secno">3.3</span> <span class="content">Interaction with Sized Delete</span></a>
      <li><a href="#advantages"><span class="secno">3.4</span> <span class="content">Advantages</span></a>
     </ol>
    <li><a href="#new"><span class="secno">4</span> <span class="content">New Expressions</span></a>
    <li><a href="#rel"><span class="secno">5</span> <span class="content">Related work</span></a>
    <li>
     <a href="#history"><span class="secno">6</span> <span class="content">History</span></a>
     <ol class="toc">
      <li><a href="#R6"><span class="secno">6.1</span> <span class="content">R6 → R7</span></a>
      <li><a href="#R6①"><span class="secno">6.2</span> <span class="content">R5 → R6</span></a>
      <li><a href="#R5"><span class="secno">6.3</span> <span class="content">R4 → R5</span></a>
      <li><a href="#R4"><span class="secno">6.4</span> <span class="content">R3 → R4</span></a>
      <li><a href="#R3"><span class="secno">6.5</span> <span class="content">R2 → R3</span></a>
      <li><a href="#R2"><span class="secno">6.6</span> <span class="content">R1 → R2</span></a>
      <li><a href="#R1"><span class="secno">6.7</span> <span class="content">R0 → R1</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="mot"><span class="secno">1. </span><span class="content">Motivation</span><a class="self-link" href="#mot"></a></h2>
    <em>Throughout this document "malloc" refers to the</em> <strong>implementation</strong> <em>of</em> <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> <em>both as fairly standard practice for implementers, and to
make clear the distinction between the interface and the implementation.</em> 
   <p>Everyone’s favorite dynamic data structure, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code>, allocates memory with
code that looks something like this (with many details, like <code class="highlight"><c- n>Allocator</c-></code>,
templating for non <code class="highlight"><c- b>char</c-></code>, and
exception safety, elided):</p>
<pre class="highlight"><c- b>void</c-> <c- n>vector</c-><c- o>::</c-><c- n>reserve</c-><c- p>(</c-><c- b>size_t</c-> <c- n>new_cap</c-><c- p>)</c-> <c- p>{</c->
  <c- k>if</c-> <c- p>(</c-><c- n>capacity_</c-> <c- o>>=</c-> <c- n>new_cap</c-><c- p>)</c-> <c- k>return</c-><c- p>;</c->
  <c- k>const</c-> <c- b>size_t</c-> <c- n>bytes</c-> <c- o>=</c-> <c- n>new_cap</c-><c- p>;</c->
  <c- b>void</c-> <c- o>*</c-><c- n>newp</c-> <c- o>=</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>new_cap</c-><c- p>);</c->
  <c- n>memcpy</c-><c- p>(</c-><c- n>newp</c-><c- p>,</c-> <c- n>ptr_</c-><c- p>,</c-> <c- n>capacity_</c-><c- p>);</c->
  <c- n>ptr_</c-> <c- o>=</c-> <c- n>newp</c-><c- p>;</c->
  <c- n>capacity_</c-> <c- o>=</c-> <c- n>bytes</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>Consider the sequence of calls:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>char</c-><c- o>></c-> <c- n>v</c-><c- p>;</c->
<c- n>v</c-><c- p>.</c-><c- n>reserve</c-><c- p>(</c-><c- mi>37</c-><c- p>);</c->
<c- c1>// ...</c->
<c- n>v</c-><c- p>.</c-><c- n>reserve</c-><c- p>(</c-><c- mi>38</c-><c- p>);</c->
</pre>
   <p>All reasonable implementations of malloc round sizes, both for alignment
requirements and improved performance.  It is extremely unlikely that malloc
provided us exactly 37 bytes.  We do not need to invoke the allocator
here...except that we don’t know that for sure, and to use the 38th byte would
be undefined behavior.  We would like that 38th byte to be usable without a
roundtrip through the allocator.</p>
   <p>This paper proposes an API making it safe to use that
byte, and explores many of the design choices (not all of which are obvious
without implementation experience.)</p>
   <h3 class="heading settled" data-level="1.1" id="nallocx"><span class="secno">1.1. </span><span class="content">nallocx: not as awesome as it looks</span><a class="self-link" href="#nallocx"></a></h3>
   <p>The simplest way to help here is to provide an informative API answering the
question "If I ask for N bytes, how many do I actually get?" <a data-link-type="biblio" href="#biblio-jemalloc">[jemalloc]</a> and <a data-link-type="biblio" href="#biblio-tcmalloc">[TCMalloc]</a> call this <code class="highlight"><c- n>nallocx</c-></code>.  We can then use that hint as a smarter
parameter for operator new:</p>
<pre class="highlight"><c- b>void</c-> <c- n>vector</c-><c- o>::</c-><c- n>reserve</c-><c- p>(</c-><c- b>size_t</c-> <c- n>new_cap</c-><c- p>)</c-> <c- p>{</c->
  <c- k>if</c-> <c- p>(</c-><c- n>capacity_</c-> <c- o>>=</c-> <c- n>new_cap</c-><c- p>)</c-> <c- k>return</c-><c- p>;</c->
  <c- k>const</c-> <c- b>size_t</c-> <c- n>bytes</c-> <c- o>=</c-> <c- n>nallocx</c-><c- p>(</c-><c- n>new_cap</c-><c- p>,</c-> <c- mi>0</c-><c- p>);</c->
  <c- b>void</c-> <c- o>*</c-><c- n>newp</c-> <c- o>=</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>bytes</c-><c- p>);</c->
  <c- n>memcpy</c-><c- p>(</c-><c- n>newp</c-><c- p>,</c-> <c- n>ptr_</c-><c- p>,</c-> <c- n>capacity_</c-><c- p>);</c->
  <c- n>ptr_</c-> <c- o>=</c-> <c- n>newp</c-><c- p>;</c->
  <c- n>capacity_</c-> <c- o>=</c-> <c- n>bytes</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>This is a good start, and does in fact work to allow vector and friends to use
the true extent of returned objects.  But there are three significant problems
with this approach.</p>
   <h4 class="heading settled" data-level="1.1.1" id="whatval"><span class="secno">1.1.1. </span><span class="content">nallocx must give a conservative answer</span><a class="self-link" href="#whatval"></a></h4>
   <p>While many allocators have a deterministic map from requested size to allocated
size, it is by no means guaranteed that all do.  Presumably they can make a
reasonably good guess, but if two calls to <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- mi>37</c-><c- p>)</c-></code> might return 64
and 128 bytes, we’d definitely rather know the right answer, not a conservative
approximation.</p>
   <h4 class="heading settled" data-level="1.1.2" id="speed"><span class="secno">1.1.2. </span><span class="content">nallocx duplicates work</span><a class="self-link" href="#speed"></a></h4>
   <p>Allocation is often a crucial limit on performance.  Most allocators compute
the returned size of an object as part of fulfilling that allocation...but if
we make a second call to <code class="highlight"><c- n>nallocx</c-></code>, we duplicate all that communication, and
also the overhead of the function call.</p>
   <h4 class="heading settled" data-level="1.1.3" id="feedback"><span class="secno">1.1.3. </span><span class="content">nallocx hides information from malloc</span><a class="self-link" href="#feedback"></a></h4>
   <p>The biggest problem (for the authors) is that <code class="highlight"><c- n>nallocx</c-></code> discards information
malloc finds valuable (the user’s intended allocation size.)  That is: in our
running example, malloc normally knows that the user wants 37 bytes (then 38),
but with <code class="highlight"><c- n>nallocx</c-></code>, we will only ever be told that they want 40 (or 48, or
whatever <code class="highlight"><c- n>nallocx</c-><c- p>(</c-><c- mi>37</c-><c- p>)</c-></code> returns.)</p>
   <p>Google’s malloc implementation (<a data-link-type="biblio" href="#biblio-tcmalloc">[TCMalloc]</a>) rounds requests to one of a small
(&lt;100) number of <em>sizeclasses</em>: we maintain local caches of appropriately sized
objects, and cannot do this for every possible size of object.  Originally,
these sizeclasses were just reasonably evenly spaced among the range they
cover.  Since then, we have used extensive telemetry on allocator use in the
wild to tune these choices. In particular, as we know (approximately) how many
objects of any given size are requested, we can solve a fairly simple
optimization problem to minimize the total internal fragmentation for any
choice of N sizeclasses.</p>
   <p>Widespread use of <code class="highlight"><c- n>nallocx</c-></code> breaks this. By the time TCMalloc’s telemetry sees
a request that was hinted by nallocx, to the best of our knowledge the user <em>wants</em> exactly as many bytes as we currently provide them.  If a huge number
of callers wanted 40 bytes but were currently getting 48, we’d lose the ability
to know that and optimize for it.</p>
   <p>Note that we can’t take the same telemetry from <code class="highlight"><c- n>nallocx</c-></code> calls: we have no
idea how many times the resulting hint will be used (we might not allocate at
all, or we might cache the result and make a million allocations guided by it.)
We would also lose important information in the stack traces we collect from
allocation sites.</p>
   <p>Optimization guided by malloc telemetry has been one of our most effective
tools in improving allocator performance.  It is important that we fix this
issue <em>without</em> losing the ground truth of what a caller of <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> wants.</p>
   <p>These three issues explain why we don’t believe <code class="highlight"><c- n>nallocx</c-></code> is a sufficient
solution here.</p>
   <h3 class="heading settled" data-level="1.2" id="afteralloc"><span class="secno">1.2. </span><span class="content">after allocation is too late</span><a class="self-link" href="#afteralloc"></a></h3>
   <p>Another obvious suggestion is to add a way to inspect the size of an object
returned by <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>. Most mallocs provide a way to do this; <a data-link-type="biblio" href="#biblio-jemalloc">[jemalloc]</a> calls it <code class="highlight"><c- n>sallocx</c-></code>.  Vector would look like:</p>
<pre class="highlight"><c- b>void</c-> <c- n>vector</c-><c- o>::</c-><c- n>reserve</c-><c- p>(</c-><c- b>size_t</c-> <c- n>new_cap</c-><c- p>)</c-> <c- p>{</c->
  <c- k>if</c-> <c- p>(</c-><c- n>capacity_</c-> <c- o>>=</c-> <c- n>new_cap</c-><c- p>)</c-> <c- k>return</c-><c- p>;</c->
