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  <title>p1099R2: Using Enum</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">p1099R2<br>Using Enum</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-09-01">1 September 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://github.com/atomgalaxy/using-enum/using-enum.bs">https://github.com/atomgalaxy/using-enum/using-enum.bs</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Gašper Ažman &lt;gasper.azman@gmail.com></span>
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Jonathan Müller &lt;jonathan.mueller@foonathan.net></span>
     <dt>Audience:
     <dd>SG1, EWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Class enums are restricted namespaces. Let’s extend the using declaration to them.</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="#history"><span class="secno">1</span> <span class="content">Revision History</span></a>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#rationale"><span class="secno">3</span> <span class="content">Rationale</span></a>
     <ol class="toc">
      <li><a href="#consistency"><span class="secno">3.1</span> <span class="content">Consistency</span></a>
      <li><a href="#betterids"><span class="secno">3.2</span> <span class="content">Better Identifiers</span></a>
      <li><a href="#need"><span class="secno">3.3</span> <span class="content">Evidence of Need</span></a>
     </ol>
    <li>
     <a href="#proposal"><span class="secno">4</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#using-enum-id"><span class="secno">4.1</span> <span class="content">Syntax: <code class="highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">IDENTIFIER</span></code></span></a>
      <li><a href="#using-id"><span class="secno">4.2</span> <span class="content">Syntax: <code class="highlight"><span class="k">using</span> <span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span></code></span></a>
     </ol>
    <li>
     <a href="#faq"><span class="secno">5</span> <span class="content">Frequently Asked Questions</span></a>
     <ol class="toc">
      <li><a href="#faq-old-enum"><span class="secno">5.1</span> <span class="content">Can I do this with unscoped enums?</span></a>
      <li><a href="#faq-ns-alias"><span class="secno">5.2</span> <span class="content">Are you proposing mirroring the namespace alias syntax as well?</span></a>
      <li><a href="#faq-using-enum-struct"><span class="secno">5.3</span> <span class="content">Why not allow <code class="highlight"><span class="k">using</span> <span class="k">enum</span> <span class="k">struct</span><span class="o">/</span><span class="k">class</span> <span class="nc">ENUM_ID</span><span class="p">;</span></code>?</span></a>
      <li><a href="#faq-using-decl-why"><span class="secno">5.4</span> <span class="content">Why propose <code class="highlight"><span class="k">using</span> <span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span></code> at all?</span></a>
     </ol>
    <li>
     <a href="#wording"><span class="secno">6</span> <span class="content">Proposed Wording</span></a>
     <ol class="toc">
      <li><a href="#wording-preface"><span class="secno">6.1</span> <span class="content">Preface</span></a>
      <li><a href="#wording-enumdirect"><span class="secno">6.2</span> <span class="content">Changes required to add <code class="highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">IDENTIFIER</span><span class="p">;</span></code></span></a>
      <li><a href="#wording-enumdecl"><span class="secno">6.3</span> <span class="content">Changes required to add <code class="highlight"><span class="k">using</span> <span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span><span class="p">;</span></code></span></a>
     </ol>
    <li><a href="#ack"><span class="secno">7</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="history"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#history"></a></h2>
   <ul>
    <li data-md="">
     <p>r1: Typos. Changed target from LEWG to EWG.</p>
    <li data-md="">
     <p>r2: Added FAQ and wording, and extended to regular enums.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <blockquote>
    <p>The single biggest deterrent to use of scoped enumerations is the inability to associate them with a <em>using directive</em>.</p>
    <p>    — Dan Saks</p>
   </blockquote>
   <p>Consider an enum class:</p>
<pre class="language-cpp highlight"><span class="k">enum</span> <span class="k">class</span> <span class="nc">rgba_color_channel</span> <span class="p">{</span> <span class="n">red</span><span class="p">,</span> <span class="n">green</span><span class="p">,</span> <span class="n">blue</span><span class="p">,</span> <span class="n">alpha</span> <span class="p">};</span>
</pre>
   <p>Currently, a switch using this enum looks as follows:</p>
<pre class="language-cpp highlight"><span class="n">std</span><span class="o">::</span><span class="n">string_view</span> <span class="n">to_string</span><span class="p">(</span><span class="n">rgba_color_channel</span> <span class="n">channel</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">switch</span> <span class="p">(</span><span class="n">channel</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">case</span> <span class="n">rgba_color_channel</span><span class="o">::</span><span class="nl">red</span><span class="p">:</span>   <span class="k">return</span> <span class="s">"red"</span><span class="p">;</span>
