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<!doctype html public "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<head>
<title>
std::format() fill character allowances;
proposed resolution for LWG issues 3576 and 3639
</title>
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<body style="max-width: 8.5in">
<table id="header">
<tr>
<th>Document Number:</th>
<td>P2572R0</td>
</tr>
<tr>
<th>Date:</th>
<td>2022-06-11</td>
</tr>
<tr>
<th>Audience:</th>
<td>SG16, LEWG</td>
</tr>
<tr>
<th>Reply-to:</th>
<td>Tom Honermann <tom@honermann.net></td>
</tr>
</table>
<h1 style="margin-bottom: 0.0em"><tt>std::format()</tt> fill character allowances</h1>
<h2 style="margin-top: 0.0em">(Proposed resolution for LWG issues 3576 and 3639)</h2>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#design">Design considerations</a>
<ul>
<li><a href="#design-char-restrict">Character encoding restrictions</li>
<li><a href="#design-width-restrict">Estimated display width restrictions</li>
</ul>
</li>
<li><a href="#existing-practice">Existing practice</a></li>
<li><a href="#proposal">Proposal</a></li>
<li><a href="#future">Future considerations and ABI</a></li>
<li><a href="#implementation-exp">Implementation experience</a></li>
<li><a href="#impact">Implementation impact</a></li>
<li><a href="#ack">Acknowledgements</a></li>
<li><a href="#references">References</a></li>
<li><a href="#wording">Wording</a></li>
</ul>
<h1 id="introduction">Introduction</h1>
<p>
Presented is a proposed resolution for the following LWG issues concerning
the specification of fill characters in <tt>std::format()</tt>.
<ul>
<li><a href="https://wg21.link/lwg3576">LWG issue 3576: Clarifying fill character in std::format</a></li>
<li><a href="https://wg21.link/lwg3639">LWG issue 3639: Handling of fill character width is underspecified in std::format</a></li>
</ul>
</p>
<p>
This proposal follows prior discussion as recorded in the:
<ul>
<li><a href="https://github.com/sg16-unicode/sg16-meetings/blob/master/README-2021.md#august-25th-2021">SG16 meeting summary for 2021-08-25</a>.</li>
<li><a href="https://lists.isocpp.org/sg16/2021/11/2851.php">SG16 mailing list archives beginning 2021-11-27 with subject "Agenda for the 2021-12-01 SG16 telecon"</a>.</li>
<li><a href="https://lists.isocpp.org/sg16/2021/11/2867.php">SG16 mailing list archives beginning 2021-12-01 with subject "Proposed resolution for LWG3639: Handling of fill character width is underspecified in std::format"</a>.</li>
<li><a href="https://github.com/sg16-unicode/sg16-meetings/blob/master/README-2021.md#december-1st-2021">SG16 meeting summary for 2021-12-01</a>.</li>
<li><a href="https://lists.isocpp.org/sg16/2021/12/2887.php">SG16 mailing list archives beginning 2021-12-02 with subject "Use of U+3000 IDEOGRAPHIC SPACE is common practice"</a>.</li>
<li><a href="https://lists.isocpp.org/sg16/2021/12/2889.php">SG16 mailing list archives beginning 2021-12-02 with subject "More Ruminations about fill characters and alignement (LWG3639)"</a>.</li>
</ul>
</p>
<p>
The current wording in
<a href="http://eel.is/c++draft/format.string.std#1">[format.string.std]p1</a>
restricts fill characters to
"any character other than <tt>{</tt> or <tt>}</tt>".
Depending on how "character" is interpreted, this may permit
characters with a negative display width,
characters with no display width,
characters with a display width greater than one,
chraracters with a varying display width,
characters with an actual display width that differs from their estimated width,
combining characters (with or without a non-combining lead character),
decomposed characters,
characters with right-to-left directionality,
control characters,
formatting characters, and
emoji.
The following table presents some examples of such characters.
