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 <head>
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  <title>P3178R1: Retrieval of Exception Information</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
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * 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)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * 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
 *
 * Outdated warning for old specs
 *
 * 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)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	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;

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) 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: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		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;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

	/* 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: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		/* 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 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);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) 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);
			color: var(--tocsidebar-heading-text);
		}
		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);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) 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);
			color: var(--tocsidebar-heading-text);
		}

		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;
		color: var(--heading-text);
	}

	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,
	#profile-and-date {
		/* #profile-and-date 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) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		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;
	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 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: inherit;
	}
	dfn var {
		font-style: normal;
	}

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

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

/** 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;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	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;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

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

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

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

	/* 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;
		height: 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,
	.amendment, .correction, .addition {
		margin: 1em auto;
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

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

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

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

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* 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;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* 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;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

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

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

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

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** 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: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    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;
		border-bottom: 1px solid var(--defrow-border);
	}

	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-color: var(--datacell-border);
		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 {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		border-top: 1px solid var(--datacell-border);
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
<style>
th, td, table {
    border: 1px solid var(--text);
}
th, td {
    border-left-width: 0;
    border-right-width: 0;
    text-align: center;
}

.indent {
    margin-left: 2em;
}

ins {
    background: rgba(136, 255, 93, 0.2);
    color: inherit;
    text-decoration: underlined;
}
del {
    background: rgba(255, 93, 93, 0.2);
    color: inherit;
    text-decoration: strikethrough;
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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">P3178R1<br>Retrieval of Exception Information</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">New Proposal, <time class="dt-updated" datetime="2024-05-30">2024-05-30</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://virjacode.com/papers/p3178r1.htm" target="_blank">http://virjacode.com/papers/p3178r1.htm</a>
     <dt>Latest version:
     <dd><a class="u-url" href="http://virjacode.com/papers/p3178.htm" target="_blank">http://virjacode.com/papers/p3178.htm</a>
     <dt class="editor">Author:
     <dd>Thomas PK Healy &lt;healytpk@vir7ja7code7.com></a> <i>(Remove all sevens from email address)</i>
     <dt>Audience:
     <dd>SG18 Library Evolution Working Group (LEWG)
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </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>
   Add two new functions to the standard library to facilitate the retrieval of further information about the currently handled exception, or from an <code class="highlight"><c- b>exception_ptr</c-></code>.<BR>
   <pre class="highlight line-numbered">
   <span class="line-no"></span><small><small><span class="line">&nbsp;<code>Declared in header</code> <code class="highlight"><c- k>&lt;exception&gt;</c-></code></span></small></small>
   <span class="line-no"></span><span class="line">type_info const &amp;exception_typeid() noexcept;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<sup><small>(1) <font color="green">(since C++26)</font></small></sup></span>
   <span class="line-no"></span><span class="line">type_info const &amp;exception_typeid(exception_ptr const &amp;p) noexcept;&nbsp;&nbsp;&nbsp;&nbsp;<sup><small>(2) <font color="green">(since C++26)</font></small></sup></span>