    <span class="k">case</span> <span class="n">rgba_color_channel</span><span class="o">::</span><span class="nl">green</span><span class="p">:</span> <span class="k">return</span> <span class="s">"green"</span><span class="p">;</span>
    <span class="k">case</span> <span class="n">rgba_color_channel</span><span class="o">::</span><span class="nl">blue</span><span class="p">:</span>  <span class="k">return</span> <span class="s">"blue"</span><span class="p">;</span>
    <span class="k">case</span> <span class="n">rgba_color_channel</span><span class="o">::</span><span class="nl">alpha</span><span class="p">:</span> <span class="k">return</span> <span class="s">"alpha"</span><span class="p">;</span>
  <span class="p">}</span>
<span class="p">}</span>
</pre>
   <p>The necessary repetition of the class enum name reduces legibility by introducing noise in contexts where the enum class is obvious.</p>
   <p>To eliminate the noise penalty for introducing long (but descriptive) <code class="highlight"><span class="k">enum</span> <span class="k">class</span></code> names, this paper proposes that the statement</p>
<pre class="language-cpp highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">rgba_color_channel</span><span class="p">;</span>
</pre>
   <p>introduce the enum member identifiers into the local scope so that they may be referred to unqualified.</p>
   <p>Furthermore, the syntax</p>
<pre class="language-cpp highlight"><span class="k">using</span> <span class="n">rgba_color_channel</span><span class="o">::</span><span class="n">red</span><span class="p">;</span>
</pre>
   <p>should bring the identifier <code class="highlight"><span class="n">red</span></code> into the local scope, so that it may be used unqualified.</p>
   <p>The above example would then probably be written as</p>
<pre class="language-cpp highlight"><span class="n">std</span><span class="o">::</span><span class="n">string_view</span> <span class="n">to_string</span><span class="p">(</span><span class="n">rgba_color_channel</span> <span class="n">channel</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">switch</span> <span class="p">(</span><span class="n">my_channel</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">using</span> <span class="k">enum</span> <span class="n">rgba_color_channel</span><span class="p">;</span>
    <span class="k">case</span> <span class="nl">red</span><span class="p">:</span>   <span class="k">return</span> <span class="s">"red"</span><span class="p">;</span>
    <span class="k">case</span> <span class="nl">green</span><span class="p">:</span> <span class="k">return</span> <span class="s">"green"</span><span class="p">;</span>
    <span class="k">case</span> <span class="nl">blue</span><span class="p">:</span>  <span class="k">return</span> <span class="s">"blue"</span><span class="p">;</span>
    <span class="k">case</span> <span class="nl">alpha</span><span class="p">:</span> <span class="k">return</span> <span class="s">"alpha"</span><span class="p">;</span>
  <span class="p">}</span>
<span class="p">}</span>
</pre>
   <h2 class="heading settled" data-level="3" id="rationale"><span class="secno">3. </span><span class="content">Rationale</span><a class="self-link" href="#rationale"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="consistency"><span class="secno">3.1. </span><span class="content">Consistency</span><a class="self-link" href="#consistency"></a></h3>
   <p><code class="highlight"><span class="k">enum</span> <span class="k">class</span></code>es, as well as plain old <code class="highlight"><span class="k">enum</span></code>s, are not classes - they seem to be closer to namespaces consisting of <code class="highlight"><span class="k">static</span> <span class="k">constexpr</span></code> inline variables. The familiar <code class="highlight"><span class="k">using</span></code> syntax that works for namespaces should therefore apply to them as well, in some fashion.</p>
   <h3 class="heading settled" data-level="3.2" id="betterids"><span class="secno">3.2. </span><span class="content">Better Identifiers</span><a class="self-link" href="#betterids"></a></h3>
   <p>The introduction of this feature would allow better naming of enums. Currently, enums are named with as short an identifier as possible, often to the point of absurdity, when they are reduced to completely nondescriptive abbreviations that only hint at their proper meaning. (Just what does <code class="highlight"><span class="n">zfqc</span><span class="o">::</span><span class="n">add_op</span></code> really mean?)</p>
   <p>With this feature, identifiers become available to unqualified lookup in local contexts where their source is obvious, giving control of lookup style back to the user of the enum, instead of baking name semantics into the type of the enum.</p>
   <h3 class="heading settled" data-level="3.3" id="need"><span class="secno">3.3. </span><span class="content">Evidence of Need</span><a class="self-link" href="#need"></a></h3>
    At a casual search, we were able to locate this <a href="https://stackoverflow.com/questions/9450338/equivalent-of-using-namespace-x-for-scoped-enumerations">thread on stackoverflow</a>. 