<table border="1">
<tr>
<th>Glyph</th>
<th>Estimated width</th>
<th>Code point(s)</th>
<th>Character name</th>
</tr>
<tr>
<td><tt>><</tt></td>
<td>1</td>
<td>U+0007</td>
<td>BELL</td>
</tr>
<tr>
<td><tt>><</tt></td>
<td>1</td>
<td>U+0008</td>
<td>BACKSPACE</td>
</tr>
<tr>
<td><tt>><</tt></td>
<td>1</td>
<td>U+007F</td>
<td>DELETE</td>
</tr>
<tr>
<td><tt>>	<</tt></td>
<td>1</td>
<td>U+0009</td>
<td>CHARACTER TABULATION</td>
</tr>
<tr>
<td><tt>>​<</tt></td>
<td>1</td>
<td>U+200B</td>
<td>ZERO WIDTH SPACE</td>
</tr>
<tr>
<td><tt>>́<</tt></td>
<td>1</td>
<td>U+0301</td>
<td>COMBINING ACCUTE ACCENT</td>
</tr>
<tr>
<td><tt>>é<</tt></td>
<td>1</td>
<td>U+00E9</td>
<td>LATIN SMALL LETTER E WITH ACUTE</td>
</tr>
<tr>
<td><tt>>é<</tt></td>
<td>1</td>
<td>U+0065<br/>U+0301</td>
<td>LATIN SMALL LETTER E<br/>COMBINING ACCUTE ACCENT</td>
</tr>
<tr>
<td><tt>>è́̂̃̄<</tt></td>
<td>1</td>
<td>U+0065<br/>U+0300<br/>U+0301<br/>U+0302<br/>U+0303<br/>U+0304</td>
<td>LATIN SMALL LETTER E<br/>COMBINING GRAVE ACCENT<br/>COMBINING ACUTE ACCENT<br/>COMBINING CIRCUMFLEX ACCENT<br/>COMBINING TILDE<br/>COMBINING MACRON</td>
</tr>
<tr>
<td><tt>>ｅ<</tt></td>
<td>2</td>
<td>U+FF45</td>
<td>FULLWIDTH LATIN SMALL LETTER E</td>
</tr>
<tr>
<td><tt>>ェ<</tt></td>
<td>2</td>
<td>U+30A7</td>
<td>KATAKANA LETTER SMALL E</td>
</tr>
<tr>
<td><tt>>ｪ<</tt></td>
<td>1</td>
<td>U+FF6A</td>
<td>HALFWIDTH KATAKANA LETTER SMALL E</td>
</tr>
<tr>
<td><tt>>　<</tt></td>
<td>2</td>
<td>U+3000</td>
<td>IDEOGRAPHIC SPACE</td>
</tr>
<tr>
<td><tt>>ת<</tt></td>
<td>1</td>
<td>U+05EA</td>
<td>HEBREW LETTER TAV (a right-to-left character)</td>
</tr>
<tr>
<td><tt>>🤡<</tt></td>
<td>2</td>
<td>U+1F921</td>
<td>CLOWN FACE</td>
</tr>
<tr>
<td><tt>>﷽<</tt></td>
<td>1</td>
<td>U+FDFD</td>
<td><spane style="white-space:nowrap;">ARABIC LIGATURE BISMILLAH AR-RAHMAN AR-RAHEEM</span></td>
</tr>
</table>
[ <em>Note:</em>
The glyphs are presented in monospace font and may render inconsistently across
browsers and operating systems.
The glyphs are displayed between '>' and '<' characters to make it easier
to see their presentation width.
The estimated width value corresponds to
<a href="http://eel.is/c++draft/format.string.std#11">[format.string.std]p11</a>;
that paragraph associates an estimated width of either one or two with all
characters.
In each case, the code point sequence constitutes a single extended grapheme
cluster.
— <em>end note</em> ]
</p>
<p>
It is likely that the displayed character differs from the estimated width for
at least some cases above; most likely the last case.
Unfortunately, there is no specification currently available that governs
character display width; actual width may vary based on font selection.
</p>
<p>
Use of a fill character with a display width other than one potentially
prevents a <tt>std::format()</tt> implementation from properly aligning
fields.
Consider a format specification for a field of width four and a field argument
with an estimated field width of one.
The implementation is expected to insert fill characters to consume an estimated
field width of three, but that is not possible if the fill character has an
estimated field width of two.
Portable behavior requires that the standard clarify the intended behavior
for such characters.
</p>
<p>
A <tt>std::format()</tt> implementation must store or reference a fill character
in some way.
