<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P3374R0: Adding formatter for fpos&lt;mbstate_t></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;
			*/
		}
	}
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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">P3374R0<br>Adding formatter for fpos&lt;mbstate_t></h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-08-01">2024-08-01</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://extra-creativity.github.io/public/cpp/proposals/add%20formatter%20for%20fpos.html">https://extra-creativity.github.io/public/cpp/proposals/add%20formatter%20for%20fpos.html</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:liangjiaming@stu.pku.edu.cn">Liang Jiaming</a> (<a class="p-org org" href="https://www.pku.edu.cn/">Peking University</a>)
     <dt>Audience:
     <dd>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>This paper aims to add formatter for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>fpos</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>mbstate_t</c-><c- o>></c-></code> widely used by position indicator APIs in stream to make it convenient and robust to output.</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="#Intro"><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>
    <li>
     <a href="#Design"><span class="secno">3</span> <span class="content">Design Decision</span></a>
     <ol class="toc">
      <li>
       <a href="#Problem"><span class="secno">3.1</span> <span class="content">Core Problem</span></a>
       <ol class="toc">
        <li><a href="#not-an-integer-only-convertible"><span class="secno">3.1.1</span> <span class="content">Not an integer, only convertible</span></a>
        <li><a href="#not-only-the-integer-though-convertible"><span class="secno">3.1.2</span> <span class="content">Not only the integer, though convertible</span></a>
       </ol>
      <li><a href="#proposed-solution"><span class="secno">3.2</span> <span class="content">Proposed Solution</span></a>
      <li><a href="#Evolution"><span class="secno">3.3</span> <span class="content">Possible Future Evolution</span></a>
     </ol>
    <li><a href="#Wording"><span class="secno">4</span> <span class="content">Standard Wording</span></a>
    <li><a href="#Impact"><span class="secno">5</span> <span class="content">Impact on Existing Code</span></a>
    <li><a href="#Implementation"><span class="secno">6</span> <span class="content">Implementation</span></a>
    <li><a href="#Acknowledgement"><span class="secno">7</span> <span class="content">Acknowledgement</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="Intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#Intro"></a></h2>
   <p>Stream-based I/O is designed since C++98 to give a thorough abstraction to I/O. An important part of the abstraction is position indicator, which is used to tell and seek the current position. Such position is most commonly conveyed by <code class="highlight"><c- n>char_traits</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>pos_type</c-></code>, which is <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>char_traits</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>state_type</c-><c- o>></c-></code> by default and the only instantiation used in the standard library is <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code>. It’s widely implemented to be able to be output by stream <code class="highlight"><c- k>operator</c-><c- o>&lt;&lt;</c-></code>; however, it cannot be output directly by <code class="highlight"><c- n>print</c-></code> in C++23 astonishingly due to lack of <code class="highlight"><c- n>formatter</c-></code>. Moreover, the state of the position is usually neglected, making it not robust in some cases to be output by <code class="highlight"><c- k>operator</c-><c- o>&lt;&lt;</c-></code>. This paper aims to add a specialization for <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code> to solve these problems.</p>
