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 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P0901R2: 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)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

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

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

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

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

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

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

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

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

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

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

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

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

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

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

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

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

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

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

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

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

  /* Do something nice. */

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

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

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

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

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

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

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

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

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

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

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

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

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

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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/******************************************************************************/
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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">P0901R2<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="2018-11-25">2018-11-25</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0901R2">http://wg21.link/P0901R2</a>
     <dt>Authors:
     <dd>
      <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>
      <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>EWG
     <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>
     <ol class="toc">
      <li><a href="#splode"><span class="secno">2.1</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">2.2</span> <span class="content">Implementation difficulty</span></a>
      <li><a href="#sizeddelete"><span class="secno">2.3</span> <span class="content">Interaction with Sized Delete</span></a>
      <li><a href="#advantages"><span class="secno">2.4</span> <span class="content">Advantages</span></a>
     </ol>
    <li><a href="#new"><span class="secno">3</span> <span class="content">New Expressions</span></a>
    <li><a href="#rel"><span class="secno">4</span> <span class="content">Related work</span></a>
    <li>
     <a href="#history"><span class="secno">5</span> <span class="content">History</span></a>
     <ol class="toc">
      <li><a href="#R2"><span class="secno">5.1</span> <span class="content">R1 → R2</span></a>
      <li><a href="#R1"><span class="secno">5.2</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> calls
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 (TCMalloc) 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> 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
implementation.</p>
<pre class="highlight"><c- k>struct</c-> <c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-> <c- p>{};</c->

<c- k>struct</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_ptr_t</c-> <c- p>{</c->
  <c- b>void</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->

<c- n>std</c-><c- o>::</c-><c- n>sized_ptr_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>sized_ptr_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>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>sized_ptr_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- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-> <c- o>&amp;</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>sized_ptr_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>al</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- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->

<c- n>std</c-><c- o>::</c-><c- n>sized_ptr_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>sized_ptr_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>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>sized_ptr_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- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-> <c- o>&amp;</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>sized_ptr_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>al</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- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
</pre>
   <p>Additionally, we amend 15.5.4.6 (Replacement functions), wording relative to <a data-link-type="biblio" href="#biblio-n4762">[N4762]</a>:</p>
<pre class="highlight"><c- k>operator</c-> <c- nf>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- n>std</c-><c- o>::</c-><c- n>sized_ptr_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>sized_ptr_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>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>sized_ptr_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- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-> <c- o>&amp;</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>sized_ptr_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>al</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- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
</ins>
<c- k>operator</c-> <c- nf>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-><c- p>)</c->
<c- k>operator</c-> <c- k>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>(</c-><c- b>void</c-><c- o>*</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->

<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- n>std</c-><c- o>::</c-><c- n>sized_ptr_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>sized_ptr_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>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>sized_ptr_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- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>nothrow_t</c-> <c- o>&amp;</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>sized_ptr_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>al</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- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
</ins>
<c- k>operator</c-> <c- k>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</c-><c- p>)</c->
<c- k>operator</c-> <c- k>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</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>delete</c-><c- p>[](</c-><c- b>void</c-><c- o>*</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->
</pre>
   <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- 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. Candidate (rough) language for the first overload
would be:</p>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]]</c-> <c- n>std</c-><c- o>::</c-><c- n>sized_ptr_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- k>const</c->
<c- n>std</c-><c- o>::</c-><c- n>return_size_t</c-><c- p>);</c->
</pre>
   <blockquote>
    <p><em>Effects</em>: returns a pair (p, n) with <code class="highlight"><c- n>n</c-> <c- o>>=</c-> <c- n>size</c-></code>.
Behaves as if <code class="highlight"><c- n>p</c-></code> was the return value of a call to <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-><c- p>(</c-><c- n>n</c-><c- p>)</c-></code>.</p>
   </blockquote>
   <p>The intention is quite simple: we return the "actual" size of the allocation,
and rely on "as if" to do the heavy lifting that lets us use more than <code class="highlight"><c- n>size</c-></code> bytes of the resulting allocation. <strong>In particular, this means at no point do
we risk undefined behavior from using more bytes than</strong> <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code> <strong>was
called with.</strong></p>
   <h3 class="heading settled" data-level="2.1" id="splode"><span class="secno">2.1. </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-p0401">[P0401]</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>
   <h3 class="heading settled" data-level="2.2" id="trivial"><span class="secno">2.2. </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_ptr_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 a (currently internal) implementation.  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="2.3" id="sizeddelete"><span class="secno">2.3. </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_ptr_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="2.4" id="advantages"><span class="secno">2.4. </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 with <code class="highlight"><c- n>nallocx</c-></code> or the like.  We pay almost nothing in speed to return an actual-size
parameter; allocator telemetry knows actual request sizes exactly; and we are
told exactly the size we have, without risk of UB.</p>
   <h2 class="heading settled" data-level="3" id="new"><span class="secno">3. </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>, pointers to the object created and the end of the allocation.</p>
    <li data-md>
     <p>For <code class="highlight"><c- k>new</c-><c- p>[]</c-></code>, pointers to the initial element of the array and one past the
last element of the array.</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_t</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.</p>
   </ul>
   <p>We considered alternatives for returning the size.</p>
   <ul>
    <li data-md>
     <p>We could return the size in units of <em>bytes</em> (minus the array allocation
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_t</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>
    <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>s</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="4" id="rel"><span class="secno">4. </span><span class="content">Related work</span><a class="self-link" href="#rel"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-allocatorext">[AllocatorExt]</a> considered 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: 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>
   <p>If this proposal is accepted, it’s likely that <a data-link-type="biblio" href="#biblio-allocatorext">[AllocatorExt]</a> should be taken
up again.</p>
   <h2 class="heading settled" data-level="5" id="history"><span class="secno">5. </span><span class="content">History</span><a class="self-link" href="#history"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="R2"><span class="secno">5.1. </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="5.2" id="R1"><span class="secno">5.2. </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">§2.1 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">§3 New Expressions</a>, as requested by EWG.</p>
   </ul>
   <p>Additionally, a discussion of <a href="#sizeddelete">§2.3 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-allocatorext">[AllocatorExt]
   <dd>Jonathan Wakely. <a href="http://wg21.link/P0401R0">Extensions to the Allocator interface</a>. 2015-07-08. URL: <a href="http://wg21.link/P0401R0">http://wg21.link/P0401R0</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-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-n4762">[N4762]
   <dd><a href="https://wg21.link/N4762">Working Draft, Standard for Programming Language C++</a>. 2018-07-07. URL: <a href="https://wg21.link/N4762">https://wg21.link/N4762</a>
   <dt id="biblio-p0401">[P0401]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0401r0.html">Extensions to the Allocator interface</a>. 2015-07-08. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0401r0.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0401r0.html</a>
   <dt id="biblio-sizeddelete">[SizedDelete]
   <dd>L; et al. <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>
  </dl>