<!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>P0401R3: Providing size feedback in the Allocator interface</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">P0401R3<br>Providing size feedback in the Allocator interface</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2020-03-01">2020-03-01</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0401R3">http://wg21.link/P0401R3</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:cxx@kayari.org">Jonathan Wakely</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>LWG
     <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>Utilize size feedback from Allocator to reduce spurious reallocations</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="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#reallocation"><span class="secno">2.1</span> <span class="content">Why not <code class="highlight"><c- n>realloc</c-></code></span></a>
      <li><a href="#size"><span class="secno">2.2</span> <span class="content">Why not ask the allocator for the size</span></a>
     </ol>
    <li><a href="#proposal"><span class="secno">3</span> <span class="content">Proposal</span></a>
    <li>
     <a href="#design"><span class="secno">4</span> <span class="content">Design Considerations</span></a>
     <ol class="toc">
      <li><a href="#overloads"><span class="secno">4.1</span> <span class="content"><code class="highlight"><c- n>allocate</c-></code> selection</span></a>
      <li><a href="#returnvalue"><span class="secno">4.2</span> <span class="content">Size Return Value</span></a>
      <li><a href="#deallocate"><span class="secno">4.3</span> <span class="content"><code class="highlight"><c- n>deallocate</c-></code> changes</span></a>
      <li><a href="#pmr"><span class="secno">4.4</span> <span class="content">Interaction with <code class="highlight"><c- n>polymorphic_allocator</c-></code></span></a>
      <li><a href="#zero"><span class="secno">4.5</span> <span class="content">Zero-Sized Requests</span></a>
     </ol>
    <li>
     <a href="#revisions"><span class="secno">5</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#R3"><span class="secno">5.1</span> <span class="content">R2 → R3</span></a>
      <li><a href="#R2"><span class="secno">5.2</span> <span class="content">R1 → R2</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="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>This is a library paper to describe how size feedback can be used with allocators:</p>
   <ul>
    <li data-md>
     <p>In the case of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocator</c-></code>, the language feature proposed in <a data-link-type="biblio" href="#biblio-p0901r5">[P0901R5]</a> could be used.</p>
    <li data-md>
     <p>In the case of custom allocators, the availability of the traits  interface
allows standard library containers to leverage this functionality.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Consider code adding elements to <code class="highlight"><c- n>vector</c-></code>:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- p>{</c-><c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-><c- p>};</c->
<c- c1>// Expected: v.capacity() == 3</c->

<c- c1>// Add an additional element, triggering a reallocation.</c->
<c- n>v</c-><c- p>.</c-><c- n>push_back</c-><c- p>(</c-><c- mi>4</c-><c- p>);</c->
</pre>
   <p>Many allocators only allocate in fixed-sized chunks of memory, rounding up
requests.  Our underlying heap allocator received a request for 12 bytes (<code class="highlight"><c- mi>3</c-> <c- o>*</c-> <c- k>sizeof</c-><c- p>(</c-><c- b>int</c-><c- p>)</c-></code>) while constructing <code class="highlight"><c- n>v</c-></code>.  For several implementations, this request
is turned into a 16 byte region.</p>
   <h3 class="heading settled" data-level="2.1" id="reallocation"><span class="secno">2.1. </span><span class="content">Why not <code class="highlight"><c- n>realloc</c-></code></span><a class="self-link" href="#reallocation"></a></h3>
   <p><code class="highlight"><c- n>realloc</c-></code> poses several problems problems:</p>
   <ul>
    <li data-md>
     <p>We have unclear strategies for growing a container.  Consider the two
options (<code class="highlight"><c- n>a</c-></code> and <code class="highlight"><c- n>b</c-></code>) outlined in the comments of the example below.
