<!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>p1010R0: Container support for implicit lifetime types</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,
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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">p1010R0<br>Container support for implicit lifetime types</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Proposal, <time class="dt-updated" datetime="2018-05-06">6 May 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1010r0">http://wg21.link/p1010r0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:mzeren@vmware.com">Mark Zeren</a> (<span class="p-org org">VMware</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>LEWG, LWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
     <dt>Source:
     <dd><a href="https://github.com/mzeren-vmw/iso/blob/master/resize/p1010r0.bs">https://github.com/mzeren-vmw/iso/blob/master/resize/p1010r0.bs</a>
    </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>Extend allocators, <code class="highlight"><span class="n">allocator_traits</span></code>, and <code class="highlight"><span class="n">vector</span></code> to allow efficient addition of elements of <em>implicit lifetime</em> type.</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="#summary"><span class="secno">1</span> <span class="content">Summary</span></a>
     <ol class="toc">
      <li><a href="#approach"><span class="secno">1.1</span> <span class="content">Approach</span></a>
     </ol>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivating examples</span></a>
     <ol class="toc">
      <li><a href="#network"><span class="secno">2.1</span> <span class="content">Example: reading from the network</span></a>
      <li><a href="#example-stamping-a-pattern"><span class="secno">2.2</span> <span class="content">Example: stamping a pattern</span></a>
     </ol>
    <li>
     <a href="#related"><span class="secno">3</span> <span class="content">Related work</span></a>
     <ol class="toc">
      <li><a href="#google"><span class="secno">3.1</span> <span class="content">Google <code class="highlight"><span class="n">basic_string</span></code></span></a>
      <li><a href="#boost"><span class="secno">3.2</span> <span class="content">Boost containers</span></a>
      <li><a href="#mongodb"><span class="secno">3.3</span> <span class="content">MongoDB</span></a>
      <li><a href="#vmware"><span class="secno">3.4</span> <span class="content">VMware string builders</span></a>
      <li><a href="#std-proposals"><span class="secno">3.5</span> <span class="content">Discussion on std-proposals</span></a>
      <li><a href="#p1020r0"><span class="secno">3.6</span> <span class="content">P1020R0</span></a>
     </ol>
    <li>
     <a href="#alternatives"><span class="secno">4</span> <span class="content">Alternatives Considered</span></a>
     <ol class="toc">
      <li><a href="#free"><span class="secno">4.1</span> <span class="content">Free function <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">implicit_construct</span></code></span></a>
      <li><a href="#insert_from_capacity"><span class="secno">4.2</span> <span class="content"><code class="highlight"><span class="n">insert_from_capacity</span></code></span></a>
      <li><a href="#implicit_construct"><span class="secno">4.3</span> <span class="content"><code class="highlight"><span class="n">implicit_construct</span></code></span></a>
      <li><a href="#uninitialized_data"><span class="secno">4.4</span> <span class="content"><code class="highlight"><span class="n">uninitialized_data</span></code></span></a>
     </ol>
    <li><a href="#issues"><span class="secno">5</span> <span class="content">Open Issues</span></a>
    <li>
     <a href="#wording"><span class="secno">6</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#requirements"><span class="secno">6.1</span> <span class="content">[<strong>allocator.requirements</strong>]</span></a>
      <li><a href="#traits"><span class="secno">6.2</span> <span class="content">[<strong>allocator.traits</strong>]</span></a>
      <li><a href="#general"><span class="secno">6.3</span> <span class="content">[<strong>container.requirements.general</strong>]</span></a>
      <li><a href="#vector"><span class="secno">6.4</span> <span class="content">[<strong>vector</strong>]</span></a>
     </ol>
    <li><a href="#acknowledgments"><span class="secno">7</span> <span class="content">Acknowledgments</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>
   <hr>
   <blockquote> ... there is a family of types for which programmers <br> assume they do not need to explicitly create objects ... <br> </blockquote>
    <a data-link-type="biblio" href="#biblio-p0593r2">[P0593R2]</a> 
   <hr>
   <h2 class="heading settled" data-level="1" id="summary"><span class="secno">1. </span><span class="content">Summary</span><a class="self-link" href="#summary"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0593r2">[P0593R2]</a> Gives us defined behavior for creating objects from raw, application
initialized, memory. These <em>implicit lifetime</em> types include:</p>
   <ul>
    <li> Scalar types 
    <li> Array types (with any element type) 
    <li> Class types with a trivial destructor and a trivial constructor (of any
     kind) 
   </ul>
   <p>When working with implicit lifetime types applications may be able to bulk
initialize objects without value initialization (zeroing) or explicit calls to
constructors. For example, the application may read objects from the network, or
it may stamp out a pattern of objects, etc.. This proposal enables these
optimizations for allocator aware containers, and extends <code class="highlight"><span class="n">vector</span></code> to allow
them. Prior work in this area has relied on trivial default construction.
However, <a data-link-type="biblio" href="#biblio-p0593r2">[P0593R2]</a> allows class types with <em>non-trivial</em> default constructors
that have trivial move or copy constructors to be implicit lifetime types. This
proposal includes this alternate set of types. Default construction typically
leaves indeterminate values in elements controlled by the <code class="highlight"><span class="n">vector</span></code>, exposing it
to undefined behavior until the application initializes all such elements. This
proposal avoids this "trap state" for the container by not incorporating
elements into the container until they have been initialized.</p>
   <p>We will propose related changes to <code class="highlight"><span class="n">basic_string</span></code> in a separate paper <a data-link-type="biblio" href="#biblio-p1072r0">[P1072R0]</a>.</p>
   <h3 class="heading settled" data-level="1.1" id="approach"><span class="secno">1.1. </span><span class="content">Approach</span><a class="self-link" href="#approach"></a></h3>
   <p>Allocator aware containers must cooperate with their allocator for object
construction and destruction. Working with the broader set of implicit lifetime
types requires that the container use a two step interaction with the
application. First, the container exposes memory that the application
initializes. Second, the application tells the container how many objects were
initialized. The container can then tell the allocator about the newly created
objects.</p>
   <p>References and wording are relative to <a data-link-type="biblio" href="#biblio-n4741">[N4741]</a>.</p>
   <p>Starting with [<strong>allocator.requirements</strong>] (Table 31), we add:</p>
   <ul>
    <li> <code class="highlight"><span class="n">a</span><span class="p">.</span><span class="n">implicit_construct</span><span class="p">(</span><span class="n">c</span><span class="p">)</span></code> - This expression informs the allocator <em>post
     facto</em> of an object of implicit lifetime type that has been initialized and
     implicitly created by the application. This member function, if provided,
     does not participate in overload resolution unless <code class="highlight"><span class="n">C</span></code> is an implicit
     lifetime type. By default it does nothing. 