   </pre>
   <pre class="highlight line-numbered">   
   <span class="line-no"></span><span class="line">void *exception_object() noexcept;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<sup><small>(3) <font color="green">(since C++26)</font></small></sup></span>
   <span class="line-no"></span><span class="line">void *exception_object(exception_ptr const &amp;p) noexcept;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<sup><small>(4) <font color="green">(since C++26)</font></small></sup></span>
   </pre>
   1) Retrieve the <code class="highlight"><c- b>std::type_info</c-></code> for the currently handled exception<BR>
   2) Retrieve the <code class="highlight"><c- b>std::type_info</c-></code> for the exception referred by <b>p</b><BR>
   3) Retrieve the address of the exception object for the currently handled exception<BR>
   4) Retrieve the address of the exception object for the exception referred by <b>p</b>
   <BR>
   <BR>
   <big><b>Parameters</b></big><BR>
   <b>p</b>&nbsp;&nbsp;&nbsp;-&nbsp;&nbsp;&nbsp;null or non-null <code class="highlight"><c- b>std::exception_ptr</c-></code>
   <BR>
   <BR>
   <big><b>Return value</b></big>
   <BR>
   1,2) The <code class="highlight"><c- b>std::type_info</c-></code> of the exception, otherwise <code class="highlight"><c- b>typeid(void)</c-></code> if unavailable.<BR>
   3,4) The address of the exception object, otherwise <code class="highlight"><c- b>nullptr</c-></code> if unavailable.
   </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="#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#motivation1"><span class="secno">2.1</span> <span class="content"><code class="highlight"><c- k>catch</c-><c- p>(...)</c-></code></span></a>
      <li><a href="#motivation2"><span class="secno">2.2</span> <span class="content">Runtime plugins</span></a>
      <li><a href="#motivation3"><span class="secno">2.3</span> <span class="content">As a stepping stone</span></a>
     </ol>
    <li><a href="#impact-on-the-standard"><span class="secno">3</span> <span class="content">Impact on the standard</span></a>
    <li><a href="#impact-on-existing-code"><span class="secno">4</span> <span class="content">Impact on existing code</span></a>
    <li><a href="#design-considerations"><span class="secno">5</span> <span class="content">Design considerations</span></a>
     <ol class="toc">
      <li><a href="#consideration1"><span class="secno">5.1</span> <span class="content">Microsoft SEH</span></a>
      <li><a href="#consideration2"><span class="secno">5.2</span> <span class="content">Forced unwinding</span></a>
     </ol>
    <li><a href="#implementations"><span class="secno">6</span> <span class="content">Possible implementations</span></a>
     <ol class="toc">
      <li><a href="#implementation1"><span class="secno">6.1</span> <span class="content">Apple macOS, Linux, FreeBSD</span></a>
      <li><a href="#implementation2"><span class="secno">6.2</span> <span class="content">Microsoft Windows</span></a>
     </ol>
    <li><a href="#proposed-wording"><span class="secno">7</span> <span class="content">Proposed wording</span></a>
    <li><a href="#alternatives"><span class="secno">8</span> <span class="content">Alternatives</span></a>
     <ol class="toc">
      <li><a href="#alternative1"><span class="secno">8.1</span> <span class="content"><code class="highlight"><c- k>std</c-><c- p>::</c-><c- k>exception_ptr</c-><c- p>::</c-><c- k>type</c-><c- p>()</c-></code></span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">9</span> <span class="content">Acknowledgements</span></a>
    <li><a href="#revisions"><span class="secno">10</span> <span class="content">Revision history</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <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="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
   <p>Inside a <code class="highlight"><c- k>catch</c-><c- p>(...)</c-></code> block, we should have access to the <code class="highlight"><c- b>type_info</c-></code> of what was thrown. Currently there is no portable way to get the <code class="highlight"><c- b>type_info</c-></code> from an <code class="highlight"><c- b>exception_ptr</c-></code>, and so if an object is thrown of intrinsic type or a non-polymorphic class type, and caught in a <code class="highlight"><c- k>catch(...)</c-></code> block, the type of the exception object is unknown. It is also impossible to get the address of the exception object itself.
   <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>
   <h3 class="heading settled" data-level="2.1" id="motivation1"><span class="secno">2.1. </span><span class="content"><code class="highlight"><c- k>catch</c-><c- p>(...)</c-></code></span><a class="self-link" href="#motivation1"></a></h3>
   <p>A C++ program can link with a shared library which has incomplete or inaccurate documentation for the exceptions it throws, or the program may link at runtime using <code class="highlight"><c- b>LoadLibrary</c-></code> or <code class="highlight"><c- b>dlopen</c-></code> to link with a library which it knows very little about. Code in the main program may handle an exception thrown from within the shared library as follows:
<pre class="highlight line-numbered"><code>#include &lt;exception&gt;  // exception, exception_ptr
#include &lt;new&gt;        // bad_alloc
#include &lt;typeinfo&gt;   // typeid, type_info

extern void SomeLibraryFunction(void) noexcept(false);

int main(void)
{
    try
    {
        SomeLibraryFunction();
    }
    catch(std::bad_alloc const&)
    {
        // Attempt safe shutdown when memory is depleted
    }
    catch(std::exception const &e)
    {
        std::type_info const &ti = typeid(e);
        // As 'std::exception' has a virtual destructor, it is
        // a polymorphic type, and so 'typeid' will give us the
        // RTTI of the derived class.
    }
    catch(...)
    {
        std::exception_ptr const p = std::current_exception();
        // We can get a pointer to the exception object, but
        // we have no idea of the type of what was thrown
    }
}
</code>
</pre
   </p>

