<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P1453R0: Modularizing the Standard Library is a Reorganization Opportunity</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,
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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">P1453R0<br>Modularizing the Standard Library is a Reorganization Opportunity</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-01-21">2019-01-21</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:brycelelbach@gmail.com">Bryce Adelstein Lelbach</a> (<span class="p-org org">NVIDIA</span>)
     <dt>Source:
     <dd><a href="https://github.com/brycelelbach/wg21_p1453_modularizing_the_standard_library_is_a_reorganization_opportunity/blob/master/modularizing_the_standard_library_is_a_reorganization_opportunity.bs">GitHub</a>
     <dt>Issue Tracking:
     <dd><a href="https://github.com/brycelelbach/wg21_p1453_modularizing_the_standard_library_is_a_reorganization_opportunity/issues">GitHub</a>
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Audience:
     <dd>LEWG
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </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="#need-for-reorganization"><span class="secno">2</span> <span class="content">The Need for Reorganization</span></a>
    <li>
     <a href="#opportunity"><span class="secno">3</span> <span class="content">A Rare Opportunity</span></a>
     <ol class="toc">
      <li><a href="#time-limited"><span class="secno">3.1</span> <span class="content">Time is Limited</span></a>
     </ol>
    <li>
     <a href="#cpp20-options"><span class="secno">4</span> <span class="content">Options for a C++20 Modular Standard Library</span></a>
     <ol class="toc">
      <li><a href="#cpp20-dont-modularize"><span class="secno">4.1</span> <span class="content">Don’t Modularize the Standard Library in C++20</span></a>
      <li><a href="#cpp20-modularize-existing-structure"><span class="secno">4.2</span> <span class="content">Modularize the Existing Structure in C++20 and Reorganize Later</span></a>
      <li><a href="#cpp20-one-module"><span class="secno">4.3</span> <span class="content">One Big Module for C++20</span></a>
      <li><a href="#cpp20-reorganize"><span class="secno">4.4</span> <span class="content">Attempt to Reorganize the Entire Standard Library in C++20</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>Modules are coming to C++.
After the San Diego 2018 C++ committee meeting, it seems quite likely that we
  will have modules in C++20.
This feature will have a transformative impact on almost all C++ code.
Consider the typical "hello world" program in C++:</p>
<pre class="highlight"><c- cp>#include</c-> &lt;iostream>
<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"hello world</c-><c- se>\n</c-><c- s>"</c-><c- p>;</c-> <c- p>}</c->
</pre>
   <p>"hello world" has remained constant from C++98 through C++17.
With C++20, however, it may finally change:</p>
<pre class="highlight"><c- n>import</c-> <c- d>/* ??? */</c-><c- p>;</c->
<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"hello world</c-><c- se>\n</c-><c- s>"</c-><c- p>;</c-> <c- p>}</c->
</pre>
   <p>But, what will <code class="highlight"><c- d>/* ??? */</c-></code> be?</p>
   <p>While we appear to have achieved consensus on a design for the modules language
  feature, our plan for how and when the C++ standard library will be modularized
  are not as mature.
Some proposals have been made (<a data-link-type="biblio" href="#biblio-p0581r1">[P0581R1]</a> and <a data-link-type="biblio" href="#biblio-p1212r0">[P1212R0]</a>) and preliminary
  discussions have taken place (<a data-link-type="biblio" href="#biblio-2018-jacksonville-lewg-p0581r0-minutes">[2018-Jacksonville-LEWG-P0581R0-Minutes]</a> and <a data-link-type="biblio" href="#biblio-2018-san-diego-ewg-p1212r0-minutes">[2018-San-Diego-EWG-P1212R0-Minutes]</a>), but we haven’t committed to a path
  yet.
Given that the C++20 cycle is nearly over, it’s time for us to make a decision
  on our strategy for standard library modules in C++20.</p>
   <h2 class="heading settled" data-level="2" id="need-for-reorganization"><span class="secno">2. </span><span class="content">The Need for Reorganization</span><a class="self-link" href="#need-for-reorganization"></a></h2>
   <p>Modules are a sorely needed opportunity to reorganize the standard library.
Today’s header-based partitioning of the standard library has not aged well
  over the past two decades.
Some of the pain points include:</p>
   <ul>
    <li data-md>
     <p><strong>Transitive Includes:</strong> Standard library headers are allowed to include other
  standard library headers.
There is often implementation divergence in the use of transitive includes. <a data-link-type="dfn" href="https://stackoverflow.com/questions/40819566/why-clang-on-mac-automatically-includes-some-missing-headers#">When users rely on transitive includes, they can end up with non portable code.</a></p>
    <li data-md>
     <p><strong>Overly Coarse Includes:</strong> Some standard library headers have evolved into
  repositories of facilities that do not necessarily belong together.