   </ul>
   <p>Then in [<strong>allocator.traits</strong>] we add a new <em>optional</em> member:</p>
   <ul>
    <li>
      <code class="highlight"><span class="n">implicit_construct</span><span class="p">(</span><span class="n">Alloc</span><span class="o">&amp;</span> <span class="n">a</span><span class="p">,</span> <span class="n">T</span><span class="o">*</span> <span class="n">p</span><span class="p">)</span></code> - This member function: 
     <ul>
      <li> Calls <code class="highlight"><span class="n">a</span><span class="p">.</span><span class="n">implicit_construct</span><span class="p">(</span><span class="n">p</span><span class="p">)</span></code> if it is well-formed, otherwise ... 
      <li> Does nothing if T is an implicit lifetime type and <code class="highlight"><span class="n">a</span><span class="p">.</span><span class="n">construct</span><span class="p">(</span><span class="n">p</span><span class="p">)</span></code> is <em>not</em> well-formed, otherwise ... 
      <li> Does not participate in overload resolution.
     </ul>
     <p>(The intent is to leave the meaning of allocators which define <code class="highlight"><span class="n">construct</span><span class="p">(</span><span class="n">T</span><span class="o">*</span><span class="p">)</span></code> unchanged, but to allow those that don’t, including the
     default allocator, to support <code class="highlight"><span class="n">implicit_construct</span></code> implicitly.)</p>
   </ul>
   <p>Finally, in [<strong>vector</strong>] we add two member functions:</p>
   <ul>
    <li>
      <code class="highlight"><span class="n">T</span><span class="o">*</span> <span class="n">uninitialized_data</span><span class="p">()</span></code> - Returns a pointer to storage that
     would back elements [<code class="highlight"><span class="n">size</span><span class="p">()</span></code>, <code class="highlight"><span class="n">capacity</span><span class="p">()</span></code>). Note
     that this storage may contain indeterminate values ([<strong>dcl.init</strong>]p12). The
     application may initialize the memory by casting to <code class="highlight"><span class="kt">void</span><span class="o">*</span></code> and then to <code class="highlight"><span class="kt">char</span><span class="o">*</span></code>, <code class="highlight"><span class="kt">unsigned</span> <span class="kt">char</span><span class="o">*</span></code>, or <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">byte</span><span class="o">*</span></code>. ([<strong>basic.life</strong>]p6.4). 
     <p>Does not participate in overload resolution if <code class="highlight"><span class="n">allocator_traits</span><span class="o">&lt;</span><span class="n">Allocator</span><span class="o">>::</span><span class="n">rebind_traits</span><span class="o">&lt;</span><span class="n">U</span><span class="o">>::</span><span class="n">implicit_construct</span><span class="p">(</span><span class="n">U</span> <span class="o">*</span><span class="p">)</span></code> is
     not well-formed.</p>
    <li>
      <code class="highlight"><span class="n">insert_from_capacity</span><span class="p">(</span><span class="n">size_type</span> <span class="n">n</span><span class="p">)</span></code> - Appends <code class="highlight"><span class="n">n</span></code> elements from capacity. The application must have initialized the storage
     backing these elements otherwise the behavior is undefined. 
     <p>Does not participate in overload resolution if <code class="highlight"><span class="n">allocator_traits</span><span class="o">&lt;</span><span class="n">Allocator</span><span class="o">>::</span><span class="n">rebind_traits</span><span class="o">&lt;</span><span class="n">U</span><span class="o">>::</span><span class="n">implicit_construct</span><span class="p">(</span><span class="n">U</span> <span class="o">*</span><span class="p">)</span></code> is
     not well-formed.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivating examples</span><a class="self-link" href="#motivation"></a></h2>
   <p>The extra overhead described in the examples below is often small, yet the
optimization can be significant in performance critical execution paths.</p>
   <p>Casts, using declarations, and other details have been elided to keep the
examples simple.</p>
   <h3 class="heading settled" data-level="2.1" id="network"><span class="secno">2.1. </span><span class="content">Example: reading from the network</span><a class="self-link" href="#network"></a></h3>
   <p>The current <code class="highlight"><span class="n">vector</span></code> interface forces a copy when reading objects from the
network, or a file, etc.. (<code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">byte</span></code> keeps the example simple, but the
principle applies to user defined implicit lifetime types as well.):</p>
<pre class="highlight"><span class="k">using</span> <span class="n">ByteVec</span> <span class="o">=</span> <span class="n">vector</span><span class="o">&lt;</span><span class="n">byte</span><span class="o">></span><span class="p">;</span>

<span class="k">class</span> <span class="nc">Socket</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
  <span class="kt">size_t</span> <span class="n">Read</span><span class="p">(</span><span class="n">byte</span><span class="o">*</span> <span class="n">buf</span><span class="p">,</span> <span class="kt">size_t</span> <span class="n">size</span><span class="p">);</span>
  <span class="p">...</span>
<span class="p">};</span>

<span class="kt">unsigned</span> <span class="nf">ReadSome</span><span class="p">(</span><span class="n">ByteVec</span><span class="o">*</span> <span class="n">out</span><span class="p">,</span> <span class="n">Socket</span><span class="o">&amp;</span> <span class="n">socket</span><span class="p">)</span>
<span class="p">{</span>
  <span class="n">byte</span> <span class="n">buf</span><span class="p">[</span><span class="n">kBufferSize</span><span class="p">];</span>
  <span class="k">auto</span> <span class="n">size</span> <span class="o">=</span> <span class="n">socket</span><span class="p">.</span><span class="n">Read</span><span class="p">(</span><span class="o">&amp;</span><span class="n">buf</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="n">kBufferSize</span><span class="p">);</span>
