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  <title>P1682R0: std::to_underlying for enumerations</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">P1682R0<br>std::to_underlying for enumerations</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-06-17">2019-06-17</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:phdofthehouse@gmail.com">JeanHeyd Meneide</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Latest:
     <dd><a href="https://thephd.github.io/vendor/future_cxx/papers/d1379.html">https://thephd.github.io/vendor/future_cxx/papers/d1379.html</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>A proposal to add a short utility function to handle going from an enumeration to its underlying integral value for safety and ease of use.</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="#changelog"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#changelog-r0"><span class="secno">1.1</span> <span class="content">Revision 0 - June 17th, 2019</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li><a href="#design"><span class="secno">3</span> <span class="content">Design</span></a>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Proposing Wording</span></a>
     <ol class="toc">
      <li><a href="#wording-feature"><span class="secno">4.1</span> <span class="content">Proposed Feature Test Macro</span></a>
      <li><a href="#wording-intent"><span class="secno">4.2</span> <span class="content">Intent</span></a>
      <li><a href="#wording-library"><span class="secno">4.3</span> <span class="content">Proposed Library Wording</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">5</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="changelog"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="changelog-r0"><span class="secno">1.1. </span><span class="content">Revision 0 - June 17th, 2019</span><a class="self-link" href="#changelog-r0"></a></h3>
   <ul>
    <li data-md>
     <p>Initial release.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Many codebases write a version of a small utility function converting an enumeration to its underlying type. The reason for this function is very simple: applying <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>/ <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- b>unsigned</c-> <c- b>long</c-><c- o>></c-></code> (or similar) to change an enumeration to its underlying type makes it harder to quickly read and maintain places where the user explicitly converts from a strongly-typed enumeration to its underlying value. For the purposes of working with an untyped API or similar, casts just look like any old cast, making it harder to read code and potentially incorrect when enumeration types are changed from signed / unsigned or similar.</p>
   <p>Much of the same rationale is why this is Item 10 in Scott Meyers' Effective Modern C++. In Around Christmas of 2016, the number of these function invocations for C++ was around 200 including both to_underlying/to_underlying_type/toUtype (the last in that list being the way it was spelled by Scott Meyers). As of June 17th, 2019, the collective hits on GitHub and other source engines totals well over 1,000 hits, disregarding duplication from common base frameworks such as the realm mobile app database and more. The usefulness of this function appears in Loggers for enumerations, casting for C APIs, stream operations, and more.</p>
   <p>We are seeing an explosive move and growth in usage of Modern C++ utilities, and the growth in this usage clearly indicates that the foresight and advice of Scott Meyers is being taken seriously by the full gamut of hobbyist to commercial software engineers. Therefore, it would seem prudent to make the spelling and semantics of this oft-reached-for utility standard in C++.</p>
   <p>Typical casts can also mask potential bugs from size/signed-ness changes and hide programmer intent. For example, going from this code,</p>
<pre class="highlight"><c- k>enum</c-> <c- k>class</c-> <c- nc>ABCD</c-> <c- p>{</c->
	<c- n>A</c-> <c- o>=</c-> <c- mh>0x1012</c-><c- p>,</c->
	<c- n>B</c-> <c- o>=</c-> <c- mh>0x405324</c-><c- p>,</c->
	<c- n>C</c-> <c- o>=</c-> <c- n>A</c-> <c- o>&amp;</c-> <c- n>B</c->
<c- p>};</c->

<c- c1>// sometime later ...</c->

<c- b>void</c-> <c- nf>do_work</c-><c- p>(</c-><c- n>ABCD</c-> <c- n>some_value</c-><c- p>)</c-> <c- p>{</c->
	<c- c1>// no warning, no visual indication,</c->
	<c- c1>// is this what the person wanted,</c->
	<c- c1>// what was the original intent in this</c->
	<c- c1>// 'harmless' code?</c->
	<c- n>internal_untyped_api</c-><c- p>(</c-><c- k>static_cast</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>some_value</c-><c- p>));</c->
<c- p>}</c->
</pre>
   <p>To this code:</p>
<pre class="highlight"><c- cp>#include</c-> &lt;cstdint>

