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 <head>
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  <title>P1874R1: Dynamic Initialization Order of Non-Local Variables in Modules</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

  /* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1874R1<br>Dynamic Initialization Order of Non-Local Variables in Modules</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-11-08">2019-11-08</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1874r1">http://wg21.link/p1874r1</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:<michael_spencer@apple.com>">Michael Spencer</a> (<span class="p-org org">Apple</span>)
     <dt>Audience:
     <dd>EWG, SG2
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>The order of dynamic initialization of non-local variables with static

          storage duration (globals) imported from interface and header units is
          indeterminately sequenced relative to those of their importer. This
          has the potential to break code, including code which includes <code class="highlight"><c- o>&lt;</c-><c- n>iostream</c-><c- o>></c-></code>, when moving to C++20. This paper
          explores what existing implementations do in this case, and defines
          an ordering as is currently done in both Clang and GCC. This paper
          resolves US082.</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="#tony-table"><span class="secno">1</span> <span class="content">Effects of This Paper</span></a>
    <li><a href="#problem"><span class="secno">2</span> <span class="content">The Problem</span></a>
    <li>
     <a href="#solution"><span class="secno">3</span> <span class="content">The Solution</span></a>
     <ol class="toc">
      <li><a href="#catch"><span class="secno">3.1</span> <span class="content">The Catch</span></a>
     </ol>
    <li><a href="#implementation-units"><span class="secno">4</span> <span class="content">Implementation Units?</span></a>
    <li><a href="#target"><span class="secno">5</span> <span class="content">Ship Vehicle</span></a>
    <li><a href="#polls"><span class="secno">6</span> <span class="content">Proposed Polls</span></a>
    <li>
     <a href="#wording"><span class="secno">7</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#basic-start-dynamic"><span class="secno">7.1</span> <span class="content">[basic.start.dynamic]</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="tony-table"><span class="secno">1. </span><span class="content">Effects of This Paper</span><a class="self-link" href="#tony-table"></a></h2>
   <p class="note" role="note"><span>Note:</span> In all of the examples in this paper it is assumed that <code class="highlight"><c- cp>#include</c-></code> translation does not occur.</p>
   <table style="margin-left:auto;margin-right:auto;">
    <thead>
     <tr>
      <th>Status Quo
      <th>This Paper
    <tbody>
     <tr>
      <td>
<pre class="highlight"><c- n>import</c-> <c- o>&lt;</c-><c- n>iostream</c-><c- o>></c-><c- p>;</c->

<c- k>struct</c-> <c- n>G</c-> <c- p>{</c->
  <c- n>G</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>"Constructing</c-><c- se>\n</c-><c- s>"</c-><c- p>;</c->
  <c- p>}</c->
<c- p>}</c-> <c- n>g</c-><c- p>;</c->
<c- c1>// ☠️ std::Init may not have been initialized yet,</c->
<c- c1>// so std::cout may not have been initialized yet.</c->
</pre>
      <td>
<pre class="highlight"><c- n>import</c-> <c- o>&lt;</c-><c- n>iostream</c-><c- o>></c-><c- p>;</c->