Assume, as with the <a href="#motivation">§2 Motivation</a> example, that there is a 16 byte size
class for our underlying allocator.</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- p>{</c-><c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-><c- p>};</c->
<c- c1>// Expected: v.capacity() == 3</c->

<c- c1>// Add an additional element.  Does the vector:</c->
<c- c1>// a) realloc(sizeof(int) * 4), adding a single element</c->
<c- c1>// b) realloc(sizeof(int) * 6), attempt to double, needing reallocation</c->
<c- n>v</c-><c- p>.</c-><c- n>push_back</c-><c- p>(</c-><c- mi>4</c-><c- p>);</c->

<c- c1>// Add another element.</c->
<c- c1>// a) realloc(sizeof(int) * 5), fails in-place, requires reallocation</c->
<c- c1>// b) size + 1 &lt;= capacity, no reallocation required</c->
<c- n>v</c-><c- p>.</c-><c- n>push_back</c-><c- p>(</c-><c- mi>5</c-><c- p>);</c->
</pre>
     <p>Option <code class="highlight"><c- n>a</c-></code> requires every append (after the initial allocation) try to <code class="highlight"><c- n>realloc</c-></code>, requiring an interaction with the allocator each time.</p>
     <p>Option <code class="highlight"><c- n>b</c-></code> requires guesswork to probe the allocator’s true size.  By
doubling (the typical growth pattern for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code> in libc++ and
libstdc++), we skip past the existing allocation’s size boundary.  While we
can use a reduced growth rate, doing so requires more interactions with the
allocator (calls to <code class="highlight"><c- n>realloc</c-></code>), as well as copies (when we cannot resize
in-place).</p>
     <p>Both of these options add extra branching on the control flow for growing
the buffer.</p>
    <li data-md>
     <p>When the allocator cannot resize the buffer in-place, it allocates new
memory and <code class="highlight"><c- n>memcpy</c-></code>'s the contents.  For non-trivially copyable types, this
is disastrous.  The <em>language</em> lacks support for reallocating in-place <em>without</em> moving.  While this function could be implemented as magic
library function, it could not interact with many underlying <code class="highlight"><c- n>malloc</c-></code> implementations (libc, in the case of libc++ and libstdc++) used to build <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code>, nor could it interact with user-provided replacement
functions.</p>
     <p>In-place reallocation has been considered in <a data-link-type="biblio" href="#biblio-n3495">[N3495]</a> (throwing <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bad_alloc</c-></code> when unable) and <a data-link-type="biblio" href="#biblio-p0894r1">[P0894R1]</a> (returning <code class="highlight">false</code> when
unable).  This has sometimes been referred to as an "extend" or an "expand"
interface.  Extending an existing allocation requires a round-trip to the
allocator (and its requisite branch/call overheads) to determine whether
the allocation <em>could</em> be expanded.  As discussed previously, we need to
probe for the true size of the backing object (until we find extend fails)
until we give up and apply a multiplicative expansion factor.</p>
   </ul>
   <p>For fixed-size allocators it makes more sense, and is much simpler for
containers to adapt to, if the allocator is able to over-allocate on the
initial request and inform the caller how much memory was made available.</p>
   <h3 class="heading settled" data-level="2.2" id="size"><span class="secno">2.2. </span><span class="content">Why not ask the allocator for the size</span><a class="self-link" href="#size"></a></h3>
   <p>We could also explore APIs that answer the question: "If I ask for <code class="highlight"><c- n>N</c-></code> bytes,
how many do I actually get?" <a data-link-type="biblio" href="#biblio-jemalloc">[jemalloc]</a> and <a data-link-type="biblio" href="#biblio-tcmalloc">[TCMalloc]</a> call this <code class="highlight"><c- n>nallocx</c-></code>.</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>nallocx</c-></code> requires an extra call / size calculation.  This requires we
duplicate work, as the size calculation is performed as part of allocation
itself.</p>
    <li data-md>
     <p><code class="highlight"><c- n>nallocx</c-></code> impairs telemetry for monitoring allocation request sizes.  If a <code class="highlight"><c- n>nallocx</c-></code> return value is cached, the user appears to be asking for exactly
as many bytes as <code class="highlight"><c- n>nallocx</c-></code> said they would get.</p>
   </ul>
   <p>See also <a data-link-type="biblio" href="#biblio-p0901r5">[P0901R5]</a>'s discussion "<code class="highlight"><c- n>nallocx</c-></code>: not as awesome as it looks."</p>
   <h2 class="heading settled" data-level="3" id="proposal"><span class="secno">3. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>Wording relative to <a data-link-type="biblio" href="#biblio-n4800">[N4800]</a>.</p>
   <p>We propose a free function to return the allocation size simultaneously with an
allocation.</p>
   <blockquote>
    <p>Amend [allocator.requirements]:</p>
    <ins>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>Pointer</c-><c- o>></c->
<c- k>struct</c-> <c- n>allocation_result</c-> <c- p>{</c->
  <c- n>Pointer</c-> <c- n>ptr</c-><c- p>;</c->
  <c- b>size_t</c-> <c- n>count</c-><c- p>;</c->
<c- p>};</c->
</pre>
     <p> The class template <code class="highlight"><c- n>allocation_result</c-></code> has the template parameters, data
members, and special members specified above. It has no base classes or members
other than those specified. </p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>Allocator</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>allocation_result</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>::</c-><c- n>pointer</c-><c- o>></c-> <c- n>allocate_at_least</c-><c- p>(</c-><c- n>Allocator</c-><c- o>&amp;</c-> <c- n>a</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>n</c-><c- p>);</c->
</pre>
    </ins>
    <ul>
     <li>
      <ins>
       <em>Effects:</em> Let <code class="highlight"><c- n>s</c-></code> be the result of <code class="highlight"><c- n>allocate_at_least</c-><c- p>(</c-><c- n>a</c-><c- p>,</c-> <c- n>n</c-><c- p>)</c-></code>.