  <c- b>void</c-> <c- o>*</c-><c- n>newp</c-> <c- o>=</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>new_cap</c-><c- p>);</c->
  <c- k>const</c-> <c- b>size_t</c-> <c- n>bytes</c-> <c- o>=</c-> <c- n>sallocx</c-><c- p>(</c-><c- n>newp</c-><c- p>);</c->
  <c- n>memcpy</c-><c- p>(</c-><c- n>newp</c-><c- p>,</c-> <c- n>ptr_</c-><c- p>,</c-> <c- n>capacity_</c-><c- p>);</c->
  <c- n>ptr_</c-> <c- o>=</c-> <c- n>newp</c-><c- p>;</c->
  <c- n>capacity_</c-> <c- o>=</c-> <c- n>bytes</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>This is worse than nallocx. It fixes the non-constant size problem, and avoids
a feedback loop, but the performance issue is worse (this is the major issue <em>fixed</em> by <a data-link-type="biblio" href="#biblio-sizeddelete">[SizedDelete]</a>!), and what’s worse, the above code invokes UB as
soon as we touch byte <code class="highlight"><c- n>new_cap</c-><c- o>+</c-><c- mi>1</c-></code>.  We could in principle change the standard,
but this would be an implementation nightmare.</p>
   <h3 class="heading settled" data-level="1.3" id="realloc"><span class="secno">1.3. </span><span class="content">realloc’s day has passed</span><a class="self-link" href="#realloc"></a></h3>
   <p>We should also quickly examine why the classic C API <code class="highlight"><c- n>realloc</c-></code> is insufficient.</p>
<pre class="highlight"><c- b>void</c-> <c- n>vector</c-><c- o>::</c-><c- n>reserve</c-><c- p>(</c-><c- b>size_t</c-> <c- n>new_cap</c-><c- p>)</c-> <c- p>{</c->
  <c- k>if</c-> <c- p>(</c-><c- n>capacity_</c-> <c- o>>=</c-> <c- n>new_cap</c-><c- p>)</c-> <c- k>return</c-><c- p>;</c->
  <c- n>ptr_</c-> <c- o>=</c-> <c- n>realloc</c-><c- p>(</c-><c- n>ptr_</c-><c- p>,</c-> <c- n>new_cap</c-><c- p>);</c->
  <c- n>capacity_</c-> <c- o>=</c-> <c- n>new_cap</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>In principle a realloc from 37 to 38 bytes wouldn’t carry the full cost of
allocation. But it’s dramatically more expensive than making no call at all.
What’s more, there are a number of more complicated dynamic data structures that
store variable-sized chunks of data but are never actually resized. These data
structures still deserve the right to use all the memory they’re paying for.</p>
   <p>Furthermore, <code class="highlight"><c- n>realloc</c-></code>'s original purpose was not to allow the use of more bytes
the caller already had, but to (hopefully) extend an allocation in place to
adjacent free space.  In a classic malloc implementation this would actually be
possible...but most modern allocators use variants of slab allocation. Even if
the 65th byte in a 64-byte allocation isn’t in use, they cannot be combined into
a single object; it’s almost certainly required to be used for the next 64-byte
allocation.  In the modern world, <code class="highlight"><c- n>realloc</c-></code> serves little purpose.</p>
   <h2 class="heading settled" data-level="2" id="prop"><span class="secno">2. </span><span class="content">Proposal</span><a class="self-link" href="#prop"></a></h2>
   <p>We propose adding new overloads of <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> (size-returning allocation
functions) that directly inform the user of the size available to them.  C++
makes <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> replaceable (15.5.4.6), allowing a program to provide
its own version different from the standard library’s implementation.</p>
   <p>We propose wording, relative to <a data-link-type="biblio" href="#biblio-n4791">[N4791]</a>:</p>
   <ul>
    <li data-md>
     <p>Amend [basic.stc.dynamic.allocation] (6.6.5.4.1) paragraph 1:</p>
   </ul>
   <blockquote>
     An allocation function shall be a class member function or a global function; a
program is ill-formed if an allocation function is declared in a namespace
scope other than global scope or declared static in global scope.
    <ins>
     <p>An allocation function is a <em>size-returning allocation function</em> if it has a
second parameter of type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-></code>, or it has a second parameter of
type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-></code> and a third parameter of type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-></code>.</p>
    </ins>
    The return type shall be 
    <ins><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-></code> if the allocation
function is a size-returning allocation function and </ins>
    <code class="highlight"><c- b>void</c-><c- o>*</c-></code>
    <ins> otherwise</ins>
    . The first parameter shall have type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- b>size_t</c-></code> ([support.types]).  The first parameter shall not have an associated default
argument ([dcl.fct.default]).  The value of the first parameter is interpreted
as the requested size of the allocation. [...] 
   </blockquote>
   <ul>
    <li data-md>
     <p>Amend [expr.new] (7.6.2.4) paragraph 5:</p>
   </ul>
   <blockquote>
     Objects created by a <em>new-expression</em> have dynamic storage duration
([basic.stc.dynamic]). [ Note: The lifetime of such an object is not
necessarily restricted to the scope in which it is created. — end note ] 
    <p>When the allocated object is not an array, the result of the new-expression is</p>
    <ul>
     <li data-md>
      <ins>if the new-expression is a size-returning placement new expression (see
below), an object of type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-></code> ([new.syn]) whose <code class="highlight"><c- n>p</c-></code> member points to the object created and whose <code class="highlight"><c- n>n</c-></code> member is the <code class="highlight"><c- n>n</c-></code> member
of the value returned by the size-returning allocation function;</ins>
     <li data-md>
      <ins>otherwise,</ins>
       a pointer to the object created. 
    </ul>
   </blockquote>
   <ul>
    <li data-md>
     <p>Amend [expr.new] (7.6.2.4) paragraph 6:</p>
   </ul>
   <blockquote>
     When the allocated object is an array (that is, the <em>noptr-new-declarator</em> syntax is used or the <em>new-type-id</em> or <em>type-id</em> denotes an array type), the <em>new-expression</em> yields 
    <ul>
     <li data-md>
      <ins>if the new-expression is a size-returning placement new expression (see
below), an object of type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-></code> ([new.syn]) whose <code class="highlight"><c- n>p</c-></code> member points to the object created and whose <code class="highlight"><c- n>n</c-></code> member is the <code class="highlight"><c- n>n</c-></code> member
of the value returned by the size-returning allocation function less any
array allocation overhead;</ins>
     <li data-md>
      <ins>otherwise,</ins>
       a pointer to the initial element (if any) of the array. 
    </ul>
    <p>[ Note: Both <code class="highlight"><c- k>new</c-> <c- b>int</c-></code> and <code class="highlight"><c- k>new</c-> <c- b>int</c-><c- p>[</c-><c- mi>10</c-><c- p>]</c-></code> have type <code class="highlight"><c- b>int</c-><c- o>*</c-></code> and the type of <code class="highlight"><c- k>new</c-> <c- b>int</c-><c- p>[</c-><c- n>i</c-><c- p>][</c-><c- mi>10</c-><c- p>]</c-></code> is <code class="highlight"><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)[</c-><c- mi>10</c-><c- p>]</c-></code> — end note ] The <em>attribute-specifier-seq</em> in a <em>noptr-new-declarator</em> appertains to the associated array type.</p>
   </blockquote>
   <ul>
    <li data-md>
     <p>Amend [expr.new] (7.6.2.4) paragraph 15:</p>
   </ul>
   <blockquote>
     Overload resolution is performed on a function call created by assembling an
argument list. The first argument is the amount of space requested, and has
type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- b>size_t</c-></code>. If the type of the allocated object has new-extended
alignment, the next argument is the type’s alignment, and has type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-></code>. If the <em>new-placement</em> syntax is used, the
initializer-clauses in its expression-list are the succeeding arguments.
If no matching function is found and the allocated object type has new-extended
alignment, the alignment argument is removed from the argument list, and
overload resolution is performed again. 
    <ins> A <em>size-returning placement new expression</em> is one whose selected allocation
function is a <em>size-returning allocation function</em>. </ins>
   </blockquote>
   <ul>
    <li data-md>
     <p>Amend [expr.new] (7.6.2.4) paragraph 25:</p>
   </ul>
   <blockquote>
     If a <em>new-expression</em> calls a deallocation function, it passes 
    <ul>
     <li data-md>
      <ins>the <code class="highlight"><c- n>p</c-></code> member of the object returned by the <em>size-returning allocation function</em>;</ins>
     <li data-md>
      <ins>otherwise,</ins>
      the value returned from the allocation function call 
    </ul>
    <p>
     as the first argument of type <code class="highlight"><c- b>void</c-><c- o>*</c-></code>. If a placement deallocation
function is called, it is passed the same additional arguments as were
passed to the placement allocation function, that is, the same arguments
as those specified with the <em>new-placement</em> syntax. 
     <ins>If no matching
function is found and one of the arguments to the <em>new-placement</em> syntax
has type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-></code>, that argument is removed from the
argument list, and overload resolution is performed again.</ins>
      If the
implementation is allowed to introduce a temporary object or make a copy
of any argument as part of the call to the allocation function, it is
unspecified whether the same object is used in the call to both the
allocation and deallocation functions.
    </p>
   </blockquote>
   <ul>
    <li data-md>
     <p>Amend [replacement.functions] (15.5.4.6) paragraph 2:</p>
   </ul>
<pre class="highlight"><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c->