   <p>This proposal will allow us to access the <code class="highlight"><c- k>type_info</c-></code> of the exception object inside a <code class="highlight"><c- k>catch(...)</c-></code> block, as follows:</p>
<pre class="highlight line-numbered"><code>catch(...)
{
    std::type_info const &amp;ti = std::exception_typeid();
}
</code></pre>
   <p>And also to get the <code class="highlight"><c- k>type_info</c-></code> from any <code class="highlight"><c- k>exception_ptr</c-></code>, as follows:</p>
<pre class="highlight line-numbered"><code>void Func(std::exception_ptr const &amp;p)
{
    std::type_info const &amp;ti = std::exception_typeid(p);
    std::puts( ti.what() );
}

int main(void)
{
    std::exception_ptr p;

    try
    {
        throw std::runtime_error("resources depleted");
    }
    catch(...)
    {
        p = std::current_exception();
    }

    Func(p);
}
</code></pre>
   <p>Once we have the <code class="highlight"><c- k>type_info</c-></code>, we can perform checks on the name:</p>
<pre class="highlight line-numbered"><code>catch(...)
{
    std::type_info const &amp;ti = std::exception_typeid();
    if ( std::strstr( ti.name(), "FieryShadow" ) ) return EXIT_FAILURE;
}
</code></pre>
   <p>The name of the type will of course be mangled and therefore be different on different compilers, but if a class is named '<b>FieryShadow</b>', then its mangled name will contain '<b>FieryShadow</b>', hence the function <code class="highlight"><c- b>std::strstr</c-></code> will work here. However, until a name mangling library is added to the standard library, code to distinguish specifically between type names (e.g. where one class is inside an anonymous namespace) will be platform-specific and non-portable.</p>
   <h3 class="heading settled" data-level="2.2" id="motivation2"><span class="secno">2.2. </span><span class="content">Runtime plugins</span><a class="self-link" href="#motivation2"></a></h3>
   <p>
   A massive program such as 3D modelling software for a desktop computer may allow the installation of 3rd party plugins. These plugins come in the form of a dynamic shared library (e.g. '<B>.dll</B>' on <i>Microsoft Windows</i>, or '<B>.so</B>' on <i>Apple macOS</i>) which is loaded at runtime using <code class="highlight"><c- b>LoadLibrary</c-></code> or <code class="highlight"><c- b>dlopen</c-></code>, and the plugin must export a list of functions expected by the main program, e.g. <code class="highlight"><c- b>Plugin_GetName()</c-></code> and <code class="highlight"><c- b>Plugin_GetCategory()</c-></code>. One of these exported functions could be <code class="highlight"><c- b>Plugin_GetExceptions()</c-></code> as follows:
   <pre class="highlight line-numbered">
   <span class="line-no"></span><span class="line">std::type_info const *const *Plugin_GetExceptions();</span>
   </pre>
   <p><code class="highlight"><c- b>Plugin_GetExceptions()</c-></code> returns a pointer to a null-terminated array of pointers to the <code class="highlight"><c- b>type_info</c-></code>'s of bespoke exceptions thrown from the plugin. When the main program first loads the plugin, the main program populates a global array of these exception types, and the main program consults this array when an exception is caught:</p>
   </p>
<pre class="highlight line-numbered"><code>catch(...)
{
    std::type_info const &amp;ti = std::exception_typeid();
    if ( nullptr != plugin_exceptions.find(&amp;ti) )
    {
        // We have caught one of the 'registered' exception types
    }
}
</code></pre>
<p>From here, the main program can perform tasks specific to the exception type, such as calling a specific handler function exported by the plugin:</p>
<pre class="highlight line-numbered"><code>catch(...)
{
    std::type_info const &amp;ti = std::exception_typeid();
    if ( nullptr != plugin_exceptions.find(&amp;ti) )
    {
        void (*const handler)(void*) = handlers[ std::type_index(ti) ];
        handler( std::exception_object() );
    }
}
</code></pre>
   <h3 class="heading settled" data-level="2.3" id="motivation3"><span class="secno">2.3. </span><span class="content">As a stepping stone</span><a class="self-link" href="#motivation3"></a></h3>
   <p>
   There has been recent discussion about making major additions to <code class="highlight"><c- b>std::type_info</c-></code>, for example to record the <code class="highlight"><c- b>sizeof</c-></code>, <code class="highlight"><c- b>alignof</c-></code> and <code class="highlight"><c- b>std::is_polymorphic_v</c-></code> of the type. There is the hope that if this current proposal is accepted, it will be used as a stepping stone for future papers, for example to log unknown exceptions and give a hexdump of the exception object. For example we could get a print-out such as the following:
<pre class="highlight line-numbered"><code>Type of exception object: 5Dummy
Size of exception object: 32
  Hex: 46726f6773313233000000000000000000000000000000000000000000000000
ASCII: Frogs123  
</code></pre>
from the following code:
<pre class="highlight line-numbered"><code>struct Dummy {
    char buf[32];
    Dummy(void) : buf("Frogs123") {}
};

