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<!DOCTYPE html>
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<html lang="en">
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<title>Issue 4210: Issue with cache_latest_view::iterator's reference type </title>
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<meta property="og:title" content="Issue 4210: Issue with cache_latest_view::iterator's reference type ">
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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="4210"><a href="lwg-active.html#4210">4210</a>. Issue with <code>cache_latest_view::<i>iterator</i></code>'s reference type </h3>
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<p><b>Section:</b> 25.7.34.3 <a href="https://wg21.link/range.cache.latest.iterator">[range.cache.latest.iterator]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-02-09 <b>Last modified:</b> 2025-02-09</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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<code>cache_latest_view::<i>iterator</i></code> can be roughly regarded as an iterator that
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transforms the original iterator's reference, because its reference is <i>always</i> an lvalue
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which is <code>range_reference_t&lt;V&gt;&amp;</code>, even if the original iterator returns
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an rvalue.
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<p/>
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In this case, it seems a bit odd to still specialize <code>iter_move</code> and return the rvalue
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reference type of the original iterator, because the original reference has changed, so the former
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may not form a valid common reference with the original rvalue reference.
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<p/>
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That is, in some rare cases even if the <code>cache_latest_view</code> can be legally constructed,
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it may not be an <code>input_range</code> as <code>indirectly_readable</code> is not satisfied.
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<p/>
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A contrived example could be:
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</p>
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<blockquote><pre>
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struct I {
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using value_type = int;
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using difference_type = int;
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struct move_only_ref {
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move_only_ref(const int&amp;);
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move_only_ref(move_only_ref&amp;&amp;) = default;
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};
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move_only_ref operator*() const;
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I&amp; operator++();
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void operator++(int);
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};
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using R = ranges::cache_latest_view&lt;ranges::subrange&lt;I, unreachable_sentinel_t&gt;&gt;; // ok
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static_assert(input_iterator&lt;I&gt;);
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static_assert(ranges::input_range&lt;R&gt;); // failed
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</pre></blockquote>
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<p>
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It's unclear whether we should change the reference type to <code>range_reference_t&lt;V&gt;&amp;&amp;</code>
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to preserve the original ref-qualifiers as much as possible (I don't see any discussion of this option
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in the original paper), or not provide a specialized <code>iter_move</code>, since it's intuitive to have
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the default <code>iter_move</code> return an rvalue when it always returns an lvalue.
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</p>
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<p id="res-4210"><b>Proposed resolution:</b></p>
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</html>

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