<c- k>struct</c-> <c- n>G</c-> <c- p>{</c->
  <c- n>G</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>"Constructing</c-><c- se>\n</c-><c- s>"</c-><c- p>;</c->
  <c- p>}</c->
<c- p>}</c-> <c- n>g</c-><c- p>;</c->
<c- c1>// ✅ std::Init has been initialized,</c->
<c- c1>// thus std::cout has been initialized.</c->
</pre>
   </table>
   <h2 class="heading settled" data-level="2" id="problem"><span class="secno">2. </span><span class="content">The Problem</span><a class="self-link" href="#problem"></a></h2>
   <p>[basic.start.dynamic] states:</p>
   <blockquote>
    <p>Dynamic initialization of non-local variables <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> with static storage duration are ordered as follows:</p>
    <ul>
     <li data-md>
      <p>If <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> have ordered initialization and <code class="highlight"><c- n>V</c-></code> is defined before <code class="highlight"><c- n>W</c-></code> within a single translation unit, the initialization of <code class="highlight"><c- n>V</c-></code> is sequenced before the initialization of <code class="highlight"><c- n>W</c-></code>.</p>
     <li data-md>
      <p>If <code class="highlight"><c- n>V</c-></code> has partially-ordered initialization, <code class="highlight"><c- n>W</c-></code> does not have unordered initialization, and <code class="highlight"><c- n>V</c-></code> is defined before <code class="highlight"><c- n>W</c-></code> in every translation unit in which <code class="highlight"><c- n>W</c-></code> is defined, then</p>
      <ul>
       <li data-md>
        <p>if the program starts a thread ([intro.multithread]) other than the main thread ([basic.start.main]), the initialization of <code class="highlight"><c- n>V</c-></code> strongly happens before the initialization of <code class="highlight"><c- n>W</c-></code>;</p>
       <li data-md>
        <p>otherwise, the initialization of <code class="highlight"><c- n>V</c-></code> is sequenced before the initialization of <code class="highlight"><c- n>W</c-></code>.</p>
      </ul>
     <li data-md>
      <p>Otherwise, if the program starts a thread other than the main thread before either <code class="highlight"><c- n>V</c-></code> or <code class="highlight"><c- n>W</c-></code> is initialized, it is unspecified in which threads the initializations of <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> occur; the initializations are unsequenced if they occur in the same thread.</p>
     <li data-md>
      <p>Otherwise, the initializations of <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> are indeterminately sequenced.</p>
    </ul>
   </blockquote>
   <p>With textual <code class="highlight"><c- cp>#include</c-></code>s the first rule holds for global initaliziers, but this
breaks when moving to <code class="highlight"><c- n>import</c-></code>, either explicitly or via <code class="highlight"><c- cp>#include</c-></code> translation. This happens
because header units are their own translation units, and thus fall into the last
rule. A similar problem exists with module interface units which transitively <code class="highlight"><c- cp>#include</c-></code> or <code class="highlight"><c- n>import</c-></code> such a header.</p>
   <h2 class="heading settled" data-level="3" id="solution"><span class="secno">3. </span><span class="content">The Solution</span><a class="self-link" href="#solution"></a></h2>
   <p>Clang already ran into this issue and has a simple fix. Run all global
initializers from translation units which are transitively imported when they
are imported.</p>
<pre class="highlight"><c- c1>// H1.h</c->
<c- kr>inline</c-> <c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->

<c- c1>// H2.h</c->
<c- kr>inline</c-> <c- b>int</c-> <c- n>b</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->

<c- c1>// TU.cpp</c->
<c- b>int</c-> <c- n>c</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->
<c- n>import</c-> <c- s>"H1.h"</c-><c- p>;</c->
<c- n>import</c-> <c- s>"H2.h"</c-><c- p>;</c->
</pre>
   <p>Clang modules tried to mimic includes as closely as possible, thus in the above
example Clang will initialize <code class="highlight"><c- n>c</c-></code> first, and then <code class="highlight"><c- n>a</c-></code> before <code class="highlight"><c- n>b</c-></code>. If you
flip the order of the imports, then it would initialize <code class="highlight"><c- n>b</c-></code> before <code class="highlight"><c- n>a</c-></code>. While
this is a reasonable model for Clang modules, which was intended to mimic <code class="highlight"><c- cp>#include</c-></code>s, it is a poor fit for C++20 modules, as it violates the rule that
the order of imports doesn’t matter.</p>
   <p>This paper explores a simpler user model based on dependency order. All global initaliziers from
translation units which are transitively imported are run before any globals in
the translation unit which imports them. For the above example it would mean
that the initialization of <code class="highlight"><c- n>a</c-></code> and <code class="highlight"><c- n>b</c-></code> is sequenced before the initialization of <code class="highlight"><c- n>c</c-></code>, but that the initializations of <code class="highlight"><c- n>a</c-></code> and <code class="highlight"><c- n>b</c-></code> are indeterminately sequenced.</p>
   <h3 class="heading settled" data-level="3.1" id="catch"><span class="secno">3.1. </span><span class="content">The Catch</span><a class="self-link" href="#catch"></a></h3>
   <p>Initializers of inline variable <code class="highlight"><c- n>V</c-></code> and variable <code class="highlight"><c- n>W</c-></code> are ordered when "<code class="highlight"><c- n>V</c-></code> is
defined before <code class="highlight"><c- n>W</c-></code> in every translation unit in which <code class="highlight"><c- n>W</c-></code> is defined." If the
above solution is applied blindly, then cycles can be created.</p>
<pre class="highlight"><c- c1>// H1.h</c->
<c- b>int</c-> <c- nf>external</c-><c- p>(</c-><c- b>int</c-><c- p>);</c->

