<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>p0901r0: 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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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">p0901r0<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="2018-02-05">5 February 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0901R0">http://wg21.link/P0901R0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ahh@google.com">Andrew Hunter</a> (<span class="p-org org">Google</span>)
     <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 JTC1/SC22/WG21: Programming Language C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>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 operator news?</span></a>
      <li><a href="#trivial"><span class="secno">2.2</span> <span class="content">Implementation difficulty</span></a>
      <li><a href="#advantages"><span class="secno">2.3</span> <span class="content">Advantages</span></a>
     </ol>
    <li><a href="#rel"><span class="secno">3</span> <span class="content">Related work</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="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"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span></code> <em>both as fairly standard practice for implementors, and to
make clear the distinction between the interface and the implementation.</em> 
   <p>Everyone’s favorite dynamic data structure, <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">vector</span></code>, allocates memory with
code that looks something like this (with many details, like <code class="highlight"><span class="n">Allocator</span></code>,
templating for non <code class="highlight"><span class="kt">char</span></code>, and
exception safety, elided):</p>
<pre class="highlight"><span class="kt">void</span> <span class="n">vector</span><span class="o">::</span><span class="n">reserve</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">new_cap</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">if</span> <span class="p">(</span><span class="n">capacity_</span> <span class="o">>=</span> <span class="n">new_cap</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span>
  <span class="k">const</span> <span class="kt">size_t</span> <span class="n">bytes</span> <span class="o">=</span> <span class="n">new_cap</span><span class="p">;</span>
  <span class="kt">void</span> <span class="o">*</span><span class="n">newp</span> <span class="o">=</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="n">new_cap</span><span class="p">);</span>
  <span class="n">memcpy</span><span class="p">(</span><span class="n">newp</span><span class="p">,</span> <span class="n">ptr_</span><span class="p">,</span> <span class="n">capacity_</span><span class="p">);</span>
  <span class="n">ptr_</span> <span class="o">=</span> <span class="n">newp</span><span class="p">;</span>
  <span class="n">capacity_</span> <span class="o">=</span> <span class="n">bytes</span><span class="p">;</span>
<span class="p">}</span>
</pre>
   <p>Consider the sequence of calls:</p>
<pre class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="kt">char</span><span class="o">></span> <span class="n">v</span><span class="p">;</span>
<span class="n">v</span><span class="p">.</span><span class="n">reserve</span><span class="p">(</span><span class="mi">37</span><span class="p">);</span>
<span class="c1">// ...</span>
<span class="c1"></span><span class="n">v</span><span class="p">.</span><span class="n">reserve</span><span class="p">(</span><span class="mi">38</span><span class="p">);</span>
</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>
    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"><span class="n">nallocx</span></code>.  We can then use that hint as a smarter parameter for operator
new: 
<pre class="highlight"><span class="kt">void</span> <span class="n">vector</span><span class="o">::</span><span class="n">reserve</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">new_cap</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">if</span> <span class="p">(</span><span class="n">capacity_</span> <span class="o">>=</span> <span class="n">new_cap</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span>
  <span class="k">const</span> <span class="kt">size_t</span> <span class="n">bytes</span> <span class="o">=</span> <span class="n">nallocx</span><span class="p">(</span><span class="n">new_cap</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span>
  <span class="kt">void</span> <span class="o">*</span><span class="n">newp</span> <span class="o">=</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="n">bytes</span><span class="p">);</span>
  <span class="n">memcpy</span><span class="p">(</span><span class="n">newp</span><span class="p">,</span> <span class="n">ptr_</span><span class="p">,</span> <span class="n">capacity_</span><span class="p">);</span>
  <span class="n">ptr_</span> <span class="o">=</span> <span class="n">newp</span><span class="p">;</span>
  <span class="n">capacity_</span> <span class="o">=</span> <span class="n">bytes</span><span class="p">;</span>
<span class="p">}</span>
</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>
    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"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="mi">37</span><span class="p">)</span></code> might return 64
and 128 bytes, we’d definitely rather know the right answer, not a conservative approximation. 
   <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>
    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"><span class="n">nallocx</span></code>, we duplicate all that communication, and also
the overhead of the function call. 