For example, in C++98, <code class="highlight"><c- o>&lt;</c-><c- n>functional</c-><c- o>></c-></code> was a simple header with a simple
  mission: provide unary and binary function objects for operators.
Including <code class="highlight"><c- o>&lt;</c-><c- n>functional</c-><c- o>></c-></code> with GCC 4.1 compiling in C++03 mode added 600
  lines to your code.
Today, <code class="highlight"><c- o>&lt;</c-><c- n>functional</c-><c- o>></c-></code> contains many things, including <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>function</c-></code> (which needs type traits and memory management facilities) and <code class="highlight"><c- n>std</c-><c- o>::*</c-><c- n>_searcher</c-></code>s (which need string processing facilities).
Including <code class="highlight"><c- o>&lt;</c-><c- n>functional</c-><c- o>></c-></code> with GCC 8 compiling in C++11 mode adds 44,691
  lines to your code.
That’s a lot if all you needed was <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>plus</c-></code>.</p>
    <li data-md>
     <p><strong>Lack of Logical Partitioning:</strong> It is common for C++ programmers to think of
  each standard library header as encapsulating a component of the standard
  library.
For example, the regex library lives in <code class="highlight"><c- o>&lt;</c-><c- n>regex</c-><c- o>></c-></code>, the filesystem library
  lives in <code class="highlight"><c- o>&lt;</c-><c- n>filesystem</c-><c- o>></c-></code>, and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple</c-></code> lives in <code class="highlight"><c- o>&lt;</c-><c- n>tuple</c-><c- o>></c-></code>.
However, many components of the standard library are spread across multiple
  headers, such as the sequence algorithms.
Most sequence algorithms are found in <code class="highlight"><c- o>&lt;</c-><c- n>algorithm</c-><c- o>></c-></code>, but some are found in <code class="highlight"><c- o>&lt;</c-><c- n>numeric</c-><c- o>></c-></code> and <code class="highlight"><c- o>&lt;</c-><c- n>memory</c-><c- o>></c-></code>.
This has even been confusing to the committee, which has often forgot to
  apply changes applicable to all algorithms to the ones in <code class="highlight"><c- o>&lt;</c-><c- n>numeric</c-><c- o>></c-></code> and <code class="highlight"><c- o>&lt;</c-><c- n>memory</c-><c- o>></c-></code>.
We even moved <a data-link-type="dfn" href="http://eel.is/c++draft/numeric.ops.overview">the section in
  the standard for the <code class="highlight"><c- o>&lt;</c-><c- n>numeric</c-><c- o>></c-></code> header</a> into the <code class="highlight"><c- p>[</c-><c- n>algorithms</c-><c- p>]</c-></code> clause recently so that it will no longer be overlooked.
Additionally, some standard library facilities that live in omnibus headers
  and can be difficult to locate.
Where is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pair</c-></code>? In <code class="highlight"><c- o>&lt;</c-><c- n>utility</c-><c- o>></c-></code>, not <code class="highlight"><c- o>&lt;</c-><c- n>pair</c-><c- o>></c-></code>.
Where is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-></code>? In <code class="highlight"><c- o>&lt;</c-><c- n>memory</c-><c- o>></c-></code>, not <code class="highlight"><c- o>&lt;</c-><c- n>unique_ptr</c-><c- o>></c-></code>.</p>
    <li data-md>
     <p><strong>Freestanding:</strong> There is general agreement on the committee that the current
  definition of the freestanding standard library subset is not particularly
  useful.
At the same time, there are a number of constituencies (embedded,
  kernel/OS development, parallel programming) that desire a useful
  freestanding standard library and would like to see freestanding
  overhauled in a future standard (<a data-link-type="biblio" href="#biblio-p0829r3">[P0829R3]</a>, <a data-link-type="biblio" href="#biblio-p1212r0">[P1212R0]</a>, <a data-link-type="biblio" href="#biblio-p1376r0">[P1376R0]</a>).</p>
   </ul>
   <p>All of these issues could be solved by a careful reorganization of the standard
  library.
The only reason we have not undertaken such a reorganization has been a lack of
  opportunity before modules.</p>
   <h2 class="heading settled" data-level="3" id="opportunity"><span class="secno">3. </span><span class="content">A Rare Opportunity</span><a class="self-link" href="#opportunity"></a></h2>
   <p>This opportunity to reorganize the standard library is a rare one.
Standard library implementations make strong backwards compatibility guarantees,
  so we cannot just decide to move entities from one header to another.</p>
   <p>Modules, however, move us away from a header-based model entirely.
When we modularize the standard library, we are not bound to the existing
  structure of our current header-based model.
Thee moduarlized standard library can be repartition into more sensible subsets.</p>
   <p>However, we only have one chance to do this.
Once we ship a modularized standard library, we will find ourselves in the same
  situation that we have today.