<c- c1>// changed enumeration, underlying type</c->
<c- k>enum</c-> <c- k>class</c-> <c- nc>ABCD</c-> <c- o>:</c-> <c- b>uint32_t</c-> <c- p>{</c->
	<c- n>A</c-> <c- o>=</c-> <c- mh>0x1012</c-><c- p>,</c->
	<c- n>B</c-> <c- o>=</c-> <c- mh>0x405324</c-><c- p>,</c->
	<c- n>C</c-> <c- o>=</c-> <c- n>A</c-> <c- o>&amp;</c-> <c- n>B</c-><c- p>,</c->
	<c- n>D</c-> <c- o>=</c-> <c- mh>0xFFFFFFFF</c-> <c- c1>// !!</c->
<c- p>};</c->

<c- c1>// from before:</c->

<c- b>void</c-> <c- nf>do_work</c-><c- p>(</c-><c- n>ABCD</c-> <c- n>some_value</c-><c- p>)</c-> <c- p>{</c->
	<c- c1>// no warning, no visual indication,</c->
	<c- c1>// is this what the person wanted,</c->
	<c- c1>// what was the original intent in this</c->
	<c- c1>// 'harmless' code?</c->
	<c- n>internal_untyped_api</c-><c- p>(</c-><c- k>static_cast</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>some_value</c-><c- p>));</c->
<c- p>}</c->
</pre>
   <p>is dangerous, but the <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> is seen by the compiler as intentional by the user.</p>
   <p>Calling <code class="highlight"><c- n>do_work</c-><c- p>(</c-><c- n>ABCD</c-><c- o>::</c-><c- n>D</c-><c- p>);</c-></code> is a code smell internally because the cast is the wrong one for the enumeration. If the internal untyped API takes an integral value larger than the size of <code class="highlight"><c- b>int</c-></code> and friends, then this code might very well pass a bit pattern that will be interpreted as the wrong value inside of the <code class="highlight"><c- n>internal_untyped_api</c-></code>, too. Of course, this change does not trigger warnings or errors: <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> is a declaration of intent that says "I meant to do this cast", even if that cast was done before any changes or refactoring was performed on the enumeration.</p>
   <p>Doing it the right way is also cumbersome:</p>
<pre class="highlight"><c- b>void</c-> <c- nf>do_work</c-><c- p>(</c-><c- n>ABCD</c-> <c- n>some_value</c-><c- p>)</c-> <c- p>{</c->
	<c- c1>// will produce proper warnings,</c->
	<c- c1>// but is cumbersome to type</c->
	<c- n>internal_untyped_api</c-><c- p>(</c-><c- k>static_cast</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>underlying_type_t</c-><c- o>&lt;</c-><c- n>ABCD</c-><c- o>>></c-><c- p>(</c-><c- n>some_value</c-><c- p>));</c->
<c- p>}</c->
</pre>
   <p>It is also vulnerable to the parameter’s type changing from an enumeration to another type that is convertible to an integer. Because it is still a <code class="highlight"><c- k>static_cast</c-></code>, unless someone changes the type for <code class="highlight"><c- n>do_work</c-></code> while also deleting <code class="highlight"><c- n>ABCD</c-></code>, that code will still compile:</p>
<pre class="highlight"><c- b>void</c-> <c- nf>do_work</c-><c- p>(</c-><c- n>OtherEnumeration</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
	<c- c1>// no warnings, no errors, ouch!</c->
	<c- n>internal_untyped_api</c-><c- p>(</c-><c- k>static_cast</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>underlying_type_t</c-><c- o>&lt;</c-><c- n>ABCD</c-><c- o>>></c-><c- p>(</c-><c- n>some_value</c-><c- p>));</c->
<c- p>}</c->
</pre>
   <p>We propose an intent-preserving function used in many codebases across C++ called <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>to_underlying</c-></code>, to be used with enumeration values.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design</span><a class="self-link" href="#design"></a></h2>
   <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>to_underlying</c-></code> completely avoids all of the above-mentioned problems related to code reuse and refactoring. It makes it harder to write bugs when working with strongly-typed enumerations into untyped APIs such with things such as C code and similar. It only works on enumeration types. It will <code class="highlight"><c- k>static_cast</c-></code> the enumeration to integral representation with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>underlying_type_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>. This means that the value passed into the function provides the type information, and the type information is provided by the compiler, not by the user.</p>
   <p>This makes it easy to find conversion points for "unsafe" actions, reducing search and refactoring area. It also puts the <code class="highlight"><c- k>static_cast</c-></code> inside of a utility function, meaning that warnings relating to size and signed-ness differences can still be caught in many cases since the result’s usage comes from a function, not from an explicitly inserted user cast.</p>
<pre class="highlight"><c- cp>#include</c-> &lt;utility>

