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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Issue 4211: IANA time zone database allows links to refer to links</title>
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<meta property="og:title" content="Issue 4211: IANA time zone database allows links to refer to links">
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<meta property="og:description" content="C++ library issue. Status: New">
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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>
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<h3 id="4211"><a href="lwg-active.html#4211">4211</a>. IANA time zone database allows links to refer to links</h3>
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<p><b>Section:</b> 30.11.2.1 <a href="https://wg21.link/time.zone.db.tzdb">[time.zone.db.tzdb]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-02-22 <b>Last modified:</b> 2025-02-22</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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Since
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<a href="https://github.com/eggert/tz/commit/4878b644b25020568d4dda346c5cea91826c0c7c">October 2022</a>
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the IANA Time Zone Database's <code class='backtick'>tzfile</code> data format allows a Link to refer
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to another Link, instead of referring directly to a Zone.
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The <a href="https://man7.org/linux/man-pages/man8/zic.8.html#FILES">zic(8)</a>
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man page says:
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<blockquote>
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The TARGET field should appear as the NAME field in some zone line
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or as the LINK-NAME field in some link line.
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The LINK-NAME field is used as an alternative name for that zone;
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it has the same syntax as a zone line's NAME field.
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Links can chain together, although the behavior is unspecified if
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a chain of one or more links does not terminate in a Zone name.
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</blockquote>
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Because chains of links were introduced to <code class='backtick'>tzfile</code> after <code class='backtick'>chrono::tzdb</code>
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was standardized, C++ does not properly support chains of links.
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</p>
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<p>
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The <code class='backtick'>time_zone_link::target()</code> member function is required to return
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"The name of the <code class='backtick'>time_zone</code> for which this <code class='backtick'>time_zone_link</code> provides
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an alternative name."
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This doesn't allow returning the name of another <code class='backtick'>time_zone_link</code>,
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which implies that the implementation should flatten a series of links so
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that the target is a <code class='backtick'>time_zone</code>. However, this discards information which
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is present in the <code class='backtick'>tzdb</code>, so that the information exposed to a C++ program
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does not preserve the original structure of a chain of links.
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</p>
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<p>
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The standard could be adjusted to support chains of links by allowing
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<code class='backtick'>time_zone_link::target()</code> to refer to another link, and then altering the
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algorithm used by <code class='backtick'>tzdb::locate_zone(string_view)</code> to find a <code class='backtick'>time_zone</code> from
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a name that might be a zone or a link.
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</p>
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<p id="res-4211"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5001">N5001</a>.
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</p>
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<ol>
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<li>
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Modify 30.11.2.1 <a href="https://wg21.link/time.zone.db.tzdb">[time.zone.db.tzdb]</a> as indicated:
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<blockquote>
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<pre><code>
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const time_zone* locate_zone(string_view tz_name) const;
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</code></pre>
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<p>
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-2- <i>Returns</i>:
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<ol style="list-style-type:none">
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<li>
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(2.1) &mdash;
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If <code class='backtick'>zones</code> contains an element <code class='backtick'>tz</code> for which <code class='backtick'>tz.name() == tz_name</code>,
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a pointer to <code class='backtick'>tz</code>;
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</li>
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<li>
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(2.2) &mdash;
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otherwise, if <code class='backtick'>links</code> contains an element <code class='backtick'>tz_l</code> for which
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<code class='backtick'>tz_l.name() == tz_name</code>, then
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<del>a pointer to the element <code class='backtick'>tz</code> of zones for which
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<code class='backtick'>tz.name() == tz_l.target()</code></del>
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<ins>
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the result of <code class='backtick'>locate_zone(tz_l.target())</code>.
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</ins>.
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</li>
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</ol>
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</p>
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<p>
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[<i>Note 1</i>: A <code class='backtick'>time_zone_link</code> specifies an alternative name for a
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<code class='backtick'>time_zone</code>. &mdash; <i>end note</i>]
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</p>
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<p>
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-3- <i>Throws</i>:
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If a <code class='backtick'>const time_zone*</code> cannot be found as described in the <i>Returns</i>:
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element, throws a <code class='backtick'>runtime_error</code>.
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</p>
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<p>
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[<i>Note 2</i>: On non-exceptional return, the return value is always a
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pointer to a valid <code class='backtick'>time_zone</code>. &mdash; <i>end note</i>]
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</p>
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<p>
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<ins>
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-?- <i>Remarks</i>:
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If both <code class='backtick'>zones</code> and <code class='backtick'>links</code> contain elements that match <code class='backtick'>tz_name</code> then
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it is unspecified whether <code>&amp;tz</code>
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or <code>locate_zone(tz_l.target())</code> is returned.
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</ins>
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</p>
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<p><i>[Drafting note:
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This gives flexibility how to implement the lookup in <code class='backtick'>locate_zone</code>.
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]</i></p>
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</blockquote>
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</li>
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<li>
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Modify 30.11.9.1 <a href="https://wg21.link/time.zone.link.overview">[time.zone.link.overview]</a> as indicated:
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<blockquote>
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<p>
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-1-
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A <code class='backtick'>time_zone_link</code> specifies an alternative name for a <code class='backtick'>time_zone</code>.
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<code class='backtick'>time_zone_links</code> are constructed when the time zone database is initialized.
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<ins>
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The name of a <code class='backtick'>time_zone_link</code> can be used as an argument to
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<code class='backtick'>tzdb::locate_zone</code> (30.11.2.1 <a href="https://wg21.link/time.zone.db.tzdb">[time.zone.db.tzdb]</a>).
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A <code class='backtick'>time_zone_link</code> can refer directly to a <code class='backtick'>time_zone</code>,
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or to another <code class='backtick'>time_zone_link</code>, forming a chain of links.
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</ins>
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</p>
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</blockquote>
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</li>
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<li>
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Modify 30.11.9.2 <a href="https://wg21.link/time.zone.link.members">[time.zone.link.members]</a> as indicated:
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<blockquote>
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<pre><code>
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string_view name() const noexcept;
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</code></pre>
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-1- <i>Returns</i>:
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The alternative name for the time zone.
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<pre><code>
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string_view target() const noexcept;
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</code></pre>
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<p>
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-2- <i>Returns</i>:
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The name of the <code class='backtick'>time_zone</code>
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for which this <code class='backtick'>time_zone_link</code> provides an alternative name
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<ins>or the name of another <code class='backtick'>time_zone_link</code></ins>.
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</p>
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<p>
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<ins>
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[<i>Note 1</i>:
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<code class='backtick'>tzdb::locate_zone</code> follows a chain of links formed when
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a link's target is the name of a <code class='backtick'>time_zone_link</code>,
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throwing an exception if the
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chain of links does not terminate in a <code class='backtick'>time_zone</code>.
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&mdash; <i>end note</i>]
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</ins>
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</p>
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</blockquote>
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</li>
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</html>

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