   <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"><span class="n">nallocx</span></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"><span class="n">nallocx</span></code>, we will only ever be told that they want 40 (or 48, or whatever <code class="highlight"><span class="n">nallocx</span><span class="p">(</span><span class="mi">37</span><span class="p">)</span></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.  The exact results we’ve achieved are confidential, but the
savings are significant.</p>
   <p>(sufficiently widespread use of) <code class="highlight"><span class="n">nallocx</span></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"><span class="n">nallocx</span></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"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span></code> wants.</p>
   <p>These three issues explain why we don’t believe <code class="highlight"><span class="n">nallocx</span></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>
    Another obvious suggestion is to add a way to inspect the size of an object returend
by <code class="highlight"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span></code>. Most mallocs provide a way to do this; jemalloc calls it <code class="highlight"><span class="n">sallocx</span></code>.  Vector would look like: 
<pre class="highlight"><span class="kt">void</span> <span class="n">vector</span><span class="o">::</span><span class="n">reserve</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">new_cap</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">if</span> <span class="p">(</span><span class="n">capacity_</span> <span class="o">>=</span> <span class="n">new_cap</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span>
  <span class="kt">void</span> <span class="o">*</span><span class="n">newp</span> <span class="o">=</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="n">new_cap</span><span class="p">);</span>
  <span class="k">const</span> <span class="kt">size_t</span> <span class="n">bytes</span> <span class="o">=</span> <span class="n">sallocx</span><span class="p">(</span><span class="n">newp</span><span class="p">);</span>
  <span class="n">memcpy</span><span class="p">(</span><span class="n">newp</span><span class="p">,</span> <span class="n">ptr_</span><span class="p">,</span> <span class="n">capacity_</span><span class="p">);</span>
  <span class="n">ptr_</span> <span class="o">=</span> <span class="n">newp</span><span class="p">;</span>
  <span class="n">capacity_</span> <span class="o">=</span> <span class="n">bytes</span><span class="p">;</span>
<span class="p">}</span>
</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"><span class="n">new_cap</span><span class="o">+</span><span class="mi">1</span></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>
    We should also quickly examine why the classic C API <code class="highlight"><span class="n">realloc</span></code> is insufficient. 
<pre class="highlight"><span class="kt">void</span> <span class="n">vector</span><span class="o">::</span><span class="n">reserve</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">new_cap</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">if</span> <span class="p">(</span><span class="n">capacity_</span> <span class="o">>=</span> <span class="n">new_cap</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span>
  <span class="n">ptr_</span> <span class="o">=</span> <span class="n">realloc</span><span class="p">(</span><span class="n">ptr_</span><span class="p">,</span> <span class="n">new_cap</span><span class="p">);</span>
  <span class="n">capacity_</span> <span class="o">=</span> <span class="n">new_cap</span><span class="p">;</span>
<span class="p">}</span>
</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"><span class="n">realloc</span></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"><span class="n">realloc</span></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"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span></code> that directly inform the user of the size available to them.</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span> <span class="p">{};</span>

<span class="k">struct</span> <span class="n">std</span><span class="o">::</span><span class="n">sized_ptr_t</span> <span class="p">{</span>
  <span class="kt">void</span> <span class="o">*</span><span class="n">p</span><span class="p">;</span>
  <span class="kt">size_t</span> <span class="n">n</span><span class="p">;</span>
<span class="p">};</span>

<span class="n">std</span><span class="o">::</span><span class="n">sized_ptr_t</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">size</span><span class="p">,</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="o">&amp;</span><span class="p">);</span>
<span class="n">std</span><span class="o">::</span><span class="n">sized_ptr_t</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">size</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">align_val_t</span> <span class="n">al</span><span class="p">,</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="o">&amp;</span><span class="p">);</span>
<span class="n">std</span><span class="o">::</span><span class="n">sized_ptr_t</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">size</span><span class="p">,</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">nothrow_t</span> <span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="o">&amp;</span><span class="p">);</span>
<span class="n">std</span><span class="o">::</span><span class="n">sized_ptr_t</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">size</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">align_val_t</span> <span class="n">al</span><span class="p">,</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">nothrow_t</span> <span class="o">&amp;</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="o">&amp;</span><span class="p">);</span>
</pre>