   <p>Anecdotally, 100% of people the authors have shown this to (~30) at CppCon have displayed a very enthusiastic response, with frequent comments of "I’d use enum classes but they are too verbose, this solves my problem!"</p>
   <h2 class="heading settled" data-level="4" id="proposal"><span class="secno">4. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="using-enum-id"><span class="secno">4.1. </span><span class="content">Syntax: <code class="highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">IDENTIFIER</span></code></span><a class="self-link" href="#using-enum-id"></a></h3>
   <p>We propose the addition of a new <code class="highlight"><span class="k">using</span> <span class="k">enum</span></code> statement:</p>
<pre class="language-cpp highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">IDENTIFIER</span><span class="p">;</span>
</pre>
   <p>The above statement introduces the members of the enumeration <code class="highlight"><span class="n">IDENTIFIER</span></code> into the local namespace, thus enabling lookup without qualification. As usual, a name lookup ambiguity makes the program ill-formed (diagnostic required).</p>
   <h3 class="heading settled" data-level="4.2" id="using-id"><span class="secno">4.2. </span><span class="content">Syntax: <code class="highlight"><span class="k">using</span> <span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span></code></span><a class="self-link" href="#using-id"></a></h3>
   <p>We propose to allow the syntax of</p>
<pre class="language-cpp highlight"><span class="k">using</span> <span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span>
</pre>
   <p>to introduce the <code class="highlight"><span class="n">IDENTIFIER</span></code> into the local namespace, aliasing <code class="highlight"><span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span></code>.</p>
   <p>This would mirror the current syntax for introducing namespaced names into the current scope.</p>
   <p class="note" role="note"><span>Note:</span> this does not conflict with <a data-link-type="biblio" href="#biblio-p0945r0">[P0945R0]</a>, because that paper only deals with the syntax <code class="highlight"><span class="k">using</span> <span class="n">name</span> <span class="o">=</span> <span class="n">id</span><span class="o">-</span><span class="n">expression</span></code>, which duplicates the enumerator name.</p>
   <h2 class="heading settled" data-level="5" id="faq"><span class="secno">5. </span><span class="content">Frequently Asked Questions</span><a class="self-link" href="#faq"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="faq-old-enum"><span class="secno">5.1. </span><span class="content">Can I do this with unscoped enums?</span><a class="self-link" href="#faq-old-enum"></a></h3>
   <p>Yes. The motivation for that is the pattern</p>
<pre class="language-cpp highlight"><span class="k">class</span> <span class="nc">foo</span> <span class="p">{</span>
   <span class="k">enum</span> <span class="n">bar</span> <span class="p">{</span>
     <span class="n">A</span><span class="p">,</span>
     <span class="n">B</span><span class="p">,</span>
     <span class="n">C</span>
   <span class="p">};</span>
<span class="p">};</span>
</pre>
   <p>which was superceeded by scoped enums. With the feature this paper proposes one can bring <code class="highlight"><span class="n">A</span></code>, <code class="highlight"><span class="n">B</span></code> and <code class="highlight"><span class="n">C</span></code> into the local scope by invoking:</p>
<pre class="language-cpp highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">foo</span><span class="o">::</span><span class="n">bar</span><span class="p">;</span>
</pre>
   <h3 class="heading settled" data-level="5.2" id="faq-ns-alias"><span class="secno">5.2. </span><span class="content">Are you proposing mirroring the namespace alias syntax as well?</span><a class="self-link" href="#faq-ns-alias"></a></h3>
   <p>No. We already have a way to do that, and it looks like this:</p>
<pre class="language-cpp highlight"><span class="k">using</span> <span class="n">my_alias</span> <span class="o">=</span> <span class="n">my</span><span class="o">::</span><span class="n">name_space</span><span class="o">::</span><span class="n">enum_name</span><span class="p">;</span>
</pre>