  <span class="n">out</span><span class="o">-></span><span class="n">insert</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span> <span class="o">&amp;</span><span class="n">buff</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="o">&amp;</span><span class="n">buff</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">+</span> <span class="n">size</span><span class="p">);</span> <span class="c1">// BAD: Copies.</span>
<span class="c1"></span>  <span class="k">return</span> <span class="n">size</span><span class="p">;</span>
<span class="p">}</span>
</pre>
   <p>With the changes proposed in this paper, the above example would be optimized
as:</p>
<pre class="highlight"><span class="kt">unsigned</span> <span class="nf">ReadSome</span><span class="p">(</span><span class="n">ByteVec</span><span class="o">*</span> <span class="n">out</span><span class="p">,</span> <span class="n">Socket</span><span class="o">&amp;</span> <span class="n">socket</span><span class="p">)</span>
<span class="p">{</span>
  <span class="n">out</span><span class="p">.</span><span class="n">reserve</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">+</span> <span class="n">kBufferSize</span><span class="p">);</span>
  <span class="k">auto</span> <span class="n">size</span> <span class="o">=</span> <span class="n">socket</span><span class="p">.</span><span class="n">Read</span><span class="p">(</span><span class="n">out</span><span class="o">-></span><span class="n">uninitialized_data</span><span class="p">(),</span>    <span class="c1">// GOOD: No copies.</span>
<span class="c1"></span>                          <span class="n">data</span><span class="p">.</span><span class="n">capacity</span><span class="p">()</span> <span class="o">-</span> <span class="n">data</span><span class="p">.</span><span class="n">size</span><span class="p">());</span>
  <span class="n">out</span><span class="o">-></span><span class="n">insert_from_capacity</span><span class="p">(</span><span class="n">size</span><span class="p">);</span>                      <span class="c1">// GOOD: No-op.</span>
<span class="c1"></span>  <span class="k">return</span> <span class="n">size</span><span class="p">;</span>
<span class="p">}</span>
</pre>
   <p class="note" role="note"><span>Note:</span> <code class="highlight"><span class="n">reserve</span></code> bypasses <code class="highlight"><span class="n">vector</span></code>'s normal growth algorithm which can
lead to unnecessary allocations. Resolving that issue is out of scope for
this proposal.</p>
   <h3 class="heading settled" data-level="2.2" id="example-stamping-a-pattern"><span class="secno">2.2. </span><span class="content">Example: stamping a pattern</span><a class="self-link" href="#example-stamping-a-pattern"></a></h3>
   <p>For a second example, consider stamping a repeating pattern of elements. <code class="highlight"><span class="n">vector</span></code>'s interface offers two options, neither optimal:</p>
   <ol>
    <li> Call <strong><code class="highlight"><span class="n">resize</span></code></strong> and write directly into the
     container. However, this <em>value initializes</em> elements, typically writing
     zeros: 
<pre class="highlight"><span class="k">using</span> <span class="n">IntVec</span> <span class="o">=</span> <span class="n">vector</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">></span><span class="p">;</span>

<span class="kt">void</span> <span class="nf">AppendPattern</span><span class="p">(</span><span class="n">IntVec</span><span class="o">&amp;</span> <span class="n">out</span><span class="p">,</span> <span class="n">span</span><span class="o">&lt;</span><span class="k">const</span> <span class="kt">int</span><span class="o">></span> <span class="n">pattern</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">count</span><span class="p">)</span>
<span class="p">{</span>
  <span class="k">auto</span> <span class="n">start</span> <span class="o">=</span> <span class="n">out</span><span class="p">.</span><span class="n">size</span><span class="p">();</span>
  <span class="k">auto</span> <span class="n">step</span> <span class="o">=</span> <span class="n">pattern</span><span class="p">.</span><span class="n">size</span><span class="p">();</span>
  <span class="n">out</span><span class="p">.</span><span class="n">resize</span><span class="p">(</span><span class="n">start</span> <span class="o">+</span> <span class="n">step</span> <span class="o">*</span> <span class="n">count</span><span class="p">);</span>    <span class="c1">// BAD: Zeros.</span>
<span class="c1"></span>  <span class="k">for</span> <span class="p">(</span><span class="k">auto</span> <span class="n">cur</span> <span class="o">=</span> <span class="n">out</span><span class="p">.</span><span class="n">begin</span><span class="p">()</span> <span class="o">+</span> <span class="n">start</span><span class="p">;</span>
       <span class="n">cur</span> <span class="o">&lt;</span> <span class="n">out</span><span class="p">.</span><span class="n">end</span><span class="p">();</span> <span class="n">cur</span> <span class="o">+=</span> <span class="n">step</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">memcpy</span><span class="p">(</span><span class="o">&amp;*</span><span class="n">cur</span><span class="p">,</span> <span class="n">pattern</span><span class="p">.</span><span class="n">data</span><span class="p">(),</span>      <span class="c1">// GOOD: No bookkeeping.</span>
<span class="c1"></span>           <span class="n">step</span> <span class="o">*</span> <span class="k">sizeof</span><span class="p">(</span><span class="kt">int</span><span class="p">)));</span>
  <span class="p">}</span>
<span class="p">}</span>
</pre>
    <li> Call <strong><code class="highlight"><span class="n">reserve</span></code></strong> and then <code class="highlight"><span class="n">insert</span></code> in
     a loop. However, this incurs bookkeeping overhead in each insert: 