   <h2 class="heading settled" data-level="2" id="Motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#Motivation"></a></h2>
   <p>Let’s see tony table to illustrate it directly:</p>
   <table>
    <tbody>
     <tr>
      <th> Before 
     <tr>
      <td valign="top">
<pre class="highlight" lang="cpp"><c- n>std</c-><c- o>::</c-><c- n>ofstream</c-> <c- n>s</c-><c- p>{</c-><c- s>"some_file"</c-><c- p>};</c->
<c- c1>// Do some output...</c->
<c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>s</c-><c- p>.</c-><c- n>tellg</c-><c- p>();</c->        <c- c1>// ❓Yes on almost all implementations, but not robust</c->
<c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"{}"</c-><c- p>,</c-> <c- n>s</c-><c- p>.</c-><c- n>tellg</c-><c- p>());</c-> <c- c1>// ❌Compile error...</c->
<c- c1>// To make it work same as operator&lt;&lt;, we have to write...</c->
<c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"{}"</c-><c- p>,</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>streamoff</c-><c- p>)</c-><c- n>s</c-><c- p>.</c-><c- n>tellg</c-><c- p>());</c->    <c- c1>// 😢What?</c->
<c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"{}"</c-><c- p>,</c-> <c- n>s</c-><c- p>.</c-><c- n>tellg</c-><c- p>()</c-> <c- o>-</c-> <c- n>std</c-><c- o>::</c-><c- n>streampos</c-><c- p>{});</c-> <c- c1>// 😢No way...</c->
</pre>
     <tr>
      <th> After 
     <tr>
      <td valign="top">
<pre class="highlight" lang="cpp"><c- n>std</c-><c- o>::</c-><c- n>ofstream</c-> <c- n>s</c-><c- p>{</c-><c- s>"some_file"</c-><c- p>};</c->
<c- c1>// Do some output...</c->
<c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>s</c-><c- p>.</c-><c- n>tellg</c-><c- p>();</c->           <c- c1>// ❓Yes on almost all implementations, but not robust</c->
<c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"{}"</c-><c- p>,</c-> <c- n>s</c-><c- p>.</c-><c- n>tellg</c-><c- p>());</c->    <c- c1>// ✅Yes and robust!</c->
<c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"{:d}"</c-><c- p>,</c-> <c- n>s</c-><c- p>.</c-><c- n>tellg</c-><c- p>());</c->  <c- c1>// ✅Non-robust way can be controlled by users explicitly.</c->
<c- n>std</c-><c- o>::</c-><c- n>stringstream</c-> <c- n>s2</c-><c- p>{</c-><c- s>"ABC"</c-><c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"{:d}"</c-><c- p>,</c-> <c- n>s2</c-><c- p>.</c-><c- n>tellg</c-><c- p>());</c-> <c- c1>// ✅Especially for streams that don’t need codecvt.</c->
</pre>
   </table>
   <h2 class="heading settled" data-level="3" id="Design"><span class="secno">3. </span><span class="content">Design Decision</span><a class="self-link" href="#Design"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="Problem"><span class="secno">3.1. </span><span class="content">Core Problem</span><a class="self-link" href="#Problem"></a></h3>
   <h4 class="heading settled" data-level="3.1.1" id="not-an-integer-only-convertible"><span class="secno">3.1.1. </span><span class="content">Not an integer, only convertible</span><a class="self-link" href="#not-an-integer-only-convertible"></a></h4>
   <p>For C programmers and those who don’t take a thorough look at the design of stream, they’re likely to regard the position as an integer directly since <code class="highlight"><c- n>ftell</c-></code> just returns so. Actually, in C++ it’s designed as follows:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>CharTraits</c-> <c- o>=</c-> <c- n>char_traits</c-><c- o>&lt;</c-><c- n>CharT</c-><c- o>>></c->
<c- k>class</c-> <c- nc>basic_iostream</c->
<c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- k>using</c-> <c- n>pos_type</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>CharTraits</c-><c- o>::</c-><c- n>pos_type</c-><c- p>;</c->
    <c- n>pos_type</c-> <c- nf>tellg</c-><c- p>();</c->
    <c- n>pos_type</c-> <c- nf>tellp</c-><c- p>();</c->
