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<!DOCTYPE html>
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<html lang="en">
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<meta charset="utf-8">
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<title>Issue 4237: The standard library iterator adaptor does not handle iterator_category correctly</title>
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<meta property="og:title" content="Issue 4237: The standard library iterator adaptor does not handle iterator_category correctly">
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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="4237"><a href="lwg-active.html#4237">4237</a>. The standard library iterator adaptor does not handle <code>iterator_category</code> correctly</h3>
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<p><b>Section:</b> 24.5.3.3 <a href="https://wg21.link/const.iterators.iterator">[const.iterators.iterator]</a>, 25.7 <a href="https://wg21.link/range.adaptors">[range.adaptors]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-03-27 <b>Last modified:</b> 2025-03-29</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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Currently, <code>basic_const_iterator</code>, and several range adaptors such as
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<code>filter_view</code>'s iterators provide <code>iterator_category</code> only when the
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underlying iterator models <code>forward_iterator</code>, implying that they expect those
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iterators should have a valid <code>iterator_category</code>.
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</p>
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<p>
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However, this is incorrect because being a <code>forward_iterator</code> does not
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necessarily mean it is a <i>Cpp17InputIterator</i>, it just means that it <i>probably</i>
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meets the syntactic requirements of <i>Cpp17InputIterator</i>.
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</p>
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<p>
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Any iterator that specializes <code>iterator_traits</code> and provides only
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<code>iterator_concept</code> without <code>iterator_category</code> is not a
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<i>Cpp17InputIterator</i>, for example, <code>common_iterator</code> with a
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<code>difference_type</code> of integer-class type.
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</p>
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<p>
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In this case, instantiating these iterator adaptors will result in a hard error because the
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<code>iterator_category</code> they expect does not exist. The following illustrates the
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problem (<a href="https://godbolt.org/z/j7x8bKzo4">demo</a>):
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</p>
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<blockquote><pre>
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#include &lt;iterator&gt;
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#include &lt;ranges&gt;
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int main() {
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auto r = std::views::iota(0ULL)
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| std::views::take(5)
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| std::views::common;
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static_assert(std::ranges::forward_range&lt;decltype(r)&gt;);
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std::basic_const_iterator ci(r.begin()); // <span style="color:red;font-weight:bolder">'iterator_category': is not a member of 'std::iterator_traits'</span>
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auto f = r | std::views::filter([](auto) { return true; });
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auto b = f.begin(); // <span style="color:red;font-weight:bolder">'iterator_category': is not a member of 'std::iterator_traits'</span>
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}
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</pre></blockquote>
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<p>
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I believe that checking if the underlying iterator is a <code>forward_iterator</code> is not an appropriate
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mechanism to provide <code>iterator_category</code>, but rather checking if its <code>iterator_traits</code>
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specialization provides <code>iterator_category</code>.
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</p>
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<p>
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This issue is somewhat related to LWG <a href="lwg-active.html#3763" title="Should range adaptor iterators only provide iterator_category when its
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difference_type is not an integer-class type? (Status: New)">3763</a><sup><a href="https://cplusplus.github.io/LWG/issue3763" title="Latest snapshot">(i)</a></sup>, which is a further consideration after LWG
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<a href="lwg-defects.html#3749" title="common_iterator should handle integer-class difference types (Status: WP)">3749</a><sup><a href="https://cplusplus.github.io/LWG/issue3749" title="Latest snapshot">(i)</a></sup> has been resolved.
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</p>
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<p id="res-4237"><b>Proposed resolution:</b></p>
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</body>
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</html>

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