<ins>
<c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>al</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>al</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c->
</ins>

<c- p>[...]</c->

<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c->

<ins>
<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>al</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c->
<c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>al</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c->
</ins>

<c- p>[...]</c->
</pre>
   <ul>
    <li data-md>
     <p>Amend header <code class="highlight"><c- o>&lt;</c-><c- k>new</c-><c- o>></c-></code> synopsis in [new.syn] (16.6.1):</p>
   </ul>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>class</c-> <c- nc>bad_alloc</c-><c- p>;</c->
  <c- k>class</c-> <c- nc>bad_array_new_length</c-><c- p>;</c->

  <c- k>struct</c-> <c- n>destroying_delete_t</c-> <c- p>{</c->
    <c- k>explicit</c-> <c- n>destroying_delete_t</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
  <c- p>};</c->
  <c- kr>inline</c-> <c- k>constexpr</c-> <c- n>destroying_delete_t</c-> <c- n>destroying_delete</c-><c- p>{};</c->

<ins>
  <c- k>struct</c-> <c- n>return_size_t</c-> <c- p>{</c->
    <c- k>explicit</c-> <c- n>return_size_t</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
  <c- p>};</c->

  <c- kr>inline</c-> <c- k>constexpr</c-> <c- n>return_size_t</c-> <c- n>return_size</c-><c- p>{};</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-> <c- o>=</c-> <c- b>void</c-><c- o>></c->
  <c- k>struct</c-> <c- n>sized_allocation_t</c-> <c- p>{</c->
    <c- n>T</c-> <c- o>*</c-><c- n>p</c-><c- p>;</c->
    <c- b>size_t</c-> <c- n>n</c-><c- p>;</c->
  <c- p>};</c->
</ins>

  <c- k>enum</c-> <c- k>class</c-> <c- nc>align_val_t</c-> <c- o>:</c-> <c- b>size_t</c-> <c- p>{};</c->

  <c- p>[...]</c->
<c- p>}</c->

<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c->
                                 <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<ins>
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
        <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
<c- p>[...]</c->

<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c->
                                   <c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<ins>
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
        <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
<c- p>[...]</c->