<c- b>void</c-> <c- nf>do_work</c-><c- p>(</c-><c- n>MyEnum</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
	<c- c1>// changes to match its value,</c->
	<c- c1>// proper warnings for signed/unsigned mismatch,</c->
	<c- c1>// and ease-of-use!</c->
	<c- n>internal_untyped_api</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>to_underlying</c-><c- p>(</c-><c- n>some_value</c-><c- p>));</c->
<c- p>}</c->
</pre>
   <h2 class="heading settled" data-level="4" id="wording"><span class="secno">4. </span><span class="content">Proposing Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>The wording proposed here is relative to <a data-link-type="biblio" href="#biblio-n4800">[n4800]</a>.</p>
   <h3 class="heading settled" data-level="4.1" id="wording-feature"><span class="secno">4.1. </span><span class="content">Proposed Feature Test Macro</span><a class="self-link" href="#wording-feature"></a></h3>
   <p>The proposed library feature test macro is <code class="highlight"><c- n>__cpp_lib_to_underlying</c-></code>.</p>
   <h3 class="heading settled" data-level="4.2" id="wording-intent"><span class="secno">4.2. </span><span class="content">Intent</span><a class="self-link" href="#wording-intent"></a></h3>
   <p>The intent of this wording is to introduce 1 function into the <code class="highlight"><c- o>&lt;</c-><c- n>utility</c-><c- o>></c-></code> header called <code class="highlight"><c- n>to_underlying</c-></code>. If the input to the function is not an enumeration, then the program is ill-formed.</p>
   <h3 class="heading settled" data-level="4.3" id="wording-library"><span class="secno">4.3. </span><span class="content">Proposed Library Wording</span><a class="self-link" href="#wording-library"></a></h3>
   <p>Append to §17.3.1 General [<strong>support.limits.general</strong>]'s <strong>Table 35</strong> one additional entry:</p>
   <blockquote>
    <table>
     <tbody>
      <tr>
       <th>Macro name
       <th>Value
      <tr>
       <td>
        <ins>__cpp_lib_to_underlying</ins>
       <td>
        <ins>202002L</ins>
    </table>
   </blockquote>
   <p>Add the following into §20.2.1 Header <code class="highlight"><c- o>&lt;</c-><c- n>utility</c-><c- o>></c-></code> [utility.syn] synopsis:</p>
   <blockquote>
    <ins>
<pre class="highlight"><c- c1>// [utility.underlying], to_underlying</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>std</c-><c- o>::</c-><c- n>underlying_type_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>to_underlying</c-><c- p>(</c-><c- n>T</c-> <c- n>value</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
   </blockquote>
   <p>Add a new section §20.2.7 Function template <code class="highlight"><c- n>to_underlying</c-></code> [utility.underlying]:</p>
   <blockquote>
     <strong>20.2.7</strong> Function template <code class="highlight"><c- n>to_underlying</c-></code> <span float="right">[<strong>utility.underlying</strong>]</span> 
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
	<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->
	<c- k>constexpr</c-> <c- n>std</c-><c- o>::</c-><c- n>underlying_type_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>to_underlying</c-><c- p>(</c-> <c- n>T</c-> <c- n>value</c-> <c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>}</c->
</pre>
    <p><sup>1</sup> Constraints: <code class="highlight"><c- n>T</c-></code> shall satisfy <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_enum_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>.</p>
    <p><sup>2</sup> Returns: <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>underlying_type_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>(</c-><c- n>value</c-><c- p>)</c-></code>.</p>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="acknowledgements"><span class="secno">5. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to Rein Halbersma for bringing this up as part of the things that would make programming in his field easier and the others who chimed in. Thanks to Walter E. Brown for the encouragement to Rein Halbersma to get this paper moving.</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-n4800">[N4800]
   <dd>ISO/IEC JTC1/SC22/WG21 - The C++ Standards Committee; Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4800.pdf">N4800 - Working Draft, Standard for Programming Language C++</a>. January 21st, 2019. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4800.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4800.pdf</a>
  </dl>