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<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
<meta charset="utf-8" />
<meta name="generator" content="mpark/wg21" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2022-05-16" />
<title>Formatting Ranges</title>
<style>
code{white-space: pre-wrap;}
span.smallcaps{font-variant: small-caps;}
span.underline{text-decoration: underline;}
div.column{display: inline-block; vertical-align: top; width: 50%;}
div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
ul.task-list{list-style: none;}
pre > code.sourceCode { white-space: pre; position: relative; }
pre > code.sourceCode > span { display: inline-block; line-height: 1.25; }
pre > code.sourceCode > span:empty { height: 1.2em; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
{ counter-reset: source-line 0; }
pre.numberSource code > span
{ position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
{ content: counter(source-line);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
color: #aaaaaa;
}
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
div.sourceCode
{ background-color: #f6f8fa; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span. { } /* Normal */
code span.al { color: #ff0000; } /* Alert */
code span.an { } /* Annotation */
code span.at { } /* Attribute */
code span.bn { color: #9f6807; } /* BaseN */
code span.bu { color: #9f6807; } /* BuiltIn */
code span.cf { color: #00607c; } /* ControlFlow */
code span.ch { color: #9f6807; } /* Char */
code span.cn { } /* Constant */
code span.co { color: #008000; font-style: italic; } /* Comment */
code span.cv { color: #008000; font-style: italic; } /* CommentVar */
code span.do { color: #008000; } /* Documentation */
code span.dt { color: #00607c; } /* DataType */
code span.dv { color: #9f6807; } /* DecVal */
code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #9f6807; } /* Float */
code span.fu { } /* Function */
code span.im { } /* Import */
code span.in { color: #008000; } /* Information */
code span.kw { color: #00607c; } /* Keyword */
code span.op { color: #af1915; } /* Operator */
code span.ot { } /* Other */
code span.pp { color: #6f4e37; } /* Preprocessor */
code span.re { } /* RegionMarker */
code span.sc { color: #9f6807; } /* SpecialChar */
code span.ss { color: #9f6807; } /* SpecialString */
code span.st { color: #9f6807; } /* String */
code span.va { } /* Variable */
code span.vs { color: #9f6807; } /* VerbatimString */
code span.wa { color: #008000; font-weight: bold; } /* Warning */
code.diff {color: #898887}
code.diff span.va {color: #006e28}
code.diff span.st {color: #bf0303}
</style>
<style type="text/css">
body {
margin: 5em;
font-family: serif;
hyphens: auto;
line-height: 1.35;
}
div.wrapper {
max-width: 60em;
margin: auto;
}
ul {
list-style-type: none;
padding-left: 2em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
a {
text-decoration: none;
color: #4183C4;
}
a.hidden_link {
text-decoration: none;
color: inherit;
}
li {
margin-top: 0.6em;
margin-bottom: 0.6em;
}
h1, h2, h3, h4 {
position: relative;
line-height: 1;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
font-family: sans-serif;
font-weight: normal;
font-size: 83%;
}
a.self-link:hover { opacity: 1; }
a.self-link::before { content: "§"; }
ul > li:before {
content: "\2014";
position: absolute;
margin-left: -1.5em;
}
:target { background-color: #C9FBC9; }
:target .codeblock { background-color: #C9FBC9; }
:target ul { background-color: #C9FBC9; }
.abbr_ref { float: right; }
.folded_abbr_ref { float: right; }
:target .folded_abbr_ref { display: none; }
:target .unfolded_abbr_ref { float: right; display: inherit; }
.unfolded_abbr_ref { display: none; }
.secnum { display: inline-block; min-width: 35pt; }
.header-section-number { display: inline-block; min-width: 35pt; }
.annexnum { display: block; }
div.sourceLinkParent {
float: right;
}
a.sourceLink {
position: absolute;
opacity: 0;
margin-left: 10pt;
}
a.sourceLink:hover {
opacity: 1;
}
a.itemDeclLink {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
opacity: 0;
}
a.itemDeclLink:hover { opacity: 1; }
span.marginalizedparent {
position: relative;
left: -5em;
}
li span.marginalizedparent { left: -7em; }
li ul > li span.marginalizedparent { left: -9em; }
li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
div.footnoteNumberParent {
position: relative;
left: -4.7em;
}
a.marginalized {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
}
a.enumerated_item_num {
position: relative;
left: -3.5em;
display: inline-block;
margin-right: -3em;
text-align: right;
width: 3em;
}
div.para { margin-bottom: 0.6em; margin-top: 0.6em; text-align: justify; }
div.section { text-align: justify; }
div.sentence { display: inline; }
span.indexparent {
display: inline;
position: relative;
float: right;
right: -1em;
}
a.index {
position: absolute;
display: none;
}
a.index:before { content: "⟵"; }
a.index:target {
display: inline;
}
.indexitems {
margin-left: 2em;
text-indent: -2em;
}
div.itemdescr {
margin-left: 3em;
}
.bnf {
font-family: serif;
margin-left: 40pt;
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.ncbnf {
font-family: serif;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
}
.ncsimplebnf {
font-family: serif;
font-style: italic;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
background: inherit;
}
span.textnormal {
font-style: normal;
font-family: serif;
white-space: normal;
display: inline-block;
}
span.rlap {
display: inline-block;
width: 0px;
}
span.descr { font-style: normal; font-family: serif; }
span.grammarterm { font-style: italic; }
span.term { font-style: italic; }
span.terminal { font-family: monospace; font-style: normal; }
span.nonterminal { font-style: italic; }
span.tcode { font-family: monospace; font-style: normal; }
span.textbf { font-weight: bold; }
span.textsc { font-variant: small-caps; }
a.nontermdef { font-style: italic; font-family: serif; }
span.emph { font-style: italic; }
span.techterm { font-style: italic; }
span.mathit { font-style: italic; }
span.mathsf { font-family: sans-serif; }
span.mathrm { font-family: serif; font-style: normal; }
span.textrm { font-family: serif; }
span.textsl { font-style: italic; }
span.mathtt { font-family: monospace; font-style: normal; }
span.mbox { font-family: serif; font-style: normal; }
span.ungap { display: inline-block; width: 2pt; }
span.textit { font-style: italic; }
span.texttt { font-family: monospace; }
span.tcode_in_codeblock { font-family: monospace; font-style: normal; }
span.phantom { color: white; }
span.math { font-style: normal; }
span.mathblock {
display: block;
margin-left: auto;
margin-right: auto;
margin-top: 1.2em;
margin-bottom: 1.2em;
text-align: center;
}
span.mathalpha {
font-style: italic;
}
span.synopsis {
font-weight: bold;
margin-top: 0.5em;
display: block;
}
span.definition {
font-weight: bold;
display: block;
}
.codeblock {
margin-left: 1.2em;
line-height: 127%;
}
.outputblock {
margin-left: 1.2em;
line-height: 127%;
}
div.itemdecl {
margin-top: 2ex;
}
code.itemdeclcode {
white-space: pre;
display: block;
}
span.textsuperscript {
vertical-align: super;
font-size: smaller;
line-height: 0;
}
.footnotenum { vertical-align: super; font-size: smaller; line-height: 0; }
.footnote {
font-size: small;
margin-left: 2em;
margin-right: 2em;
margin-top: 0.6em;
margin-bottom: 0.6em;
}
div.minipage {
display: inline-block;
margin-right: 3em;
}
div.numberedTable {
text-align: center;
margin: 2em;
}
div.figure {
text-align: center;
margin: 2em;
}
table {
border: 1px solid black;
border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none;
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
}
td.empty {
padding: 0px;
padding-left: 1px;
}
td.left {
text-align: left;
}
td.right {
text-align: right;
}
td.center {
text-align: center;
}
td.justify {
text-align: justify;
}
td.border {
border-left: 1px solid black;
}
tr.rowsep, td.cline {
border-top: 1px solid black;
}
tr.even, tr.odd {
border-bottom: 1px solid black;
}
tr.capsep {
border-top: 3px solid black;
border-top-style: double;
}
tr.header {
border-bottom: 3px solid black;
border-bottom-style: double;
}
th {
border-bottom: 1px solid black;
}
span.centry {
font-weight: bold;
}
div.table {
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
}
span.indented {
display: block;
margin-left: 2em;
margin-bottom: 1em;
margin-top: 1em;
}
ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }
code.sourceCode > span { display: inline; }
</style>
<style type="text/css">a {
color : #4183C4;
text-decoration: underline;
}
a.marginalized {
text-decoration: none;
}
a.self-link {
text-decoration: none;
}
h1#toctitle {
border-bottom: 1px solid #cccccc;
}
#TOC li {
margin-top: 1px;