<pre class="highlight"><span class="kt">void</span> <span class="nf">AppendPattern</span><span class="p">(</span><span class="n">IntVec</span><span class="o">&amp;</span> <span class="n">out</span><span class="p">,</span> <span class="n">span</span><span class="o">&lt;</span><span class="k">const</span> <span class="kt">int</span><span class="o">></span> <span class="n">pattern</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">count</span><span class="p">)</span>
<span class="p">{</span>
  <span class="n">out</span><span class="p">.</span><span class="n">reserve</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">+</span> <span class="n">pattern</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">*</span> <span class="n">count</span><span class="p">);</span> <span class="c1">// GOOD: No zeros.</span>
<span class="c1"></span>  <span class="k">for</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="n">count</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">out</span><span class="p">.</span><span class="n">insert</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span> <span class="n">pattern</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span>          <span class="c1">// BAD: Bookkeeping.</span>
<span class="c1"></span>               <span class="n">pattern</span><span class="p">.</span><span class="n">end</span><span class="p">());</span>
  <span class="p">}</span>
<span class="p">}</span>
</pre>
   </ol>
   <p>With the changes proposed in this paper the above example would be optimized
as:</p>
   <ol>
<pre class="highlight"><span class="kt">void</span> <span class="nf">AppendPattern</span><span class="p">(</span><span class="n">IntVec</span><span class="o">&amp;</span> <span class="n">out</span><span class="p">,</span> <span class="n">span</span><span class="o">&lt;</span><span class="k">const</span> <span class="kt">int</span><span class="o">></span> <span class="n">pattern</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">count</span><span class="p">)</span>
<span class="p">{</span>
  <span class="k">auto</span> <span class="n">step</span> <span class="o">=</span> <span class="n">pattern</span><span class="p">.</span><span class="n">size</span><span class="p">();</span>
  <span class="k">auto</span> <span class="n">total</span> <span class="o">=</span> <span class="n">step</span> <span class="o">*</span> <span class="n">count</span><span class="p">;</span>
  <span class="n">out</span><span class="p">.</span><span class="n">reserve</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">+</span> <span class="n">total</span><span class="p">);</span>     <span class="c1">// GOOD: No zeros.</span>
<span class="c1"></span>  <span class="kt">int</span><span class="o">*</span> <span class="n">cur</span> <span class="o">=</span> <span class="n">out</span><span class="p">.</span><span class="n">uninitialized_data</span><span class="p">();</span>
  <span class="kt">int</span><span class="o">*</span> <span class="n">end</span> <span class="o">=</span> <span class="n">cur</span> <span class="o">+</span> <span class="n">total</span><span class="p">;</span>
  <span class="k">for</span> <span class="p">(;</span><span class="n">cur</span> <span class="o">&lt;</span> <span class="n">end</span><span class="p">;</span> <span class="n">cur</span> <span class="o">+=</span> <span class="n">step</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">memcpy</span><span class="p">(</span><span class="n">cur</span><span class="p">,</span> <span class="n">pattern</span><span class="p">.</span><span class="n">data</span><span class="p">(),</span>        <span class="c1">// GOOD: No bookkeeping.</span>
<span class="c1"></span>           <span class="n">step</span> <span class="o">*</span> <span class="k">sizeof</span><span class="p">(</span><span class="kt">int</span><span class="p">)));</span>
  <span class="p">}</span>
  <span class="n">out</span><span class="p">.</span><span class="n">insert_from_capacity</span><span class="p">(</span><span class="n">total</span><span class="p">);</span>     <span class="c1">// GOOD: No-op.</span>
<span class="c1"></span><span class="p">}</span>
</pre>
   </ol>
   <h2 class="heading settled" data-level="3" id="related"><span class="secno">3. </span><span class="content">Related work</span><a class="self-link" href="#related"></a></h2>
   <p>As mentioned above, all related work to date has used default initialization.
This is the first proposal that uses implicit lifetime types.</p>
   <h3 class="heading settled" data-level="3.1" id="google"><span class="secno">3.1. </span><span class="content">Google <code class="highlight"><span class="n">basic_string</span></code></span><a class="self-link" href="#google"></a></h3>
   <p>Google has hacked their internal <code class="highlight"><span class="n">basic_string</span></code> implementation to
provide a related <code class="highlight"><span class="n">resize_uninitialized</span></code> API. They have measured performance
improvements (that are not public) that justify maintaining this extension.</p>
   <p>Google’s Abseil open source library provides hooks for other users that want to
independently apply the same hack. See: <a href="https://github.com/abseil/abseil-cpp/blob/master/absl/strings/internal/resize_uninitialized.h"> https://github.com/abseil/abseil-cpp/blob/master/absl/strings/internal/resize_uninitialized.h</a></p>
   <p>Google’s Protocol Buffers open source library takes advantage of Abseil’s hooks
to improve performance. See: <a href="https://github.com/google/protobuf/blob/master/src/google/protobuf/stubs/stl_util.h#L61"> https://github.com/google/protobuf/blob/master/src/google/protobuf/stubs/stl_util.h#L61</a></p>
   <p>The Snappy compression library has hooks for a similar hack:<br> <a href="https://github.com/google/snappy/search?q=STLStringResizeUninitialized"> https://github.com/google/snappy/search?q=STLStringResizeUninitialized</a></p>
   <p>Tensor Flow does too:<br> <a href="https://github.com/tensorflow/tensorflow/search?q=STLStringResizeUninitialized"> https://github.com/tensorflow/tensorflow/search?q=STLStringResizeUninitialized</a></p>
   <h3 class="heading settled" data-level="3.2" id="boost"><span class="secno">3.2. </span><span class="content">Boost containers</span><a class="self-link" href="#boost"></a></h3>