<c- p>};</c->
</pre>
   <p>And the most commonly used types are alias like:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>CharT</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>CharTraits</c-> <c- o>=</c-> <c- n>char_traits</c-><c- o>&lt;</c-><c- n>CharT</c-><c- o>>></c->
<c- k>class</c-> <c- nc>basic_fstream</c-> <c- o>:</c-> <c- k>public</c-> <c- n>basic_iostream</c-><c- o>&lt;</c-><c- n>CharT</c-><c- p>,</c-> <c- n>CharTraits</c-><c- o>></c-> <c- p>{</c-> <c- p>...</c-> <c- p>};</c->

<c- k>using</c-> <c- n>fstream</c-> <c- o>=</c-> <c- n>basic_iostream</c-><c- o>&lt;</c-><c- b>char</c-><c- o>></c-><c- p>;</c->
<c- k>using</c-> <c- n>wfstream</c-> <c- o>=</c-> <c- n>basic_iostream</c-><c- o>&lt;</c-><c- b>wchar_t</c-><c- o>></c-><c- p>;</c->
</pre>
   <p>So the return type of <code class="highlight"><c- n>tellg</c-><c- o>/</c-><c- n>p</c-></code> is usually determined by <code class="highlight"><c- n>char_traits</c-><c- o>&lt;</c-><c- n>CharT</c-><c- o>>::</c-><c- n>pos_type</c-></code>, where <code class="highlight"><c- n>CharT</c-></code> is <code class="highlight"><c- b>char</c-></code> or <code class="highlight"><c- b>wchar_t</c-></code>. They’re defined as <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>char_traits</c-><c- o>&lt;</c-><c- n>CharT</c-><c- o>>::</c-><c- n>state_type</c-><c- o>></c-></code>, and the only used instantiation in the standard library is <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code>. <code class="highlight"><c- n>fpos</c-></code> only supports limited integer operations, like subtracting another <code class="highlight"><c- n>fpos</c-></code> to get <code class="highlight"><c- n>streamoff</c-></code>, or adding a <code class="highlight"><c- n>streamoff</c-></code> offset to get a new <code class="highlight"><c- n>fpos</c-></code>. Particularly, <code class="highlight"><c- n>streamoff</c-></code> is regulated to be an alias of a signed integer type, and <code class="highlight"><c- n>fpos</c-></code> should be convertible to <code class="highlight"><c- n>streamoff</c-></code> to make expression <code class="highlight"><c- n>streamoff</c-><c- p>(</c-><c- n>pos</c-><c- p>)</c-></code> compile (See <a href="https://eel.is/c++draft/stream.types">[stream.types]</a> and <a href="https://eel.is/c++draft/fpos.operations">[fpos.operations]</a> in the standard).</p>
   <p>Though such requirement can be implemented as follows:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>StateT</c-><c- o>></c->
<c- k>class</c-> <c- nc>fpos</c->
<c- p>{</c->
    <c- k>explicit</c-> <c- k>operator</c-> <c- n>streamoff</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- p>...</c-> <c- p>}</c->
<c- p>};</c->
</pre>
   <p>The existing mainstream implementations, like <a href="https://github.com/microsoft/STL/blob/184ce39eef53e7ec0ab6dd16027771a120eac62f/stl/inc/iosfwd#L64">MS-STL</a>, <a href="https://github.com/gcc-mirror/gcc/blob/a945c346f57ba40fc80c14ac59be0d43624e559d/libstdc%2B%2B-v3/include/bits/postypes.h#L113">libstdc++</a>, <a href="https://github.com/llvm/llvm-project/blob/9783f28cbb155e4a8d49c12e1c60ce14dcfaf0c7/libcxx/include/__ios/fpos.h#L31">libc++</a>, and many other implmentations that support C++98 like <a href="https://github.com/apache/stdcxx/blob/trunk/include/rw/_traits.h#L249">Apache stdcxx</a>, <a href="https://sourceforge.net/projects/stlport/files/STLport/STLport-5.2.1/">STLport (stlport/stl/char_traits#92)</a>, 
all choose to make it not <code class="highlight"><c- k>explicit</c-></code>. Such tacit phenomenon make an illusion to C++ programmers that "it’s just an integer". Specifically, for output, the <code class="highlight"><c- k>operator</c-><c- o>&lt;&lt;</c-></code> has all overloads like:</p>
<pre class="highlight"><c- n>basic_ostream</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>&lt;&lt;</c-><c- p>(</c-><c- b>int</c-><c- p>);</c->