try
{
    throw Dummy();
}
catch(...)
{
    using <code class="highlight"><c- b>std::exception_object</c-></code>;
    using <code class="highlight"><c- b>std::sizeof_type_info</c-></code>;

    std::type_info const &amp;ti = std::exception_typeid();
    printf("Type of exception object: %s\n" , ti.name());

    if ( (0u==<code class="highlight"><c- b>sizeof_type_info</c-></code>(ti)) || (-1==<code class="highlight"><c- b>sizeof_type_info</c-></code>(ti)) ) return;
    printf("Size of exception object: %zu\n", <code class="highlight"><c- b>sizeof_type_info</c-></code>(ti));

    char unsigned const *const p = static_cast&lt;char unsigned*&gt;(<code class="highlight"><c- b>exception_object</c-></code>(p));
    printf("  Hex: ");
    for ( size_t i = 0u; i < sizeof_type_info(ti); ++i )
    {
        printf("%02x", static_cast<unsigned>(p[i]) );
    }
    printf("\nASCII: ");
    for ( size_t i = 0u; i < sizeof_type_info(ti); ++i )
    {
        char c = p[i];
        if ( !isprint(c) || isspace(c) ) c = ' ';
        putchar(c);
    }
    putchar('\n');
}
</code></pre>
<p>This would greatly aid in logging exceptions in both <i>Debug</i> and <i>Release</i> builds, even where C++ code is linked with another language such as Java or C#.</p>
   </p>
   <h2 class="heading settled" data-level="3" id="impact-on-the-standard"><span class="secno">3. </span><span class="content">Impact on the standard</span><a class="self-link" href="#impact-on-the-standard"></a></h2>
   <p>This proposal is a pure library extension. It does not require changes to the standard components. Four short paragraphs are to be appended to Section 17.9.7 <a href="http://eel.is/c++draft/propagation" target="_blank">[support.exception.propagation]</a>. The text addition is 139 words, and the addition has no effect on any other part of the standard.</p>
   <h2 class="heading settled" data-level="4" id="impact-on-existing-code"><span class="secno">4. </span><span class="content">Impact on existing code</span><a class="self-link" href="#impact-on-existing-code"></a></h2>
   <p>No existing code becomes ill-formed. The behaviour of all existing code is unaffected by this addition to the standard library.</p>
   <h2 class="heading settled" data-level="5" id="design-considerations"><span class="secno">5. </span><span class="content">Design considerations</span><a class="self-link" href="#design-considerations"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="consideration1"><span class="secno">5.1. </span><span class="content">Microsoft SEH</span><a class="self-link" href="#consideration1"></a></h3>
   <p>
   The Microsoft C++ compiler has two kinds of exception:<BR>
   <ul>
     <li>C++ exceptions</li>
     <li>SEH exceptions</li>
   </ul>
   SEH exceptions are not caught by a C++ <code class="highlight"><c- b>catch</c-></code> block -- not even by a <code class="highlight"><c- b>catch(...)</c-></code> block. However if you compile with the <b>/EHa</b> switch, an SEH exception will be caught by a <code class="highlight"><c- b>catch(...)</c-></code> block. In this case, <code class="highlight"><c- b>current_exception()</c-></code> returns a valid <code class="highlight"><c- b>exception_ptr</c-></code>, however there is no <code class="highlight"><c- b>type_info</c-></code> available for an SEH exception. In any circumstance where <code class="highlight"><c- b>type_info</c-></code> is unavailable, the recommendation is that <code class="highlight"><c- b>exception_typeid()</c-></code> and <code class="highlight"><c- b>exception_typeid(p)</c-></code> shall return <code class="highlight"><c- b>typeid(void)</c-></code>, or alternatively the <code class="highlight"><c- b>type_info</c-></code> of a special type such as <code class="highlight"><c- b>typeid(_s__se_exception)</c-></code>.
   <BR><BR>
   <code class="highlight"><c- b>current_exception()</c-></code> returns a valid pointer for an SEH exception, and the exception object is an <code class="highlight"><c- b>unsigned int</c-></code>. The recommendation therefore is that <code class="highlight"><c- b>std::exception_object()</c-></code> shall return the address of the <code class="highlight"><c- b>unsigned int</c-></code>.
   <BR><BR>
   There is a workaround to ensure that SEH exceptions have <code class="highlight"><c- b>type_info</c-></code>, and that is to use the function <code class="highlight"><c- b>_set_se_translator</c-></code> to convert <i>SEH exceptions</i> to <i>C++ exceptions</i> as follows:
<pre class="highlight line-numbered"><code>// Compile with Microsoft C++ compiler with option /EHa
#include &lt;cstdio&gt;      // std::puts, ::sprintf_s
#include &lt;exception&gt;   // exception
#include &lt;eh.h&gt;        // _set_se_translator, _EXCEPTION_POINTERS