margin-bottom: 1px;
}
#TOC ul>li:before { display: none; }
h3.subtitle { margin-top: -15px; }
h1:target { background-color: transparent; }
h2:target { background-color: transparent; }
h3:target { background-color: transparent; }
h4:target { background-color: transparent; }
h5:target { background-color: transparent; }
h6:target { background-color: transparent; }
code span.co { font-family: monospace; }
table tr {
background-color: white;
}
table tr:nth-child(2n) {
background-color: #f6f8fa;
}
#title-block-header > table tr:nth-child(2n) {
background-color: white;
}
td > div.sourceCode {
background-color: inherit;
}
table {
border-collapse: collapse;
}
table td, table th {
border: 1px solid #cccccc;
}
table th {
border-bottom: 1px solid black;
text-align: center;
}
table tr:first-child th {
border-top: 0;
}
table tr:last-child td {
border-bottom: 0;
}
table tr td:first-child,
table tr th:first-child {
border-left: 0;
}
table tr td:last-child,
table tr th:last-child {
border-right: 0;
}
table tbody tr:first-child td {
border-top: 1px solid black;
}
#title-block-header td { border: 0; }
@media all {
body {
margin: 2em;
}
}
@media screen and (min-width: 480px) {
body {
margin: 5em;
}
}
#refs code{padding-left: 0px; text-indent: 0px;}
:root {
--diff-ins: #e6ffed;
--diff-strongins: #acf2bd;
--diff-del: #ffdddd;
--diff-strongdel: #ff8888;
}
span.diffins {
background-color: var(--diff-strongins);
}
span.diffdel {
background-color: var(--diff-strongdel);
}
div.rm { text-decoration: line-through; }
div.rm code.sourceCode { text-decoration: line-through; }
div.addu, span.addu {
color: #006e28;
background-color: var(--diff-ins);
}
div.rm pre, div.add pre { background-color: #f6f8fa; }
div.addu pre { background-color: var(--diff-ins); }
div.add, div.add pre { background-color: var(--diff-ins); }
div.addu blockquote {
border-left: 4px solid #00a000;
padding: 0 15px;
color: #006e28;
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<h1 class="title" style="text-align:center">Formatting Ranges</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2286R8</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-05-16</td>
</tr>
<tr>
<td style="vertical-align:top">Project:</td>
<td>Programming Language C++</td>
</tr>
<tr>
<td style="vertical-align:top">Audience:</td>
<td>
LEWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a>
<ul>
<li><a href="#implementation-experience"><span class="toc-section-number">2.1</span> Implementation Experience<span></span></a></li>
</ul></li>
<li><a href="#proposal-considerations"><span class="toc-section-number">3</span> Proposal Considerations<span></span></a>
<ul>
<li><a href="#what-types-to-print"><span class="toc-section-number">3.1</span> What types to print?<span></span></a></li>
<li><a href="#detecting-whether-a-type-is-formattable"><span class="toc-section-number">3.2</span> Detecting whether a type is formattable<span></span></a></li>
<li><a href="#what-representation"><span class="toc-section-number">3.3</span> What representation?<span></span></a>
<ul>
<li><a href="#vector-and-other-ranges"><span class="toc-section-number">3.3.1</span> <code class="sourceCode cpp">vector</code> (and other ranges)<span></span></a></li>
<li><a href="#pair-and-tuple"><span class="toc-section-number">3.3.2</span> <code class="sourceCode cpp">pair</code> and <code class="sourceCode cpp">tuple</code><span></span></a></li>
<li><a href="#map-and-set-and-other-associative-containers"><span class="toc-section-number">3.3.3</span> <code class="sourceCode cpp">map</code> and <code class="sourceCode cpp">set</code> (and other associative containers)<span></span></a></li>
<li><a href="#char-and-string-and-other-string-like-types-in-ranges-or-tuples"><span class="toc-section-number">3.3.4</span> <code class="sourceCode cpp"><span class="dt">char</span></code> and <code class="sourceCode cpp">string</code> (and other string-like types) in ranges or tuples<span></span></a></li>
<li><a href="#filesystempath"><span class="toc-section-number">3.3.5</span> <code class="sourceCode cpp">filesystem<span class="op">::</span>path</code><span></span></a></li>
<li><a href="#format-specifiers"><span class="toc-section-number">3.3.6</span> Format Specifiers<span></span></a></li>
<li><a href="#explanation-of-added-specifiers"><span class="toc-section-number">3.3.7</span> Explanation of Added Specifiers<span></span></a>
<ul>
<li><a href="#the-debug-specifier"><span class="toc-section-number">3.3.7.1</span> The debug specifier <code class="sourceCode cpp"><span class="op">?</span></code><span></span></a></li>
<li><a href="#range-specifiers"><span class="toc-section-number">3.3.7.2</span> Range specifiers<span></span></a></li>
<li><a href="#pair-and-tuple-specifiers"><span class="toc-section-number">3.3.7.3</span> Pair and Tuple Specifiers<span></span></a></li>
</ul></li>
<li><a href="#escaping-behavior"><span class="toc-section-number">3.3.8</span> Escaping Behavior<span></span></a>
<ul>
<li><a href="#python"><span class="toc-section-number">3.3.8.1</span> Python<span></span></a></li>
<li><a href="#rust"><span class="toc-section-number">3.3.8.2</span> Rust<span></span></a></li>
<li><a href="#golang"><span class="toc-section-number">3.3.8.3</span> Golang<span></span></a></li>
<li><a href="#proposed-for-c"><span class="toc-section-number">3.3.8.4</span> Proposed for C++<span></span></a></li>
</ul></li>
</ul></li>
<li><a href="#implementation-challenges"><span class="toc-section-number">3.4</span> Implementation Challenges<span></span></a></li>
<li><a href="#how-to-support-those-views-which-are-not-const-iterable"><span class="toc-section-number">3.5</span> How to support those views which are not <code class="sourceCode cpp"><span class="kw">const</span></code>-iterable?<span></span></a></li>
<li><a href="#interface-of-the-proposed-solution"><span class="toc-section-number">3.6</span> Interface of the proposed solution<span></span></a></li>
<li><a href="#what-additional-functionality"><span class="toc-section-number">3.7</span> What additional functionality?<span></span></a></li>
<li><a href="#format-or-stdcout"><span class="toc-section-number">3.8</span> <code class="sourceCode cpp">format</code> or <code class="sourceCode cpp">std<span class="op">::</span>cout</code>?<span></span></a></li>
<li><a href="#what-about-vectorbool"><span class="toc-section-number">3.9</span> What about <code class="sourceCode cpp">vector<span class="op"><</span><span class="dt">bool</span><span class="op">></span></code>?<span></span></a></li>
<li><a href="#what-about-container-adaptors"><span class="toc-section-number">3.10</span> What about container adaptors?<span></span></a></li>
<li><a href="#examples-with-user-defined-types"><span class="toc-section-number">3.11</span> Examples with user-defined types<span></span></a></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">4</span> Proposal<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">5</span> Wording<span></span></a>
<ul>
<li><a href="#concept-formattable"><span class="toc-section-number">5.1</span> Concept <code class="sourceCode cpp">formattable</code><span></span></a></li>
<li><a href="#additional-formatting-support-for-characters-and-strings"><span class="toc-section-number">5.2</span> Additional formatting support for characters and strings<span></span></a></li>
<li><a href="#formatting-for-ranges"><span class="toc-section-number">5.3</span> Formatting for ranges<span></span></a>
<ul>
<li><a href="#formatting-for-specific-ranges-all-the-maps-and-sets"><span class="toc-section-number">5.3.1</span> Formatting for specific ranges: all the maps and sets<span></span></a></li>
<li><a href="#formatting-for-specific-ranges-all-the-container-adaptors"><span class="toc-section-number">5.3.2</span> Formatting for specific ranges: all the container adaptors<span></span></a></li>
</ul></li>
<li><a href="#formatting-for-pair-and-tuple"><span class="toc-section-number">5.4</span> Formatting for <code class="sourceCode cpp">pair</code> and <code class="sourceCode cpp">tuple</code><span></span></a></li>
<li><a href="#formatter-for-vectorboolreference"><span class="toc-section-number">5.5</span> Formatter for <code class="sourceCode cpp">vector<span class="op"><</span><span class="dt">bool</span><span class="op">>::</span>reference</code><span></span></a></li>
<li><a href="#feature-test-macro"><span class="toc-section-number">5.6</span> Feature-test Macro<span></span></a></li>
</ul></li>
<li><a href="#acknowledgments"><span class="toc-section-number">6</span> Acknowledgments<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">7</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2286R7">[<a href="#ref-P2286R7" role="doc-biblioref">P2286R7</a>]</span>, further wording improvements, almost exclusively around string and character escaping.</p>