   Memory is allocated for at least <code class="highlight"><c- n>n</c-></code> objects of type <code class="highlight"><c- n>T</c-></code> but objects are not
   constructed.
       <ins></ins>
      </ins>
     <li>
      <ins><em>Returns:</em> <code class="highlight"><c- n>allocation_result</c-><c- p>{</c-><c- n>ptr</c-><c- p>,</c-> <c- n>m</c-><c- p>}</c-></code>, where <code class="highlight"><c- n>ptr</c-></code> is that memory
   and <code class="highlight"><c- n>m</c-></code> is the number of objects for which memory has been allocated, such
   that <code class="highlight"><c- n>m</c-><c- o>>=</c-><c- n>n</c-></code>.</ins>
     <li>
      <ins>[ <em>Note:</em> If <code class="highlight"><c- n>s</c-><c- p>.</c-><c- n>count</c-> <c- o>==</c-> <c- mi>0</c-></code>, the value of <code class="highlight"><c- n>s</c-><c- p>.</c-><c- n>ptr</c-></code> is unspecified.
   -- end note]</ins>
    </ul>
    <p>Table 34 - Cpp17Allocator Requirements</p>
    <table>
     <tbody>
      <tr>
       <td>
        <ins><code class="highlight"><c- n>a</c-><c- p>.</c-><c- n>allocate_at_least</c-><c- p>(</c-><c- n>n</c-><c- p>)</c-></code></ins>
       <td>
        <ins><code class="highlight"><c- n>allocation_result</c-><c- o>&lt;</c-><c- n>X</c-><c- o>::</c-><c- n>pointer</c-><c- o>></c-></code></ins>
       <td>
        <ins>Returns <code class="highlight"><c- n>allocation_result</c-><c- p>{</c-><c- n>ptr</c-><c- p>,</c-> <c- n>m</c-><c- p>}</c-></code>.  Memory is allocated for at least <code class="highlight"><c- n>n</c-></code> objects of type <code class="highlight"><c- n>T</c-></code> but objects are not constructed and returned as <code class="highlight"><c- n>ptr</c-></code>.
The actual number of objects <code class="highlight"><c- n>m</c-></code>, such that <code class="highlight"><c- n>m</c-><c- o>>=</c-><c- n>n</c-></code>, that memory has been
allocated for is returned in the second value. <code class="highlight"><c- n>allocate_at_least</c-></code> may throw
an appropriate exception. [ Note: If <code class="highlight"><c- n>n</c-> <c- o>==</c-> <c- mi>0</c-></code>, the value of <code class="highlight"><c- n>ptr</c-></code> is
unspecified. — end note ]</ins>
       <td>
        <ins><code class="highlight"><c- p>{</c-><c- n>a</c-><c- p>.</c-><c- n>allocate</c-><c- p>(</c-><c- n>n</c-><c- p>),</c-> <c- n>n</c-><c- p>}</c-></code></ins>
       <td>
      <tr>
       <td><code class="highlight"><c- n>a</c-><c- p>.</c-><c- n>deallocate</c-><c- p>(</c-><c- n>p</c-><c- p>,</c-><c- n>n</c-><c- p>)</c-></code>
       <td>(not used)
       <td>
        <em>Preconditions</em>: <code class="highlight"><c- n>p</c-></code> shall be a value returned by an earlier call to <code class="highlight"><c- n>allocate</c-></code>
        <ins> or <code class="highlight"><c- n>allocate_at_least</c-></code></ins>
         that has not been invalidated by
an intervening call to <code class="highlight"><c- n>deallocate</c-></code>. 