class SEH_Exception : public std::exception {
private:
    unsigned const var_number;
    mutable char var_what[14u + 1u];  // +1 for null terminator
public:
    SEH_Exception(unsigned const arg) noexcept : var_number(arg) {}
    char const *what(void) const noexcept override
    {
        static_assert( 0xFFFFFFFF == (unsigned)-1, "unsigned int must be 32-Bit" );
        ::sprintf_s(this-&gt;var_what, "SEH 0x%08x", this-&gt;var_number);
        return this-&gt;var_what;
    }
    unsigned number(void) const noexcept { return var_number; }
};

void DivideByZero(void)     // Throws an SEH exception
{
    int volatile x, y = 0;
    x = 5 / y;              // division by zero throws 0xc0000094
}

void translator(unsigned const u, ::_EXCEPTION_POINTERS*)
{
    throw SEH_Exception(u);
}

int main(void)
{
    ::_set_se_translator( translator );

    try
    {
        DivideByZero();
    }
    catch(std::exception const &amp;e)
    {
        std::puts( e.what() );
    }
}
</code></pre>
   </p>
   <h3 class="heading settled" data-level="5.2" id="consideration2"><span class="secno">5.2. </span><span class="content">Forced unwinding</span><a class="self-link" href="#consideration2"></a></h3>
   <p>
   The throwing of an exception results in unwinding of the stack, and so normally stack unwinding is accompanied by the presence of an exception object. There are circumstances however in which stack unwinding can take place without an accompanying exception object, and one such circumstance is a <b>forced unwinding</b>. An example of a forced unwinding is where the execution of a thread is abruptly stopped by the invocation of <code class="highlight"><c- b>pthread_cancel</c-></code>, as follows:
<pre class="highlight line-numbered"><code>#include &lt;cstdio&gt;            // puts
#include &lt;exception&gt;         // exception_ptr
#include &lt;pthread.h&gt;         // pthread_create
#include &lt;unistd.h&gt;          // sleep

void *Thread_Entry_Point(void*)
{
    try
    {
        for (; /* ever */ ;)
        {
            std::puts("*");
            ::sleep(1u);
        }
    }
    catch(...)
    {
        std::exception_ptr const p = std::current_exception();
        if ( false == static_cast&lt;bool&gt;(p) )
        {
            std::puts("exception_ptr is invalid - might be forced unwinding");
            throw;
        }
    }

    return nullptr;
}

int main(void)
{
    ::pthread_t h;
    ::pthread_create(&amp;h, nullptr, Thread_Entry_Point, nullptr);
    ::sleep(3u);
    ::pthread_cancel(h);
    ::pthread_join(h, nullptr);
}
</code></pre>
In the case of a forced unwinding, the recommendation is that <code class="highlight"><c- b>exception_typeid()</c-></code> shall return either <code class="highlight"><c- b>typeid(void)</c-></code>, or the <code class="highlight"><c- b>type_info</c-></code> of a special type which indicates forced unwinding, for example <code class="highlight"><c- b>typeid(abi::__forced_unwind)</c-></code> -- even if <code class="highlight"><c- b>current_exception()</c-></code> returns a null pointer. This is a scenario in which <code class="highlight"><c- b>exception_typeid()</c-></code> might not be equivalent to <code class="highlight"><c- b>exception_typeid( current_exception() )</c-></code>.
<BR><BR>
In a forced unwinding, <code class="highlight"><c- b>current_exception()</c-></code> returns a null pointer. The recommendation is that <code class="highlight"><c- b>exception_object()</c-></code> shall return <code class="highlight"><c- b>nullptr</c-></code> in any circumstance where <code class="highlight"><c- b>current_exception()</c-></code> returns a null pointer.
   </p>
   <h2 class="heading settled" data-level="6" id="implementations"><span class="secno">6. </span><span class="content">Possible implementations</span><a class="self-link" href="#implementations"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="implementation1"><span class="secno">6.1. </span><span class="content">Apple macOS, Linux, FreeBSD</span><a class="self-link" href="#implementation1"></a></h3>
   <pre class="highlight line-numbered"><code>#include &lt;exception&gt;   // exception_ptr, rethrow_exception
#include &lt;memory&gt;      // addressof
#include &lt;typeinfo&gt;    // type_info
#include &lt;cxxabi.h&gt;    // __cxa_current_exception_type