<p>Since <span class="citation" data-cites="P2286R6">[<a href="#ref-P2286R6" role="doc-biblioref">P2286R6</a>]</span>, wording.</p>
<p>Since <span class="citation" data-cites="P2286R5">[<a href="#ref-P2286R5" role="doc-biblioref">P2286R5</a>]</span>, missing feature test macro and few wording changes, including:</p>
<ul>
<li><code class="sourceCode cpp">formatter<span class="op"><</span>R, charT<span class="op">></span></code> for ranges no longer specified to inherit from <code class="sourceCode cpp">range_formatter<span class="op"><</span>range_reference_t<span class="op"><</span>R<span class="op">>></span></code></li>
<li>the output iterator for the <code class="sourceCode cpp">formattable</code> concept is now unspecified rather than implementation-defined</li>
</ul>
<p>Since <span class="citation" data-cites="P2286R4">[<a href="#ref-P2286R4" role="doc-biblioref">P2286R4</a>]</span>, several major changes:</p>
<ul>
<li>Removed the <code class="sourceCode cpp">d</code> specifier for delimiters. This paper offers no direct support for changing delimiters (which this paper also in the wording refers to as separators).</li>
<li>Removed the extra APIs (<code class="sourceCode cpp">retargeted_format_context</code> and <code class="sourceCode cpp">end_sentry</code>), and the motivation for their existence.</li>
<li>Added clearer description of why <code class="sourceCode cpp">range_formatter</code> is desired and what its <a href="#interface-of-the-proposed-solution">exposed API is</a>.</li>
<li>Updated <a href="#escaping-behavior">escaping behavior</a> description with how some other languages do this.</li>
<li>Added section on <a href="#what-about-container-adaptors">container adaptors</a>.</li>
<li>Added wording.</li>
</ul>
<p>Since <span class="citation" data-cites="P2286R3">[<a href="#ref-P2286R3" role="doc-biblioref">P2286R3</a>]</span>, several major changes:</p>
<ul>
<li>Removed the special <code class="sourceCode cpp">pair</code>/<code class="sourceCode cpp">tuple</code> parsing for individual elements. This proved complicated and illegible, and led to having to deal with more issues that would make this paper harder to make it for C++23.</li>
<li>Adding sections on <a href="#dynamic-delimiter-for-ranges">dynamic</a> and <a href="#static-delimiter-for-ranges">static</a> delimiters for ranges. Removing <code class="sourceCode cpp">std<span class="op">::</span>format_join</code> in their favor.</li>
<li>Renaming <code class="sourceCode cpp">format_as_debug</code> to <code class="sourceCode cpp">set_debug_format</code> (since it’s not actually <em>formatting</em> anything, it’s just setting up)</li>
<li>Discussing <code class="sourceCode cpp">std<span class="op">::</span>filesystem<span class="op">::</span>path</code></li>
</ul>
<p>Since <span class="citation" data-cites="P2286R2">[<a href="#ref-P2286R2" role="doc-biblioref">P2286R2</a>]</span>, several major changes:</p>
<ul>
<li>This paper assumes the adoption of <span class="citation" data-cites="P2418R0">[<a href="#ref-P2418R0" role="doc-biblioref">P2418R0</a>]</span>, which affects how <a href="#how-to-support-those-views-which-are-not-const-iterable">non-<code class="sourceCode cpp"><span class="kw">const</span></code>-iterable views</a> are handled. This paper now introduces two concepts (<code class="sourceCode cpp">formattable</code> and <code class="sourceCode cpp">const_formattable</code>) instead of just one.</li>
<li>Extended discussion and functionality for various <a href="#what-representation">representations</a>, including how to quote strings properly and how to format associative ranges.</li>
<li>Introduction of format specifiers of all kinds and discussion of how to make them work more broadly.</li>
<li>Removed the wording, since the priority is the design.</li>
</ul>
<p>Since <span class="citation" data-cites="P2286R1">[<a href="#ref-P2286R1" role="doc-biblioref">P2286R1</a>]</span>, adding a sketch of wording.</p>
<p><span class="citation" data-cites="P2286R0">[<a href="#ref-P2286R0" role="doc-biblioref">P2286R0</a>]</span> suggested making all the formatting implementation-defined. Several people reached out to me suggesting in no uncertain terms that this is unacceptable. This revision lays out options for such formatting.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p><span class="citation" data-cites="LWG3478">[<a href="#ref-LWG3478" role="doc-biblioref">LWG3478</a>]</span> addresses the issue of what happens when you split a string and the last character in the string is the delimiter that you are splitting on. One of the things I wanted to look at in research in that issue is: what do <em>other</em> languages do here?</p>
<p>For most languages, this is a pretty easy proposition. Do the split, print the results. This is usually only a few lines of code.</p>
<p>Python:</p>
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb1-1"><a href="#cb1-1"></a><span class="bu">print</span>(<span class="st">"xyx"</span>.split(<span class="st">"x"</span>))</span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb1-1"><a href="#cb1-1"></a>['', 'y', '']</span></code></pre></div>
</blockquote>
<p>Java (where the obvious thing prints something useless, but there’s a non-obvious thing that is useful):</p>
<blockquote>
<div class="sourceCode" id="cb2"><pre class="sourceCode java"><code class="sourceCode java"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">import</span><span class="im"> java.util.Arrays;</span></span>
<span id="cb2-2"><a href="#cb2-2"></a></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="kw">class</span> Main {</span>
<span id="cb2-4"><a href="#cb2-4"></a> <span class="kw">public</span> <span class="dt">static</span> <span class="dt">void</span> <span class="fu">main</span>(<span class="bu">String</span> args[]) {</span>
<span id="cb2-5"><a href="#cb2-5"></a> <span class="bu">System</span>.<span class="fu">out</span>.<span class="fu">println</span>(<span class="st">"xyx"</span>.<span class="fu">split</span>(<span class="st">"x"</span>));</span>
<span id="cb2-6"><a href="#cb2-6"></a> <span class="bu">System</span>.<span class="fu">out</span>.<span class="fu">println</span>(<span class="bu">Arrays</span>.<span class="fu">toString</span>(<span class="st">"xyx"</span>.<span class="fu">split</span>(<span class="st">"x"</span>)));</span>
<span id="cb2-7"><a href="#cb2-7"></a> }</span>
<span id="cb2-8"><a href="#cb2-8"></a>}</span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1"></a>[Ljava.lang.String;@76ed5528</span>
<span id="cb2-2"><a href="#cb2-2"></a>[, y]</span></code></pre></div>
</blockquote>
<p>Rust (a couple options, including also another false friend):</p>
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">use</span> <span class="pp">itertools::</span>Itertools<span class="op">;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a></span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="kw">fn</span> main() <span class="op">{</span></span>
<span id="cb3-4"><a href="#cb3-4"></a> <span class="pp">println!</span>(<span class="st">"{:?}"</span><span class="op">,</span> <span class="st">"xyx"</span><span class="op">.</span>split(<span class="ch">'x'</span>))<span class="op">;</span></span>
<span id="cb3-5"><a href="#cb3-5"></a> <span class="pp">println!</span>(<span class="st">"[{}]"</span><span class="op">,</span> <span class="st">"xyx"</span><span class="op">.</span>split(<span class="ch">'x'</span>)<span class="op">.</span>format(<span class="st">", "</span>))<span class="op">;</span></span>
<span id="cb3-6"><a href="#cb3-6"></a> <span class="pp">println!</span>(<span class="st">"{:?}"</span><span class="op">,</span> <span class="st">"xyx"</span><span class="op">.</span>split(<span class="ch">'x'</span>)<span class="op">.</span><span class="pp">collect::</span><span class="op"><</span><span class="dt">Vec</span><span class="op"><</span>_<span class="op">>></span>())<span class="op">;</span></span>
<span id="cb3-7"><a href="#cb3-7"></a><span class="op">}</span></span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1"></a>Split(SplitInternal { start: 0, end: 3, matcher: CharSearcher { haystack: "xyx", finger: 0, finger_back: 3, needle: 'x', utf8_size: 1, utf8_encoded: [120, 0, 0, 0] }, allow_trailing_empty: true, finished: false })</span>
<span id="cb3-2"><a href="#cb3-2"></a>[, y, ]</span>
<span id="cb3-3"><a href="#cb3-3"></a>["", "y", ""]</span></code></pre></div>
</blockquote>
<p>Kotlin:</p>
<blockquote>
