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Re: [SG16-Unicode] [isocpp-lib] New issue: Are std::format field widths code units, code points, or something else?

From: Marshall Clow <mclow.lists_at_[hidden]>
Date: Wed, 11 Sep 2019 12:32:47 -0700
On Sat, Sep 7, 2019 at 5:13 PM Tom Honermann via Lib <lib_at_[hidden]>

> [format.string.std]p7 <http://eel.is/c++draft/format#string.std-7> states:
> The *positive-integer* in *width* is a decimal integer defining the
> minimum field width. If *width* is not specified, there is no minimum
> field width, and the field width is determined based on the content of the
> field.
> Is field width measured in code units, code points, or something else?
> Consider the following example assuming a UTF-8 locale:
> std::format("{}", "\xC3\x81"); // U+00C1 { LATIN CAPITAL
> std::format("{}", "\x41\xCC\x81"); // U+0041 U+0301 { LATIN CAPITAL
> 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.
> 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?
(Coming late to the party)
Let's ask a different question.

          std::string s = "/* some content */";
          std::ostringstream oss;
          oss << std::setw(22) << s;
          std::string result1 = oss.str();
          std::string result2 = std::format("{:22}", s);

What can we say about the contents of "result1" and "result2"?
Are they the same? Does it matter what the contents of `s` is?

-- Marshall

Received on 2019-09-11 21:33:01