   <p>(We would need the same <code class="highlight"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">[]</span></code> overloads; omitted for clarity.) Another signature we could use would be:</p>
<pre class="highlight"><span class="k">enum</span> <span class="k">class</span> <span class="nc">return_size_t</span> <span class="o">:</span> <span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="p">{};</span>
<span class="kt">void</span> <span class="o">*</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">size</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="o">&amp;</span><span class="p">);</span>
</pre>
   <p>(and so on.)  This is slightly simpler to read as a signature, but arguably
worse in usage:</p>
<pre class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">tie</span><span class="p">(</span><span class="n">obj</span><span class="p">.</span><span class="n">ptr</span><span class="p">,</span> <span class="n">obj</span><span class="p">.</span><span class="n">size</span><span class="p">)</span> <span class="o">=</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="mi">37</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="p">{});</span>

<span class="c1">// ...vs...</span>
<span class="c1"></span>
<span class="c1">// Presumably the object implementation wants to contain a size_t,</span>
<span class="c1">// not a return_size_t.</span>
<span class="c1"></span><span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span> <span class="n">rs</span><span class="p">;</span>
<span class="n">obj</span><span class="p">.</span><span class="n">ptr</span> <span class="o">=</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="mi">37</span><span class="p">,</span> <span class="n">rs</span><span class="p">);</span>
<span class="n">obj</span><span class="p">.</span><span class="n">size</span> <span class="o">=</span> <span class="n">rs</span><span class="p">;</span>
</pre>
   <p>More importantly, this form is less efficient. In practice, underlying malloc
implementations provide actual definitions of <code class="highlight"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span></code> symbols which are
called like any other function.  Passing a reference parameter requires us to
actually return the size via memory; most important 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>
   <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"><span class="p">[[</span><span class="n">nodiscard</span><span class="p">]]]</span> <span class="n">std</span><span class="o">::</span><span class="n">sized_ptr_t</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">size</span><span class="p">,</span> <span class="k">const</span>
<span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="o">&amp;</span><span class="p">);</span>
</pre>
   <blockquote>
    <p><em>Effects</em>: returns a pair (p, n) with <code class="highlight"><span class="n">n</span> <span class="o">>=</span> <span class="n">size</span></code>.
Behaves as if <code class="highlight"><span class="n">p</span></code> was the return value of a call to <code class="highlight"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="n">n</span><span class="p">)</span></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"><span class="n">size</span></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"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span></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 operator news?</span><a class="self-link" href="#splode"></a></h3>
   <p>It is unfortunate that we have so many permutations of <code class="highlight"><span class="o">::</span><span class="k">operator</span> <span class="k">new</span></code>--eight
seems like far more than we should really need!  But there really isn’t any
siginificant runtime cost for having them.</p>
   <p>Any alternate proposal that didn’t require so many trivially different
signatures would be appreciated.</p>
   <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"><span class="n">std</span><span class="o">::</span><span class="n">sized_ptr_t</span> <span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">n</span><span class="p">,</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">return_size_t</span><span class="o">&amp;</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">return</span> <span class="p">{</span><span class="o">::</span><span class="k">operator</span> <span class="k">new</span><span class="p">(</span><span class="n">n</span><span class="p">),</span> <span class="n">n</span><span class="p">};</span>
<span class="p">}</span>
</pre>
   <p>Malloc implementations are free to properly override this with a more impactful
definition, but this paper poses no significant difficulty for toolchain
implementors.</p>
   <h3 class="heading settled" data-level="2.3" id="advantages"><span class="secno">2.3. </span><span class="content">Advantages</span><a class="self-link" href="#advantages"></a></h3>
    It’s easy to see that this approach nicely solves the problems with <code class="highlight"><span class="n">nallocx</span></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. 
   <h2 class="heading settled" data-level="3" id="rel"><span class="secno">3. </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"><span class="n">Allocator</span></code> concept. Ironically, the lack of the above API was one significant problem: how
could an implementation of <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">allocator</span></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>
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    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn’t exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn’t standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don’t have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we’re at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can’t find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>
  <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-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-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>
  </dl>