   <p>Boost provides a related optimization for vector-like containers, introduced in <a href="https://github.com/boostorg/container/commit/14f092ab00def8e692b81d57d0466a617a6f6130"> [SVN r85964] </a> by Ion Gaztañaga.</p>
   <blockquote>
<pre class="highlight">Default initialization for vector-like containers
Complexity guarantees for associative container constructors and ordered input ranges
Added benchmark for associative containers
Fixes #9166
</pre>
   </blockquote>
   <p>E.g.: <a href="https://github.com/boostorg/container/blob/develop/include/boost/container/vector.hpp"> boost/container/vector.hpp</a>:</p>
<pre class="highlight"><span class="c1">//! &lt;b>Effects&lt;/b>: Constructs a vector that will use a copy of allocator a</span>
<span class="c1">//!   and inserts n default initialized values.</span>
<span class="c1">//!</span>
<span class="c1">//! &lt;b>Throws&lt;/b>: If allocator_type’s allocation</span>
<span class="c1">//!   throws or T’s default initialization throws.</span>
<span class="c1">//!</span>
<span class="c1">//! &lt;b>Complexity&lt;/b>: Linear to n.</span>
<span class="c1">//!</span>
<span class="c1">//! &lt;b>Note&lt;/b>: Non-standard extension</span>
<span class="c1"></span><span class="n">vector</span><span class="p">(</span><span class="n">size_type</span> <span class="n">n</span><span class="p">,</span> <span class="n">default_init_t</span><span class="p">);</span>
<span class="n">vector</span><span class="p">(</span><span class="n">size_type</span> <span class="n">n</span><span class="p">,</span> <span class="n">default_init_t</span><span class="p">,</span> <span class="k">const</span> <span class="n">allocator_type</span> <span class="o">&amp;</span><span class="n">a</span><span class="p">)</span>
<span class="p">...</span>
<span class="kt">void</span> <span class="n">resize</span><span class="p">(</span><span class="n">size_type</span> <span class="n">new_size</span><span class="p">,</span> <span class="n">default_init_t</span><span class="p">);</span>
<span class="p">...</span>
</pre>
   <p>These optimizations are also supported in Boost Container’s <code class="highlight"><span class="n">small_vector</span></code>, <code class="highlight"><span class="n">static_vector</span></code>, <code class="highlight"><span class="n">deque</span></code>, <code class="highlight"><span class="n">stable_vector</span></code>, and <code class="highlight"><span class="n">string</span></code>.</p>
   <h3 class="heading settled" data-level="3.3" id="mongodb"><span class="secno">3.3. </span><span class="content">MongoDB</span><a class="self-link" href="#mongodb"></a></h3>
   <p>MongoDB has a string builder that could have been implemented in terms of <code class="highlight"><span class="n">basic_string</span></code> as a return value. However, as explained by Mathias Stearn, zero
initialization was measured and was too costly. Instead a custom string builder
type is used:</p>
   <p>E.g.: <a href="https://github.com/mongodb/mongo/blob/67da083face9dabdf95a5ffb72d3da675269b4c7/src/mongo/db/fts/unicode/string.h#L128-L143"> https://github.com/mongodb/mongo/blob/master/src/mongo/db/fts/unicode/string.h </a></p>
<pre class="highlight"><span class="cm">/**</span>
<span class="cm"> * Strips diacritics and case-folds the utf8 input string, as needed to support</span>
<span class="cm"> * options.</span>
<span class="cm"> *</span>
<span class="cm"> * The options field specifies what operations to *skip*, so kCaseSensitive</span>
<span class="cm"> * means to skip case folding and kDiacriticSensitive means to skip diacritic</span>
<span class="cm"> * striping. If both flags are specified, the input utf8 StringData is returned</span>
<span class="cm"> * directly without any processing or copying.</span>
<span class="cm"> *</span>
<span class="cm"> * If processing is performed, the returned StringData will be placed in buffer.</span>
<span class="cm"> * buffer’s contents (if any) will be replaced. Since we may return the input</span>
<span class="cm"> * unmodified the returned StringData’s lifetime is the shorter of the input</span>
<span class="cm"> * utf8 and the next modification to buffer. The input utf8 must not point into</span>
<span class="cm"> * buffer.</span>
<span class="cm"> */</span>
<span class="k">static</span> <span class="n">StringData</span> <span class="nf">caseFoldAndStripDiacritics</span><span class="p">(</span><span class="n">StackBufBuilder</span><span class="o">*</span> <span class="n">buffer</span><span class="p">,</span>
                                             <span class="n">StringData</span> <span class="n">utf8</span><span class="p">,</span>
                                             <span class="n">SubstrMatchOptions</span> <span class="n">options</span><span class="p">,</span>
                                             <span class="n">CaseFoldMode</span> <span class="n">mode</span><span class="p">);</span>
</pre>
   <p>(Comments re-wrapped.)</p>
   <h3 class="heading settled" data-level="3.4" id="vmware"><span class="secno">3.4. </span><span class="content">VMware string builders</span><a class="self-link" href="#vmware"></a></h3>
   <p>VMware has an internal string builder implementation that avoids <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">string</span></code> due, in part, to <code class="highlight"><span class="n">reserve</span></code>’s zero-writing
behavior. This is similar in spirit to the MongoDB example above.</p>
   <h3 class="heading settled" data-level="3.5" id="std-proposals"><span class="secno">3.5. </span><span class="content">Discussion on std-proposals</span><a class="self-link" href="#std-proposals"></a></h3>
   <p>This topic was discussed in 2013 on std-proposals in a thread titled "Add
basic_string::resize_uninitialized (or a similar mechanism)":<br> <a href="https://groups.google.com/a/isocpp.org/forum/#!topic/std-proposals/XIO4KbBTxl0"> https://groups.google.com/a/isocpp.org/forum/#!topic/std-proposals/XIO4KbBTxl0</a></p>
   <h3 class="heading settled" data-level="3.6" id="p1020r0"><span class="secno">3.6. </span><span class="content">P1020R0</span><a class="self-link" href="#p1020r0"></a></h3>
    See also <a data-link-type="biblio" href="#biblio-p1020r0">[P1020R0]</a> "Smart pointer creation functions for default initialization". 