<div class="sourceCode" id="cb4"><pre class="sourceCode kotlin"><code class="sourceCode kotlin"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">fun</span> <span class="fu">main</span>() {</span>
<span id="cb4-2"><a href="#cb4-2"></a> println(<span class="st">"xyx"</span>.split(<span class="st">"x"</span>));</span>
<span id="cb4-3"><a href="#cb4-3"></a>}</span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb4-1"><a href="#cb4-1"></a>[, y, ]</span></code></pre></div>
</blockquote>
<p>Go:</p>
<blockquote>
<div class="sourceCode" id="cb5"><pre class="sourceCode go"><code class="sourceCode go"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">package</span> main</span>
<span id="cb5-2"><a href="#cb5-2"></a><span class="kw">import</span> <span class="st">"fmt"</span></span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="kw">import</span> <span class="st">"strings"</span></span>
<span id="cb5-4"><a href="#cb5-4"></a></span>
<span id="cb5-5"><a href="#cb5-5"></a><span class="kw">func</span> main() {</span>
<span id="cb5-6"><a href="#cb5-6"></a> fmt.Println(strings.Split(<span class="st">"xyx"</span>, <span class="st">"x"</span>));</span>
<span id="cb5-7"><a href="#cb5-7"></a>}</span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb5-1"><a href="#cb5-1"></a>[ y ]</span></code></pre></div>
</blockquote>
<p>JavaScript:</p>
<blockquote>
<div class="sourceCode" id="cb6"><pre class="sourceCode js"><code class="sourceCode javascript"><span id="cb6-1"><a href="#cb6-1"></a><span class="bu">console</span><span class="op">.</span><span class="fu">log</span>(<span class="st">'xyx'</span><span class="op">.</span><span class="fu">split</span>(<span class="st">'x'</span>))</span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb6-1"><a href="#cb6-1"></a>[ '', 'y', '' ]</span></code></pre></div>
</blockquote>
<p>And so on and so forth. What we see across these languages is that printing the result of split is pretty easy. In most cases, whatever the print mechanism is just works and does something meaningful. In other cases, printing gave me something other than what I wanted but some other easy, provided mechanism for doing so.</p>
<p>Now let’s consider C++.</p>
<blockquote>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="pp">#include </span><span class="im"><iostream></span></span>
<span id="cb7-2"><a href="#cb7-2"></a><span class="pp">#include </span><span class="im"><string></span></span>
<span id="cb7-3"><a href="#cb7-3"></a><span class="pp">#include </span><span class="im"><ranges></span></span>
<span id="cb7-4"><a href="#cb7-4"></a><span class="pp">#include </span><span class="im"><format></span></span>
<span id="cb7-5"><a href="#cb7-5"></a></span>
<span id="cb7-6"><a href="#cb7-6"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb7-7"><a href="#cb7-7"></a> <span class="co">// need to predeclare this because we can't split an rvalue string</span></span>
<span id="cb7-8"><a href="#cb7-8"></a> std<span class="op">::</span>string s <span class="op">=</span> <span class="st">"xyx"</span>;</span>
<span id="cb7-9"><a href="#cb7-9"></a> <span class="kw">auto</span> parts <span class="op">=</span> s <span class="op">|</span> std<span class="op">::</span>views<span class="op">::</span>split<span class="op">(</span><span class="ch">'x'</span><span class="op">)</span>;</span>
<span id="cb7-10"><a href="#cb7-10"></a></span>
<span id="cb7-11"><a href="#cb7-11"></a> <span class="co">// nope</span></span>
<span id="cb7-12"><a href="#cb7-12"></a> std<span class="op">::</span>cout <span class="op"><<</span> parts;</span>
<span id="cb7-13"><a href="#cb7-13"></a></span>
<span id="cb7-14"><a href="#cb7-14"></a> <span class="co">// nope (assuming std::print from P2093)</span></span>
<span id="cb7-15"><a href="#cb7-15"></a> std<span class="op">::</span>print<span class="op">(</span><span class="st">"{}"</span>, parts<span class="op">)</span>;</span>
<span id="cb7-16"><a href="#cb7-16"></a></span>
<span id="cb7-17"><a href="#cb7-17"></a></span>
<span id="cb7-18"><a href="#cb7-18"></a> std<span class="op">::</span>cout <span class="op"><<</span> <span class="st">"["</span>;</span>
<span id="cb7-19"><a href="#cb7-19"></a> <span class="dt">char</span> <span class="kw">const</span><span class="op">*</span> delim <span class="op">=</span> <span class="st">""</span>;</span>
<span id="cb7-20"><a href="#cb7-20"></a> <span class="cf">for</span> <span class="op">(</span><span class="kw">auto</span> part <span class="op">:</span> parts<span class="op">)</span> <span class="op">{</span></span>
<span id="cb7-21"><a href="#cb7-21"></a> std<span class="op">::</span>cout <span class="op"><<</span> delim;</span>
<span id="cb7-22"><a href="#cb7-22"></a></span>
<span id="cb7-23"><a href="#cb7-23"></a> <span class="co">// still nope</span></span>
<span id="cb7-24"><a href="#cb7-24"></a> std<span class="op">::</span>cout <span class="op"><<</span> part;</span>
<span id="cb7-25"><a href="#cb7-25"></a></span>
<span id="cb7-26"><a href="#cb7-26"></a> <span class="co">// also nope</span></span>
<span id="cb7-27"><a href="#cb7-27"></a> std<span class="op">::</span>print<span class="op">(</span><span class="st">"{}"</span>, part<span class="op">)</span>;</span>
<span id="cb7-28"><a href="#cb7-28"></a></span>
<span id="cb7-29"><a href="#cb7-29"></a> <span class="co">// this finally works</span></span>
<span id="cb7-30"><a href="#cb7-30"></a> std<span class="op">::</span>ranges<span class="op">::</span>copy<span class="op">(</span>part, std<span class="op">::</span>ostream_iterator<span class="op"><</span><span class="dt">char</span><span class="op">>(</span>std<span class="op">::</span>cout<span class="op">))</span>;</span>
<span id="cb7-31"><a href="#cb7-31"></a></span>
<span id="cb7-32"><a href="#cb7-32"></a> <span class="co">// as does this</span></span>
<span id="cb7-33"><a href="#cb7-33"></a> <span class="cf">for</span> <span class="op">(</span><span class="dt">char</span> c <span class="op">:</span> part<span class="op">)</span> <span class="op">{</span></span>
<span id="cb7-34"><a href="#cb7-34"></a> std<span class="op">::</span>cout <span class="op"><<</span> c;</span>
<span id="cb7-35"><a href="#cb7-35"></a> <span class="op">}</span></span>
<span id="cb7-36"><a href="#cb7-36"></a> delim <span class="op">=</span> <span class="st">", "</span>;</span>
<span id="cb7-37"><a href="#cb7-37"></a> <span class="op">}</span></span>
<span id="cb7-38"><a href="#cb7-38"></a> std<span class="op">::</span>cout <span class="op"><<</span> <span class="st">"]</span><span class="sc">\n</span><span class="st">"</span>;</span>
<span id="cb7-39"><a href="#cb7-39"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This took me more time to write than any of the solutions in any of the other languages. Including the Go solution, which contains 100% of all the lines of Go I’ve written in my life.</p>
<p>Printing is a fairly fundamental and universal mechanism to see what’s going on in your program. In the context of ranges, it’s probably the most useful way to see and understand what the various range adapters actually do. But none of these things provides an <code class="sourceCode cpp"><span class="kw">operator</span><span class="op"><<</span></code> (for <code class="sourceCode cpp">std<span class="op">::</span>cout</code>) or a formatter specialization (for <code class="sourceCode cpp">format</code>). And the further problem is that as a user, I can’t even do anything about this. I can’t just provide an <code class="sourceCode cpp"><span class="kw">operator</span><span class="op"><<</span></code> in <code class="sourceCode cpp"><span class="kw">namespace</span> std</code> or a very broad specialization of <code class="sourceCode cpp">formatter</code> - none of these are program-defined types, so it’s just asking for clashes once you start dealing with bigger programs.</p>
<p>The only mechanisms I have at my disposal to print something like this is either</p>
<ol type="1">
<li>nested loops with hand-written delimiter handling (which are tedious and a bad solution), or</li>
<li>at least replace the inner-most loop with a <code class="sourceCode cpp">ranges<span class="op">::</span>copy</code> into an output iterator (which is more differently bad), or</li>
<li>Write my own formatting library that I <em>am</em> allowed to specialize (which is not only bad but also ridiculous)</li>
<li>Use <code class="sourceCode cpp">fmt<span class="op">::</span>format</code>.</li>
</ol>
<h2 data-number="2.1" id="implementation-experience"><span class="header-section-number">2.1</span> Implementation Experience<a href="#implementation-experience" class="self-link"></a></h2>
<p>That’s right, there’s a fourth option for C++ that I haven’t shown yet, and that’s this:</p>