Fill character allowances may impose dynamic memory management requirements or
increase the complexity of parsing standard format specifiers depending on
implementation choices.
Implementation choices may also cause fill character restrictions to be
reflected in the ABI thus making it difficult to relax restrictions later.
Portable behavior requires that the standard specify whether fill characters are
restricted to those that are encoded as, for example, a single code unit, a
single UCS scalar value, a
<a href="https://unicode.org/reports/tr15/#Stream_Safe_Text_Format">stream-safe extended grapheme cluster</a>
<sup><a title="Unicode Standard Annex #15 - Unicode Normalization Forms"
href="#ref_uax15">[UAX#15]</a></sup>,
or an extended grapheme cluster of unbounded length.
</p>
<h1 id="design">Design considerations</h1>
<h2 id="design-char-restrict">Character encoding restrictions</h2>
<p>
Fill character allowances pose a performance and overhead tradeoff.
Consider the following four options for fill character support.
<ol>
<li>Allow any extended grapheme cluster (EGC).</li>
<li>Allow any
<a href="https://unicode.org/reports/tr15/#Stream_Safe_Text_Format">stream-safe EGC</a>
<sup><a title="Unicode Standard Annex #15 - Unicode Normalization Forms"
href="#ref_uax15">[UAX#15]</a></sup>.
<li>Allow any single UCS scalar value.</li>
<li>Allow any single UCS scalar value that is encoded using a single code
unit.</li>
</ol>
</p>
<p>
The first option (any EGC) would require implementations to support EGCs that
consist of an unbounded number of code points.
This option implies dynamic memory management and would require implementations
to identify EGC boundaries in the format string; a requirement that otherwise
does not exist at present (implementations are currently required to identify
EGC boundaries in formatted field arguments for the purpose of computing the
estimated width, but not in the format string itself).
</p>
<p>
The second option (any stream-safe EGC) would require implementations to support
EGCs that consist of up to 32 code points.
This option allows an implementation to trade off dynamic memory allocation in
favor of larger data structures, but still requires EGC boundary analysis of
format strings.
</p>
<p>
The third option (any single UCS scalar value) avoids dynamic memory
requirements and significant increases to sizes of data structures;
the fill character could be stored in a single <tt>char32_t</tt> object.
</p>
<p>
The fourth option (any single code unit) reduces fill character storage
requirements to a single code unit (<tt>char</tt> or <tt>wchar_t</tt>), but has
the unfortunate side effect of making the permissible set of fill characters
dependent on encoding. For example, U+00E9 (LATIN SMALL LETTER E WITH ACUTE)
would be rejected in a UTF-8 encoded format string, but would be accepted in a
UTF-16 encoded one. Similarly, U+1F921 (CLOWN FACE) would be rejected in a
UTF-16 encoded format string, but accepted in a UTF-32 encoded one.
</p>
<h2 id="design-width-restrict">Estimated display width restrictions</h2>
<p>
The following behaviors represent possible options for formatting fields when
the fill character has an estimated width other than one.
<ol>
<li>Use an estimated width of one for the fill character regardless of the
value specified in
<a href="http://eel.is/c++draft/format.string.std#11">[format.string.std]p11</a>.</li>
<li>Overfill<br/>
Insert fill characters until the estimated width of the formatted field
argument and the fill characters meets or exceeds the field width.</li>
<li>Underfill<br/>
Insert fill characters so long as the estimated width of the formatted
field argument and the fill characters does not exceed the field
width.</li>
<li>Pad with a different fill character<br/>
Insert an alternate fill character known to have an estimated width of
one when inserting the requested fill character would have caused the
field width to be exceeded.</li>
<li>Undefined, unspecified, or implementation-defined behavior<br/>
Impose no portable behavior.</li>
<li>Error unconditionally<br/>
Throw a <tt>format_error</tt> exception.</li>
<li>Error if the alignment to the field width is not possible<br/>
Throw a <tt>format_error</tt> exception if the remainder of the field
width after subtracting the estimated width of the formatted field
argument is not evenly divisible by the estimated width of the fill
character.</li>
</ol>
</p>
<p>
The following table illustrates the above options for
<tt>std::format(">{:🤡^4}<\n", 'X')</tt>.