<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c->  <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>void</c-><c- o>*</c-> <c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c->  <c- k>operator</c-> <c- nf>delete</c->  <c- p>(</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- b>void</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c->  <c- k>operator</c-> <c- k>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- b>void</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <ul>
    <li data-md>
     <p>Amend [new.delete.single] (16.6.2.1)</p>
   </ul>
   <blockquote>
    <ins>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
        <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
</pre>
     <ul>
      <li data-md>
       <p><em>Effects</em>: Same as above, except these are called by a placement version of
a <em>new-expression</em> when a C++ program prefers the <em>size-returning
allocation function</em>.</p>
      <li data-md>
       <p><em>Replaceable</em>: A C++ program may define functions with either of these
function signatures, and thereby displace the default versions defined by
the C++ standard library.</p>
      <li data-md>
       <p><em>Required behavior</em>: Return a <code class="highlight"><c- n>sized_allocation_t</c-></code> whose <code class="highlight"><c- n>p</c-></code> member represents the
address of a region of <em>N</em> bytes of suitably aligned storage
([basic.stc.dynamic]) for some <em>N</em> >= <code class="highlight"><c- n>size</c-></code>, and whose <code class="highlight"><c- n>n</c-></code> member is <em>N</em>,
or else throw a <code class="highlight"><c- n>bad_alloc</c-></code> exception.  This requirement is binding on any
replacement versions of these functions.</p>
      <li data-md>
       <p><em>Default behavior</em>: Returns <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>size</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>size</c-><c- p>,</c-> <c- n>alignment</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> respectively. If a
user-provided operator new is invoked directly or indirectly, N is <code class="highlight"><c- n>size</c-></code>.</p>
     </ul>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
     <ul>
      <li data-md>
       <p><em>Effects</em>: Same as above, except these are called by a placement version of
a <em>new-expression</em> when a C++ program prefers the <em>size-returning
allocation function</em> and a null pointer result as an error indication.</p>
      <li data-md>
       <p><em>Replaceable</em>: A C++ program may define functions with either of these
function signatures, and thereby displace the default versions defined by
the C++ standard library.</p>
      <li data-md>
       <p><em>Required behavior</em>: Return a <code class="highlight"><c- n>sized_allocation_t</c-></code> whose <code class="highlight"><c- n>p</c-></code> member represents the
address of a region of <em>N</em> bytes of suitably aligned storage
([basic.stc.dynamic]) for some <em>N</em> >= <code class="highlight"><c- n>size</c-></code>, and whose <code class="highlight"><c- n>n</c-></code> member is <em>N</em>,
or else return <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>nullptr</c-><c- p>,</c-> <c- mi>0</c-><c- p>}</c-></code>.  Each of these nothrow
versions of <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code> returns a pointer obtained as if acquired from
the (possibly replaced) corresponding non-placement function.  This
requirement is binding on any replacement versions of these functions.</p>
      <li data-md>
       <p><em>Default behavior</em>: Returns <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>size</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>size</c-><c- p>,</c-> <c- n>alignment</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> respectively. If a
user-provided operator new is invoked directly or indirectly, N is <code class="highlight"><c- n>size</c-></code>.
If the call to <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code> throws, returns <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>nullptr</c-><c- p>,</c-> <c- mi>0</c-><c- p>}</c-></code>.</p>
     </ul>
    </ins>
<pre class="highlight"><c- b>void</c-> <c- k>operator</c-> <c- nf>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- k>operator</c-> <c- nf>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- k>operator</c-> <c- nf>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- k>operator</c-> <c- nf>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <p>[...]</p>
    <ul>
     <li data-md>
      <p>
       <em>Requires</em>: If the <code class="highlight"><c- n>alignment</c-></code> parameter is not present, <code class="highlight"><c- n>ptr</c-></code> shall have been
returned by an allocation function without an alignment parameter. If
present, the <code class="highlight"><c- n>alignment</c-></code> argument shall equal the alignment argument passed
to the allocation function that returned <code class="highlight"><c- n>ptr</c-></code>. If present, the <code class="highlight"><c- n>size</c-></code> argument
shall equal the <code class="highlight"><c- n>size</c-></code> argument passed to the allocation function that
returned <code class="highlight"><c- n>ptr</c-></code>
       <ins>, if <code class="highlight"><c- n>ptr</c-></code> was not allocated by a <em>size-returning
allocation function</em>.  If present, the <code class="highlight"><c- n>size</c-></code> argument shall satisify <code class="highlight"><c- n>n</c-> <c- o>>=</c-> <c- n>size</c-> <c- o>>=</c-> <c- n>requested</c-></code> if <code class="highlight"><c- n>ptr</c-></code> was allocated by a <em>size-returning allocation
function</em>, where <code class="highlight"><c- n>n</c-></code> is the size returned in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-></code> and <code class="highlight"><c- n>requested</c-></code> is the size argument passed to the allocation function</ins>
       .
      </p>
    </ul>
    <p>[...]</p>
   </blockquote>
   <ul>
    <li data-md>
     <p>Amend [new.delete.array] (16.6.2.2)</p>
   </ul>
   <blockquote>
    <ins>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
        <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
</pre>
     <ul>
      <li data-md>
       <p><em>Effects</em>: Same as above, except these are called by a placement version of
a <em>new-expression</em> when a C++ program prefers the <em>size-returning
allocation function</em>.</p>
      <li data-md>
       <p><em>Replaceable</em>: A C++ program may define functions with either of these
function signatures, and thereby displace the default versions defined by
the C++ standard library.</p>
      <li data-md>
       <p><em>Required behavior</em>: Return a <code class="highlight"><c- n>sized_allocation_t</c-></code> whose <code class="highlight"><c- n>p</c-></code> member represents the
address of a region of <em>N</em> bytes of suitably aligned storage
([basic.stc.dynamic]) for some <em>N</em> >= <code class="highlight"><c- n>size</c-></code>, and whose <code class="highlight"><c- n>n</c-></code> member is <em>N</em>,
or else throw a <code class="highlight"><c- n>bad_alloc</c-></code> exception.  This requirement is binding on any
replacement versions of these functions.</p>
      <li data-md>
       <p><em>Default behavior</em>: Returns <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>size</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>size</c-><c- p>,</c-> <c- n>alignment</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> respectively. If a
user-provided operator new is invoked directly or indirectly, N is <code class="highlight"><c- n>size</c-></code>.</p>
     </ul>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c->
  <c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
     <ul>
      <li data-md>
       <p><em>Effects</em>: Same as above, except these are called by a placement version of
a <em>new-expression</em> when a C++ program prefers the <em>size-returning
allocation function</em> and a null pointer result as an error indication.</p>
      <li data-md>
       <p><em>Replaceable</em>: A C++ program may define functions with either of these
function signatures, and thereby displace the default versions defined by
the C++ standard library.</p>
      <li data-md>
       <p><em>Required behavior</em>: Return a <code class="highlight"><c- n>sized_allocation_t</c-></code> whose <code class="highlight"><c- n>p</c-></code> member represents the
address of a region of <em>N</em> bytes of suitably aligned storage
([basic.stc.dynamic]) for some <em>N</em> >= <code class="highlight"><c- n>size</c-></code>, and whose <code class="highlight"><c- n>n</c-></code> member is <em>N</em>,
or else return <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>nullptr</c-><c- p>,</c-> <c- mi>0</c-><c- p>}</c-></code>.  This requirement is binding
on any replacement versions of these functions.</p>
      <li data-md>
       <p><em>Default behavior</em>: Returns <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>size</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>operator</c-> <c- k>new</c-><c- p>[](</c-><c- n>size</c-><c- p>,</c-> <c- n>alignment</c-><c- p>),</c-> <c- n>N</c-><c- p>}</c-></code> respectively. If a
user-provided operator new is invoked directly or indirectly, N is <code class="highlight"><c- n>size</c-></code>.
If the call to <code class="highlight"><c- k>operator</c-> <c- k>new</c-><c- p>[]</c-></code> throws, returns <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-><c- p>{</c-><c- k>nullptr</c-><c- p>,</c-> <c- mi>0</c-><c- p>}</c-></code>.</p>
     </ul>
    </ins>
<pre class="highlight"><c- b>void</c-> <c- k>operator</c-> <c- k>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- k>operator</c-> <c- k>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- k>operator</c-> <c- k>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- k>operator</c-> <c- k>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>align_val_t</c-> <c- n>alignment</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <p>[...]</p>
    <ul>
     <li data-md>
      <p>
       <em>Requires</em>: If the <code class="highlight"><c- n>alignment</c-></code> parameter is not present, <code class="highlight"><c- n>ptr</c-></code> shall have been
returned by an allocation function without an <code class="highlight"><c- n>alignment</c-></code> parameter. If
present, the <code class="highlight"><c- n>alignment</c-></code> argument shall equal the <code class="highlight"><c- n>alignment</c-></code> argument passed
to the allocation function that returned <code class="highlight"><c- n>ptr</c-></code>. If present, the <code class="highlight"><c- n>size</c-></code> argument
shall equal the <code class="highlight"><c- n>size</c-></code> argument passed to the allocation function that
returned <code class="highlight"><c- n>ptr</c-></code>
       <ins>, if <code class="highlight"><c- n>ptr</c-></code> was not allocated by a <em>size-returning
allocation function</em>.  If present, the <code class="highlight"><c- n>size</c-></code> argument shall satisify <code class="highlight"><c- n>n</c-> <c- o>>=</c-> <c- n>size</c-> <c- o>>=</c-> <c- n>requested</c-></code> if <code class="highlight"><c- n>ptr</c-></code> was allocated by a <em>size-returning allocation
function</em>, where <code class="highlight"><c- n>n</c-></code> is the size returned in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-></code> and <code class="highlight"><c- n>requested</c-></code> is the size argument passed to the allocation function</ins>
       .
      </p>
    </ul>
    <p>[...]</p>
   </blockquote>
   <ul>
    <li data-md>
     <p>Amend Table 17 ("Feature-test macros") [cpp.predefined] (15.10)</p>
   </ul>
   <blockquote>
    <table>
     <tbody>
      <tr>
       <th>Name
       <th>Value
      <tr>
       <td>
        <ins><code class="highlight"><c- n>__cpp_size_returning_new</c-></code></ins>
       <td>
        <ins>PLACEHOLDER DATE</ins>
    </table>
   </blockquote>
   <h2 class="heading settled" data-level="3" id="alternatives"><span class="secno">3. </span><span class="content">Alternative Designs Considered</span><a class="self-link" href="#alternatives"></a></h2>
   <p>Another signature we could use would be:</p>
<pre class="highlight"><c- k>enum</c-> <c- k>class</c-> <c- nc>return_size_t</c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- p>{};</c->
<c- b>void</c-><c- o>*</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- b>size_t</c-> <c- n>size</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- o>&amp;</c-><c- p>);</c->
</pre>
   <p>(and so on.)  This is slightly simpler to read as a signature, but arguably
worse in usage:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tie</c-><c- p>(</c-><c- n>obj</c-><c- p>.</c-><c- n>ptr</c-><c- p>,</c-> <c- n>obj</c-><c- p>.</c-><c- n>size</c-><c- p>)</c-> <c- o>=</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- mi>37</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>{});</c->