We will be unable to reorganize things without breaking backwards
  compatibility.</p>
   <p>This may be our only chance in the lifetime of C++ to reorganize the standard
  library.
Thus, it is imperative that we get it right.</p>
   <h3 class="heading settled" data-level="3.1" id="time-limited"><span class="secno">3.1. </span><span class="content">Time is Limited</span><a class="self-link" href="#time-limited"></a></h3>
   <p>We are nearing the end of the C++20 development cycle.
We plan to ship a committee draft of the C++20 standard six months
  from the authoring of this paper.</p>
   <p>Integrating new language features into the standard library is one of the harder
  parts of our standardization work.
Recently, we have wisely decided to exercise caution when rolling out new
  language features in the standard library.</p>
   <p>Take concepts as an example.
In C++20, we have a core concepts library, and a major new piece of the standard
  library, ranges, uses concepts.
However, we have not attempted to go through the entire standard library and
  deploy concepts everywhere we can for C++20.
Additionally, we have been conservative in the use of concepts for new library
  features for C++20.
As far as this author is aware, only the new ranges library will use concepts.
It seems likely that in future standards, in places where proper constrained
  template parameters are desirable, we will either update existing facilities
  to use concepts or introduce new improved versions of those facilities.</p>
   <p>While it may seem counter-intuitive for the standard library to not aggressively
  embrace new language features, it is in fact the right thing to do in many
  cases.
While the impact of new language feature on the library should be considered
  and evaluated, we should neither make new language features wait for deployment
  within the standard library nor rush that deployment to not delay the feature.
When a language feature is ready, so long as we are confident that we will be
  able to complete deployment of the feature in the standard library in the
  future, we should be ready to ship it.
We should not let the perfect be the enemy of the good.</p>
   <h2 class="heading settled" data-level="4" id="cpp20-options"><span class="secno">4. </span><span class="content">Options for a C++20 Modular Standard Library</span><a class="self-link" href="#cpp20-options"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="cpp20-dont-modularize"><span class="secno">4.1. </span><span class="content">Don’t Modularize the Standard Library in C++20</span><a class="self-link" href="#cpp20-dont-modularize"></a></h3>
   <p>The first and most conservative option would be to not modularize the standard
  library in C++20.
Users would access the standard library via legacy imports or coontinue <code class="highlight"><c- cp>#include</c-></code>s.</p>
   <p><strong>Pros:</strong> This approach preserves the most freedom for future modularization and
  presents no risk of shipping something we are unable to change later.</p>
   <p><strong>Cons:</strong> The lack of a modularized standard library may inhibit adoption of
  modules or lead to implementation-specific standard library modules.</p>
   <h3 class="heading settled" data-level="4.2" id="cpp20-modularize-existing-structure"><span class="secno">4.2. </span><span class="content">Modularize the Existing Structure in C++20 and Reorganize Later</span><a class="self-link" href="#cpp20-modularize-existing-structure"></a></h3>
   <p>In this approach, we would introduce standard library modules in C++20 that map
  to the existing standard library headers.
For each standard library header <code class="highlight"><c- o>&lt;</c-><c- n>foo</c-><c- o>></c-></code>, we would introduce a <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>header</c-><c- p>.</c-><c- n>foo</c-></code> module.</p>
   <p>The use of the <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>header</c-></code> prefix is intended to reserve syntactic space for the
  future.
For example, we might want to have a <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>algorithm</c-></code> module in the future which
  contains all the sequence algorithms, while the <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>header</c-><c- p>.</c-><c- n>algorithm</c-></code> module
  would contain just the sequence algorithms in <code class="highlight"><c- o>&lt;</c-><c- n>algorithm</c-><c- o>></c-></code>.
This prefix could be spelled differently, possibly <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>legacy</c-></code>, <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>old</c-></code>, or <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>v1</c-></code>.</p>
   <p><strong>Pros:</strong> This approach provides standard library modules in C++20, preserves a
  great deal of freedom for future modularization, and presents an acceptably
  small amount of risk of shipping something we are unable to change later.</p>
   <p><strong>Cons:</strong> The <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>header</c-></code> modules we’d be shipping would be difficult to get rid
  of. They’d co-exist with whatever new standard library modules we introduce in
  the future and potentially preserve some of the problems with the existing
  standard library structure identified in <a href="#need-for-reorganization">§2 The Need for Reorganization</a>.</p>
   <p>There is one major question with this approach.
Will we be able to move the definition of an entity between modules without
  breaking ABI compatibility?