        <ins>If this memory was obtained by a call
to <code class="highlight"><c- n>allocate</c-></code>,</ins>
         <code class="highlight"><c- n>n</c-></code> shall match the value passed to <code class="highlight"><c- n>allocate</c-></code> to obtain
this memory. 
        <ins>Otherwise, <code class="highlight"><c- n>n</c-></code> shall satisfy <code class="highlight"><c- n>capacity</c-> <c- o>>=</c-> <c- n>n</c-> <c- o>>=</c-> <c- n>requested</c-></code> where <code class="highlight"><c- p>[</c-><c- n>p</c-><c- p>,</c-> <c- n>capacity</c-><c- p>]</c-> <c- o>=</c-> <c- n>allocate_at_least</c-><c- p>(</c-><c- n>requested</c-><c- p>)</c-></code> was used to obtain this memory.</ins>
        <p><em>Throws</em>: Nothing.</p>
       <td>
    </table>
   </blockquote>
   <p>Amend [allocator.members]</p>
   <blockquote>
    <p>Except for the destructor, member functions of the default allocator shall
not introduce data races as a result of concurrent calls to those member
functions from different threads. Calls to these functions that allocate or
deallocate a particular unit of storage shall occur in a single total order,
and each such deallocation call shall happen before the next allocation (if
any) in this order.</p>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- n>allocate</c-><c- p>(</c-><c- b>size_t</c-> <c- n>n</c-><c- p>);</c-></pre>
    <ul>
     <li><em>Returns:</em> A pointer to the initial element of an array of storage of size <code class="highlight"><c- n>n</c-> <c- o>*</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code>, aligned appropriately for objects of type <code class="highlight"><c- n>T</c-></code>.
     <li><em>Remarks:</em> the storage is obtained by calling <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>, but it is
     unspecified when or how often this function is called.
     <li><em>Throws:</em> <code class="highlight"><c- n>bad_array_new_length</c-></code> if <code class="highlight"><c- n>SIZE_MAX</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- o>&lt;</c-> <c- n>n</c-></code>, or <code class="highlight"><c- n>bad_alloc</c-></code> if the storage cannot be obtained.
    </ul>
<pre class="highlight"><ins><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- k>constexpr</c-> <c- n>allocation_result</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-> <c- n>allocate_at_least</c-><c- p>(</c-><c- b>size_t</c-> <c- n>n</c-><c- p>);</c-></ins></pre>
    <ul>
     <li>
      <ins><em>Returns:</em> <code class="highlight"><c- n>allocation_result</c-><c- p>{</c-><c- n>ptr</c-><c- p>,</c-> <c- n>count</c-><c- p>}</c-></code>, where <code class="highlight"><c- n>ptr</c-></code> is a pointer to the
    initial element of an array of storage of size <code class="highlight"><c- n>count</c-> <c- o>*</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code>, aligned
    appropriately for objects of type <code class="highlight"><c- n>T</c-></code>, and <code class="highlight"><c- n>count</c-> <c- o>>=</c-> <c- n>n</c-></code>.</ins>
     <li>
      <ins><em>Remarks:</em> the storage is obtained by calling <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>new</c-></code>, but it
    is unspecified when or how often this function is called.</ins>
     <li>
      <ins><em>Throws:</em> <code class="highlight"><c- n>bad_array_new_length</c-></code> if <code class="highlight"><c- n>SIZE_MAX</c-> <c- o>/</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- o>&lt;</c-> <c- n>n</c-></code>, or <code class="highlight"><c- n>bad_alloc</c-></code> if the storage cannot be obtained.</ins>
    </ul>
<pre class="highlight"><c- k>constexpr</c-> <c- b>void</c-> <c- nf>deallocate</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>p</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>n</c-><c- p>);</c-></pre>
    <ul>
     <li>
      <em>Preconditions:</em> <code class="highlight"><c- n>p</c-></code> shall be a pointer value obtained from <code class="highlight"><c- n>allocate</c-><c- p>()</c-></code>
      <ins> or <code class="highlight"><c- n>allocate_at_least</c-></code></ins>
      . 