<c- n>basic_ostream</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>&lt;&lt;</c-><c- p>(</c-><c- b>long</c-><c- p>);</c->
<c- n>basic_ostream</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>&lt;&lt;</c-><c- p>(</c-><c- b>long</c-> <c- b>long</c-><c- p>);</c->
</pre>
   <p>This enables implicit conversion from <code class="highlight"><c- n>fpos</c-></code> to <code class="highlight"><c- n>streamoff</c-></code> to match one of the overloads and makes output successful. However, it fails to work when it comes to <code class="highlight"><c- n>format</c-></code>, since template doesn’t try to do implicit conversions in most cases. The <code class="highlight"><c- n>formatter</c-></code> specialization for <code class="highlight"><c- b>int</c-></code>, <code class="highlight"><c- b>long</c-></code> and <code class="highlight"><c- b>long</c-> <c- b>long</c-></code> cannot be utilized by <code class="highlight"><c- n>fpos</c-></code> without explicit conversion.</p>
   <h4 class="heading settled" data-level="3.1.2" id="not-only-the-integer-though-convertible"><span class="secno">3.1.2. </span><span class="content">Not only the integer, though convertible</span><a class="self-link" href="#not-only-the-integer-though-convertible"></a></h4>
   <p>Another problem that’s usually neglected is that <code class="highlight"><c- n>fpos</c-></code> doesn’t merely contain the position integer; it also has a <code class="highlight"><c- n>state</c-></code> as conveyed by the template parameter, most typically <code class="highlight"><c- b>mbstate_t</c-></code>. It’s used to determine the current state of character conversion, like for <code class="highlight"><c- n>codecvt</c-></code> in locale. By default, <code class="highlight"><c- n>fpos</c-></code> assigned by a <code class="highlight"><c- n>streamoff</c-></code> will have a value-initialized state, which means the initial state. However, sometimes it’s possibly not "initial", making such restoration unsafe and incorrect.</p>
   <p>For instance, though rarely happen, if some derived class of <code class="highlight"><c- n>basic_streambuf</c-></code> allows partial conversion when overriding <code class="highlight"><c- n>overflow</c-></code> (since it’s only regulated to prepare space for at least one <code class="highlight"><c- n>CharT</c-></code>), <code class="highlight"><c- n>tellp</c-></code> of its stream may also report a <code class="highlight"><c- n>fpos</c-></code> with partial status. Anyway, it’s incomplete to only report the position integer in some cases.</p>
   <h3 class="heading settled" data-level="3.2" id="proposed-solution"><span class="secno">3.2. </span><span class="content">Proposed Solution</span><a class="self-link" href="#proposed-solution"></a></h3>
   <p>To make it both safe and convenient, we propose to add formatter specialization for <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code>. Considering that almost all behaviors of <code class="highlight"><c- b>mbstate_t</c-></code> are implementation-defined, it seems meaningless to regulate its format specifications. However, the state should be output in a way that can fully convey its information, so if <a data-link-type="biblio" href="#biblio-p1729" title="Text Parsing">[P1729]</a> is accepted, a corresponding scanner can be defined to do a round trip.</p>
   <p>So to be specific, the formatter specialization of <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code> should behave as follows:</p>
   <ul>
    <li data-md>
     <p>When no specification is given (like <code class="highlight"><c- p>{}</c-></code> or <code class="highlight"><c- p>{</c-><c- o>:</c-><c- p>}</c-></code>), <code class="highlight"><c- n>format</c-></code> should produce <code class="highlight"><c- s>"(position, mbstate descriptor)"</c-></code>, where <code class="highlight"><c- n>mbstate</c-> <c- n>descriptor</c-></code> can be used to fully restore the state of <code class="highlight"><c- n>fpos</c-></code>;</p>
