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<div class="moz-cite-prefix">On 9/7/19 8:27 PM, Tony V E wrote:<br>
</div>
<blockquote type="cite"
cite="mid:20190908002724.5218384.37561.86238@gmail.com">
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<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>
</blockquote>
I agree that would be ideal, but...<br>
<blockquote type="cite"
cite="mid:20190908002724.5218384.37561.86238@gmail.com">
<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>
</blockquote>
I don't know how to achieve that. Field width doesn't really work
for alignment unless one assumes a monospace font. We could measure
in terms of extended grapheme clusters, but EGCS width has changed
over time (e.g., family emoji). That makes alignment dependent on
both display properties and Unicode version. And, of course, this
would drag in locale dependence as well.
<blockquote type="cite"
cite="mid:20190908002724.5218384.37561.86238@gmail.com">
<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>
</blockquote>
<p>Perhaps we could (I'm not sure how to specify that), but then we
end up with the locale dependency, at least for char and wchar_t
(which is all that is supported right now).<br>
</p>
<p>Tom.</p>
<blockquote type="cite"
cite="mid:20190908002724.5218384.37561.86238@gmail.com">
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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>
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<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;
<a class="moz-txt-link-abbreviated" href="mailto:unicode@isocpp.open-std.org">unicode@isocpp.open-std.org</a></div>
<div><b>Reply To: </b><a class="moz-txt-link-abbreviated" href="mailto:lib@lists.isocpp.org">lib@lists.isocpp.org</a></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>
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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>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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