   <p>In addition, <a data-link-type="biblio" href="#biblio-p0945r0">[P0945R0]</a> proposes deprecating namespace aliases in favor of generalized <code class="highlight"><span class="k">using</span> <span class="n">name</span> <span class="o">=</span> <span class="n">id_expression</span></code>, so doing this would go counter the current movement of the standard.</p>
   <h3 class="heading settled" data-level="5.3" id="faq-using-enum-struct"><span class="secno">5.3. </span><span class="content">Why not allow <code class="highlight"><span class="k">using</span> <span class="k">enum</span> <span class="k">struct</span><span class="o">/</span><span class="k">class</span> <span class="nc">ENUM_ID</span><span class="p">;</span></code>?</span><a class="self-link" href="#faq-using-enum-struct"></a></h3>
   <p>Because would have been a needless complication and would introduce another layer of "<code class="highlight"><span class="k">struct</span></code> and <code class="highlight"><span class="k">class</span></code> don’t match" linter errors that current <code class="highlight"><span class="k">class</span></code>es and <code class="highlight"><span class="k">struct</span></code>s already have with forward declarations.</p>
   <h3 class="heading settled" data-level="5.4" id="faq-using-decl-why"><span class="secno">5.4. </span><span class="content">Why propose <code class="highlight"><span class="k">using</span> <span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span></code> at all?</span><a class="self-link" href="#faq-using-decl-why"></a></h3>
   <p>... given that the following already works:</p>
<pre class="language-cpp highlight">  <span class="k">constexpr</span> <span class="k">auto</span> <span class="n">red</span> <span class="o">=</span> <span class="n">rgba_color_channel</span><span class="o">::</span><span class="n">red</span><span class="p">;</span>
</pre>
   <p>and that, given <a data-link-type="biblio" href="#biblio-p0945r0">[P0945R0]</a>, this will work:</p>
<pre class="language-cpp highlight">  <span class="k">using</span> <span class="n">red</span> <span class="o">=</span> <span class="n">rgba_color_channel</span><span class="o">::</span><span class="n">red</span><span class="p">;</span>
</pre>
   <p>The reason is "DRY" - don’t repeat yourself - one is forced to repeat the name of the enumerator. That said, the authors are perfectly willing to throw this part of the paper out if the <code class="highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">ENUM_ID</span></code> piece gets consensus and this is the stumbling block.</p>
   <h2 class="heading settled" data-level="6" id="wording"><span class="secno">6. </span><span class="content">Proposed Wording</span><a class="self-link" href="#wording"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="wording-preface"><span class="secno">6.1. </span><span class="content">Preface</span><a class="self-link" href="#wording-preface"></a></h3>
    The authors are new at this, and welcome suggestions for wording. 
   <p>The intention is to effectively mirror the wording for namespaces from [namespace.udir], and pare it down to only things that apply to enumerators.</p>
   <p>All wording is relative to the working draft of the ISO/IEC IS 14882: N4765.</p>
   <h3 class="heading settled" data-level="6.2" id="wording-enumdirect"><span class="secno">6.2. </span><span class="content">Changes required to add <code class="highlight"><span class="k">using</span> <span class="k">enum</span> <span class="n">IDENTIFIER</span><span class="p">;</span></code></span><a class="self-link" href="#wording-enumdirect"></a></h3>
   <p>In chapter [dcl.dcl], in [dcl.enum], add section titled "Using Directive", with the stable reference "[enum.udir]".</p>
   <p>Add:</p>
   <div class="ins">
     <i>using-enum-directive:</i><br> <i>    attribute-specifier-seq<sub>opt</sub></i> 
    <tt>using enum</tt>
     <i>nested-name-specifier<sub>opt</sub></i> <i>enum-name</i> 
    <ol>
     <li data-md="">
      <p>A <i>using-enum-directive</i> shall not appear in class scope, but may appear in namespace scope or in block scope. The optional <i>attribute-specifier-seq</i> appertains to the <i>using-enum-directive</i>. The <i>enum-name</i> in a <i>using-enum-directive</i> shall name a scoped enumeration or an unscoped enumeration.</p>
     <li data-md="">
      <p>A <i>using-enum-directive</i> specifies that the <i>enumerator</i>s in the nominated scoped enumeration or unscoped enumeration can be used in the scope in which the <i>using-enum-directive</i> appears after the <i>using-enum-directive</i>.  During unqualified name lookup ([basic.lookup.unqual]), the names appear as if they were declared in the scope where the <i>using-enum-directive</i> appears.</p>