    <li data-md>
     <p>When some specifications are given (e.g. <code class="highlight"><c- p>{</c-><c- o>:</c-><c- n>d</c-><c- p>}</c-></code>), only the position will be output in the way determined by the format specifications.</p>
   </ul>
   <h3 class="heading settled" data-level="3.3" id="Evolution"><span class="secno">3.3. </span><span class="content">Possible Future Evolution</span><a class="self-link" href="#Evolution"></a></h3>
   <p>An important aspect to consider is whether we need to change the behavior of <code class="highlight"><c- k>operator</c-><c- o>&lt;&lt;</c-></code>, like making conversion operator to <code class="highlight"><c- n>streamoff</c-></code> explicit or overloading <code class="highlight"><c- k>operator</c-><c- o>&lt;&lt;</c-></code> for <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code>. Such breaking change may or may not be expected by many.</p>
   <p>Besides, it’s worthwhile to have a discussion about whether we need to leave some way for the <code class="highlight"><c- n>formatter</c-></code> specialization to check whether <code class="highlight"><c- b>mbstate_t</c-></code> is in its initial state by <code class="highlight"><c- n>mbsinit</c-></code> to boost safety in some cases.</p>
   <p>It could also be talked about whether the formatter should be generalized for <code class="highlight"><c- n>fpos</c-></code> to support any <code class="highlight"><c- n>StateT</c-></code> that is formattable, and whether <code class="highlight"><c- b>mbstate_t</c-></code> should support <code class="highlight"><c- n>formatter</c-></code> itself instead of relying on <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code>. It may be also worthy to discuss whether ambiguity is introduced when parsing <code class="highlight"><c- p>(</c-><c- n>position</c-><c- p>,</c-> <c- n>mbstate</c-> <c- n>descriptor</c-><c- p>)</c-></code> to restore the <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code> if the descriptor is not constrained at all.</p>
   <h2 class="heading settled" data-level="4" id="Wording"><span class="secno">4. </span><span class="content">Standard Wording</span><a class="self-link" href="#Wording"></a></h2>
   <p>We propose to add wording in <a href="https://eel.is/c++draft/fpos.operations">[fpos.operations]</a>:</p>
   <blockquote>
     2.Stream operations that return a value of type traits::pos_type return P(O(-1)) as an invalid value to signal an error. If this value is used as an argument to any istream, ostream, or streambuf member that accepts a value of type traits::pos_type then the behavior of that function is undefined.<br> 
    <ins>3. Assuming that mbstate_t can be fully restored by a <code class="highlight"><c- n>basic_string</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-></code> called mbstate descriptor, the formatter of <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code> should behave as follows:</ins>
<pre class="highlight" lang="cpp" style="background-color: #CCFFCC;"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->
 <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- o>></c->
 <c- k>struct</c-> <c- nc>formatter</c-><c- o>&lt;</c-><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-><c- p>,</c-> <c- n>charT</c-><c- o>></c-> <c- p>{</c->
 <c- k>private</c-><c- o>:</c->
  <c- n>formatter</c-><c- o>&lt;</c-><c- n>streamoff</c-><c- p>,</c-> <c- n>charT</c-><c- o>></c-> <c- n>underlying_</c-><c- p>;</c->    <c- c1>// exposition only</c->    <br>
  <c- b>bool</c-> <c- n>need_state_</c-> <c- o>=</c-> false<c- p>;</c->     <c- c1>// exposition only</c->    <br>
 <c- k>public</c-><c- o>:</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
   <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