Font selection will determine to what degree the results shown deviate from
the reference alignment.
<table border="1">
<tr>
<th>Behavioral choice</th>
<th>Result</th>
</tr>
<tr>
<td>(reference alignment)
<td><tt>>-X--<</tt></td>
</tr>
<tr>
<td>Use an estimated width of one</th>
<td><tt>>🤡X🤡🤡<</tt></td>
</tr>
<tr>
<td>Overfill</th>
<td><tt>>🤡X🤡<</tt></td>
</tr>
<tr>
<td>Underfill</th>
<td><tt>>X🤡<</tt></td>
</tr>
<tr>
<td>Pad with a different fill character (space)</th>
<td><tt>> X🤡<</tt></td>
</tr>
<tr>
<td>Undefined, unspecified, or implementation-defined behavior</th>
<td>???</td>
</tr>
<tr>
<td>Error (unconditionally or due to inability to align)</th>
<td>N/A</td>
</tr>
</table>
</p>
<h1 id="existing-practice">Existing practice</h1>
<p>
The following table illustrates existing behavior for several
<tt>std::format()</tt> implementations when the example characters from the
<a href="#introduction">introduction</a>
are used as the fill character with a directionally neutral field argument of
<tt>'#'</tt> (the directionality affects the behavior of the U+05EA example).
The first row illustrates a reference alignment.
<table border="`">
<tr>
<th>Code point(s)</th>
<th>Format string</th>
<th>Clang 15 trunk<br/>with libc++</th>
<th>Gcc 12.1<br/>with fmt 8.1.1</th>
<th>MSVC 19.31</th>
</tr>
<tr>
<td>U+002D HYPHEN-MINUS</td>
<td><tt>">{:-^4}<"</tt></td>
<td><tt>>-#--<</tt></td>
<td><tt>>-#--<</tt></td>
<td><tt>>-#--<</tt></td>
</tr>
<tr>
<td>U+0007 BELL</td>
<td><tt>">{:^4}<"</tt></td>
<td><tt>>#<</tt></td>
<td><tt>>#<</tt></td>
<td><tt>>#<</tt></td>
</tr>
<tr>
<td>U+0008 BACKSPACE</td>
<td><tt>">{:^4}<"</tt></td>
<td><tt>>#<</tt></td>
<td><tt>>#<</tt></td>
<td><tt>>#<</tt></td>
</tr>
<tr>
<td>U+007F DELETE</td>
<td><tt>">{:^4}<"</tt></td>
<td><tt>>#<</tt></td>
<td><tt>>#<</tt></td>
<td><tt>>#<</tt></td>
</tr>
<tr>
<td>U+0009 CHARACTER TABULATION</td>
<td><tt>">{:	^4}<"</tt></td>
<td><tt>><span style="white-space: pre">	#		</span><</tt></td>
<td><tt>><span style="white-space: pre">	#		</span><</tt></td>
<td><tt>><span style="white-space: pre">	#		</span><</tt></td>
</tr>
<tr>
<td>U+200B ZERO WIDTH SPACE</td>
<td><tt>">{:​^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>​#​​<</tt></td>
<td><tt>>​#​​<</tt></td>
</tr>
<tr>
<td>U+0301 COMBINING ACCUTE ACCENT</td>
<td><tt>">{:́^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>́#́́<</tt></td>
<td><tt>>́#́́<</tt></td>
</tr>
<tr>
<td>U+00E9 LATIN SMALL LETTER E WITH ACUTE</td>
<td><tt>">{:é^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>é#éé<</tt></td>
<td><tt>>é#éé<</tt></td>
</tr>
<tr>
<td>U+0065 LATIN SMALL LETTER E<br/>U+0301 COMBINING ACCUTE ACCENT</td>
<td><tt>">{:é^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><b>Error</b><sup>2</sup></td>
<td><b>Error</b><sup>3</sup></td>
</tr>
<tr>
<td>U+0065 LATIN SMALL LETTER E<br/>U+0300 COMBINING GRAVE ACCENT<br/>U+0301 COMBINING ACUTE ACCENT<br/>U+0302 COMBINING CIRCUMFLEX ACCENT<br/>U+0303 COMBINING TILDE<br/>U+0304 COMBINING MACRON</td>
<td><tt>">{:è́̂̃̄^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><b>Error</b><sup>2</sup></td>
<td><b>Error</b><sup>3</sup></td>
</tr>
<tr>
<td>U+FF45 FULLWIDTH LATIN SMALL LETTER E</td>
<td><tt>">{:ｅ^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>ｅ#ｅｅ<</tt></td>
