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| <!DOCTYPE html> | ||
| <html lang="en"> | ||
| <head> | ||
| <meta charset="utf-8"> | ||
| <title>Issue 4070: Transcoding by std::formatter<std::filesystem::path></title> | ||
| <meta property="og:title" content="Issue 4070: Transcoding by std::formatter<std::filesystem::path>"> | ||
| <meta property="og:description" content="C++ library issue. Status: New"> | ||
| <meta property="og:url" content="https://cplusplus.github.io/LWG/issue4070.html"> | ||
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| <p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p> | ||
| <h3 id="4070"><a href="lwg-active.html#4070">4070</a>. Transcoding by <code>std::formatter<std::filesystem::path></code></h3> | ||
| <p><b>Section:</b> 99 [fs.path.fmtr.funcs] <b>Status:</b> <a href="lwg-active.html#New">New</a> | ||
| <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2024-04-19 <b>Last modified:</b> 2024-04-19</p> | ||
| <p><b>Priority: </b>Not Prioritized | ||
| </p> | ||
| <p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p> | ||
| <p><b>Discussion:</b></p> | ||
| <p> | ||
| 99 [fs.path.fmtr.funcs] says: | ||
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| <blockquote> | ||
| If <code class='backtick'>charT</code> is <code class='backtick'>char</code>, <code class='backtick'>path::value_type</code> is <code class='backtick'>wchar_t</code>, | ||
| and the literal encoding is UTF-8, then the escaped path is | ||
| transcoded from the native encoding for wide character strings to UTF-8 | ||
| with maximal subparts of ill-formed subsequences substituted with | ||
| <span style="font-variant:small-caps">u+fffd</span> | ||
| replacement character per the Unicode Standard [...]. | ||
| Otherwise, transcoding is implementation-defined. | ||
| </blockquote> | ||
| </p> | ||
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| <p> | ||
| This seems to mean that the Unicode substitutions are only done | ||
| for an escaped path, i.e. when the <code class='backtick'>?</code> option is used. Otherwise, the form | ||
| of transcoding is completely implementation-defined. | ||
| However, this makes no sense. | ||
| An escaped string will have no ill-formed subsequences, because they will | ||
| already have been replaced as per 22.14.6.4 <a href="https://wg21.link/format.string.escaped">[format.string.escaped]</a>: | ||
| <blockquote> | ||
| Otherwise (<em>X</em> is a sequence of ill-formed code units), | ||
| each code unit <em>U</em> is appended to <em>E</em> in order as | ||
| the sequence <code>\x{<em>hex-digit-sequence</em>}</code>, | ||
| where <code><em>hex-digit-sequence</em></code> is the shortest hexadecimal | ||
| representation of <em>U</em> using lower-case hexadecimal digits. | ||
| </blockquote> | ||
| </p> | ||
| <p> | ||
| So only unescaped strings can have ill-formed sequences by the time | ||
| we do transcoding to <code class='backtick'>char</code>, but whether or not any | ||
| <span style="font-variant:small-caps">u+fffd</span> substitution | ||
| occurs is just implementation-defined. | ||
| </p> | ||
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| <p> | ||
| I believe we want to specify the substitutions are done when transcoding | ||
| an <em>unescaped</em> path (and it doesn't matter whether we specify it | ||
| for escaped paths, because it's a no-op if escaping happens first, | ||
| as is apparently intended). | ||
| </p> | ||
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| <p> | ||
| It does matter whether we escape first or perform substitutions first. | ||
| If we escape first then every code unit in an ill-formed sequence is | ||
| individually escaped as <code class='backtick'>\x{hex-digit-sequence}</code>. | ||
| So an ill-formed sequence of two <code class='backtick'>wchar_t</code> values will be escaped as | ||
| two <code class='backtick'>\x{...}</code> strings, which are then transcoded to UTF-8. | ||
| If we transcode (with substitutions first) then the entire | ||
| ill-formed sequence is replaced with a single replacement character, | ||
| which will then be escaped as <code class='backtick'>\x{fffd}</code>. | ||
| SG16 should be asked to confirm that escaping first is intended, | ||
| so that an escaped string shows the original invalid code units. | ||
| For a non-escaped string, we want the ill-formed sequence to be | ||
| formatted as �, which the proposed resolution tries to ensure. | ||
| </p> | ||
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| <p id="res-4070"><b>Proposed resolution:</b></p> | ||
| <p> | ||
| This wording is relative to <a href="https://wg21.link/N4981">N4981</a>. | ||
| </p> | ||
| <ol> | ||
| <li><p>Modify 99 [fs.path.fmtr.funcs] as indicated:</p> | ||
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| <blockquote> | ||
| <pre><code> | ||
| template<class FormatContext> | ||
| typename FormatContext::iterator | ||
| format(const filesystem::path& p, FormatContext& ctx) const; | ||
| </code></pre> | ||
| <blockquote>-5- | ||
| <em>Effects</em>: | ||
| Let <code class='backtick'>s</code> be <code>p.generic_string<filesystem::path::value_type>()</code> | ||
| if the <code class='backtick'>g</code> option is used, otherwise <code class='backtick'>p.native()</code>. | ||
| Writes <code class='backtick'>s into </code>ctx.out()`, adjusted according to the path-format-spec. | ||
| If <code class='backtick'>charT</code> is <code class='backtick'>char</code>, <code class='backtick'>path::value_type</code> is <code class='backtick'>wchar_t</code>, | ||
| and the literal encoding is UTF-8, then the | ||
| <del>escaped path</del> | ||
| <ins>(possible escaped) string</ins> | ||
| is transcoded from the native encoding for wide character strings to UTF-8 | ||
| with maximal subparts of ill-formed subsequences substituted with | ||
| <span style="font-variant:small-caps">u+fffd</span> replacement character per | ||
| the Unicode Standard, Chapter 3.9 <span style="font-variant:small-caps">u+fffd</span> | ||
| Substitution in Conversion. | ||
| If <code class='backtick'>charT</code> and <code class='backtick'>path::value_type</code> are the same then no transcoding is performed. | ||
| Otherwise, transcoding is implementation-defined | ||
| </blockquote> | ||
| </blockquote> | ||
| </li> | ||
| <li> | ||
| Modify the entry in the index of implementation-defined behavior as indicated: | ||
| <blockquote> | ||
| transcoding of a formatted <code class='backtick'>path</code> when <code class='backtick'>charT</code> and <code class='backtick'>path::value_type</code> differ | ||
| <ins>and not converting from <code class='backtick'>wchar_t</code> to UTF-8</ins> | ||
| </blockquote> | ||
| </li> | ||
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| </body> | ||
| </html> |
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