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<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;
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 </head><body class="h-entry toc-sidebar"><p id="toc-nav"><a id="toc-jump" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#toc"><span aria-hidden="true">↑</span> <span>Jump to Table of Contents</span></a><a id="toc-toggle" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#toc"><span aria-hidden="true">←</span> <span>Collapse Sidebar</span></a></p>
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1749R0<br>Access control for reflection</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Proposal, <time class="dt-updated" datetime="2019-06-14">2019-06-14</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     </dt><dd><a class="u-url" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html">https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html</a>
     </dd><dt>Author:
     </dt><dd>
      </dd><dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:YehezkelShB@gmail.com">Yehezkel Bernat</a>
     </dd><dt>Audience:
     </dt><dd>SG7 (REFLECTION)
     </dd><dt>Project:
     </dt><dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dd></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>P1749 claims that reflection must be constrained in regular code and obey access control and scope rules</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="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#problem"><span class="secno">1</span> <span class="content">Problem statement</span></a>
     <ol class="toc">
      <li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#odr"><span class="secno">1.1</span> <span class="content">ODR - One <em>Declaration</em> Rule</span></a>
      </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#api"><span class="secno">1.2</span> <span class="content">Backward compatibility and API breaking</span></a>
      </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#portability"><span class="secno">1.3</span> <span class="content">Portability and standardization issues</span></a>
      </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#readability"><span class="secno">1.4</span> <span class="content">Effect on readability</span></a>
      </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#techniques"><span class="secno">1.5</span> <span class="content">Encouraging bad code techniques</span></a>
      </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#inconsistency"><span class="secno">1.6</span> <span class="content">Inconsistency with other parts of the standard</span></a>
     </li></ol>
    </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#solution"><span class="secno">2</span> <span class="content">Proposed solution</span></a>
    </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#acknowledgements"><span class="secno">3</span> <span class="content">Acknowledgements</span></a>
    </li><li><a href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#ref"><span class="secno">4</span> <span class="content">References</span></a>
   </li></ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="problem"><span class="secno">1. </span><span class="content">Problem statement</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#problem"></a></h2>
   <p>In "The Design &amp; Evolution of C++", there is a section about "Keyword Arguments"
(section 6.5.1, page 153), a language extension proposed in the process of the
first standardization of C++. It was about "a mechanism for specifying function
arguments by name" in the call site, e.g. <code class="highlight"><c- k="">new</c-> <c- nf="">window</c-><c- p="">(</c-><c- nl="">Color</c-><c- p="">:</c-><c- o="">=</c-><c- n="">green</c-><c- p="">,</c-> <c- nl="">ysize</c-><c- p="">:</c-><c- o="">=</c-><c- mi="">150</c-><c- p="">);</c-></code>.
Bjarne Stroustrup mentioned there were a few concerns and issues, which were the
reasons to reject this extension (pg. 155 and on). Reading it, the author found
that some of the arguments are still relevant today and apply to reflection.</p>
   <p>Here are some of those arguments and some comments, perspectives and thoughts
about them and how they are relevant for reflection.</p>
   <h3 class="heading settled" data-level="1.1" id="odr"><span class="secno">1.1. </span><span class="content">ODR - One <em>Declaration</em> Rule</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#odr"></a></h3>
   <blockquote>
    <p>The first serious problem discovered with the proposal was that keyword
arguments would introduce a new form of binding between a calling interface
and an implementation:</p>
    <p>[1] An argument must have the same name in a function declaration as in the
function definition.</p>
    <p>[...]</p>
    <p>Worse, this turned our to be a compatibility problem of significant magnitude.
Some organizations recommend a style with "long, informative" argument names
in header files, and "short, convenient" names in the definitions. For example:</p>
<pre class="highlight"><c- b="">void</c-> <c- nf="">reverse</c-><c- p="">(</c-><c- b="">int</c-><c- o="">*</c-> <c- n="">elements</c-><c- p="">,</c-> <c- b="">int</c-> <c- n="">length_of_elements_array</c-><c- p="">);</c->
<c- c1="">// ...</c->
<c- b="">void</c-> <c- nf="">reverse</c-><c- p="">(</c-><c- b="">int</c-><c- o="">*</c-> <c- n="">v</c-><c- p="">,</c-> <c- b="">int</c-> <c- n="">n</c-><c- p="">)</c->
<c- p="">{</c->
    <c- c1="">// ...</c->
<c- p="">}</c->
</pre>
    Naturally, some people find that style abhorrent, whereas others (including me)
find it quite reasonable. Apparently, significant amounts of such code exist. 