<td><tt>>ｅ#ｅｅ<</tt></td>
</tr>
<tr>
<td>U+30A7 KATAKANA LETTER SMALL E</td>
<td><tt>">{:ェ^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>ェ#ェェ<</tt></td>
<td><tt>>ェ#ェェ<</tt></td>
</tr>
<tr>
<td>U+FF6A HALFWIDTH KATAKANA LETTER SMALL E</td>
<td><tt>">{:ｪ^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>ｪ#ｪｪ<</tt></td>
<td><tt>>ｪ#ｪｪ<</tt></td>
</tr>
<tr>
<td>U+3000 IDEOGRAPHIC SPACE</td>
<td><tt>">{:　^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>　#　　<</tt></td>
<td><tt>>　#　　<</tt></td>
</tr>
<tr>
<td>U+05EA HEBREW LETTER TAV</td>
<td><tt>">{:ת‎^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>ת#תת<‎<sup>4</sup></tt><br/>
<tt>>תXתת<‎<sup>4</sup></tt></td>
<td><tt>>ת#תת<‎<sup>4</sup></tt><br/>
<tt>>תXתת<‎<sup>4</sup></tt></td>
</tr>
<tr>
<td>U+1F921 CLOWN FACE</td>
<td><tt>">{:🤡^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>🤡#🤡🤡<</tt></td>
<td><tt>>🤡#🤡🤡<</tt></td>
</tr>
<tr>
<td><span style="white-space:nowrap;">U+FDFD ARABIC LIGATURE BISMILLAH AR-RAHMAN AR-RAHEEM</span></td>
<td><tt>">{:﷽^4}<"</tt></td>
<td><b>Error</b><sup>1</sup></td>
<td><tt>>﷽#﷽﷽<</tt></td>
<td><tt>>﷽#﷽﷽<</tt></td>
</tr>
</table>
<p>
1) Clang with libc++ restricts fill characters to characters that are encoded
as a single code unit; an exception is thrown that, if not caught, aborts
the process with the following error message.
<div style="margin-left:1em; background-color:lemonchiffon"><tt>
libc++abi: terminating with uncaught exception of type std::__1::format_error: The format-spec should consume the input or end with a '}'
</tt></div>
</p>
<p>
2) Gcc with fmt restricts fill characters to characters that are encoded as a
single code point. Compilation fails with the following error message.
<div style="margin-left:1em; background-color:lemonchiffon"><tt>
error: call to non-'constexpr' function 'void fmt::v8::detail::error_handler::on_error(const char*)'
</tt></div>
</p>
<p>
3) MSVC restricts fill characters to characters that are encoded as a single
code point. Compilation is successful, but program execution terminates
with an exit code of
3221226505 (0xC0000409: <tt>STATUS_STACK_BUFFER_OVERRUN</tt>).
The buffer overflow has been corrected for the next MSVC release and these
cases are now rejected with the following error message.
<div style="margin-left:1em; background-color:lemonchiffon"><tt>
error C7595: 'std::_Basic_format_string<char,char>::_Basic_format_string': call to immediate function is not a constant expression<br/>
.../msvc/14.32.31302/include\format(1378): note: failure was caused by evaluating a throw sub-expression
</tt></div>
</p>
<p>
4) Use of a fill character with right-to-left directionality potentially causes
the formatted field to be rendered right to left depending on the formatted
field argument.
Two examples are provided, one in which the directionally neutral character
<tt>'#'</tt> is used as the formatted field argument and one in which the
left-to-right character <tt>'X'</tt> is used.
U+200E LEFT-TO-RIGHT MARK characters have been inserted by the paper
author to negate the right-to-left effect on surrounding text.
In practice, the right-to-left directionality may affect how surrounding
text from the format string or other format fields are presented.