    <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->    <br>
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>FormatContext</c-><c- o>></c->
   <c- k>typename</c-> <c- nc>FormatContext</c-><c- o>::</c-><c- n>iterator</c->
    <c- n>format</c-><c- p>(</c-><c- k>const</c-> <c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>>&amp;</c-> <c- n>ref</c-><c- p>,</c-> <c- n>FormatContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
 <c- p>};</c->
<c- p>}</c->    <br>
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
 <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
  <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->
</pre>
    <div style="background-color: #CCFFCC;">
     <div class="itemdescr" style="background-color: #CCFFCC;"> <i>Effects:</i> Sets <span class="texttt">need_state_</span> to <span class="texttt">true</span> if <i>format-specifier</i> or <i>format-spec</i> ([format.string.general]) is not present, otherwise same as <span class="texttt">underlying_.format(ctx)</span>.<br> <i>Returns:</i> An iterator past the end of <i>format-spec</i>.<br> </div>
      <br> 
    </div>
<pre class="highlight" lang="cpp" style="background-color: #CCFFCC;"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>FormatContext</c-><c- o>></c->
 <c- k>typename</c-> <c- nc>FormatContext</c-><c- o>::</c-><c- n>iterator</c->
  <c- n>format</c-><c- p>(</c-><c- k>const</c-> <c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>>&amp;</c-> <c- n>ref</c-><c- p>,</c-> <c- n>FormatContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
</pre>
    <div style="background-color: #CCFFCC;">
     <div class="itemdescr">
       <i>Effects:</i> Writes the following into <span class="texttt">ctx.out()</span>: 
      <ul>
       <li>If <span class="texttt">need_state_</span> is <span class="texttt">false</span>, then as if <span class="texttt">underlying_.format(static_cast&lt;streamoff>(ref), ctx)</span>;
       <li>
        Otherwise, 
        <ul>
         <li><i>STATICALLY-WIDEN</i><span class="texttt">&lt;charT>("(")</span>,
         <li>the result of writing <span class="texttt">static_cast&lt;streamoff>(ref)</span> via <span class="texttt">underlying_</span>,
         <li><i>STATICALLY-WIDEN</i><span class="texttt">&lt;charT>(", ")</span>,
         <li>the mbstate descriptor of <span class="texttt">ref.state()</span>,
         <li><i>STATICALLY-WIDEN</i><span class="texttt">&lt;charT>(")")</span>.
        </ul>
      </ul>
       <i>Returns:</i> An iterator past the end of the output range.<br> 
     </div>
    </div>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="Impact"><span class="secno">5. </span><span class="content">Impact on Existing Code</span><a class="self-link" href="#Impact"></a></h2>
   <p>This is a pure extension to the standard library so there won’t be severe conflicts with the existing code. The only possible conflict is that some existing code has already added specialization on <code class="highlight"><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-></code>, but it seems that no open-source code on Github does so.</p>
   <h2 class="heading settled" data-level="6" id="Implementation"><span class="secno">6. </span><span class="content">Implementation</span><a class="self-link" href="#Implementation"></a></h2>
   <p>Since <code class="highlight"><c- b>mbstate_t</c-></code> is implementation-defined and the internal state is undocumented, we may only be able to write some pseudo-code that’s close to the final implementation. The easiest implementation may make use of inheritance:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>CharT</c-><c- o>></c->
<c- k>struct</c-> <c- nc>formatter</c-><c- o>&lt;</c-><c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>></c-><c- p>,</c-> <c- n>CharT</c-><c- o>></c-> <c- o>:</c-> <c- n>formatter</c-><c- o>&lt;</c-><c- n>streamoff</c-><c- p>,</c-> <c- n>CharT</c-><c- o>></c->