E.g. If entity <code class="highlight"><c- n>foo</c-></code> is currently defined in <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>header</c-><c- p>.</c-><c- n>bar</c-></code>, in the future can
  we introduce a new module <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>foo</c-></code> which defines <code class="highlight"><c- n>foo</c-></code>, and modify <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>header</c-><c- p>.</c-><c- n>bar</c-></code> to simply re-export <code class="highlight"><c- n>foo</c-></code>?</p>
   <h3 class="heading settled" data-level="4.3" id="cpp20-one-module"><span class="secno">4.3. </span><span class="content">One Big Module for C++20</span><a class="self-link" href="#cpp20-one-module"></a></h3>
   <p>One option would be to introduce a <code class="highlight"><c- n>std</c-></code> module that includes everything in the
  standard library.
For finer grained usage, users could either use the modules from <a href="#cpp20-modularize-existing-structure">§4.2 Modularize the Existing Structure in C++20 and Reorganize Later</a>, use legacy imports, or continue
  using <code class="highlight"><c- cp>#include</c-></code>s.</p>
   <p><strong>Pros:</strong> This would be easy to use and would still allow us to reorganize the
  standard library into finer grained modules in the future.</p>
   <p><strong>Cons:</strong> The standard library has a number of global dynamic constructors
  (such as the ones in iostreams) and auxiliary dependencies (filesystem
  libraries, regex libraries, etc). It would be undesirable to encourage people
  to pay for these as they may not be using them.</p>
   <p>As with <a href="#cpp20-modularize-existing-structure">§4.2 Modularize the Existing Structure in C++20 and Reorganize Later</a>, this approach is only sensible
  if we are able to move the definition of a standard library entity from one
  module to another without breaking ABI compatibility.</p>
   <h3 class="heading settled" data-level="4.4" id="cpp20-reorganize"><span class="secno">4.4. </span><span class="content">Attempt to Reorganize the Entire Standard Library in C++20</span><a class="self-link" href="#cpp20-reorganize"></a></h3>
   <p>In this approach, we would use the remaining time in the C++20 cycle to
  reorganize the structure of the standard library to address the problems
  identified in <a href="#need-for-reorganization">§2 The Need for Reorganization</a>.
This effort would likely need to include a redefinition of the freestanding
  standard library subset, as this would likely be our only chance to do so.
So, this would probably be a combination of <a data-link-type="biblio" href="#biblio-p0581r1">[P0581R1]</a>, <a data-link-type="biblio" href="#biblio-p0829r3">[P0829R3]</a>, and <a data-link-type="biblio" href="#biblio-p1376r0">[P1376R0]</a>.</p>
   <p><strong>Pros:</strong> We would have a complete solution in C++20 instead of a middleground
  approach with limitations.</p>
   <p><strong>Cons:</strong> This option has significant schedule risk, limited time for us to
  obtain field experience, closes the door for any post C++20
  reorganization, and presents the highest risk of shipping something that we
  are unable to change later.</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-2018-jacksonville-lewg-p0581r0-minutes">[2018-Jacksonville-LEWG-P0581R0-Minutes]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21jacksonville2018/P0581">2018 Jacksonville C++ Committee Meeting - LEWG P0581R0 Minutes</a>. URL: <a href="http://wiki.edg.com/bin/view/Wg21jacksonville2018/P0581">http://wiki.edg.com/bin/view/Wg21jacksonville2018/P0581</a>
   <dt id="biblio-2018-san-diego-ewg-p1212r0-minutes">[2018-San-Diego-EWG-P1212R0-Minutes]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21sandiego2018/P1212R0-San18">2018 San Diego C++ Committee Meeting - EWG P1212R0 Minutes</a>. URL: <a href="http://wiki.edg.com/bin/view/Wg21sandiego2018/P1212R0-San18">http://wiki.edg.com/bin/view/Wg21sandiego2018/P1212R0-San18</a>
   <dt id="biblio-p0581r1">[P0581R1]
   <dd>Marshall Clow, Beman Dawes, Gabriel Dos Reis, Stephan T. Lavavej, Billy O’Neal, Bjarne Stroustrup, Jonathan Wakely. <a href="https://wg21.link/p0581r1">Standard Library Modules</a>. 11 February 2018. URL: <a href="https://wg21.link/p0581r1">https://wg21.link/p0581r1</a>
   <dt id="biblio-p0829r3">[P0829R3]
   <dd>Ben Craig. <a href="https://wg21.link/p0829r3">Freestanding Proposal</a>. 6 October 2018. URL: <a href="https://wg21.link/p0829r3">https://wg21.link/p0829r3</a>
   <dt id="biblio-p1212r0">[P1212R0]
   <dd>Ben Craig. <a href="https://wg21.link/p1212r0">Modules and Freestanding</a>. 6 October 2018. URL: <a href="https://wg21.link/p1212r0">https://wg21.link/p1212r0</a>
   <dt id="biblio-p1376r0">[P1376R0]
   <dd>Ben Craig. <a href="https://wg21.link/p1376r0">Summary of freestanding evening session discussions</a>. 24 November 2018. URL: <a href="https://wg21.link/p1376r0">https://wg21.link/p1376r0</a>
  </dl>