   <h2 class="heading settled" data-level="4" id="alternatives"><span class="secno">4. </span><span class="content">Alternatives Considered</span><a class="self-link" href="#alternatives"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="free"><span class="secno">4.1. </span><span class="content">Free function <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">implicit_construct</span></code></span><a class="self-link" href="#free"></a></h3>
   <p>This proposal adds an optional member to <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">allocator_traits</span></code>. In <a data-link-type="biblio" href="#biblio-p0653r2">[P0653R2]</a>, which added the optional member <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">pointer_traits</span><span class="o">::</span><span class="n">to_address</span></code>,
the author also added a free function <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">to_address</span></code>. The free function
provides uniform access to user specializations of <code class="highlight"><span class="n">pointer_traits</span></code> that do not
define <code class="highlight"><span class="n">to_address</span></code>. A free function is not necessary or applicable here because
containers need to directly test for well-formedness of <code class="highlight"><span class="n">allocator_traits</span><span class="o">&lt;</span><span class="n">Allocator</span><span class="o">>::</span><span class="n">rebind_traits</span><span class="o">&lt;</span><span class="n">U</span><span class="o">>::</span><span class="n">implicit_construct</span><span class="p">(</span><span class="n">U</span> <span class="o">*</span><span class="p">)</span></code> to
correctly enable / disable methods in their interface.</p>
   <p>Also consider that <code class="highlight"><span class="n">allocator_traits</span></code> is designed to provide uniform operations
over custom allocators. [<strong>allocator.requirements</strong>]p2 does allow for user
specialization of <code class="highlight"><span class="n">allocator_traits</span></code>, but do users need this additional
customization point? Should <code class="highlight"><span class="n">allocator_traits</span></code> have been named <code class="highlight"><span class="n">allocator_adapter</span></code>? Should we discourage or deprecate user specialization?</p>
   <h3 class="heading settled" data-level="4.2" id="insert_from_capacity"><span class="secno">4.2. </span><span class="content"><code class="highlight"><span class="n">insert_from_capacity</span></code></span><a class="self-link" href="#insert_from_capacity"></a></h3>
    What name and return type should we use? <br>  
   <table>
    <tbody>
     <tr>
      <th>Return type
      <th>Name
      <th>Comments
     <tr>
     <tr>
      <td><code class="highlight"><span class="n">iterator</span></code>
      <td><code class="highlight"><span class="n">insert_from_capacity</span><span class="p">(</span><span class="n">n</span><span class="p">)</span></code>
      <td>as proposed
     <tr>
      <td><code class="highlight"><span class="n">vector</span><span class="o">&amp;</span></code>
      <td><code class="highlight"><span class="n">append_from_capacity</span><span class="p">(</span><span class="n">n</span><span class="p">)</span></code>
      <td>like <code class="highlight"><span class="n">basic_string</span><span class="o">::</span><span class="n">append</span></code>. <code class="highlight"><span class="n">vector</span></code> does not have <code class="highlight"><span class="n">append</span></code>.
     <tr>
      <td><code class="highlight"><span class="kt">void</span></code>
      <td><code class="highlight"><span class="n">resize_uninitialized</span><span class="p">(</span><span class="n">new_size</span><span class="p">)</span></code>
      <td>but the elements <em>are</em> initialized
     <tr>
      <td><code class="highlight"><span class="kt">void</span></code>
      <td><code class="highlight"><span class="n">resize_from_capacity</span><span class="p">(</span><span class="n">new_size</span><span class="p">)</span></code>
      <td><code class="highlight"><span class="n">resize</span></code> implies / is associated with reallocation, but this operation
    cannot reallocate
     <tr>
      <td><code class="highlight"><span class="kt">void</span></code>
      <td><code class="highlight"><span class="n">extend</span><span class="p">(</span><span class="n">n</span><span class="p">)</span></code>, <code class="highlight"><span class="n">grow</span><span class="p">(</span><span class="n">n</span><span class="p">)</span></code>, etc.
      <td>some new term...
   </table>
   <h3 class="heading settled" data-level="4.3" id="implicit_construct"><span class="secno">4.3. </span><span class="content"><code class="highlight"><span class="n">implicit_construct</span></code></span><a class="self-link" href="#implicit_construct"></a></h3>
    What name should we use? <br>  
   <table>
    <tbody>
     <tr>
      <th>Return type
      <th>Name
      <th>Comments
     <tr>
     <tr>
      <td><code class="highlight"><span class="kt">void</span></code>
      <td><code class="highlight"><span class="n">implicit_construct</span><span class="p">(</span><span class="n">T</span> <span class="o">*</span><span class="p">)</span></code>
      <td>as proposed
     <tr>
      <td><code class="highlight"><span class="kt">void</span></code>
      <td><code class="highlight"><span class="n">bless</span><span class="p">(</span><span class="n">T</span> <span class="o">*</span><span class="p">)</span></code>
      <td>From <a data-link-type="biblio" href="#biblio-p0593r2">[P0593R2]</a>.
   </table>
   <h3 class="heading settled" data-level="4.4" id="uninitialized_data"><span class="secno">4.4. </span><span class="content"><code class="highlight"><span class="n">uninitialized_data</span></code></span><a class="self-link" href="#uninitialized_data"></a></h3>
   <p>What name and return type should we use? <br> </p>
   <table>
    <tbody>
     <tr>
      <th>Return type
      <th>Name
      <th>Comments
     <tr>
     <tr>
      <td><code class="highlight"><span class="n">T</span><span class="o">*</span></code>
      <td><code class="highlight"><span class="n">uninitialized_data</span><span class="p">()</span></code>
      <td>as proposed
     <tr>
      <td><code class="highlight"><span class="n">span</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code>
      <td><code class="highlight"><span class="n">uninitialized_data</span><span class="p">()</span></code>
      <td> Nevin commented: the current proposal makes <code class="highlight"><span class="n">capacity</span><span class="p">()</span></code> a salient property
     of <code class="highlight"><span class="n">vector</span></code> and that that may be unsirable from the LEWG point of view.