<c- c1>// ...vs...</c->

<c- c1>// Presumably the object implementation wants to contain a size_t,</c->
<c- c1>// not a return_size_t.</c->
<c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-> <c- n>rs</c-><c- p>;</c->
<c- n>obj</c-><c- p>.</c-><c- n>ptr</c-> <c- o>=</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- mi>37</c-><c- p>,</c-> <c- n>rs</c-><c- p>);</c->
<c- n>obj</c-><c- p>.</c-><c- n>size</c-> <c- o>=</c-> <c- n>rs</c-><c- p>;</c->
</pre>
   <p>More importantly, this form is less efficient. In practice, underlying malloc
implementations provide actual definitions of <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> symbols which
are called like any other function.  Passing a reference parameter requires us
to actually return the size via memory.</p>
   <ul>
    <li data-md>
     <p>Linux ABIs support returning at least two scalar values in registers (even if
they’re members of a trivially copyable struct) which can be dramatically
more efficient.</p>
    <li data-md>
     <p>The <a data-link-type="biblio" href="#biblio-microsoftabi">[MicrosoftABI]</a> returns large types by pointer, but this is no worse
than making the reference parameter an inherent part of the API.</p>
   </ul>
   <p>Whether we use a reference parameter or a second returned value, the
interpretation is the same.</p>
   <h3 class="heading settled" data-level="3.1" id="splode"><span class="secno">3.1. </span><span class="content"><em>How</em> many <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>'s?</span><a class="self-link" href="#splode"></a></h3>
   <p>It is unfortunate that we have so many permutations of <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>--eight
seems like far more than we should really need!  But there really isn’t any
significant runtime cost for having them.  Use of raw calls to <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> is relatively rare:  It’s a <em>building block</em> for low-level libraries, allocators
(<a data-link-type="biblio" href="#biblio-p0401r1">[P0401R1]</a>), and so on, so the cognitive burden on C++ users is low.</p>
   <p>The authors have considered other alternatives to the additional overloads.  At
the Jacksonville meeting, EWG suggested looking at parameter packs.</p>
   <ul>
    <li data-md>
     <p>Parameter packs do not reduce the number of symbols introduced.
Implementers still need to provide implementations each of the n overloads.</p>
    <li data-md>
     <p>Retrofitting parameter packs leaves us with <em>more</em> mangled variants.
Implementers need to provide both the legacy symbols as well as the
parameter pack-mangled symbols.</p>
   </ul>
   <p>The authors have also considered APIs where all parameters are passed, thereby
requiring a single new overload.  This adds further overhead for
implementations, as it moves compile-time decisions (is the alignment at or
below the minimum guaranteed by <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code>) into runtime ones.</p>
   <p>The alternative to modifying the handling of <em>new-expressions</em> invoking
deallocation functions (when an exception is thrown) would require additional
overloads for <code class="highlight"><c- k>operator</c-> <c- k>delete</c-></code> / <code class="highlight"><c- k>operator</c-> <c- k>delete</c-><c- p>[]</c-></code> whose sole purpose would
be to accept and discard the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-></code>.</p>
   <h3 class="heading settled" data-level="3.2" id="trivial"><span class="secno">3.2. </span><span class="content">Implementation difficulty</span><a class="self-link" href="#trivial"></a></h3>
   <p>It’s worth reiterating that there’s a perfectly good trivial implementation of these
functions:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sized_allocation_t</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- b>size_t</c-> <c- n>n</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>)</c-> <c- p>{</c->
  <c- k>return</c-> <c- p>{</c-><c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>n</c-><c- p>),</c-> <c- n>n</c-><c- p>};</c->
<c- p>}</c->
</pre>
   <p>Malloc implementations are free to properly override this with a more impactful
definition, but this paper poses no significant difficulty for toolchain
implementers.</p>
   <p>Implementation Experience:</p>
   <ul>
    <li data-md>
     <p>TCMalloc has developed an implementation opensourced on GitHub
(<a data-link-type="biblio" href="#biblio-mallocextension">[MallocExtension]</a>).  While this requires mapping from an integer size
class to the true number of bytes, combining this lookup with the
allocation is more efficient as we avoid recomputing the sizeclass itself
(given a request) or deriving it from the object’s address.</p>
    <li data-md>
     <p>jemalloc is prototyping a <code class="highlight"><c- n>smallocx</c-></code> function providing a C API for this
functionality <a data-link-type="biblio" href="#biblio-smallocx">[smallocx]</a>.</p>
   </ul>
   <h3 class="heading settled" data-level="3.3" id="sizeddelete"><span class="secno">3.3. </span><span class="content">Interaction with Sized Delete</span><a class="self-link" href="#sizeddelete"></a></h3>
   <p>For allocations made with <code class="highlight"><c- n>sized_allocation_t</c-></code>-returning <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>, we need to
relax <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>delete</c-></code>'s size argument (16.6.2.1 and 16.6.2.2).  For
allocations of <code class="highlight"><c- n>T</c-></code>, the size quanta used by the allocator may not be a multiple
of <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code>, leading to both the original and returned sizes being
unrecoverable at the time of deletion.</p>
   <p>Consider the memory allocated by:</p>
<pre class="highlight"><c- k>using</c-> <c- n>T</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>aligned_storage</c-><c- o>&lt;</c-><c- mi>16</c-><c- p>,</c-> <c- mi>8</c-><c- o>>::</c-><c- n>type</c-><c- p>;</c->