namespace std {
    type_info const &amp;exception_typeid() noexcept
    {
        type_info const *const p = abi::__cxa_current_exception_type();
        if ( nullptr == p ) return typeid(void);
        return *p;
    }

    type_info const &amp;exception_typeid(exception_ptr const &amp;p) noexcept
    {
        if ( false == static_cast&lt;bool&gt;(p) ) return typeid(void);
        try { rethrow_exception(p); }
        catch(...) { return exception_typeid(); }
        return typeid(void);  // not needed - but belt and braces
    }

    void *exception_object(exception_ptr const &amp;p) noexcept
    {
        if ( false == static_cast&lt;bool&gt;(p) ) return nullptr;
        return *(void**)addressof(p);
    }

    void *exception_object() noexcept
    {
        return exception_object( current_exception() );
    }
}
</code></pre>

   </p>
   <h3 class="heading settled" data-level="6.2" id="implementation2"><span class="secno">6.2. </span><span class="content">Microsoft Windows</span><a class="self-link" href="#implementation2"></a></h3>
<pre class="highlight line-numbered"><code>#include &lt;cstdint&gt;       // uintptr_t
#include &lt;exception&gt;     // exception_ptr, rethrow_exception
#include &lt;memory&gt;        // addressof
#include &lt;typeinfo&gt;      // type_info

#include &lt;Windows.h&gt;     // AddVectoredExceptionHandler
#include &lt;ehdata.h&gt;      // ThrowInfo, CatchableType, EXCEPTION_POINTERS

thread_local std::type_info const *pti = &amp;typeid(void);

namespace std {
    type_info const &amp;exception_typeid() noexcept
    {
        return *pti;
    }

    type_info const &amp;exception_typeid(exception_ptr const &amp;p) noexcept
    {
        if ( false == static_cast&lt;bool&gt;(p) ) return typeid(void);
        try { rethrow_exception(p); }
        catch(...) { return exception_typeid(); }
        return typeid(void);  // not needed - but belt and braces
    }

    void *exception_object(exception_ptr const &amp;p) noexcept
    {
        if ( false == static_cast&lt;bool&gt;(p) ) return nullptr;
        return *(void**)addressof(p);
    }

    void *exception_object() noexcept
    {
        return exception_object( current_exception() );
    }
}

namespace detail {

using std::type_info;

type_info const *VectoredExceptionHandler_Proper(EXCEPTION_POINTERS const *const arg)
{
    using std::uintptr_t;

    if ( nullptr == arg ) return nullptr;

    EXCEPTION_RECORD const *const pexc = arg-&gt;ExceptionRecord;

    if ( nullptr == pexc ) return nullptr;

    switch ( pexc-&gt;ExceptionCode )
    {
    case 0x40010006 /*EXCEPTION_OUTPUT_DEBUG_STRING*/:
    case 0x406D1388 /*EXCEPTION_THREAD_NAME        */:
        return nullptr;
    }

    if ( 0x19930520 /* magic number */ != pexc-&gt;ExceptionInformation&#91;0&#93; )
        return nullptr;

    if ( pexc-&gt;NumberParameters &lt; 3u ) return nullptr;

    uintptr_t const module =   (pexc-&gt;NumberParameters &gt;= 4u)
                             ? pexc-&gt;ExceptionInformation&#91;3u&#93;
                             : 0u;