<c- c1>// H2.h</c->
<c- cp>#include</c-> "H1.h"
<c- kr>inline</c-> <c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- n>external</c-><c- p>(</c-><c- mi>0</c-><c- p>);</c->

<c- c1>// H3.h</c->
<c- cp>#include</c-> "H1.h"
<c- kr>inline</c-> <c- b>int</c-> <c- n>b</c-> <c- o>=</c-> <c- n>external</c-><c- p>(</c-><c- mi>1</c-><c- p>);</c->

<c- c1>// M1.cppm</c->
<c- n>module</c-><c- p>;</c->
<c- cp>#include</c-> "H2.h"
<c- k>export</c-> <c- n>module</c-> <c- n>M1</c-><c- p>;</c->
<c- n>import</c-> <c- s>"H3.h"</c-><c- p>;</c->

<c- c1>// M2.cppm</c->
<c- n>module</c-><c- p>;</c->
<c- cp>#include</c-> "H3.h"
<c- k>export</c-> <c- n>module</c-> <c- n>M2</c-><c- p>;</c->
<c- n>import</c-> <c- s>"H2.h"</c-><c- p>;</c->
</pre>
   <p>This example has the following sequenced before graph:</p>
   <div class="dot">
    <svg height="116pt" viewBox="0.00 0.00 62.00 116.00" width="62pt" xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg">
     <g class="graph" id="graph0" transform="scale(1 1) rotate(0) translate(4 112)">
      <title>Example</title>
      <polygon fill="#ffffff" points="-4,4 -4,-112 58,-112 58,4 -4,4" stroke="transparent"></polygon>
      <g class="node" id="node1">
       <title>b</title>
       <ellipse cx="27" cy="-90" fill="none" rx="27" ry="18" stroke="#000000"></ellipse>
       <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-86.3">b</text>
      </g>
      <g class="node" id="node2">
       <title>a</title>
       <ellipse cx="27" cy="-18" fill="none" rx="27" ry="18" stroke="#000000"></ellipse>
       <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-14.3">a</text>
      </g>
      <g class="edge" id="edge1">
       <title>b->a</title>
       <path d="M21.0843,-72.2022C20.2798,-64.1807 20.0571,-54.5231 20.4162,-45.5957" fill="none" stroke="#000000"></path>
       <polygon fill="#000000" points="23.9097,-45.8099 21.105,-35.593 16.9262,-45.3289 23.9097,-45.8099" stroke="#000000"></polygon>
      </g>
      <g class="edge" id="edge2">
       <title>a->b</title>
       <path d="M32.895,-35.593C33.7094,-43.5865 33.942,-53.2363 33.5928,-62.1759" fill="none" stroke="#000000"></path>
       <polygon fill="#000000" points="30.0976,-61.9891 32.9157,-72.2022 37.0817,-62.4608 30.0976,-61.9891" stroke="#000000"></polygon>
      </g>
     </g>
    </svg>
   </div>
   <p>M1 has an interface dependency on the the header unit "H3.h",
and contains a definition of <code class="highlight"><c- n>b</c-></code>, thus <code class="highlight"><c- n>b</c-></code>s initialization is sequenced before <code class="highlight"><c- n>a</c-></code>'s initialization. However; M2 has an interface dependency on the header
unit "M2.h" and contains a definition of <code class="highlight"><c- n>a</c-></code>, thus <code class="highlight"><c- n>a</c-></code>s initialization is
sequenced before <code class="highlight"><c- n>b</c-></code>'s initialization. This is a contradiction.</p>
   <p>The ordering guarantees will need to be weakened for this case.</p>
   <h2 class="heading settled" data-level="4" id="implementation-units"><span class="secno">4. </span><span class="content">Implementation Units?</span><a class="self-link" href="#implementation-units"></a></h2>
   <p>If we’re defining an ordering for interfaces, why not also define one for module
implementation units and solve the init order problem once and for all? While
there are plausible ways to define an ordering for implementation units, there
are several reasons why this direction was not chosen at this time:</p>
   <ul>
    <li data-md>
     <p>At best it can only order between different modules, not between
implementation units of a single module.</p>
    <li data-md>
     <p>Implementation units can form circular dependencies between modules.</p>
    <li data-md>
     <p>It has not been implemented, and has an unknown runtime impact, potentially
requring adding a global constructor to every module.</p>
    <li data-md>
     <p>It is not required to fix the <code class="highlight"><c- o>&lt;</c-><c- n>iostream</c-><c- o>></c-></code> issue, and thus out of scope for
C++20.</p>
   </ul>
   <h2 class="heading settled" data-level="5" id="target"><span class="secno">5. </span><span class="content">Ship Vehicle</span><a class="self-link" href="#target"></a></h2>
   <p>This paper is targeting C++20 as the standard library is subtly broken without
it.</p>
   <h2 class="heading settled" data-level="6" id="polls"><span class="secno">6. </span><span class="content">Proposed Polls</span><a class="self-link" href="#polls"></a></h2>
   <ol>
    <li data-md>
     <p>Pick a model. In the graphs that follow a directed arrow represents a
sequenced before relationship. Initializers with no forward path between them are
indeterminately sequenced.</p>
   </ol>
   <p class="note" role="note"><span>Note:</span> SG2 and EWG approved the Relaxed Clang Model, and the wording below implements that.</p>
   <table id="polls-table">
    <tbody>
     <tr>
      <td colspan="3">
<pre class="highlight"><c- c1>// H1.h</c->
<c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->