     <li data-md="">
      <p>A <i>using-enum-directive</i> does not add any members to the declarative region in which it appears. [ <i>Example:</i></p>
<pre class="language-cpp highlight">    <span class="k">namespace</span> <span class="n">A</span> <span class="p">{</span>
      <span class="k">enum</span> <span class="k">class</span> <span class="nc">e</span> <span class="p">{</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span> <span class="p">};</span>
      <span class="k">using</span> <span class="k">enum</span> <span class="n">a</span><span class="o">::</span><span class="n">e</span><span class="p">;</span> <span class="c1">// make e::i visible in A</span>
<span class="c1"></span>      <span class="k">namespace</span> <span class="n">B</span> <span class="p">{</span>
        <span class="k">enum</span> <span class="k">class</span> <span class="nc">f</span> <span class="p">{</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">1</span> <span class="p">};</span>
        <span class="k">using</span> <span class="k">enum</span> <span class="n">A</span><span class="o">::</span><span class="n">B</span><span class="o">::</span><span class="n">f</span><span class="p">;</span>
        <span class="kt">void</span> <span class="nf">f1</span><span class="p">()</span> <span class="p">{</span>
          <span class="n">i</span><span class="p">;</span>        <span class="c1">// OK, A::B::f::i visible in B and hides A::e::i</span>
<span class="c1"></span>        <span class="p">}</span>
      <span class="p">}</span>
    <span class="p">}</span>
    <span class="k">namespace</span> <span class="n">C</span> <span class="p">{</span>
      <span class="k">using</span> <span class="k">enum</span> <span class="n">A</span><span class="o">::</span><span class="n">e</span><span class="p">;</span>
      <span class="k">using</span> <span class="k">enum</span> <span class="n">A</span><span class="o">::</span><span class="n">B</span><span class="o">::</span><span class="n">f</span><span class="p">;</span>
      <span class="kt">void</span> <span class="nf">f2</span><span class="p">()</span> <span class="p">{</span>
        <span class="n">i</span><span class="p">;</span>        <span class="c1">// ambiguous, A::e::i or A::B::f::i?</span>
<span class="c1"></span>      <span class="p">}</span>
    <span class="p">}</span>
</pre>
      — end example ] 
    </ol>
   </div>
   <p>Under [basic.def], add (just after <i>using-directive</i>)  (and renumber section):</p>
   <div class="ins"> 2.17. — it is a <i>using-enum-directive</i> </div>
   <h3 class="heading settled" data-level="6.3" id="wording-enumdecl"><span class="secno">6.3. </span><span class="content">Changes required to add <code class="highlight"><span class="k">using</span> <span class="n">ENUM_ID</span><span class="o">::</span><span class="n">IDENTIFIER</span><span class="p">;</span></code></span><a class="self-link" href="#wording-enumdecl"></a></h3>
   <p>In chapter [namespace.udecl], remove:</p>
   <div class="del">
    <ol start="7">
     <li data-md="">
      <p>A <i>using-declaration</i> shall not name a scoped enumerator.</p>
    </ol>
   </div>
   <p>Then add:</p>
   <div class="ins">
    <ol start="7">
     <li data-md="">
      <p>A <i>using-declaration</i> that names a scoped or unscoped enumerator shall become a synonim for that enumerator in the <i>using-declaration</i>’s declarative region. [ <i>Note:</i> this is equivalent to declaring that enumerator in the <i>using-declaration</i>’s declarative region. — <i>end note</i>]</p>
    </ol>
   </div>
   <h2 class="heading settled" data-level="7" id="ack"><span class="secno">7. </span><span class="content">Acknowledgements</span><a class="self-link" href="#ack"></a></h2>
   <p>The authors would like to thank Marcel Ebmer and Lisa Lippincott for early feedback, and the members of the BSI C++ WG for further feedback, especially Graham Haynes and Barry Revzin. Even further feedback was provided by Tomas Puverle, who encouraged us to extend it to <code class="highlight"><span class="k">enum</span></code>s, and Dan Saks for the permission to include a quotation from him.</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-p0945r0">[P0945R0]
   <dd>Richard Smith. <a href="https://wg21.link/p0945r0">Generalizing alias declarations</a>. URL: <a href="https://wg21.link/p0945r0">https://wg21.link/p0945r0</a>
  </dl>