<blockquote>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="pp">#include </span><span class="im"><ranges></span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="pp">#include </span><span class="im"><string></span></span>
<span id="cb8-3"><a href="#cb8-3"></a><span class="pp">#include </span><span class="im"><fmt/ranges.h></span></span>
<span id="cb8-4"><a href="#cb8-4"></a></span>
<span id="cb8-5"><a href="#cb8-5"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb8-6"><a href="#cb8-6"></a> std<span class="op">::</span>string s <span class="op">=</span> <span class="st">"xyx"</span>;</span>
<span id="cb8-7"><a href="#cb8-7"></a> <span class="kw">auto</span> parts <span class="op">=</span> s <span class="op">|</span> std<span class="op">::</span>views<span class="op">::</span>split<span class="op">(</span><span class="ch">'x'</span><span class="op">)</span>;</span>
<span id="cb8-8"><a href="#cb8-8"></a></span>
<span id="cb8-9"><a href="#cb8-9"></a> fmt<span class="op">::</span>print<span class="op">(</span><span class="st">"{}</span><span class="sc">\n</span><span class="st">"</span>, parts<span class="op">)</span>;</span>
<span id="cb8-10"><a href="#cb8-10"></a> fmt<span class="op">::</span>print<span class="op">(</span><span class="st">"<<{}>></span><span class="sc">\n</span><span class="st">"</span>, fmt<span class="op">::</span>join<span class="op">(</span>parts, <span class="st">"--"</span><span class="op">))</span>;</span>
<span id="cb8-11"><a href="#cb8-11"></a><span class="op">}</span></span></code></pre></div>
<p>outputting</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb9-1"><a href="#cb9-1"></a>[[], ['y'], []]</span>
<span id="cb9-2"><a href="#cb9-2"></a><<[]--['y']--[]>></span></code></pre></div>
</blockquote>
<p>And this is great! It’s a single, easy line of code to just print arbitrary ranges (include ranges of ranges).</p>
<p>And, if I want to do something more involved, there’s also <code class="sourceCode cpp">fmt<span class="op">::</span>join</code>, which lets me specify both a format specifier and a delimiter. For instance:</p>
<blockquote>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a>std<span class="op">::</span>vector<span class="op"><</span><span class="dt">uint8_t</span><span class="op">></span> mac <span class="op">=</span> <span class="op">{</span><span class="bn">0xaa</span>, <span class="bn">0xbb</span>, <span class="bn">0xcc</span>, <span class="bn">0xdd</span>, <span class="bn">0xee</span>, <span class="bn">0xff</span><span class="op">}</span>;</span>
<span id="cb10-2"><a href="#cb10-2"></a>fmt<span class="op">::</span>print<span class="op">(</span><span class="st">"{:02x}</span><span class="sc">\n</span><span class="st">"</span>, fmt<span class="op">::</span>join<span class="op">(</span>mac, <span class="st">":"</span><span class="op">))</span>;</span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb11-1"><a href="#cb11-1"></a>aa:bb:cc:dd:ee:ff</span></code></pre></div>
</blockquote>
<p><code class="sourceCode cpp">fmt<span class="op">::</span>format</code> (and <code class="sourceCode cpp">fmt<span class="op">::</span>print</code>) solves my problem completely. <code class="sourceCode cpp">std<span class="op">::</span>format</code> does not, and it should.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="proposal-considerations"><span class="header-section-number">3</span> Proposal Considerations<a href="#proposal-considerations" class="self-link"></a></h1>
<p>The Ranges Plan for C++23 <span class="citation" data-cites="P2214R1">[<a href="#ref-P2214R1" role="doc-biblioref">P2214R1</a>]</span> listed as one of its top priorities for C++23 as the ability to format all views. Let’s go through the issues we need to address in order to get this functionality.</p>
<h2 data-number="3.1" id="what-types-to-print"><span class="header-section-number">3.1</span> What types to print?<a href="#what-types-to-print" class="self-link"></a></h2>
<p>The standard library is the only library that can provide formatting support for standard library types and other broad classes of types like ranges. In addition to ranges (both the conrete containers like <code class="sourceCode cpp">vector<span class="op"><</span>T<span class="op">></span></code> and the range adaptors like <code class="sourceCode cpp">views<span class="op">::</span>split</code>), there are several very commonly used types that are currently not printable.</p>
<p>The most common and important such types are <code class="sourceCode cpp">pair</code> and <code class="sourceCode cpp">tuple</code> (which ties back into Ranges even more closely once we adopt <code class="sourceCode cpp">views<span class="op">::</span>zip</code> and <code class="sourceCode cpp">views<span class="op">::</span>enumerate</code>). <code class="sourceCode cpp">fmt</code> currently supports printing such types as well:</p>
<blockquote>
<div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a>fmt<span class="op">::</span>print<span class="op">(</span><span class="st">"{}</span><span class="sc">\n</span><span class="st">"</span>, std<span class="op">::</span>pair<span class="op">(</span><span class="dv">1</span>, <span class="dv">2</span><span class="op">))</span>;</span></code></pre></div>
<p>outputs</p>
<div class="sourceCode" id="cb13"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb13-1"><a href="#cb13-1"></a>(1, 2)</span></code></pre></div>
</blockquote>
<p>Another common and important set of types are <code class="sourceCode cpp">std<span class="op">::</span>optional<span class="op"><</span>T<span class="op">></span></code> and <code class="sourceCode cpp">std<span class="op">::</span>variant<span class="op"><</span>Ts<span class="op">...></span></code>. <code class="sourceCode cpp">fmt</code> does not support printing any of the sum types. There is not an obvious representation for them in C++ as there might be in other languages (e.g. in Rust, an <code class="sourceCode cpp">Option<span class="op"><</span>i32<span class="op">></span></code> prints as either <code class="sourceCode cpp">Some<span class="op">(</span><span class="dv">42</span><span class="op">)</span></code> or <code class="sourceCode cpp">None</code>, which is also the same syntax used to construct them).</p>
<p>However, the point here isn’t necessarily to produce the best possible representation (users who have very specific formatting needs will need to write custom code anyway), but rather to provide something useful. And it’d be useful to print these types as well. However, given that <code class="sourceCode cpp">optional</code> and <code class="sourceCode cpp">variant</code> are both less closely related to Ranges than <code class="sourceCode cpp">pair</code> and <code class="sourceCode cpp">tuple</code> and also have less obvious representation, they are less important.</p>
<h2 data-number="3.2" id="detecting-whether-a-type-is-formattable"><span class="header-section-number">3.2</span> Detecting whether a type is formattable<a href="#detecting-whether-a-type-is-formattable" class="self-link"></a></h2>
<p>We need to be able to conditionally provide formatters for generic types. <code class="sourceCode cpp">vector<span class="op"><</span>T<span class="op">></span></code> needs to be formattable when <code class="sourceCode cpp">T</code> is formattable. <code class="sourceCode cpp">pair<span class="op"><</span>T, U<span class="op">></span></code> needs to be formattable when <code class="sourceCode cpp">T</code> and <code class="sourceCode cpp">U</code> are formattable. In order to do this, we need to provide a proper <code class="sourceCode cpp"><span class="kw">concept</span></code> version of the formatter requirements that we already have.</p>
<p>This paper suggests the following:</p>
<blockquote>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">class</span> T, <span class="kw">class</span> charT<span class="op">></span></span>
<span id="cb14-2"><a href="#cb14-2"></a><span class="kw">concept</span> formattable <span class="op">=</span></span>
<span id="cb14-3"><a href="#cb14-3"></a> semiregular<span class="op"><</span>formatter<span class="op"><</span>remove_cvref_t<span class="op"><</span>T<span class="op">></span>, charT<span class="op">>></span> <span class="op">&&</span></span>
<span id="cb14-4"><a href="#cb14-4"></a> <span class="kw">requires</span> <span class="op">(</span>formatter<span class="op"><</span>remove_cvref_t<span class="op"><</span>T<span class="op">></span>, charT<span class="op">></span> f,</span>