</p>
<p>
All surveyed implementations assume an estimated width of 1 for fill characters
regardless of the estimated width values specified in
<a href="http://eel.is/c++draft/format.string.std#11">[format.string.std]p11</a>.
</p>
<h1 id="proposal">Proposal</h1>
<p>
Standardize the behavior exhibited by gcc with fmt and by MSVC:
<ul>
<li>Restrict fill characters to a single UCS scalar value.<br/>
(This restriction can be lifted in the future if motivation arises for
support of EGCs that contain multiple UCS scalar values but may require
an ABI break depending on implementation details)</li>
<li>Always use an estimated width of one for fill characters.<br/>
(Ignore
<a href="http://eel.is/c++draft/format.string.std#11">[format.string.std]p11</a>
when determining how many fill characters to insert)</li>
<li>Add a note that alignment options have no effect if the estimated width
of the formatted field argument exceeds the field width.</li>
<li>Clarify wording to explicitly describe how fill characters are inserted
in order to achieve field alignment.</li>
</ul>
</p>
<h1 id="future">Future considerations and ABI</h1>
<p>
Programmers may find use cases where it is necessary for the number of inserted
fill characters to depend on the estimated width of the fill character.
Some of those use cases may warrant support in the standard.
If such motivation arises, there are at least two methods by which support
could be added.
<ol>
<li>The
<a href="http://eel.is/c++draft/format.string.std"><em>std-format-spec</em> format specification</a>
could be extended to allow an additional option to be specified to opt-in
to the desired behavior.</li>
<li>Specializations of <tt>std::formatter</tt> could be defined to provide
custom formatting on a per-type basis as is done for the chrono
library.</li>
</ol>
</p>
<p>
Motivation may arise in the future to permit the use of an EGC that consists of
multiple code points as a fill character.
Implementations that store a single <tt>char32_t</tt> or short sequence of code
units in their <tt>formatter</tt> class specializations
(<a href="http://eel.is/c++draft/format#formatter.spec">[format.formatter.spec]</a>)
may be unable to accommodate such a change without an ABI break.
Implementations are encouraged to instead store a view (an iterator pair, start
and end index, or start index and length) into the <em>std-format-spec</em>
(<a href="http://eel.is/c++draft/format#string.std-1">[format.string.std]p1</a>)
string so that code unit sequences of arbitrary length can be referenced.
However, since format strings are evaluated at compile-time, there is currently
no need for them to be persisted until run-time, so storing a view may impose
storage overhead.
</p>
<p>
It appears that the Microsoft implementation is currently susceptible to such
ABI breaks based on the implementation of their
<a href="https://github.com/microsoft/STL/blob/1a20fe1133d711a647bbb135d98743f91b7be323/stl/inc/format#L1327-L1340"><tt>_Basic_format_specs</tt> class template</a>.
Specializations of <tt>_Basic_format_specs</tt> form the base class of their
<a href="https://github.com/microsoft/STL/blob/1a20fe1133d711a647bbb135d98743f91b7be323/stl/inc/format#L1342-L1349"><tt>_Dynamic_format_specs</tt> class template</a>
for which a specialization is stored in their
<a href="https://github.com/microsoft/STL/blob/1a20fe1133d711a647bbb135d98743f91b7be323/stl/inc/format#L3263-L3297"><tt>_Formatter_base</tt> class template</a>
that forms the base class of their
<a href="https://github.com/microsoft/STL/blob/1a20fe1133d711a647bbb135d98743f91b7be323/stl/inc/format#L3299-L3347"><tt>std::formatter</tt> specializations</a>.
Microsoft is already shipping their implementation and is thus already locked
into their current ABI.
</p>
<p>
The author has not researched the ABI break susceptibility of other
implementations.
</p>
<h1 id="implementation-exp">Implementation experience</h1>
<p>
This proposal standardizes the behavior exhibited by both gcc with fmt and MSVC
and therefore reflects existing practice.
However the ABI mitigations described in the prior section are not known to have
been implemented.
</p>
<h1 id="impact">Implementation impact</h1>
<p>
Some implementations, libc++ for example, will require changes to allow any
single UCS scalar value to be specified as a fill character.
This may impose new encoding awareness requirements on format string parsers so
that fill characters encoded with more than one code unit are correctly decoded.