<c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>v</c-><c- p>(</c-><c- mi>4</c-><c- p>);</c->
</pre>
   <p>The underlying heap allocation is made with <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- mi>64</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>)</c-></code>.</p>
   <ul>
    <li data-md>
     <p>The memory allocator may return a 72 byte object:  Since there is no <code class="highlight"><c- n>k</c-></code> such that <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- o>*</c-> <c- n>k</c-> <c- o>=</c-> <c- mi>72</c-></code>, we can’t provide that value to <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-><c- p>,</c-> <c- b>size_t</c-><c- p>)</c-></code>.  The only option would be storing 72 explicitly,
which would be wasteful.</p>
    <li data-md>
     <p>The memory allocator may instead return an 80 byte object (5 <code class="highlight"><c- n>T</c-></code>'s):  We
now cannot represent the original request when deallocating without
additional storage.</p>
   </ul>
   <p>For allocations made with</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tie</c-><c- p>(</c-><c- n>p</c-><c- p>,</c-> <c- n>m</c-><c- p>)</c-> <c- o>=</c-> <c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>n</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>{});</c->
</pre>
   <p>we permit <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>delete</c-><c- p>(</c-><c- n>p</c-><c- p>,</c-> <c- n>s</c-><c- p>)</c-></code> where <code class="highlight"><c- n>n</c-> <c- o>&lt;=</c-> <c- n>s</c-> <c- o>&lt;=</c-> <c- n>m</c-></code>.</p>
   <p>This behavior is consistent with <a data-link-type="biblio" href="#biblio-jemalloc">[jemalloc]</a>'s <code class="highlight"><c- n>sdallocx</c-></code>, where the
deallocation size must fall between the request (<code class="highlight"><c- n>n</c-></code>) and the actual allocated
size (<code class="highlight"><c- n>m</c-></code>) inclusive.</p>
   <h3 class="heading settled" data-level="3.4" id="advantages"><span class="secno">3.4. </span><span class="content">Advantages</span><a class="self-link" href="#advantages"></a></h3>
   <p>It’s easy to see that this approach nicely solves the problems posed by other methods:</p>
   <ul>
    <li data-md>
     <p>We pay almost nothing in speed to return an actual-size parameter.  For
 TCMalloc and jemalloc, this is typically a load from to map from <em>sizeclass</em> to size.  This cost is strictly smaller than with <code class="highlight"><c- n>nallocx</c-></code> or
 the like, as that same translation must be performed in addition to the
 duplicative work previously discussed.</p>
    <li data-md>
     <p>We are told exactly the size we have, without risk of UB.  We can avoid
 subsequent reallocations when growing to a buffer to an already-allocated
 size.</p>
    <li data-md>
     <p>Allocator telemetry knows actual request sizes exactly.</p>
   </ul>
   <h2 class="heading settled" data-level="4" id="new"><span class="secno">4. </span><span class="content">New Expressions</span><a class="self-link" href="#new"></a></h2>
   <p>Additionally, we propose expanding this functionality to <code class="highlight"><c- k>new</c-></code> expressions by returning:</p>
   <ul>
    <li data-md>
     <p>For <code class="highlight"><c- k>new</c-></code>, a pointer to the object created and the size of the allocation in bytes.</p>
    <li data-md>
     <p>For <code class="highlight"><c- k>new</c-><c- p>[]</c-></code>, a pointers to the initial element of the array and the size of
the allocation in bytes (less overhead):</p>
<pre class="highlight"><c- k>auto</c-> <c- p>[</c-><c- n>p</c-><c- p>,</c-> <c- n>sz</c-><c- p>]</c-> <c- o>=</c-> <c- k>new</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>return_size</c-><c- p>)</c-> <c- n>T</c-><c- p>[</c-><c- mi>5</c-><c- p>];</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>5</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>sz</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>);</c-> <c- n>i</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
  <c- k>new</c-> <c- p>(</c-><c- n>p</c-><c- p>[</c-><c- n>i</c-><c- p>])</c-> <c- n>T</c-><c- p>;</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>sz</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>);</c-> <c- n>i</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
  <c- n>p</c-><c- p>[</c-><c- n>i</c-><c- p>].</c-><c- n>DoStuff</c-><c- p>();</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>5</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>sz</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>);</c-> <c- n>i</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
  <c- n>p</c-><c- p>[</c-><c- n>i</c-><c- p>].</c-><c- o>~</c-><c- n>T</c-><c- p>();</c->
<c- p>}</c->
<c- k>delete</c-><c- p>[]</c-> <c- n>p</c-><c- p>;</c->
</pre>
   </ul>
   <p>We considered alternatives for returning the size.</p>
   <ul>
    <li data-md>
     <p>We could return two pointers, the initial object and one past the end of
the array (minus the array allocation overhead).</p>
<pre class="highlight"><c- k>auto</c-> <c- p>[</c-><c- n>start</c-><c- p>,</c-> <c- n>end</c-><c- p>]</c-> <c- o>=</c-> <c- k>new</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>return_size</c-><c- p>)</c-> <c- n>T</c-><c- p>[</c-><c- mi>5</c-><c- p>];</c->
<c- k>for</c-> <c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- n>start</c-> <c- o>+</c-> <c- mi>5</c-><c- p>;</c-> <c- n>p</c-> <c- o>!=</c-> <c- n>end</c-><c- p>;</c-> <c- n>p</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
    <c- k>new</c-> <c- p>(</c-><c- n>p</c-><c- p>)</c-> <c- n>T</c-><c- p>;</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- n>start</c-><c- p>;</c-> <c- n>p</c-> <c- o>!=</c-> <c- n>end</c-><c- p>;</c-> <c- n>p</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
    <c- n>p</c-><c- o>-></c-><c- n>DoStuff</c-><c- p>();</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- n>start</c-> <c- o>+</c-> <c- mi>5</c-><c- p>;</c-> <c- n>p</c-> <c- o>!=</c-> <c- n>end</c-><c- p>;</c-> <c- n>p</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
    <c- n>p</c-><c- o>->~</c-><c- n>T</c-><c- p>();</c->
<c- p>}</c->
<c- k>delete</c-><c- p>[]</c-> <c- n>start</c-><c- p>;</c->
</pre>
     <p>The pair of pointers provides convience for use with iterator-oriented
algorithms.  The problem we foresee is that a size-returning allocation
function may not provide a size that is an appropriate multiple of <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code> (or fail to be a multiple of <code class="highlight"><c- k>alignof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code>, a possible, but
unlikely scenario).  This imposes a need for more extensive logic around
the <em>new-expression</em> in handling the returned allocation size.</p>
    <li data-md>
     <p>We could return the size in units of <code class="highlight"><c- n>T</c-></code>, this leads to an inconsistency
between the expected usage for <code class="highlight"><c- k>new</c-></code> and <code class="highlight"><c- k>new</c-><c- p>[]</c-></code>:</p>
     <ul>
      <li data-md>
       <p>For <code class="highlight"><c- k>new</c-></code>, we may only end up fitting a single <code class="highlight"><c- n>T</c-></code> into an allocator size
quanta, so the extra space remains unusable.  If we can fit multiple <code class="highlight"><c- n>T</c-></code> into a single allocator size quanta, we now have an array from what was a
scalar allocation site.  This cannot be foreseen by the compiler as <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> is a replaceable function.</p>
      <li data-md>
       <p>For <code class="highlight"><c- k>new</c-><c- p>[]</c-></code>, the size in units of <code class="highlight"><c- n>T</c-></code> can easily be derived from the
returned size in bytes.</p>
     </ul>
    <li data-md>
     <p>We could pass the size in units of <code class="highlight"><c- n>T</c-></code> or bytes to the constructor of <code class="highlight"><c- n>T</c-></code>:</p>