<span id="cb14-5"><a href="#cb14-5"></a> <span class="kw">const</span> formatter<span class="op"><</span>remove_cvref_t<span class="op"><</span>T<span class="op">></span>, charT<span class="op">></span> cf,</span>
<span id="cb14-6"><a href="#cb14-6"></a> T t,</span>
<span id="cb14-7"><a href="#cb14-7"></a> basic_format_context<span class="op"><</span><em>fmt-iter-for</em><span class="op"><</span>charT<span class="op">></span>, charT<span class="op">></span> fc,</span>
<span id="cb14-8"><a href="#cb14-8"></a> basic_format_parse_context<span class="op"><</span>charT<span class="op">></span> pc<span class="op">)</span> <span class="op">{</span></span>
<span id="cb14-9"><a href="#cb14-9"></a> <span class="op">{</span> f<span class="op">.</span>parse<span class="op">(</span>pc<span class="op">)</span> <span class="op">}</span> <span class="op">-></span> same_as<span class="op"><</span>basic_format_parse_context<span class="op"><</span>charT<span class="op">>::</span>iterator<span class="op">></span>;</span>
<span id="cb14-10"><a href="#cb14-10"></a> <span class="op">{</span> cf<span class="op">.</span>format<span class="op">(</span>t, fc<span class="op">)</span> <span class="op">}</span> <span class="op">-></span> same_as<span class="op"><</span><em>fmt-iter-for</em><span class="op"><</span>charT<span class="op">>></span>;</span>
<span id="cb14-11"><a href="#cb14-11"></a> <span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>The broad shape of this concept is just taking the Formatter requirements and turning them into code. There are a few important things to note though:</p>
<ul>
<li>We don’t specify what the iterator type is of <code class="sourceCode cpp">format_context</code> or <code class="sourceCode cpp">wformat_context</code>, the expectation is that formatters accept any iterator. As such, it is unspecified in the concept <em>which</em> iterator will be checked - simply that it is <em>some</em> <code class="sourceCode cpp">output_iterator<span class="op"><</span>charT <span class="kw">const</span><span class="op">&></span></code>. Implementations could use <code class="sourceCode cpp">format_context<span class="op">::</span>iterator</code> and <code class="sourceCode cpp">wformat_context<span class="op">::</span>iterator</code>, or they could have a bespoke minimal iterator dedicated for concept checking.</li>
<li><code class="sourceCode cpp">cf<span class="op">.</span>format<span class="op">(</span>t, fc<span class="op">)</span></code> is called on a <code class="sourceCode cpp"><span class="kw">const</span></code> <code class="sourceCode cpp">formatter</code> (see <span class="citation" data-cites="LWG3636">[<a href="#ref-LWG3636" role="doc-biblioref">LWG3636</a>]</span>)</li>
<li><code class="sourceCode cpp">cf<span class="op">.</span>format<span class="op">(</span>t, fc<span class="op">)</span></code> is called specifically on <code class="sourceCode cpp">T</code>, not a <code class="sourceCode cpp"><span class="kw">const</span> T</code>. Even if the typical formatter specialization will take its object as <code class="sourceCode cpp"><span class="kw">const</span> T<span class="op">&</span></code>. This is to handle cases like ranges that are not <code class="sourceCode cpp"><span class="kw">const</span></code>-iterable.</li>
<li><code class="sourceCode cpp">formattable<span class="op"><</span>T, <span class="dt">char</span><span class="op">></span></code> and <code class="sourceCode cpp">formattable<span class="op"><</span>T <span class="kw">const</span>, <span class="dt">char</span><span class="op">></span></code> could be different, which is important in order to probably know when a range or a <code class="sourceCode cpp">tuple</code> can be <code class="sourceCode cpp">formattable</code>.</li>
</ul>
<h2 data-number="3.3" id="what-representation"><span class="header-section-number">3.3</span> What representation?<a href="#what-representation" class="self-link"></a></h2>
<p>There are several questions to ask about what the representation should be for printing. I’ll go through each kind in turn.</p>
<h3 data-number="3.3.1" id="vector-and-other-ranges"><span class="header-section-number">3.3.1</span> <code class="sourceCode cpp">vector</code> (and other ranges)<a href="#vector-and-other-ranges" class="self-link"></a></h3>
<p>Should <code class="sourceCode cpp">std<span class="op">::</span>vector<span class="op"><</span><span class="dt">int</span><span class="op">>{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span></code> be printed as <code class="sourceCode cpp"><span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span></code> or <code class="sourceCode cpp"><span class="op">[</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">]</span></code>? At the time of <span class="citation" data-cites="P2286R1">[<a href="#ref-P2286R1" role="doc-biblioref">P2286R1</a>]</span>, <code class="sourceCode cpp">fmt</code> used <code class="sourceCode cpp"><span class="op">{}</span></code>s but changed to use <code class="sourceCode cpp"><span class="op">[]</span></code>s for consistency with Python (<a href="https://github.com/fmtlib/fmt/commit/400b953fbb420ff1e47565303c64223445a51955">400b953f</a>).</p>
<p>Even though in C++ we initialize <code class="sourceCode cpp">vector</code>s (and, generally, other containers as well) with <code class="sourceCode cpp"><span class="op">{}</span></code>s while Python’s uses <code class="sourceCode cpp"><span class="op">[</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">]</span></code> (and likewise Rust has <code class="sourceCode cpp">vec<span class="op">![</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">]</span></code>), <code class="sourceCode cpp"><span class="op">[]</span></code> is typical representationally so seems like the clear best choice here.</p>
<h3 data-number="3.3.2" id="pair-and-tuple"><span class="header-section-number">3.3.2</span> <code class="sourceCode cpp">pair</code> and <code class="sourceCode cpp">tuple</code><a href="#pair-and-tuple" class="self-link"></a></h3>
<p>Should <code class="sourceCode cpp">std<span class="op">::</span>pair<span class="op"><</span><span class="dt">int</span>, <span class="dt">int</span><span class="op">>{</span><span class="dv">4</span>, <span class="dv">5</span><span class="op">}</span></code> be printed as <code class="sourceCode cpp"><span class="op">{</span><span class="dv">4</span>, <span class="dv">5</span><span class="op">}</span></code> or <code class="sourceCode cpp"><span class="op">(</span><span class="dv">4</span>, <span class="dv">5</span><span class="op">)</span></code>? Here, either syntax can claim to be the syntax used to initialize the <code class="sourceCode cpp">pair</code>/<code class="sourceCode cpp">tuple</code>. <code class="sourceCode cpp">fmt</code> has always printed these types with <code class="sourceCode cpp"><span class="op">()</span></code>s, and this is also how Python and Rust print such types. As with using <code class="sourceCode cpp"><span class="op">[]</span></code> for ranges, <code class="sourceCode cpp"><span class="op">()</span></code> seems like the common representation for tuples and so seems like the clear best choice.</p>
<h3 data-number="3.3.3" id="map-and-set-and-other-associative-containers"><span class="header-section-number">3.3.3</span> <code class="sourceCode cpp">map</code> and <code class="sourceCode cpp">set</code> (and other associative containers)<a href="#map-and-set-and-other-associative-containers" class="self-link"></a></h3>
<p>Should <code class="sourceCode cpp">std<span class="op">::</span>map<span class="op"><</span><span class="dt">int</span>, <span class="dt">int</span><span class="op">>{</span>{<span class="dv">1</span>, <span class="dv">2</span><span class="op">}</span>, <span class="op">{</span><span class="dv">3</span>, <span class="dv">4</span><span class="op">}}</span></code> be printed as <code class="sourceCode cpp"><span class="op">[(</span><span class="dv">1</span>, <span class="dv">2</span><span class="op">)</span>, <span class="op">(</span><span class="dv">3</span>, <span class="dv">4</span><span class="op">)]</span></code> (as follows directly from the two previous choices) or as <code class="sourceCode cpp"><span class="op">{</span><span class="dv">1</span><span class="op">:</span> <span class="dv">2</span>, <span class="dv">3</span><span class="op">:</span> <span class="dv">4</span><span class="op">}</span></code> (which makes the <em>association</em> clearer in the printing)? Both Python and Rust print their associating containers this latter way.</p>