</p>
<h1 id="ack">Acknowledgements</h1>
<p>
Thank you to Victor Zverovich, Corentin Jabot, Peter Brett, and Mark de Wever
for their insights; their commentary shaped much of this proposal.
</p>
<h1 id="references">References</h1>
<table id="references">
<tr>
<td id="ref_n4910"><sup>[N4910]</sup></td>
<td>
"Working Draft, Standard for Programming Language C++", N4910, 2022.<br/>
<a href="https://wg21.link/n4910">
https://wg21.link/n4910</a></td>
</tr>
<tr>
<td id="ref_uax15"><sup>[UAX#15]</sup></td>
<td>
Ken Whistler,<br/>
"Unicode Standard Annex #15 - Unicode Normalization Forms",<br/>
Revision 51, Unicode 14.0.0, 2021.<br/>
<a href="https://www.unicode.org/reports/tr15/tr15-51.html">https://www.unicode.org/reports/tr15/tr15-51.html</a>
</td>
</tr>
</table>
<h1 id="wording">Wording</h1>
<p>These changes are relative to
<a title="Working Draft, Standard for Programming Language C++"
href="https://wg21.link/n4910">
N4910</a>
<sup><a title="Working Draft, Standard for Programming Language C++"
href="#ref_n4910">[N4910]</a></sup>.
</p>
<input type="checkbox" id="hideins">Hide inserted text</input><br/>
<input type="checkbox" id="hidedel">Hide deleted text</input>
<p>
Change in
<a href="http://eel.is/c++draft/format.string.std#2">22.14.2.2 [format.string.std] paragraph 2</a>:
<blockquote>
<ins>The <em>fill character</em> is the character denoted by the <em>fill</em>
option or, if the <em>fill</em> option is absent, the space
character.
For a format specification in a Unicode encoding, the fill character is a
single UCS scalar value.
<br/>
<br/>
</ins>
[<em>Note 2</em>:
<del>The <em>fill</em> character can be any character other than { or }.</del>
The presence of a
<del>fill character</del><ins><em>fill</em> option</ins>
is signaled by the character following it,
which must be one of the alignment options.
If the second character of <em>std-format-spec</em> is not a valid alignment
option, then it is assumed that
<del>both the fill character and the alignment option are</del><ins>the
<em>fill-and-align</em> option is</ins>
absent. — <em>end note</em>]
</blockquote>
</p>
<p>
Change in
<a href="http://eel.is/c++draft/format.string.std#3">22.14.2.2 [format.string.std] paragraph 3</a>:<br/>
<em>Drafting note:</em> The change of "specifier" to "option" for the
<em>align</em> grammar element is intended to improve consistency with
other grammar elements.
Inconsistencies remain however.
The terms currently used for each of the grammar elements are below.
LWG may wish to review further.
<ul>
<li><em>fill-and-align</em>: none (neither specifier nor option)</li>
<li><em>fill</em>: none (neither specifier nor option)</li>
<li><em>align</em>: specifier</li>
<li><em>sign</em>: option</li>
<li><em>#</em>: option</li>
<li><em>0</em>: none (neither specifier nor option).</li>
<li><em>width</em>: none (neither specifier nor option)</li>
<li><em>precision</em>: none (neither specifier nor option)</li>
<li><em>L</em>: option</li>
<li><em>type</em>: option</li>
</ul>
<blockquote>
The <em>align</em> <del>specifier</del><ins>option</ins> applies to all
argument types.
The meaning of the various alignment options is as specified in
<a href="http://eel.is/c++draft/format.string.std#tab:format.align">Table 64</a>.