      <ins>If this memory was obtained by a call to <code class="highlight"><c- n>allocate</c-></code>,</ins>
       <code class="highlight"><c- n>n</c-></code> shall equal the value passed as the first argument to
    the invocation of allocate which returned <code class="highlight"><c- n>p</c-></code>. 
      <ins>Otherwise, <code class="highlight"><c- n>n</c-></code> shall
    satisfy <code class="highlight"><c- n>count</c-> <c- o>>=</c-> <c- n>n</c-> <c- o>>=</c-> <c- n>requested</c-></code> where <code class="highlight"><c- p>[</c-><c- n>p</c-><c- p>,</c-> <c- n>count</c-><c- p>]</c-> <c- o>=</c-> <c- n>allocate_at_least</c-><c- p>(</c-><c- n>requested</c-><c- p>)</c-></code> was used to obtain this memory.</ins>
     <li><em>Effects:</em> Deallocates the storage referenced by <code class="highlight"><c- n>p</c-></code>.
     <li><em>Remarks:</em> Uses <code class="highlight"><c- o>::</c-><c- k>operator</c-> <c- k>delete</c-></code>, but it is unspecified when this
    function is called.
    </ul>
   </blockquote>
   <h2 class="heading settled" data-level="4" id="design"><span class="secno">4. </span><span class="content">Design Considerations</span><a class="self-link" href="#design"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="overloads"><span class="secno">4.1. </span><span class="content"><code class="highlight"><c- n>allocate</c-></code> selection</span><a class="self-link" href="#overloads"></a></h3>
   <p>There are multiple approaches here:</p>
   <ul>
    <li data-md>
     <p>Return a pointer-size pair, as presented.</p>
    <li data-md>
     <p>Overload <code class="highlight"><c- n>allocate</c-></code> and return via a reference parameter.  This potentially
hampers optimization depending on ABI, as the value is returned via memory
rather than a register.</p>
   </ul>
   <h3 class="heading settled" data-level="4.2" id="returnvalue"><span class="secno">4.2. </span><span class="content">Size Return Value</span><a class="self-link" href="#returnvalue"></a></h3>
   <p>In <a data-link-type="biblio" href="#biblio-prague">[Prague]</a>, LEWG discussed the return value.  For compatibility with the
existing allocator APIs, which work in units of <em>objects</em> rather than bytes,
this proposal chooses to continue to return an integer number of objects.</p>
   <p>Additionally, for types with non-trivial alignment requirements, we must
allocate storage for objects, rather than bytes, as raw bytes do not convey the
appropriate alignment needs to the allocator.</p>
   <p>For example:  In the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> case, many implementations use 3 pointers
for representing the state of the container (begin, end, and capacity).  If we
preserved the precise value returned by the underlying allocator, we may not be
able to legally form the capacity pointer.  For these implementations,
replacing the capacity pointer with a capacity in bytes would be an ABI break.</p>
   <h3 class="heading settled" data-level="4.3" id="deallocate"><span class="secno">4.3. </span><span class="content"><code class="highlight"><c- n>deallocate</c-></code> changes</span><a class="self-link" href="#deallocate"></a></h3>
   <p>We now require flexibility on the size we pass to <code class="highlight"><c- n>deallocate</c-></code>.  For container
types using this allocator interface, they are faced with the choice of storing <em>both</em> the original size request as well as the provided size (requiring
additional auxillary storage), or being unable to reproduce one during the call
to <code class="highlight"><c- n>deallocate</c-></code>.</p>
   <p>As the true size is expected to be useful for the <code class="highlight"><c- n>capacity</c-></code> of a <code class="highlight"><c- n>string</c-></code> or <code class="highlight"><c- n>vector</c-></code> instance, the returned size is available without additional storage
requirements.  The original (requested) size is unavailable, so we relax the <code class="highlight"><c- n>deallocate</c-></code> size parameter requirements for these allocations.</p>
   <h3 class="heading settled" data-level="4.4" id="pmr"><span class="secno">4.4. </span><span class="content">Interaction with <code class="highlight"><c- n>polymorphic_allocator</c-></code></span><a class="self-link" href="#pmr"></a></h3>
   <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>memory_resource</c-></code> is implemented using virtual functions.  Adding new
methods, such as the proposed allocate API would require taking an ABI break.</p>
   <h3 class="heading settled" data-level="4.5" id="zero"><span class="secno">4.5. </span><span class="content">Zero-Sized Requests</span><a class="self-link" href="#zero"></a></h3>
   <p>In Prague, LEWG discussed the behavior of <code class="highlight"><c- n>allocate_at_least</c-><c- p>(</c-><c- n>allocator</c-><c- p>,</c-> <c- mi>0</c-><c- p>)</c-></code>.