<p>The same question holds for sets as well as maps, it’s just a question for whether <code class="sourceCode cpp">std<span class="op">::</span>set<span class="op"><</span><span class="dt">int</span><span class="op">>{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span></code> prints as <code class="sourceCode cpp"><span class="op">[</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">]</span></code> (i.e. as any other range of <code class="sourceCode cpp"><span class="dt">int</span></code>) or <code class="sourceCode cpp"><span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span></code>?</p>
<p>If we print <code class="sourceCode cpp">map</code>s as any other range of pairs, there’s nothing left to do. If we print <code class="sourceCode cpp">map</code>s as associations, then we additionally have to answer the question of how user-defined associative containers can get printed in the same way. This paper proposes printing the standard library maps as <code class="sourceCode cpp"><span class="op">{</span><span class="dv">1</span><span class="op">:</span> <span class="dv">2</span>, <span class="dv">3</span>, <span class="dv">4</span><span class="op">}</span></code> and the standard library sets as <code class="sourceCode cpp"><span class="op">{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span></code>.</p>
<h3 data-number="3.3.4" id="char-and-string-and-other-string-like-types-in-ranges-or-tuples"><span class="header-section-number">3.3.4</span> <code class="sourceCode cpp"><span class="dt">char</span></code> and <code class="sourceCode cpp">string</code> (and other string-like types) in ranges or tuples<a href="#char-and-string-and-other-string-like-types-in-ranges-or-tuples" class="self-link"></a></h3>
<p>Should <code class="sourceCode cpp">pair<span class="op"><</span><span class="dt">char</span>, string<span class="op">>(</span><span class="ch">'x'</span>, <span class="st">"hello"</span><span class="op">)</span></code> print as <code class="x">(x, hello)</code> or <code class="x">('x', "hello")</code>? Should <code class="sourceCode cpp">pair<span class="op"><</span><span class="dt">char</span>, string<span class="op">>(</span><span class="ch">'y'</span>, <span class="st">"with</span><span class="sc">\n\"</span><span class="st">quotes</span><span class="sc">\"</span><span class="st">"</span><span class="op">)</span></code> print as:</p>
<blockquote>
<div class="sourceCode" id="cb15"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb15-1"><a href="#cb15-1"></a>(y, with</span>
<span id="cb15-2"><a href="#cb15-2"></a>"quotes")</span></code></pre></div>
</blockquote>
<p>or</p>
<blockquote>
<div class="sourceCode" id="cb16"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb16-1"><a href="#cb16-1"></a>('y', "with\n\"quotes\"")</span></code></pre></div>
</blockquote>
<p>While <code class="sourceCode cpp"><span class="dt">char</span></code> and <code class="sourceCode cpp">string</code> are typically printed unquoted, it is quite common to print them quoted when contained in tuples and ranges. This makes it obvious what the actual elements of the range and tuple are even when the string/char contains characters like comma or space. Python, Rust, and <code class="sourceCode cpp">fmt</code> all do this. Rust escapes internal strings, so prints as <code class="sourceCode cpp"><span class="op">(</span><span class="ch">'y'</span>, <span class="st">"with</span><span class="sc">\n\"</span><span class="st">quotes</span><span class="sc">\"</span><span class="st">"</span><span class="op">)</span></code> (the Rust implementation of <code class="sourceCode cpp">Debug</code> for <code class="sourceCode cpp">str</code> can be found <a href="https://doc.rust-lang.org/src/core/fmt/mod.rs.html#2129-2151">here</a> which is implemented in terms of <a href="https://doc.rust-lang.org/src/core/char/methods.rs.html#417-432"><code class="sourceCode cpp">escape_debug_ext</code></a>). Following discussion of this paper and this design, Victor Zverovich implemented in this <code class="sourceCode cpp">fmt</code> as well.</p>
<p>Escaping is the most desirable default behavior, and the specific escaping behavior is described <a href="#escaping-behavior">here</a>.</p>
<p>Also, <code class="sourceCode cpp">std<span class="op">::</span>string</code> isn’t the only string-like type: if we decide to print strings quoted, how do users opt in to this behavior for their own string-like types? And <code class="sourceCode cpp"><span class="dt">char</span></code> and <code class="sourceCode cpp">string</code> aren’t the only types that may desire to have some kind of <em>debug</em> format and some kind of regular format, how to differentiate those?</p>
<p>Moreover, it’s all well and good to have the default formatting option for a range or tuple of strings to be printing those strings escaped. But what if users want to print a range of strings <em>unescaped</em>? I’ll get back to this.</p>
<h3 data-number="3.3.5" id="filesystempath"><span class="header-section-number">3.3.5</span> <code class="sourceCode cpp">filesystem<span class="op">::</span>path</code><a href="#filesystempath" class="self-link"></a></h3>
<p>We have a paper, <span class="citation" data-cites="P1636R2">[<a href="#ref-P1636R2" role="doc-biblioref">P1636R2</a>]</span>, that proposes <code class="sourceCode cpp">formatter</code> specializations for a different subset of library types: <code class="sourceCode cpp">basic_streambuf</code>, <code class="sourceCode cpp">bitset</code>, <code class="sourceCode cpp">complex</code>, <code class="sourceCode cpp">error_code</code>, <code class="sourceCode cpp">filesystem<span class="op">::</span>path</code>, <code class="sourceCode cpp">shared_ptr</code>, <code class="sourceCode cpp">sub_match</code>, <code class="sourceCode cpp">thread<span class="op">::</span>id</code>, and <code class="sourceCode cpp">unique_ptr</code>. Most of those are neither ranges nor tuples, so that paper doesn’t overlap with this one.</p>
<p>Except for one: <code class="sourceCode cpp">filesystem<span class="op">::</span>path</code>.</p>
<p>During the <a href="https://github.com/sg16-unicode/sg16-meetings#september-22nd-2021">SG16 discussion of P1636</a>, they took a poll that:</p>
<blockquote>
<p>Poll 1: Recommend removing the filesystem::path formatter from P1636 “Formatters for library types”, and specifically disabling filesystem::path formatting in P2286 “Formatting ranges”, pending a proposal with specific design for how to format paths properly.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>SF</strong>
</div></th>
<th><div style="text-align:center">
<strong>F</strong>
</div></th>
<th><div style="text-align:center">
<strong>A</strong>
</div></th>
<th><div style="text-align:center">
<strong>N</strong>
</div></th>
<th><div style="text-align:center">
<strong>SA</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>5</td>
<td>5</td>
<td>1</td>
<td>0</td>
<td>0</td>
</tr>
</tbody>
</table>
</blockquote>
<p><code class="sourceCode cpp">filesystem<span class="op">::</span>path</code> is kind of an interesting range, since it’s a range of <code class="sourceCode cpp">path</code>. As such, checking to see if it would be formattable as this paper currently does would lead to constraint recursion:</p>
<blockquote>
<div class="sourceCode" id="cb17"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb17-1"><a href="#cb17-1"></a><span class="kw">template</span> <span class="op"><</span>input_range R<span class="op">></span></span>
<span id="cb17-2"><a href="#cb17-2"></a> <span class="kw">requires</span> formattable<span class="op"><</span>range_reference_t<span class="op"><</span>R<span class="op">>></span></span>
<span id="cb17-3"><a href="#cb17-3"></a><span class="kw">struct</span> formatter<span class="op"><</span>R<span class="op">></span></span>
<span id="cb17-4"><a href="#cb17-4"></a> <span class="op">:</span> range_formatter<span class="op"><</span>range_reference_t<span class="op"><</span>R<span class="op">>></span></span>
<span id="cb17-5"><a href="#cb17-5"></a><span class="op">{</span> <span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>For <code class="sourceCode cpp">R<span class="op">=</span>filesystem<span class="op">::</span>path</code>, <code class="sourceCode cpp">range_reference_t<span class="op"><</span>R<span class="op">></span></code> is also <code class="sourceCode cpp">filesystem<span class="op">::</span>path</code>. Which means that our constraint for <code class="sourceCode cpp">formatter<span class="op"><</span>fs<span class="op">::</span>path<span class="op">></span></code> requires <code class="sourceCode cpp">formattable<span class="op"><</span>fs<span class="op">::</span>path<span class="op">></span></code> Looking at the <a href="#detecting-whether-a-type-is-formattable">suggested concept</a>, the first check we will do is to verify that <code class="sourceCode cpp">formatter<span class="op"><</span>fs<span class="op">::</span>path<span class="op">></span></code> is <code class="sourceCode cpp">semiregular</code>. But we’re currently in the process of instantiating <code class="sourceCode cpp">formatter<span class="op"><</span>fs<span class="op">::</span>path<span class="op">></span></code>, it is still incomplete. Hard error.</p>