<br/><br/>
[ <em>Example 1</em>:
<div style="margin-left: 1em;"><pre>
char c = 120;
string s0 = format("{:6}", 42); // value of s0 is " 42"
string s1 = format("{:6}", 'x'); // value of s1 is "x "
string s2 = format("{:*<6}", 'x'); // value of s2 is "x*****"
string s3 = format("{:*>6}", 'x'); // value of s3 is "*****x"
string s4 = format("{:*^6}", 'x'); // value of s4 is "**x***"
string s5 = format("{:6d}", c); // value of s5 is " 120"
string s6 = format("{:6}", true); // value of s6 is "true "
<ins>string s7 = format("{:🤡^6}", "x"); // value of s7 is "🤡🤡x🤡🤡🤡"</ins>
<ins>string s8 = format("{:*^6}", "🤡🤡🤡"); // value of s8 is "🤡🤡🤡"</ins>
<ins>string s9 = format("{:*>6}", "12345678"); // value of s9 is "12345678"</ins>
</pre></div>
— <em>end example</em> ]
<br/><br/>
[ <em>Note 3</em>:
<del>Unless a minimum field width is defined, the field width is determined by
the size of the content and the alignment option has no effect.</del><ins>If
the <em>width</em> option is absent, then the field width is the estimated
width of the formatted argument and the alignment option has no effect.
If the estimated width of the formatted argument matches or exceeds the field
width, then both the alignment and width options have no effect.
The width of any fill character is assumed to be 1.
The 🤡 (U+1F921 CLOWN FACE) emoji has an estimated width of 2.
The examples above that include that character illustrate the effect of the
estimated width when that character is used as a fill character as opposed to
when it is used as a formatting argument.</ins>
— <em>end note</em> ]
<br/><br/>
<table>
<tr>
<td>
<table style="width:100%">
<tr>
<td style="text-align:center">Table
<a href="http://eel.is/c++draft/format.string.std#tab:format.align">64</a>:
Meaning of align options</td>
<td>[<a href="http://eel.is/c++draft/tab:format.align">tab:format.align</a>]</td>
</tr>
</table>
</td>
</tr>
<tr>
<td>
<table style="width:100%; border:1px solid black">
<tr style="border:1px solid black">
<th style="border-bottom:1px solid black">Option</th>
<th style="border-bottom:1px solid black">Meaning</th>
</tr>
<tr>
<td style="border-bottom:1px solid black"><</td>
<td style="border-bottom:1px solid black">
Forces the field to be aligned to the start of the available
space<ins> by inserting <em>n</em> fill characters after the
formatted argument where <em>n</em> is the field width minus the
estimated width of the formatted argument or, if the subtraction
results in a negative value, 0</ins>.
This is the default for non-arithmetic non-pointer types,
<tt>charT</tt>, and <tt>bool</tt>, unless an integer presentation
type is specified.</td>
</tr>
<tr>
<td style="border-bottom:1px solid black">></td>
<td style="border-bottom:1px solid black">
Forces the field to be aligned to the end of the available
space<ins> by inserting <em>n</em> fill characters before the
formatted argument where <em>n</em> is the field width minus the
estimated width of the formatted argument or, if the subtraction
results in a negative value, 0</ins>.
This is the default for arithmetic types other than
<tt>charT</tt> and <tt>bool</tt>, pointer types, or when an integer
presentation type is specified.</td>
</tr>
<tr>
<td style="border-bottom:1px solid black">^</td>
<td style="border-bottom:1px solid black">
Forces the field to be centered within the available space by
inserting ⌊<em>n</em>/2⌋ <ins>fill</ins> characters before and
⌈<em>n</em>/2⌉ <ins>fill </ins>characters after the
<ins>formatted argument</ins><del>value</del>, where <em>n</em> is
<del>the total number of fill characters to insert</del><ins>the
field width minus the estimated width of the formatted argument
or, if the subtraction results in a negative value, 0</ins>.</td>
</tr>
</table>
</td>
</tr>
</table>
</blockquote>
</p>
<p>
Change in
<a href="http://eel.is/c++draft/format.string.std#11">22.14.2.2 [format.string.std] paragraph 11</a>:<br/>
<em>Drafting note:</em> This change is a minor terminology correction
unrelated to the primary goals of this proposal.
<blockquote>
For a string in a Unicode encoding, implementations should estimate the width
of a string as the sum of <ins>the </ins>estimated widths of the first
<del>code points</del><ins>UCS scalar values</ins>
in its extended grapheme clusters.
The extended grapheme clusters of a string are defined by UAX #29.
The estimated width of the following
<del>code points</del><ins>UCS scalar values</ins>
is 2:<br/>
<br/>
<em>[ … ]</em><br/>
<br/>
The estimated width of other
<del>code points</del><ins>UCS scalar values</ins>
is 1.
</blockquote>
</p>
</body>