     Mathias suggested returning <code class="highlight"><span class="n">span</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code>.
   </table>
   <h2 class="heading settled" data-level="5" id="issues"><span class="secno">5. </span><span class="content">Open Issues</span><a class="self-link" href="#issues"></a></h2>
   <ul>
    <li>
      <strong>Q:</strong> Can <code class="highlight"><span class="n">implicit_construct</span></code> throw? 
     <p><strong>A:</strong> Yes.</p>
    <p></p>
    <li> It would be useful to add a discussion of the tradeoffs between implicit
     lifetime types and default initialization. 
   </ul>
   <h2 class="heading settled" data-level="6" id="wording"><span class="secno">6. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="requirements"><span class="secno">6.1. </span><span class="content">[<strong>allocator.requirements</strong>]</span><a class="self-link" href="#requirements"></a></h3>
   <p>In [<strong>allocator.requirements</strong>] Table 31 add the <code class="highlight"><span class="n">a</span><span class="p">.</span><span class="n">implicit_construct</span><span class="p">(</span><span class="n">c</span><span class="p">)</span></code> expression:</p>
   <blockquote>
    <table>
     <tbody>
      <tr>
       <th>Expression
       <th>Return type
       <th>Assertion/note <br> pre-/post-condition
       <th>Default
      <tr>
       <td>...
      <tr>
       <td><code>a.construct(c, args)</code>
       <td>(not used)
       <td><em>Effects:</em> Constructs an object of type <code>C</code> at <code>c</code>
       <td><code>::new((void*)c) C(forward<args>(args)...)</args></code>
      <tr>
       <td>
        <ins><code>a.implicit_construct(c)</code></ins>
       <td>
        <ins>(not used)</ins>
       <td>
        <ins><em>Effects:</em> Informs the allocator that an object of type <code> C </code> has been implicitly created at <code> c</code>. Only participates in overload resolution if <code> C </code> is an <em>implicit lifetime</em> type.</ins>
       <td>
        <ins>Does nothing</ins>
      <tr>
       <td><code>a.destroy(c)</code>
       <td>(not used)
       <td><em>Effects:</em> Destroys the object at <code>c</code>
       <td><code>c->~C()</code>
    </table>
     ... 
   </blockquote>
   <p>And then in [<strong>allocator.requirements</strong>]p9 add references to <code class="highlight"><span class="n">implicit_construct</span></code>:</p>
   <blockquote>
    <ol start="9">
     <li>
       An allocator may constrain the types on which it can be instantiated and
     the arguments for which its <code>construct</code>
      <ins>, <code>implicit_construct</code>,</ins>
       or <code> destroy</code> members may be called. If a type cannot be used with a
     particular allocator, the allocator class or the call to <code> construct</code>
      <ins>, <code> implicit_construct </code>,</ins>
       or <code> destroy</code> may fail to
     instantiate. 
    </ol>
   </blockquote>
   <h3 class="heading settled" data-level="6.2" id="traits"><span class="secno">6.2. </span><span class="content">[<strong>allocator.traits</strong>]</span><a class="self-link" href="#traits"></a></h3>
   <p class="note" role="note"><span>Note:</span> Following the pattern of <code class="highlight"><span class="n">pointer_traits</span><span class="o">::</span><span class="n">to_address</span></code> we do not add <code class="highlight"><span class="n">implicit_construct</span></code> to the synopsis in [<strong>allocator.traits</strong>]. Placing it there
would make it a required member of <code class="highlight"><span class="n">allocator_traits</span></code> which would break existing
user specializations.</p>
   <p>Add a new § 23.10.9.3:</p>
   <blockquote>
    <ins>
     <p><strong>23.10.9.3 Allocator traits optional static member functions
      [allocator.traits.optmem]</strong></p>
<pre>template&lt;class T, class... Args>
  static void implicit_construct(Alloc&amp; a, T* p);
</pre>
     <ol>
      <li> <em>Effects:</em> Calls <code>a.implicit_construct(p)</code> if that
     call is well-formed; otherwise, does nothing if <code>T</code> is an <em>implicit lifetime</em> type and <code>a.construct(p)</code> is <em>not</em> well-formed; otherwise, does not participate in overload
     resolution.
     </ol>
    </ins>
   </blockquote>
   <h3 class="heading settled" data-level="6.3" id="general"><span class="secno">6.3. </span><span class="content">[<strong>container.requirements.general</strong>]</span><a class="self-link" href="#general"></a></h3>
   <p>In [<strong>container.requirements.general</strong>]p3 add references to <code class="highlight"><span class="n">implicit_construct</span></code>:</p>
   <blockquote>
    <ol start="3">
     <li>
       For the components affected by this subclause that declare an <code>allocator_type</code>, objects stored in these components
     shall be constructed using the function <code> allocator_traits&lt;allocator_type>::rebind_traits&lt;U>::construct </code>
      <ins> or <code> allocator_traits&lt;allocator_type>::rebind_traits&lt;U>::implicit_construct</code></ins>
       and destroyed using the function <code> allocator_traits&lt;allocator_type>::rebind_traits&lt;U>::destroy </code> (23.10.9.2), where <code>U</code> is either <code>allocator_type::value_type</code> or an internal type used by
     the container. These functions are called only for the container’s element
     type, not for internal types used by the container. [<em>Note:</em> This means,
     for example, that a node-based container might need to construct nodes
     containing aligned buffers and call construct to place the element into the
     buffer. — <em>end note</em> ] 
    </ol>
   </blockquote>
   <h3 class="heading settled" data-level="6.4" id="vector"><span class="secno">6.4. </span><span class="content">[<strong>vector</strong>]</span><a class="self-link" href="#vector"></a></h3>
   <p>In [<strong>vector.overview</strong>] add the declaration for <code class="highlight"><span class="n">insert_from_capacity</span></code>:</p>
   <blockquote>
<pre>namespace std {
  template&lt;class T, class Allocator = allocator&lt;T>>
  class vector {

    ...