   </blockquote>
   <p><a href="https://wg21.link/p0670r2">p0670r2</a> says:</p>
   <blockquote>
    <p>This concern is mitigated in two ways:</p>
    <ol>
     <li data-md="">
      <p>Modern coding conventions have the declarations for a particular function
showing up in exactly one header file.</p>
     </li><li data-md="">
      <p>Modern coding conventions discourage the use of different argument names
between function declarations (in a header) and function definitions (in a
'.cpp' file). Dedicated compiler warnings exist to protect against this case.</p>
    </li></ol>
   </blockquote>
   <p>So we don’t think modern code is using such a coding guideline anymore.
The question is why we would want to add a new way of "ODR-violation" sort of
issues into the language, and help the potentially breakage of older code that
was written to these guidelines.</p>
   <blockquote>
    <p>Alternatively, the language shouldn’t require declarations to have the same
name for the same argument. That seemed viable to me. However, people didn’t
seem to like that variant either.</p>
    <p>There could be a noticeable impact on link times if the rule that arguments
names must match across compilation units is checked. If it isn’t checked, the
facility would not be type safe and could become a source of subtle errors.</p>
   </blockquote>
   <p>So he considered such facility type unsafe. An interesting point.</p>
   <h3 class="heading settled" data-level="1.2" id="api"><span class="secno">1.2. </span><span class="content">Backward compatibility and API breaking</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#api"></a></h3>
   <blockquote>
    <p>[2] Once a keyword argument is used, the name of that argument cannot be
changed in the function definition without breaking user code.</p>
    <p>[...]</p>
    <p>Further, an implication of keyword arguments would be that no name in a
commonly distributed head file could be changes without risking breaking code.</p>
   </blockquote>
   <p>We are making the parameter names part of the public API of any library. Library
vendors will not be able to fix a typo in the parameter name anymore, as this
will be a breaking change for the users.</p>
   <p>You may argue that any mistake in the API is hard to fix. This is true, but
usually we can have them both. We can keep the older function name with the typo
and introduce a new function with the correct name (probably <code class="highlight"><c- p="">[[</c-><c- n="">deprecate</c-><c- p="">]]</c-></code>-ing
the old one). We can add a type alias for a mistake in the type name. We can add
a new overloading for the function with stronger types for the arguments. None
of these solutions is relevant for argument names. We’ll have to introduce a new
function name (FuncEx, anyone?) just to fix the parameter name!</p>
   <h3 class="heading settled" data-level="1.3" id="portability"><span class="secno">1.3. </span><span class="content">Portability and standardization issues</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#portability"></a></h3>
   <blockquote>
    <p>Different suppliers of header files for common services (for example, Posix or
X) would also have to agree on argument names. This could easily become a
bureaucratic nightmare.</p>
   </blockquote>
   <p>Do we want to specify the parameter names of all the standard library functions
and force them on the implementations? <a href="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility">\SD-8</a> tries hard to free the standard library from various possible backward
compatibility issues, and allowing reflection on parameter names seems like
adding a new one.</p>
   <h3 class="heading settled" data-level="1.4" id="readability"><span class="secno">1.4. </span><span class="content">Effect on readability</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#readability"></a></h3>
   <blockquote>
    <p>Both the potential linking cost and the very real binding problem could be
easily avoided by omitting argument names in header files. A cautious user
might therefore avoid specifying argument names in header files. Thus, to
quote Bill Gibbons, "The net impact on readability of C++ might actually be
negative."</p>
   </blockquote>
   <p>Considering the previous 2 points, I think this one talks for itself.
Of course, for template functions, even this escape hatch isn’t available...</p>
   <h3 class="heading settled" data-level="1.5" id="techniques"><span class="secno">1.5. </span><span class="content">Encouraging bad code techniques</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#techniques"></a></h3>
   <blockquote>
    <p>My main worry about keyword arguments was actually that keyword arguments
might slow the gradual transition from traditional programming techniques to
data abstraction and object-oriented programming in C++. In code that I find
best written and easiest to maintain, long argument lists are very rare. In
fact, it is a common observation that a transition to a more object-oriented
style leads to a significant decrease in the length of argument lists; [...]</p>
    <p>[...]</p>
    <p>A further reduction in the number of arguments could be obtained by using a <code class="highlight"><c- n="">Point</c-></code> type rather than expressing interfaces directly in terms of coordinates.</p>
   </blockquote>
   <p>p0670r2 brings as an example the following code:</p>
<pre class="highlight"><c- b="">double</c-> <c- nf="">Gauss</c-><c- p="">(</c-><c- b="">double</c-> <c- n="">x</c-><c- p="">,</c-> <c- b="">double</c-> <c- n="">mean</c-><c- p="">,</c-> <c- b="">double</c-> <c- n="">width</c-><c- p="">,</c-> <c- b="">double</c-> <c- n="">height</c-><c- p="">);</c->
</pre>
   Do we want to encourage this style? Don’t we want to teach that usage of strong
types? This is true now more than ever as Metaclasses will allow easy creation
of strong typedefs, without resorting to more mouthful library-based options. 