<c- p>{</c->
<c- k>private</c-><c- o>:</c->
    <c- k>using</c-> <c- n>Base</c-> <c- o>=</c-> <c- n>formatter</c-><c- o>&lt;</c-><c- n>streamoff</c-><c- p>,</c-> <c- n>CharT</c-><c- o>></c-><c- p>;</c->
    <c- b>bool</c-> <c- n>need_state_</c-> <c- o>=</c-> false<c- p>;</c->

<c- k>public</c-><c- o>:</c->
    <c- k>constexpr</c-> <c- k>auto</c-> <c- n>parse</c-><c- p>(</c-><c- k>const</c-> <c- k>auto</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>)</c->
    <c- p>{</c->
        <c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>ctx</c-><c- p>.</c-><c- n>begin</c-><c- p>();</c->
        <c- k>if</c-> <c- p>(</c-><c- n>it</c-> <c- o>==</c-> <c- n>context</c-><c- p>.</c-><c- n>end</c-><c- p>()</c-> <c- o>||</c-> <c- o>*</c-><c- n>it</c-> <c- o>==</c-> <c- n>WIDEN</c-><c- p>(</c-><c- sc>'}'</c-><c- p>))</c->
        <c- p>{</c->
            <c- n>need_state_</c-> <c- o>=</c-> true<c- p>;</c->
            <c- k>return</c-> <c- n>it</c-><c- p>;</c->
        <c- p>}</c->

        <c- k>return</c-> <c- n>Base</c-><c- o>::</c-><c- n>parse</c-><c- p>(</c-><c- n>ctx</c-><c- p>);</c->
    <c- p>}</c->

    <c- k>auto</c-> <c- n>format</c-><c- p>(</c-><c- k>const</c-> <c- n>fpos</c-><c- o>&lt;</c-><c- b>mbstate_t</c-><c- o>>&amp;</c-> <c- n>value</c-><c- p>,</c-> <c- k>auto</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>)</c-> <c- k>const</c->
    <c- p>{</c->
        <c- k>if</c-> <c- p>(</c-><c- n>need_state_</c-><c- p>)</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>format_to</c-><c- p>(</c-><c- n>ctx</c-><c- p>.</c-><c- n>out</c-><c- p>(),</c-> <c- n>ctx</c-><c- p>.</c-><c- n>locale</c-><c- p>(),</c-> <c- n>WIDEN</c-><c- p>(</c-><c- s>"({}, {})"</c-><c- p>),</c->
                             <c- k>static_cast</c-><c- o>&lt;</c-><c- n>streamoff</c-><c- o>></c-><c- p>(</c-><c- n>value</c-><c- p>),</c->
                             <c- n>GET_MBSTATE_DESCRIPTOR</c-><c- p>(</c-><c- n>value</c-><c- p>.</c-><c- n>state</c-><c- p>()));</c->
        <c- p>}</c->
        <c- k>return</c-> <c- n>Base</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- k>static_cast</c-><c- o>&lt;</c-><c- n>streamoff</c-><c- o>></c-><c- p>(</c-><c- n>value</c-><c- p>),</c-> <c- n>ctx</c-><c- p>);</c->
    <c- p>}</c->
<c- p>};</c->
</pre>
   <p>Notice that MS-STL and libstdc++ can still utilize the implicit conversion so the <code class="highlight"><c- k>static_cast</c-></code> in <code class="highlight"><c- n>Base</c-><c- o>::</c-><c- n>format</c-></code> can be omitted. libc++ checks whether <code class="highlight"><c- n>value</c-></code> is integer in the template <code class="highlight"><c- n>Base</c-><c- o>::</c-><c- n>format</c-></code> method and thus implicit conversion cannot help.</p>
   <h2 class="heading settled" data-level="7" id="Acknowledgement"><span class="secno">7. </span><span class="content">Acknowledgement</span><a class="self-link" href="#Acknowledgement"></a></h2>
   <p>Thanks to Victor Zverovich, the author of <a data-link-type="biblio" href="#biblio-fmt" title="The {fmt} library">[fmt]</a>, and Arthur O’Dwyer for suggestions and discussions on generalization of the conversion and encouragement to post this paper. Thanks to Tom Honermann for advice on formatting the state type and assistance. I’d also like to extend my gratitude to Peking University for giving me a colorful undergraduate life in the last four years.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-fmt">[FMT]
   <dd>Victor Zverovich; et al.. <a href="https://github.com/fmtlib/fmt"><cite>The {fmt} library</cite></a>. URL: <a href="https://github.com/fmtlib/fmt">https://github.com/fmtlib/fmt</a>
   <dt id="biblio-p1729">[P1729]
   <dd>Elias Kosunen, Victor Zverovich. <a href="https://wg21.link/p1729"><cite>Text Parsing</cite></a>. URL: <a href="https://wg21.link/p1729">https://wg21.link/p1729</a>
  </dl>