    <em>// 26.3.11.4, data access</em>
    T* data() noexcept;
    const T* data() const noexcept;
    <ins>T* uninitialized_data() noexcept;</ins>

    <em>// 26.3.11.5, modifiers</em>
    template&lt;class... Args> reference emplace_back(Args&amp;&amp;... args);
    void push_back(const T&amp; x);
    void push_back(T&amp;&amp; x);
    void pop_back();

    template&lt;class... Args> iterator emplace(const_iterator position, Args&amp;&amp;... args);
    iterator insert(const_iterator position, const T&amp; x);
    iterator insert(const_iterator position, T&amp;&amp; x);
    iterator insert(const_iterator position, size_type n, const T&amp; x);
    template&lt;class InputIterator>
      iterator insert(const_iterator position, InputIterator first, InputIterator last);
    iterator insert(const_iterator position, initializer_list&lt;T> il);
    <ins>iterator insert_from_capacity(size_type n);</ins>
    iterator erase(const_iterator position);
    iterator erase(const_iterator first, const_iterator last);

    ...
</pre>
   </blockquote>
   <p>In [<strong>vector.data</strong>] add new p3-5:</p>
   <blockquote>
<pre>T*         data() noexcept;
const T*   data() const noexcept;
</pre>
    <ol>
     <li> <em>Returns:</em> A pointer such that <code>[data(), data() +
     size())</code> is a valid range. For a non-empty vector, <code> data() == addressof(front())</code>. 
     <li> <em>Complexity:</em> Constant time. 
    </ol>
    <ins>
<pre>T*         uninitialized_data() noexcept;
</pre>
     <ol start="3">
      <li> <em>Returns:</em> A pointer to uninitialized storage that would hold elements in
     the range <code>[size(), capacity())</code>. [<em>Note:</em> This
     storage may be initialized through a pointer obtained by casting <code>T*</code> to <code>void*</code> and then to <code>char*</code>, <code>unsigned char*</code>, or <code>std::byte*</code>. ([<strong>basic.life</strong>]p6.4). - <em>end note</em> ] 
      <li> <em>Remarks:</em> This member function does not participate in overload resolution
     if <code> allocator_traits&lt;Allocator>::rebind_traits&lt;U>::implicit_construct(U
     *) </code> is not well-formed. 
      <li> <em>Complexity:</em> Constant time. 
     </ol>
    </ins>
   </blockquote>
   <p>In [<strong>vector.modifiers</strong>] add new p3-6:</p>
   <blockquote>
    <ol start="2">
     <li> <em>Complexity:</em> The complexity is linear in the number of elements inserted
     plus the distance to the end of the vector.
    </ol>
    <ins>
<pre>iterator insert_from_capacity(size_type n);
</pre>
     <ol start="3">
      <li> <em>Requires:</em> - <code>n &lt;= capacity() - size()</code>. 
      <li> <em>Remarks:</em> - Appends <code class="highlight"><span class="n">n</span></code> elements by implicitly creating them
     from capacity. The application must have initialized the storage backing
     these elements otherwise the behavior is undefined. This member function
     does not participate in overload resolution if <code> allocator_traits&lt;Allocator>::rebind_traits&lt;U>::implicit_construct(U
     *) </code> is not well-formed. 
      <li> <em>Returns:</em> - an iterator to the first element inserted, otherwise <code>end()</code>. 
      <li> <em>Complexity:</em> - The complexity is linear in the number of elements
     inserted. [<em>Note:</em> For some allocators, including the default allocator,
     actual complexity is constant time. - <em>end note</em> ] 
     </ol>
    </ins>
<pre>iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);
void pop_back();
</pre>
    <ol start="7">
     <li> <em>Effects:</em> Invalidates iterators and references at or after the point of
     the erase. 
     <li> ... 
    </ol>
   </blockquote>
   <h2 class="heading settled" data-level="7" id="acknowledgments"><span class="secno">7. </span><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
   <p>Special thanks go to <strong>Glen Fernandes</strong> and <strong>Mathias Stearn</strong> for early
comments and help with wording. Thanks also for early comments from <strong>Nevin
Liber</strong>, <strong>Agustín Bergé</strong> and <strong>Arthur O’Dwyer</strong> provided guidance on object
lifetime and allocator interactions.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-n4741">[N4741]
   <dd><a href="https://wg21.link/N4741">Working Draft, Standard for Programming Language C++</a>. 2 April 2018. URL: <a href="https://wg21.link/N4741">https://wg21.link/N4741</a>
   <dt id="biblio-p0593r2">[P0593R2]
   <dd>Richard Smith. <a href="https://wg21.link/p0593r2">Implicit creation of objects for low-level object manipulation</a>. URL: <a href="https://wg21.link/p0593r2">https://wg21.link/p0593r2</a>
   <dt id="biblio-p0653r2">[P0653R2]
   <dd>Glen Joseph Fernandes. <a href="https://wg21.link/p0653r2">Utility to convert a pointer to a raw pointer</a>. URL: <a href="https://wg21.link/p0653r2">https://wg21.link/p0653r2</a>
   <dt id="biblio-p1020r0">[P1020R0]
   <dd>Glen Joseph Fernandes; Peter Dimov. <a href="https://wg21.link/P1020R0">Smart pointer creation functions for default initialization</a>. May 2018. URL: <a href="https://wg21.link/P1020R0">https://wg21.link/P1020R0</a>
   <dt id="biblio-p1072r0">[P1072R0]
   <dd>Chris Kennelly; Mark Zeren. <a href="https://wg21.link/P1071R0">Default Initialization for basic_string</a>. May 2018. URL: <a href="https://wg21.link/P1071R0">https://wg21.link/P1071R0</a>
  </dl>