     <ul>
      <li data-md>
       <p>For <code class="highlight"><c- k>new</c-></code>, this is especially useful for tail-padded arrays, but
neglects default-initialized <code class="highlight"><c- n>T</c-></code>.</p>
      <li data-md>
       <p>For <code class="highlight"><c- k>new</c-><c- p>[]</c-></code>, a common use case is expected to be the allocation of
arrays of <code class="highlight"><c- b>char</c-></code>, <code class="highlight"><c- b>int</c-></code>, etc.  The size of the overall array is
irrelevant for the individual elements.</p>
     </ul>
    <li data-md>
     <p>We could return the size via a reference parameter:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_end</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>end</c-><c- p>;</c->
<c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- k>new</c-> <c- p>(</c-><c- n>end</c-><c- p>)</c-> <c- n>T</c-><c- p>[</c-><c- mi>5</c-><c- p>];</c->
<c- k>for</c-> <c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- n>start</c-> <c- o>+</c-> <c- mi>5</c-><c- p>;</c-> <c- n>p</c-> <c- o>!=</c-> <c- n>end</c-><c- p>;</c-> <c- n>p</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
    <c- k>new</c-> <c- p>(</c-><c- n>p</c-><c- p>)</c-> <c- n>T</c-><c- p>;</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- n>start</c-><c- p>;</c-> <c- n>p</c-> <c- o>!=</c-> <c- n>end</c-><c- p>;</c-> <c- n>p</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
    <c- n>p</c-><c- o>-></c-><c- n>DoStuff</c-><c- p>();</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- n>start</c-> <c- o>+</c-> <c- mi>5</c-><c- p>;</c-> <c- n>p</c-> <c- o>!=</c-> <c- n>end</c-><c- p>;</c-> <c- n>p</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
    <c- n>p</c-><c- o>->~</c-><c- n>T</c-><c- p>();</c->
<c- p>}</c->
</pre>
     <p>or, demonstrated with bytes:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-> <c- n>size</c-><c- p>;</c->
<c- n>T</c-><c- o>*</c-> <c- n>p</c-> <c- o>=</c-> <c- k>new</c-> <c- p>(</c-><c- n>size</c-><c- p>)</c-> <c- n>T</c-><c- p>[</c-><c- mi>5</c-><c- p>];</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>5</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>size</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>);</c-> <c- n>i</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
  <c- k>new</c-> <c- p>(</c-><c- n>p</c-><c- p>[</c-><c- n>i</c-><c- p>])</c-> <c- n>T</c-><c- p>;</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>size</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>);</c-> <c- n>i</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
  <c- n>p</c-><c- p>[</c-><c- n>i</c-><c- p>].</c-><c- n>DoStuff</c-><c- p>();</c->
<c- p>}</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>5</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>size</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>);</c-> <c- n>i</c-><c- o>++</c-><c- p>)</c-> <c- p>{</c->
  <c- n>p</c-><c- p>[</c-><c- n>i</c-><c- p>].</c-><c- o>~</c-><c- n>T</c-><c- p>();</c->
<c- p>}</c->
<c- k>delete</c-><c- p>[]</c-> <c- n>p</c-><c- p>;</c->
</pre>
     <p>(Casts omitted for clarity.)</p>
     <p>As discussed for <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> in <a href="#prop">§ 2 Proposal</a>, a reference parameter poses
difficulties for optimizers and involves returning the size via
memory (depending on ABI).</p>
   </ul>
   <p>For <code class="highlight"><c- k>new</c-><c- p>[]</c-></code> expressions, we considered alternatively initializing the returned
(<code class="highlight"><c- n>sz</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code>) number of elements.</p>
   <ul>
    <li data-md>
     <p>This would avoid the need to explicitly construct / destruct the elements
with the additional returned space (if any).</p>
     <p>The <em>new-initializer</em> is invoked for the returned number of elements,
rather than the requested number of elements.  This allows <code class="highlight"><c- k>delete</c-><c- p>[]</c-></code> to
destroy the correct number of elements (by storing <code class="highlight"><c- n>sz</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code> in the
array allocation overhead).</p>
    <li data-md>
     <p>The presented proposal (leaving this space uninitialized) was chosen for
consistency with <code class="highlight"><c- k>new</c-></code>.</p>
   </ul>
   <h2 class="heading settled" data-level="5" id="rel"><span class="secno">5. </span><span class="content">Related work</span><a class="self-link" href="#rel"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0401r1">[P0401R1]</a> considers this problem at the level of the <code class="highlight"><c- n>Allocator</c-></code> concept.
Ironically, the lack of the above API was one significant problem in its
original revision: how could an implementation of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocator</c-></code> provide the
requested feedback in a way that would work with any underlying malloc
implementation?</p>
   <h2 class="heading settled" data-level="6" id="history"><span class="secno">6. </span><span class="content">History</span><a class="self-link" href="#history"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="R6"><span class="secno">6.1. </span><span class="content">R6 → R7</span><a class="self-link" href="#R6"></a></h3>
   <ul>
    <li data-md>
     <p>Removed extraneous <code class="highlight"><c- n>std</c-><c- o>::</c-></code> typo</p>
   </ul>
   <h3 class="heading settled" data-level="6.2" id="R6①"><span class="secno">6.2. </span><span class="content">R5 → R6</span><a class="self-link" href="#R6①"></a></h3>
   <p>LEWG reviewed <a data-link-type="biblio" href="#biblio-p0901r5">[P0901R5]</a> library support types at <a data-link-type="biblio" href="#biblio-prague">[Prague]</a>.</p>
   <ul>
    <li data-md>
     <p>LEWG discussed the similarity to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code>.  Participants in the
discussion were concerned about the potential library layering issue
(<code class="highlight"><c- o>&lt;</c-><c- k>new</c-><c- o>></c-></code> layers below practically everything) and that ownership is
conveyed in the return value.  In contrast, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code> is non-owning.</p>
    <li data-md>
     <p>LEWG discussed whether the type needed a name.  It needs to be named to
allow users to replace <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code> and spell the type.</p>
   </ul>
   <p>LEWG took an approval poll of different names</p>
   <blockquote>
    <table>
     <tbody>
      <tr>
       <td>5
       <td><code class="highlight"><c- n>sized_ptr_t</c-></code>
      <tr>
       <td>2
       <td><code class="highlight"><c- n>sized_ptr</c-></code>
      <tr>
       <td>3
       <td><code class="highlight"><c- n>memblock_t</c-></code>
      <tr>
       <td>7
       <td><code class="highlight"><c- n>memblock</c-></code>
      <tr>
       <td>0
       <td><code class="highlight"><c- n>memspan_t</c-></code>
      <tr>
       <td>2
       <td><code class="highlight"><c- n>memspan</c-></code>
      <tr>
       <td>8
       <td><code class="highlight"><c- n>alloc_size_t</c-></code>
      <tr>
       <td>4
       <td><code class="highlight"><c- n>alloc_size</c-></code>
      <tr>
       <td>5
       <td><code class="highlight"><c- n>alloc_t</c-></code>
      <tr>
       <td>0
       <td><code class="highlight"><c- n>alloc</c-></code>
      <tr>
       <td>13
       <td><code class="highlight"><c- n>sized_allocation_t</c-></code>
      <tr>
       <td>17
       <td><code class="highlight"><c- n>sized_allocation</c-></code>
      <tr>
       <td>0
       <td><code class="highlight"><c- n>alloc_span_t</c-></code>
      <tr>
       <td>2
       <td><code class="highlight"><c- n>alloc_span</c-></code>
      <tr>
       <td>9
       <td><code class="highlight"><c- n>allocation_t</c-></code>
      <tr>
       <td>9