    ThrowInfo const *const pthri =
        static_cast&lt;ThrowInfo const*&gt;(
            reinterpret_cast&lt;void const*&gt;(
                static_cast&lt;uintptr_t&gt;(pexc-&gt;ExceptionInformation&#91;2u&#93;)));

    if ( nullptr == pthri ) return nullptr;

    if ( 0 == pthri-&gt;pCatchableTypeArray ) return nullptr;

    if ( 0u == (0xFFFFFFFFu &amp; pthri-&gt;pCatchableTypeArray) ) return nullptr;

    _CatchableTypeArray const *const pcarr =
      static_cast&lt;_CatchableTypeArray const *&gt;(
        reinterpret_cast&lt;void const*&gt;(
          static_cast&lt;uintptr_t&gt;(module + (0xFFFFFFFFu &amp; pthri-&gt;pCatchableTypeArray))));

    if ( 0u == ( 0xFFFFFFFFu &amp;
        reinterpret_cast&lt;uintptr_t&gt;(pcarr-&gt;arrayOfCatchableTypes&#91;0u&#93;)  )  )
           return nullptr;

    CatchableType const *const pct =
      static_cast&lt;CatchableType const*&gt;(
        reinterpret_cast&lt;void const*&gt;(
          static_cast&lt;uintptr_t&gt;(module +
            (0xFFFFFFFFu &amp; reinterpret_cast&lt;uintptr_t&gt;(
              pcarr-&gt;arrayOfCatchableTypes&#91;0u&#93;)))));

    if ( 0u == (0xFFFFFFFFu &amp; pct-&gt;pType) ) return nullptr;

    return static_cast&lt;type_info const *&gt;(
             reinterpret_cast&lt;void const*&gt;(
               static_cast&lt;uintptr_t&gt;(module + (0xFFFFFFFFu &amp; pct-&gt;pType))));
}

long WINAPI VectoredExceptionHandler(EXCEPTION_POINTERS *const pointers)
{
    type_info const *const retval = VectoredExceptionHandler_Proper(pointers);

    if ( nullptr == retval ) pti = &amp;typeid(void);
                        else pti = retval;

    return EXCEPTION_CONTINUE_SEARCH;
}

// The next line ensures that 'AddVectoredExceptionHandler' is called before 'main'
void *const dummy = ::AddVectoredExceptionHandler(1u, VectoredExceptionHandler);

} // close namespace 'detail'
</code></pre>
   <h2 class="heading settled" data-level="7" id="proposed-wording"><span class="secno">7. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposed-wording"></a></h2>
   <p>The proposed wording is relative to <a data-link-type="biblio" href="#biblio-n4950" title="Working Draft, Standard for Programming Language C++">[N4950]</a>.</p>
   <p>In section 17.9.7 <a href="http://eel.is/c++draft/propagation" target="_blank">[support.exception.propagation]</a>,
append:</p>
<pre class="highlight line-numbered"><code><b>13</b>   type_info const &amp;exception_typeid() noexcept;
     Returns: A reference to a ‘type_info’ (17.7.3) object for the currently
              handled exception, or ‘typeid(void)’ (7.6.1.8) if the currently
              handled exception is lacking ‘type_info’.

<b>14</b>   type_info const &amp;exception_typeid(exception_ptr const &amp;p) noexcept;
     Returns: A reference to a ‘type_info’ (17.7.3) object for the exception
              object referred by ‘p’ (17.9.7.1), or ‘typeid(void)’ (7.6.1.8)
              if ‘p’ is a null pointer, or ‘typeid(void)’ if the exception
              is lacking ‘type_info’.

<b>15</b>   void *exception_object() noexcept;
     Returns: The address of the exception object for the currently handled
              exception, or a null pointer if the exception is lacking an
              exception object.