<c- c1>// H2.h</c->
<c- b>int</c-> <c- n>b</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->

<c- c1>// H3.h</c->
<c- b>int</c-> <c- n>c</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->

<c- c1>// TU.cpp</c->
<c- b>int</c-> <c- n>d</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->
<c- n>import</c-> <c- s>"H1.h"</c-><c- p>;</c->
<c- n>import</c-> <c- s>"H2.h"</c-><c- p>;</c->
<c- b>int</c-> <c- n>e</c-> <c- o>=</c-> <c- n>init</c-><c- p>();</c->
<c- n>import</c-> <c- s>"H3.h"</c-><c- p>;</c->
</pre>
     <tr>
      <th> Current Clang Model 
      <th> Relaxed Clang Model 
      <th> Dependency Order Model 
     <tr>
      <td>
       <div class="dot">
        <svg height="332pt" viewBox="0.00 0.00 62.00 332.00" width="62pt" xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg">
         <g class="graph" id="graph0①" transform="scale(1 1) rotate(0) translate(4 328)">
          <title>Clang Model</title>
          <polygon fill="#ffffff" points="-4,4 -4,-328 58,-328 58,4 -4,4" stroke="transparent"></polygon>
          <g class="node" id="node1①">
           <title>d</title>
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           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-302.3">d</text>
          </g>
          <g class="node" id="node2①">
           <title>a</title>
           <ellipse cx="27" cy="-234" fill="none" rx="27" ry="18" stroke="#000000"></ellipse>
           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-230.3">a</text>
          </g>
          <g class="edge" id="edge1①">
           <title>d->a</title>
           <path d="M27,-287.8314C27,-280.131 27,-270.9743 27,-262.4166" fill="none" stroke="#000000"></path>
           <polygon fill="#000000" points="30.5001,-262.4132 27,-252.4133 23.5001,-262.4133 30.5001,-262.4132" stroke="#000000"></polygon>
          </g>
          <g class="node" id="node3">
           <title>b</title>
           <ellipse cx="27" cy="-162" fill="none" rx="27" ry="18" stroke="#000000"></ellipse>
           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-158.3">b</text>
          </g>
          <g class="edge" id="edge2①">
           <title>a->b</title>
           <path d="M27,-215.8314C27,-208.131 27,-198.9743 27,-190.4166" fill="none" stroke="#000000"></path>
           <polygon fill="#000000" points="30.5001,-190.4132 27,-180.4133 23.5001,-190.4133 30.5001,-190.4132" stroke="#000000"></polygon>
          </g>
          <g class="node" id="node4">
           <title>e</title>
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           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-86.3">e</text>
          </g>
          <g class="edge" id="edge3">
           <title>b->e</title>
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           <polygon fill="#000000" points="30.5001,-118.4132 27,-108.4133 23.5001,-118.4133 30.5001,-118.4132" stroke="#000000"></polygon>
          </g>
          <g class="node" id="node5">
           <title>c</title>
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           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-14.3">c</text>
          </g>
          <g class="edge" id="edge4">
           <title>e->c</title>
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           <polygon fill="#000000" points="30.5001,-46.4132 27,-36.4133 23.5001,-46.4133 30.5001,-46.4132" stroke="#000000"></polygon>
          </g>
         </g>
        </svg>
       </div>
      <td>
       <div class="dot">
        <svg height="116pt" viewBox="0.00 0.00 206.00 116.00" width="206pt" xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg">
         <g class="graph" id="graph0②" transform="scale(1 1) rotate(0) translate(4 112)">
          <title>Relaxed Clang Model</title>
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          <g class="node" id="node1②">
           <title>e</title>
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           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="99" y="-14.3">e</text>
          </g>
          <g class="node" id="node2②">
           <title>c</title>
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           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="171" y="-14.3">c</text>
          </g>
          <g class="node" id="node3①">
           <title>d</title>
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           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-86.3">d</text>
          </g>
          <g class="edge" id="edge1②">
           <title>d->e</title>
