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<title>Issue 4080: Presumed value and difference types of an iterator type in ranges and non-ranges algorithms</title>
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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="4080"><a href="lwg-active.html#4080">4080</a>. Presumed value and difference types of an iterator type in ranges and non-ranges algorithms</h3>
<p><b>Section:</b> 24.3.2 <a href="https://wg21.link/iterator.assoc.types">[iterator.assoc.types]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-04-26 <b>Last modified:</b> 2024-06-24</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Given an iterator type <code>I</code>, currently both <code>std::iter_value_t&lt;I&gt;</code> and 
<code>std::iterator_traits&lt;I&gt;::value_type</code> can be called the value type of <code>I</code>. 
These two types can be different if one specializes <code>std::indirect_readable_traits</code> for 
<code>I</code> such that <code>std::indirect_readable_traits&lt;I&gt;::value_type</code> is different 
from <code>I::value_type</code>.
<p/>
It seems intended that ranges algorithms use <code>std::iter_value_t&lt;I&gt;</code> and non-ranges algorithms, 
along with other legacy iterator-related mechanisms, use <code>std::iterator_traits&lt;I&gt;::value_type</code>. 
But this doesn't seem clarified anywhere.
<p/>
<a href="https://wg21.link/P2408R5" title=" Ranges iterators as inputs to non-Ranges algorithms">P2408R5</a> made the situation more complicated. In C++23, the type requirements of 
non-mutating iterators passed to many legacy algorithms were changed to C++20 iterator concepts, 
which are strongly associated with <code>std::iter_value_t</code>. It didn't seem intended that 
implementations were expected to use possibly different value types since C++23, but such reading
might be suggested by the use of concepts.
<p/>
Likewise, a non-random-access iterator type <code>I</code> can have two different difference types 
if one specializes <code>std::incrementable_traits</code> for <code>I</code> such that 
<code>std::incrementable_traits&lt;I&gt;::difference_type</code> is different from <code>I::difference_type</code>. 
It's also unclear which one should be used for non-mutating iterator in algorithms.
</p>

<p><i>[2024-06-24; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-4080"><b>Proposed resolution:</b></p>





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