       <td><code class="highlight"><c- n>allocation</c-></code>
      <tr>
       <td>16
       <td><code class="highlight"><c- n>alloc_result_t</c-></code>
      <tr>
       <td>14
       <td><code class="highlight"><c- n>alloc_result</c-></code>
      <tr>
       <td>12
       <td><code class="highlight"><c- n>allocation_result_t</c-></code>
      <tr>
       <td>16
       <td><code class="highlight"><c- n>allocation_result</c-></code>
    </table>
   </blockquote>
   <p>Based on this poll, LEWG directed the paper authors to consider <code class="highlight"><c- n>sized_allocation</c-></code>, <code class="highlight"><c- n>alloc_result_t</c-></code>, and <code class="highlight"><c- n>allocation_result</c-></code>.</p>
   <p>LEWG additionally polled on whether the type should have a <code class="highlight"><c- n>_t</c-></code> suffix.</p>
   <blockquote>
    <table>
     <tbody>
      <tr>
       <th>SF
       <th>F
       <th>N
       <th>A
       <th>SA
      <tr>
       <td>0
       <td>6
       <td>7
       <td>8
       <td>1
    </table>
   </blockquote>
   <p>Based on this feedback, the authors have chosen <code class="highlight"><c- n>sized_allocation_t</c-></code>, based on
LEWG’s naming suggestions and for consistency (in choice of <code class="highlight"><c- n>_t</c-></code>) with the
other allocation library support types (<code class="highlight"><c- b>size_t</c-></code>, <code class="highlight"><c- n>align_val_t</c-></code>, <code class="highlight"><c- n>nothrow_t</c-></code>,
etc.).  We expect this to be spelled rarely.</p>
   <h3 class="heading settled" data-level="6.3" id="R5"><span class="secno">6.3. </span><span class="content">R4 → R5</span><a class="self-link" href="#R5"></a></h3>
   <p>EWG reviewed P0901R4 at <a data-link-type="biblio" href="#biblio-cologne">[Cologne]</a>.</p>
   <blockquote>
     Poll: P0901R4 as presented, forward to LEWG for C++23, <em>not</em> C++20. 
    <table>
     <tbody>
      <tr>
       <th>SF
       <th>F
       <th>N
       <th>A
       <th>SA
      <tr>
       <td>2
       <td>11
       <td>14
       <td>2
       <td>0
    </table>
   </blockquote>
   <ul>
    <li data-md>
     <p>Fixed typos in examples.</p>
    <li data-md>
     <p>Added proposed feature test macro.</p>
    <li data-md>
     <p>Added LEWG audience for library support type names.</p>
   </ul>
   <h3 class="heading settled" data-level="6.4" id="R4"><span class="secno">6.4. </span><span class="content">R3 → R4</span><a class="self-link" href="#R4"></a></h3>
   <ul>
    <li data-md>
     <p>Update reference to revised <a data-link-type="biblio" href="#biblio-p0401r1">[P0401R1]</a>.</p>
   </ul>
   <h3 class="heading settled" data-level="6.5" id="R3"><span class="secno">6.5. </span><span class="content">R2 → R3</span><a class="self-link" href="#R3"></a></h3>
   <ul>
    <li data-md>
     <p>Added proposed wording.</p>
    <li data-md>
     <p>For newly added allocation functions, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-></code> is passed by value,
rather than reference.</p>
   </ul>
   <h3 class="heading settled" data-level="6.6" id="R2"><span class="secno">6.6. </span><span class="content">R1 → R2</span><a class="self-link" href="#R2"></a></h3>
   <p>Applied feedback from San Diego Mailing</p>
   <ul>
    <li data-md>
     <p>Moved from passing <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-></code> parameter by reference to by value.
For many ABIs, this is more optimizable and to the authors' knowledge, no
worse on any other.</p>
    <li data-md>
     <p>Added rationale for not using parameter packs for this functionality.</p>
   </ul>
   <h3 class="heading settled" data-level="6.7" id="R1"><span class="secno">6.7. </span><span class="content">R0 → R1</span><a class="self-link" href="#R1"></a></h3>
   <p>Applied feedback from <a data-link-type="biblio" href="#biblio-jacksonvilleminutes">[JacksonvilleMinutes]</a>.</p>
   <ul>
    <li data-md>
     <p>Clarified in <a href="#prop">§ 2 Proposal</a> the desire to leverage the existing "replacement
functions" wording of the IS, particularly given the close interoperation
with the existing <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>/<code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>delete</c-></code> implementations.</p>
    <li data-md>
     <p>Added a discussion of the Microsoft ABI in <a href="#prop">§ 2 Proposal</a>.</p>
    <li data-md>
     <p>Noted in <a href="#splode">§ 3.1 How many ::operator new's?</a> the possibility of using a parameter pack.</p>
    <li data-md>
     <p>Added a proposal for <a href="#new">§ 4 New Expressions</a>, as requested by EWG.</p>
   </ul>
   <p>Additionally, a discussion of <a href="#sizeddelete">§ 3.3 Interaction with Sized Delete</a> has been added.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-cologne">[Cologne]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21cologne2019/P0901R4-EWG">Size feedback in operator new</a>. 2019-09-17. URL: <a href="http://wiki.edg.com/bin/view/Wg21cologne2019/P0901R4-EWG">http://wiki.edg.com/bin/view/Wg21cologne2019/P0901R4-EWG</a>
   <dt id="biblio-jacksonvilleminutes">[JacksonvilleMinutes]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21jacksonville2018/P0901R0-Jax18">Jacksonville 2018 minutes</a>. 2018-03-15. URL: <a href="http://wiki.edg.com/bin/view/Wg21jacksonville2018/P0901R0-Jax18">http://wiki.edg.com/bin/view/Wg21jacksonville2018/P0901R0-Jax18</a>
   <dt id="biblio-jemalloc">[JEMALLOC]
   <dd><a href="http://jemalloc.net/jemalloc.3.html">jemalloc(3) - Linux man page</a>. URL: <a href="http://jemalloc.net/jemalloc.3.html">http://jemalloc.net/jemalloc.3.html</a>
   <dt id="biblio-mallocextension">[MallocExtension]
   <dd><a href="https://github.com/google/tcmalloc/blob/master/tcmalloc/malloc_extension.h">TCMalloc Malloc Extensions</a>. URL: <a href="https://github.com/google/tcmalloc/blob/master/tcmalloc/malloc_extension.h">https://github.com/google/tcmalloc/blob/master/tcmalloc/malloc_extension.h</a>
   <dt id="biblio-microsoftabi">[MicrosoftABI]
   <dd><a href="https://docs.microsoft.com/en-us/cpp/build/return-values-cpp">Return Values</a>. 2016-11-03. URL: <a href="https://docs.microsoft.com/en-us/cpp/build/return-values-cpp">https://docs.microsoft.com/en-us/cpp/build/return-values-cpp</a>
   <dt id="biblio-n4791">[N4791]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4791.pdf">Working Draft, Standard for Programming Language C++</a>. 2018-12-07. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4791.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4791.pdf</a>
   <dt id="biblio-p0401r1">[P0401R1]
   <dd>Jonathan Wakely; Chris Kennelly. <a href="http://wg21.link/P0401R1">Providing size feedback in the Allocator interface</a>. 2019-06-11. URL: <a href="http://wg21.link/P0401R1">http://wg21.link/P0401R1</a>
   <dt id="biblio-p0901r5">[P0901R5]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0901r5.html">Size feedback in operator new</a>. 2019-10-06. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0901r5.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0901r5.html</a>
   <dt id="biblio-prague">[Prague]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21prague/P0901">LEWG Prague Minutes for P901R5</a>. 2020-02-10. URL: <a href="http://wiki.edg.com/bin/view/Wg21prague/P0901">http://wiki.edg.com/bin/view/Wg21prague/P0901</a>
   <dt id="biblio-sizeddelete">[SizedDelete]
   <dd>Lawrence Crowl. <a href="http://wg21.link/n3536">C++ Sized Deallocation</a>. URL: <a href="http://wg21.link/n3536">http://wg21.link/n3536</a>
   <dt id="biblio-smallocx">[SMALLOCX]
   <dd><a href="https://github.com/jemalloc/jemalloc/pull/1270">Add experimental API to support P0901r0</a>. URL: <a href="https://github.com/jemalloc/jemalloc/pull/1270">https://github.com/jemalloc/jemalloc/pull/1270</a>
   <dt id="biblio-tcmalloc">[TCMalloc]
   <dd><a href="https://github.com/google/tcmalloc">TCMalloc</a>. URL: <a href="https://github.com/google/tcmalloc">https://github.com/google/tcmalloc</a>
  </dl>