This maximizes implementation freedom.</p>
   <ul>
    <li data-md>
     <p>The return value may not be dereferenceable (even if not <code class="highlight"><c- k>nullptr</c-></code>).</p>
    <li data-md>
     <p>Other implementations, such as a stack allocator, could just return the
current offset.</p>
   </ul>
   <h2 class="heading settled" data-level="5" id="revisions"><span class="secno">5. </span><span class="content">Revision History</span><a class="self-link" href="#revisions"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="R3"><span class="secno">5.1. </span><span class="content">R2 → R3</span><a class="self-link" href="#R3"></a></h3>
   <p>Applied LEWG feedback from <a data-link-type="biblio" href="#biblio-prague">[Prague]</a>.</p>
   <ul>
    <li data-md>
     <p>Renamed <code class="highlight"><c- n>sized_ptr_t</c-></code> to <code class="highlight"><c- n>allocation_result</c-></code>, based on discussion in Prague
that this library proposal (which returns a <em>count</em> of storage for objects)
have a distinct type from the underlying language proposal (<a data-link-type="biblio" href="#biblio-p0901r5">[P0901R5]</a>,
which returns bytes).  This was one of the names bikeshedded during the
review of P0901R5 (<a data-link-type="biblio" href="#biblio-p0901r5-review">[P0901R5-Review]</a>).</p>
    <li data-md>
     <p>Renamed <code class="highlight"><c- n>n</c-></code> (the returned size) to <code class="highlight"><c- n>count</c-></code>, based on discussion in Prague
to clarify that is measured in objects, rather than bytes.  Additionally,
augmented discussion of the <code class="highlight"><c- n>deallocate</c-></code> changes for why implementations
may not be able to preserve the precise size returned by the underlying
allocator.</p>
   </ul>
   <blockquote>
     Poll: We prefer to return number of bytes. 
    <table>
     <tbody>
      <tr>
       <th>SF
       <th>F
       <th>N
       <th>A
       <th>SA
      <tr>
       <td>0
       <td>0
       <td>6
       <td>6
       <td>5
    </table>
   </blockquote>
   <ul>
    <li data-md>
     <p>Changed the template parameter to pointer type to allow for fancy pointers.</p>
   </ul>
   <blockquote>
     POLL: Change the template parameter to sized_ptr_t to pointer type (to support fancy ptr). 
    <p>Unanimous consent</p>
   </blockquote>
   <ul>
    <li data-md>
     <p>Added design intent and wording that <code class="highlight"><c- n>allocation_result</c-></code> be compatible
 with structured bindings.  This wording was adapted from the proposed
 resolution of <a data-link-type="biblio" href="#biblio-lwg3373">[LWG3373]</a>.</p>
    <li data-md>
     <p>Added <code class="highlight"><c- k>constexpr</c-></code></p>
    <li data-md>
     <p>Removed default template parameter from <code class="highlight"><c- n>allocation_result</c-></code></p>
    <li data-md>
     <p>Based on LEWG feedback, changed <code class="highlight"><c- n>allocate_at_least</c-></code> to deduce return type
 from the allocator.  Since the pointer type is chosen by the allocator,
 the underlying object type is less relevant.  This also removed the
 default template parameter.</p>
   </ul>
   <blockquote>
     POLL: We want to deduce the object type from the allocator (alloc_least_n [sic]). 
    <table>
     <tbody>
      <tr>
       <th>SF
       <th>F
       <th>N
       <th>A
       <th>SA
      <tr>
       <td>4
       <td>8
       <td>1
       <td>0
       <td>0
    </table>
   </blockquote>
   <blockquote>
     POLL: We like the design, please revise and do initial wording review (with
Pablo, and David Stone) send to LWG (tentatively Ready). 
