<html><head></head><body bgcolor="#FFFFFF" text="#000000" lang="en-US" style="background-color: rgb(255, 255, 255); line-height: initial;"> <div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);">I think we would want it to be measured in glyphs. </div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);">Are you suggesting code points because glyphs are too hard?</div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);">Should we specify glyphs anyhow and leave it to QoI?</div> <div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><br style="display:initial"></div> <div style="font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);">Sent from my BlackBerry portable Babbage Device</div> <table width="100%" style="background-color:white;border-spacing:0px;"> <tbody><tr><td colspan="2" style="font-size: initial; text-align: initial; background-color: rgb(255, 255, 255);"> <div style="border-style: solid none none; border-top-color: rgb(181, 196, 223); border-top-width: 1pt; padding: 3pt 0in 0in; font-family: Tahoma, 'BB Alpha Sans', 'Slate Pro'; font-size: 10pt;"> <div><b>From: </b>Tom Honermann via Lib</div><div><b>Sent: </b>Saturday, September 7, 2019 8:13 PM</div><div><b>To: </b>Library Working Group; unicode@isocpp.open-std.org</div><div><b>Reply To: </b>lib@lists.isocpp.org</div><div><b>Cc: </b>Tom Honermann</div><div><b>Subject: </b>[isocpp-lib] New issue: Are std::format field widths code units, code points, or something else?</div></div></td></tr></tbody></table><div style="border-style: solid none none; border-top-color: rgb(186, 188, 209); border-top-width: 1pt; font-size: initial; text-align: initial; background-color: rgb(255, 255, 255);"></div><br><div id="_originalContent" style="background-color: rgb(255, 255, 255);">
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<p><a moz-do-not-send="true" href="http://eel.is/c++draft/format#string.std-7">[format.string.std]p7</a>
states:</p>
<p>
</p><blockquote type="cite">
<p>The <i>positive-integer</i> in <i>width</i> is a decimal
integer defining the minimum field width. If <i>width</i> is
not specified, there is no minimum field width, and the field
width is determined based on the content of the field.</p>
</blockquote>
<p></p>
<p>Is field width measured in code units, code points, or something
else?</p>
<p>Consider the following example assuming a UTF-8 locale:<br>
</p>
<p><tt>std::format("{}", "\xC3\x81"); // U+00C1</tt><tt>
{ </tt><tt>LATIN CAPITAL LETTER A WITH ACUTE }</tt><br>
<tt>std::format("{}", "\x41\xCC\x81"); // U+0041 U+0301 { </tt><tt>LATIN
CAPITAL LETTER A } { </tt><tt>COMBINING ACUTE ACCENT }<br>
</tt></p>
<p>In both cases, the arguments encode the same user-perceived
character (Á). The first uses two UTF-8 code units to encode a
single code point that represents a single glyph using a composed
Unicode normalization form. The second uses three code units to
encode two code points that represent the same glyph using a
decomposed Unicode normalization form.</p>
<p>How is the field width determined? If measured in code units,
the first has a width of 2 and the second of 3. If measured in
code points, the first has a width of 1 and the second of 2. If
measured in grapheme clusters, both have a width of 1. Is the
determination locale dependent?</p>
<p><b>Proposed resolution:</b></p>
<p>Field widths are measured in code units and are not locale
dependent. Modify <a moz-do-not-send="true" href="http://eel.is/c++draft/format#string.std-7">[format.string.std]p7</a>
as follows:</p>
<p> </p>
<blockquote type="cite">
<p>The <i>positive-integer</i> in <i>width</i> is a decimal
integer defining the minimum field width. If <i>width</i> is
not specified, there is no minimum field width, and the field
width is determined based on the content of the field. <b><font color="#33cc00">Field width is measured in code units. Each
byte of a multibyte character contributes to the field
width.</font></b><br>
</p>
</blockquote>
<p>(<i>code unit</i> is not formally defined in the standard. Most
uses occur in UTF-8 and UTF-16 specific contexts, but <a moz-do-not-send="true" href="http://eel.is/c++draft/lex.ext#5">[lex.ext]p5</a>
uses it in an encoding agnostic context.)<br>
</p>
<p>Tom.<br>
</p>
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