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           <title>a</title>
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          <g class="edge" id="edge2②">
           <title>a->e</title>
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           <title>b</title>
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           <text fill="#000000" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="171" y="-86.3">b</text>
          </g>
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           <title>b->e</title>
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   <h2 class="heading settled" data-level="7" id="wording"><span class="secno">7. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>The following wording assumes that the issue with <em>interface dependency</em> not
applying to header units covered by US087 is fixed. </p>
   <h3 class="heading settled" data-level="7.1" id="basic-start-dynamic"><span class="secno">7.1. </span><span class="content">[basic.start.dynamic]</span><a class="self-link" href="#basic-start-dynamic"></a></h3>
   <blockquote>
     <sup>1+</sup>
    <ins>
     A declaration D is <em>appearance-ordered before</em> a declaration E if 
     <ul>
      <li data-md>
       <p>D appears in the same translation unit as E, or</p>
      <li data-md>
       <p>the translation unit containing E has an interface dependency on the translation unit containing D,</p>
     </ul>
     <ins>in either case prior to E.</ins>
    </ins>
    <p><sup>2</sup> Dynamic initialization of non-local variables <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> with static storage duration are ordered as follows:</p>
    <ul>
     <li data-md>
      <p>
       If <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> have ordered initialization and 
       <del><code class="highlight"><c- n>V</c-></code> is defined before <code class="highlight"><c- n>W</c-></code> within a single translation unit, the initialization of <code class="highlight"><c- n>V</c-></code> is sequenced before the initialization of <code class="highlight"><c- n>W</c-></code>.</del>
       <ins>the definition of V is appearance-ordered before the definition of W, or</ins>
       <del>If</del>
       <ins>if</ins>
        <code class="highlight"><c- n>V</c-></code> has partially-ordered initialization, <code class="highlight"><c- n>W</c-></code> does not have unordered initialization, and 
       <del><code class="highlight"><c- n>V</c-></code> is defined before <code class="highlight"><c- n>W</c-></code> in every translation unit in which <code class="highlight"><c- n>W</c-></code> is defined</del>
       <ins>for every definition E of W there exists a definition D of V such that D is appearance-ordered before E</ins>
       , then
      </p>
      <ul>
       <li data-md>
        <p>
         if the program 
         <ins>does not</ins>
          start
         <del>s</del>
          a thread ([intro.multithread]) other than the main thread ([basic.start.main]) 
         <ins>or <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> have ordered initialization and they are defined in the same translation unit,</ins>
          the initialization of <code class="highlight"><c- n>V</c-></code> 
         <del>strongly happens</del>
         <ins>is sequenced</ins>
          before the initialization of <code class="highlight"><c- n>W</c-></code>
        </p>
       <li data-md>
        <p>
         otherwise, the initialization of <code class="highlight"><c- n>V</c-></code> 
         <del>is sequenced</del>
         <ins>strongly happens</ins>
          before the initialization of <code class="highlight"><c- n>W</c-></code>.
        </p>
      </ul>
     <li data-md>
      <p>Otherwise, if the program starts a thread other than the main thread before either <code class="highlight"><c- n>V</c-></code> or <code class="highlight"><c- n>W</c-></code> is initialized, it is unspecified in which threads the initializations of <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> occur; the initializations are unsequenced if they occur in the same thread.</p>
     <li data-md>
      <p>Otherwise, the initializations of <code class="highlight"><c- n>V</c-></code> and <code class="highlight"><c- n>W</c-></code> are indeterminately sequenced.</p>
    </ul>
    <p>[ <em>Note:</em> This definition permits initialization of a sequence of ordered variables concurrently with another sequence. — <em>end note</em> ]</p>
   </blockquote>
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