   <h3 class="heading settled" data-level="1.6" id="inconsistency"><span class="secno">1.6. </span><span class="content">Inconsistency with other parts of the standard</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#inconsistency"></a></h3>
   <p>p0542 explicitly permits redeclaration of a function with contracts to have
different naming for the arguments:</p>
<pre class="highlight"><c- b="">int</c-> <c- nf="">f</c-><c- p="">(</c-><c- b="">int</c-> <c- n="">x</c-><c- p="">)</c-> 
  <c- p="">[[</c-><c- nl="">expects</c-><c- p="">:</c-> <c- n="">x</c-><c- o="">&gt;</c-><c- mi="">0</c-><c- p="">]]</c->
  <c- p="">[[</c-><c- n="">ensures</c-> <c- nl="">r</c-><c- p="">:</c-> <c- n="">r</c-><c- o="">&gt;</c-><c- mi="">0</c-><c- p="">]];</c->

<c- b="">int</c-> <c- nf="">f</c-><c- p="">(</c-><c- b="">int</c-> <c- n="">y</c-><c- p="">)</c->
  <c- p="">[[</c-><c- nl="">expects</c-><c- p="">:</c-> <c- n="">y</c-><c- o="">&gt;</c-><c- mi="">0</c-><c- p="">]]</c->    <c- c1="">// Should be OK</c->
  <c- p="">[[</c-><c- n="">ensures</c-> <c- nl="">z</c-><c- p="">:</c-> <c- n="">z</c-><c- o="">&gt;</c-><c- mi="">0</c-><c- p="">]];</c-> <c- c1="">// Should be OK</c->
</pre>
   This seems inconsistent, where in one part of the standard we explicitly allow
different naming of function arguments and in another part we forbid it. 
   <h2 class="heading settled" data-level="2" id="solution"><span class="secno">2. </span><span class="content">Proposed solution</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#solution"></a></h2>
   <p>The solution proposed here is to allow reflection in a regular code to access
only entities that are already accessible from the same code. Specifically it
means:</p>
   <ul>
    <li data-md="">
     <p>Parameter names are accessible from inside the function only (allowing the
use-case of generating diagnostic messages)</p>
    </li><li data-md="">
     <p>The regular access control is applied when accessing class members</p>
   </li></ul>
   <p>The last point comes from the observation that API issues mentioned above are
relevant for the reflection on non-public class members too.</p>
   <p>What will be allowed is to reflect on everything (including private members and
parameter names) from a metfunction or metaclass. This allows all the use-cases
of generating various language bindings etc. while preventing all the mentioned
issues from regular user code.</p>
   <p>Metaclass is a different creature anyway. It’s usually used by the type author,
and even if we allow the <code class="highlight"><c- p="">.</c-><c- n="">as</c-></code> operator to apply a metaclass on already existing
type, this is a special operation that is known to be more fragile.</p>
   <h2 class="heading settled" data-level="3" id="acknowledgements"><span class="secno">3. </span><span class="content">Acknowledgements</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#acknowledgements"></a></h2>
   <p>Thanks for David Sankel for listening to my arguments and encouraging me to
write this paper.</p>
   <h2 class="heading settled" data-level="4" id="ref"><span class="secno">4. </span><span class="content">References</span><a class="self-link" href="https://yehezkelshb.github.io/cpp_proposals/P1749-access-control-for-reflection.html#ref"></a></h2>
   <ul>
    <li data-md="">
     <p><a href="https://wg21.link/p0670r2">p0670r2</a> - Static reflection of functions</p>
    </li><li data-md="">
     <p><a href="https://wg21.link/p0542">p0542</a> - Support for contract based programming in C++</p>
    </li><li data-md="">
     <p><a href="https://isocpp.org/std/standing-documents/sd-8-standard-library-compatibility">\SD-8</a> - Standard Library Compatibility</p>
   </li></ul>
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