<b>16</b>   void *exception_object(exception_ptr const &amp;p) noexcept;
     Returns: The address of the exception object for the exception referred
              by ‘p’ (17.9.7.1), or a null pointer if ‘p’ is a null pointer,
              or a null pointer if the exception is lacking an exception object.
</code></pre>
<h2 class="heading settled" data-level="8" id="alternatives"><span class="secno">8. </span><span class="content">Alternatives</span><a class="self-link" href="#alternatives"></a></h2>
<h3 class="heading settled" data-level="8.1" id="alternative1"><span class="secno">8.1. </span><span class="content"><c- k>std</c-><c- p>::</c-><c- k>exception_ptr</c-><c- p>::</c-><c- k>type</c-><c- p>()</c-></code></span><a class="self-link" href="#alternative1"></a></h3>
<p>
It has been suggested that <code class="highlight"><c- b>std::exception_ptr</c-></code> should be given member functions, and that it should be defined in the standard as follows:
<pre class="highlight line-numbered"><code>class exception_ptr {
public:
    type_info const &amp;type() const noexcept;
    void *object() const noexcept;
};
</code></pre>
This however would prevent the compiler from simply doing:
<pre class="highlight line-numbered"><code>typedef shared_ptr&lt;void&gt; exception_ptr;
</code></pre>
Furthermore, if <code class="highlight"><c- b>exception_ptr</c-></code> is a non-polymorphic type, then all new member functions would have to be non-virtual in order to avoid an ABI break. Even if <code class="highlight"><c- b>exception_ptr</c-></code> is already a polymorphic type, adding a virtual member function could result in an ABI break for derived classes such as:
<pre class="highlight line-numbered"><code>class ExceptionHandle : public std::exception_ptr {
public:
    virtual void output(std::ostream&amp;) const;
};
</code></pre>
Adding a new virtual member function to the class <code class="highlight"><c- b>exception_ptr</c-></code> will have an effect on the index of <code class="highlight"><c- b>output</c-></code> in the vtable belonging to <code class="highlight"><c- b>ExceptionHandle</c-></code> resulting in an ABI break.
<BR>
<BR>
If we were to add a new member function to the <code class="highlight"><c- b>exception_ptr</c-></code> class instead of creating a standalone function <code class="highlight"><c- b>std::exception_typeid</c-></code>, then we would have no way of getting the <code class="highlight"><c- b>type_info</c-></code> in situations where <code class="highlight"><c- b>current_exception()</c-></code> returns a null pointer (such as a forced unwinding). Therefore even if we add a member function to <code class="highlight"><c- b>exception_ptr</c-></code>, we still need the standalone function <code class="highlight"><c- b>std::exception_typeid()</c-></code>. For the sake of simplicity it makes more sense just to have the standalone function.
</p>
<h2 class="heading settled" data-level="9" id="acknowledgements"><span class="secno">9. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
<p>
For their feedback and contributions on the mailing list <a href="https://lists.isocpp.org/std-proposals/" target="_blank">std-proposals@lists.isocpp.org</a>:<BR>
<i>Tom Honermann, Thiago Macieira, Jens Maurer, Jason McKesson, Arthur O'Dwyer, Peter Olsson, Jan Schultke, Lénárd Szolnoki, Matthew Taylor, Ville Voutilainen, Sebastian Wittmeier and <a href="https://edwardjamescatmur.muchloved.com/" target="_blank">the late Edward Catmur (1982-2024)</a></i>.
</p>

</p>
<h2 class="heading settled" data-level="10" id="revisions"><span class="secno">10. </span><span class="content">Revisions</span><a class="self-link" href="#revisions"></a></h2>
<p>
R0 => R1<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp; &bull; Revision number incremented to compensate for a corrupt upload of the original paper. (No other changes).
</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="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-n4950">[N4950]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4950" target="_blank"><cite>Working Draft, Standard for Programming Language C++</cite></a>. 10 May 2023. URL: <a href="https://wg21.link/n4950" target="_blank">https://wg21.link/n4950</a>
  </dl>
  <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-p0933r1">[P0933R1]
   <dd>Aaryaman Sagar. <a href="https://wg21.link/p0933r1" target="_blank"><cite>Runtime type introspection with std::exception_ptr</cite></a>. 7 February 2018. URL: <a href="https://wg21.link/p0933r1" target="_blank">https://wg21.link/p0933r1</a>
   <dt id="biblio-p1066r1">[P1066R1]
   <dd>Mathias Stearn. <a href="https://wg21.link/p1066r1" target="_blank"><cite>How to catch an exception_ptr without even try-ing</cite></a>. 6 October 2018. URL: <a href="https://wg21.link/p1066r1" target="_blank">https://wg21.link/p1066r1</a>
   <dt id="biblio-p2927r1">[P2927R1]
   <dd>Arthur O'Dwyer, Gor Nishanov. <a href="https://wg21.link/p2927r1" target="_blank"><cite>Inspecting exception_ptr</cite></a>. 14 February 2024. URL: <a href="https://wg21.link/p2927r1" target="_blank">https://wg21.link/p2927r1</a>
  </dl>