<p>In order to handle this case properly, we could do what SG16 suggested:</p>
<blockquote>
<div class="sourceCode" id="cb18"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb18-1"><a href="#cb18-1"></a><span class="kw">template</span> <span class="op"><></span></span>
<span id="cb18-2"><a href="#cb18-2"></a><span class="kw">struct</span> formatter<span class="op"><</span>filesystem<span class="op">::</span>path<span class="op">></span>;</span></code></pre></div>
</blockquote>
<p>But this only handles <code class="sourceCode cpp">std<span class="op">::</span>filesystem<span class="op">::</span>path</code> and would not handle other ranges-of-self (the obvious example here is <code class="sourceCode cpp">boost<span class="op">::</span>filesystem<span class="op">::</span>path</code>). So instead, this paper proposes that we first reject ranges-of-self:</p>
<blockquote>
<div class="sourceCode" id="cb19"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb19-1"><a href="#cb19-1"></a><span class="kw">template</span> <span class="op"><</span>input_range R<span class="op">></span></span>
<span id="cb19-2"><a href="#cb19-2"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">not</span> same_as<span class="op"><</span>remove_cvref_t<span class="op"><</span>range_reference_t<span class="op"><</span>R<span class="op">>></span>, R<span class="op">>)</span></span>
<span id="cb19-3"><a href="#cb19-3"></a> <span class="kw">and</span> formattable<span class="op"><</span>range_reference_t<span class="op"><</span>R<span class="op">>></span></span>
<span id="cb19-4"><a href="#cb19-4"></a><span class="kw">struct</span> formatter<span class="op"><</span>R<span class="op">></span></span>
<span id="cb19-5"><a href="#cb19-5"></a> <span class="op">:</span> range_formatter<span class="op"><</span>range_reference_t<span class="op"><</span>R<span class="op">>></span></span>
<span id="cb19-6"><a href="#cb19-6"></a><span class="op">{</span> <span class="op">}</span>;</span></code></pre></div>
</blockquote>
<h3 data-number="3.3.6" id="format-specifiers"><span class="header-section-number">3.3.6</span> Format Specifiers<a href="#format-specifiers" class="self-link"></a></h3>
<p>One of (but hardly the only) the great selling points of <code class="sourceCode cpp">format</code> over iostreams is the ability to use specifiers. For instance, from the <code class="sourceCode cpp">fmt</code> documentation:</p>
<blockquote>
<div class="sourceCode" id="cb20"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb20-1"><a href="#cb20-1"></a>fmt<span class="op">::</span>format<span class="op">(</span><span class="st">"{:<30}"</span>, <span class="st">"left aligned"</span><span class="op">)</span>;</span>
<span id="cb20-2"><a href="#cb20-2"></a><span class="co">// Result: "left aligned "</span></span>
<span id="cb20-3"><a href="#cb20-3"></a>fmt<span class="op">::</span>format<span class="op">(</span><span class="st">"{:>30}"</span>, <span class="st">"right aligned"</span><span class="op">)</span>;</span>
<span id="cb20-4"><a href="#cb20-4"></a><span class="co">// Result: " right aligned"</span></span>
<span id="cb20-5"><a href="#cb20-5"></a>fmt<span class="op">::</span>format<span class="op">(</span><span class="st">"{:^30}"</span>, <span class="st">"centered"</span><span class="op">)</span>;</span>
<span id="cb20-6"><a href="#cb20-6"></a><span class="co">// Result: " centered "</span></span>
<span id="cb20-7"><a href="#cb20-7"></a>fmt<span class="op">::</span>format<span class="op">(</span><span class="st">"{:*^30}"</span>, <span class="st">"centered"</span><span class="op">)</span>; <span class="co">// use '*' as a fill char</span></span>
<span id="cb20-8"><a href="#cb20-8"></a><span class="co">// Result: "***********centered***********"</span></span></code></pre></div>
</blockquote>
<p>Earlier revisions of this paper suggested that formatting ranges and tuples would accept no format specifiers, but there indeed are quite a few things we may want to do here (as by Tomasz Kamiński and Peter Dimov):</p>
<ul>
<li>Formatting a range of pairs as a map (the <code class="sourceCode cpp">key<span class="op">:</span> value</code> syntax rather than the <code class="sourceCode cpp"><span class="op">(</span>key, value<span class="op">)</span></code> one)</li>
<li>Formatting a range of chars as a string (i.e. to print <code class="x">hello</code> or <code class="x">"hello"</code> rather than <code class="x">['h', 'e', 'l', 'l', 'o']</code>)</li>
</ul>
<p>But these are just providing a specifier for how we format the range itself. How about how we format the elements of the range? Can I conveniently format a range of integers, printing their values as hex? Or as characters? Or print a range of chrono time points in whatever format I want? That’s fairly powerful.</p>
<p>The problem is how do we actually <em>do that</em>. After a lengthy discussion with Peter Dimov, Tim Song, and Victor Zverovich, this is what we came up with. I’ll start with a table of examples and follow up with a more detailed explanation.</p>
<p>Instead of writing a bunch of examples like <code class="sourceCode cpp">print<span class="op">(</span><span class="st">"{:?}</span><span class="sc">\n</span><span class="st">"</span>, v<span class="op">)</span></code>, I’m just displaying the format string in one column (the <code class="sourceCode cpp"><span class="st">"{:?}"</span></code> here) and the argument in another (the <code class="sourceCode cpp">v</code>):</p>
<table>
<colgroup>
<col style="width: 14%"></col>
<col style="width: 42%"></col>
<col style="width: 42%"></col>
</colgroup>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Format String</strong>
</div></th>
<th><div style="text-align:center">
<strong>Contents</strong>
</div></th>
<th><div style="text-align:center">
<strong>Formatted Output</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><code class="x">{:}</code></td>
<td><code class="sourceCode cpp"><span class="dv">42</span></code></td>
<td><code class="x">42</code></td>
</tr>
<tr class="even">
<td><code class="x">{:#x}</code></td>
<td><code class="sourceCode cpp"><span class="dv">42</span></code></td>
<td><code class="x">0x2a</code></td>
</tr>
<tr class="odd">
<td><code class="x">{}</code></td>
<td><code class="sourceCode cpp"><span class="st">"h</span><span class="sc">\t</span><span class="st">llo"</span><span class="bu">s</span></code></td>
<td><code class="x">h llo</code></td>
</tr>
<tr class="even">
<td><code class="x">{:?}</code></td>
<td><code class="sourceCode cpp"><span class="st">"h</span><span class="sc">\t</span><span class="st">llo"</span><span class="bu">s</span></code></td>
<td><code class="x">"h\tllo"</code></td>
</tr>
<tr class="odd">
<td><code class="x">{}</code></td>
<td><code class="sourceCode cpp">vector<span class="op">{</span><span class="st">"h</span><span class="sc">\t</span><span class="st">llo"</span><span class="bu">s</span>, <span class="st">"world"</span><span class="bu">s</span><span class="op">}</span></code></td>
<td><code class="x">["h\tllo", "world"]</code></td>
</tr>
<tr class="even">
<td><code class="x">{:}</code></td>
<td><code class="sourceCode cpp">vector<span class="op">{</span><span class="st">"h</span><span class="sc">\t</span><span class="st">llo"</span><span class="bu">s</span>, <span class="st">"world"</span><span class="bu">s</span><span class="op">}</span></code></td>
<td><code class="x">["h\tllo", "world"]</code></td>
</tr>
<tr class="odd">
<td><code class="x">{::}</code></td>
<td><code class="sourceCode cpp">vector<span class="op">{</span><span class="st">"h</span><span class="sc">\t</span><span class="st">llo"</span><span class="bu">s</span>, <span class="st">"world"</span><span class="bu">s</span><span class="op">}</span></code></td>
<td><code class="x">[h llo, world]</code></td>
</tr>
<tr class="even">
<td><code class="x">{:*^14}</code></td>
<td><code class="sourceCode cpp">vector<span class="op">{</span><span class="st">"he"</span><span class="bu">s</span>, <span class="st">"wo"</span><span class="bu">s</span><span class="op">}</span></code></td>
<td><code class="x">*["he", "wo"]*</code></td>
</tr>
<tr class="odd">
<td><code class="x">{::*^14}</code></td>