    <table>
     <tbody>
      <tr>
       <th>SF
       <th>F
       <th>N
       <th>A
       <th>SA
      <tr>
       <td>6
       <td>12
       <td>0
       <td>0
       <td>0
    </table>
   </blockquote>
   <h3 class="heading settled" data-level="5.2" id="R2"><span class="secno">5.2. </span><span class="content">R1 → R2</span><a class="self-link" href="#R2"></a></h3>
   <p>Applied LEWG feedback from <a data-link-type="biblio" href="#biblio-cologne">[Cologne]</a>.</p>
   <blockquote>
     Poll: We want a feature like this. 
    <table>
     <tbody>
      <tr>
       <th>SF
       <th>F
       <th>N
       <th>A
       <th>SA
      <tr>
       <td>2
       <td>9
       <td>2
       <td>0
       <td>0
    </table>
   </blockquote>
   <ul>
    <li data-md>
     <p>As users may specialize <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocator_traits</c-></code>, this revision moves to a
free function <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocate_at_least</c-></code> as suggested via a LEWG poll.</p>
   </ul>
   <blockquote>
     Poll: Name choice 
    <table>
     <tbody>
      <tr>
       <td> 5
       <td><code class="highlight"><c- n>allocate_with_size</c-></code>
      <tr>
       <td> 4
       <td><code class="highlight"><c- n>overallocate</c-></code>
      <tr>
       <td> 2
       <td><code class="highlight"><c- n>allocate_with_oversubscribe</c-></code>
      <tr>
       <td> 6
       <td><code class="highlight"><c- n>allocate_oversize</c-></code>
      <tr>
       <td>14
       <td><code class="highlight"><c- n>allocate_at_least</c-></code>
    </table>
    <p>Poll: Prefer a struct rather than a requirement of structured bindings.</p>
    <table>
     <tbody>
      <tr>
       <th>SF
       <th>F
       <th>N
       <th>A
       <th>SA
      <tr>
       <td>2
       <td>8
       <td>0
       <td>2
       <td>1
    </table>
   </blockquote>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-cologne">[Cologne]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21cologne2019/P0401">Cologne Meeting Minutes</a>. 2019-07-17. URL: <a href="http://wiki.edg.com/bin/view/Wg21cologne2019/P0401">http://wiki.edg.com/bin/view/Wg21cologne2019/P0401</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-lwg3373">[LWG3373]
   <dd><a href="https://cplusplus.github.io/LWG/lwg-active.html#3373">{to,from}_chars_result and format_to_n_result need the 'we really mean what we say' wording</a>. URL: <a href="https://cplusplus.github.io/LWG/lwg-active.html#3373">https://cplusplus.github.io/LWG/lwg-active.html#3373</a>
   <dt id="biblio-n3495">[N3495]
   <dd>Ariane van der Steldt. <a href="https://wg21.link/n3495">inplace realloc</a>. 7 December 2012. URL: <a href="https://wg21.link/n3495">https://wg21.link/n3495</a>
   <dt id="biblio-n4800">[N4800]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4800.pdf">Working Draft, Standard for Programming Language C++</a>. 2019-01-21. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4800.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4800.pdf</a>
   <dt id="biblio-p0894r1">[P0894R1]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0894r1.md">realloc for C++</a>. 2019-01-18. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0894r1.md">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0894r1.md</a>
   <dt id="biblio-p0901r5">[P0901R5]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0901r5.html">Size feedback in operator new</a>. 2019-10-03. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0901r5.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0901r5.html</a>
   <dt id="biblio-p0901r5-review">[P0901R5-Review]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21prague/P0901">P0901R5 LEWG Review Prague</a>. 2020-02-10. URL: <a href="http://wiki.edg.com/bin/view/Wg21prague/P0901">http://wiki.edg.com/bin/view/Wg21prague/P0901</a>
   <dt id="biblio-prague">[Prague]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21prague/P0401">P0401R2 LEWG Review Prague</a>. 2020-02-14. URL: <a href="http://wiki.edg.com/bin/view/Wg21prague/P0401">http://wiki.edg.com/bin/view/Wg21prague/P0401</a>
   <dt id="biblio-tcmalloc">[TCMalloc]
   <dd><a href="https://github.com/google/tcmalloc">TCMalloc</a>. URL: <a href="https://github.com/google/tcmalloc">https://github.com/google/tcmalloc</a>
  </dl>