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<title>Issue 4059: Leaky abstraction in join_with_view's iterator</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="4059"><a href="lwg-active.html#4059">4059</a>. Leaky abstraction in <code>join_with_view</code>'s iterator</h3>
<p><b>Section:</b> 25.7.15.3 <a href="https://wg21.link/range.join.with.iterator">[range.join.with.iterator]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2024-03-23 <b>Last modified:</b> 2024-08-11</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.join.with.iterator">active issues</a> in [range.join.with.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.join.with.iterator">issues</a> in [range.join.with.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>join_with_view</code>'s iterator is defined in terms of <code>variant</code>, <code>visit</code> and <code>get</code>, 
which implies that the iterator operations throw <code>bad_variant_access</code> if the underlying variant is 
valueless-by-exception (which can happen if an underlying iterator has a throwing copy constructor).
<p/>
IMO, the <code>variant</code> is an implementation detail and shouldn't be exposed. It's confusing for users to 
get <code>bad_variant_access</code> when user code does not deal with <code>variant</code>.
<p/>
The spec of <code>join_with_view</code> is also inconsistent with <code>concat_view</code> as recently added to the
working draft by <a href="https://wg21.link/P2542R8" title=" views::concat">P2542R8</a>. The latter has "<code><i>it_</i>.valueless_by_exception()</code> is 
<code>false</code>" as a precondition.
<p/>
I believe that <code>join_with_view</code> should similarly require that <code><i>inner_it_</i>.valueless_by_exception()</code> 
is <code>false</code> for each iterator operation.
<p/>
(FWIW, MSVC STL implements <code>join_with_view</code> with a trimmed-down version of <code>variant</code>, and having 
to throw <code>bad_variant_access</code> causes a small maintenance burden.)
</p>

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

<p>
Set priority to 3 after reflector poll.
We like the suggestion to require "not valueless" as a precondition.
</p>

<p><i>[2024-08-08, Inbal Levi provides wording]</i></p>



<p id="res-4059"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4988" title=" Working Draft, Programming Languages — C++">N4988</a>.
</p>

<ol>

<li><p>Modify 25.7.15.3 <a href="https://wg21.link/range.join.with.iterator">[range.join.with.iterator]</a> as indicated:</p>

<blockquote>
<pre>
constexpr <i>iterator</i>(<i>iterator</i>&lt;!Const&gt; i)
    requires Const &amp;&amp; convertible_to&lt;iterator_t&lt;V&gt;, <i>OuterIter</i>&gt; &amp;&amp;
             convertible_to&lt;iterator_t&lt;<i>InnerRng</i>&gt;, <i>InnerIter</i>&gt; &amp;&amp;
             convertible_to&lt;iterator_t&lt;Pattern&gt;, <i>PatternIter</i>&gt;;
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code><i>inner_it_</i>.valueless_by_exception()</code> is <code>false</code>.</ins>
<p/>
-10- <i>Effects</i>: [&hellip;]
</p>
</blockquote>
<pre>
constexpr decltype(auto) operator*() const;
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code><i>inner_it_</i>.valueless_by_exception()</code> is <code>false</code>.</ins>
<p/>
-12- <i>Effects</i>: Equivalent to: [&hellip;]
</p>
</blockquote>
<pre>
constexpr <i>iterator</i>&amp; operator++();
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code><i>inner_it_</i>.valueless_by_exception()</code> is <code>false</code>.</ins>
<p/>
-13- <i>Effects</i>: Equivalent to: [&hellip;]
</p>
</blockquote>
<pre>
constexpr void operator++(int);
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code><i>inner_it_</i>.valueless_by_exception()</code> is <code>false</code>.</ins>
<p/>
-14- <i>Effects</i>: Equivalent to: <code>++*this</code>.
</p>
</blockquote>
<pre>
constexpr <i>iterator</i> operator++(int)
  requires <i>ref-is-glvalue</i> &amp;&amp; forward_iterator&lt;<i>OuterIter</i>&gt; &amp;&amp; forward_iterator&lt;<i>InnerIter</i>&gt;;
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code><i>inner_it_</i>.valueless_by_exception()</code> is <code>false</code>.</ins>
<p/>
-15- <i>Effects</i>: Equivalent to: [&hellip;]
</p>
</blockquote>
<pre>
constexpr <i>iterator</i>&amp; operator--()
  requires <i>ref-is-glvalue</i> &amp;&amp; bidirectional_range&lt;<i>Base</i>&gt; &amp;&amp;
           <i>bidirectional-common</i>&lt;<i>InnerBase</i>&gt; &amp;&amp; <i>bidirectional-common</i>&lt;<i>PatternBase</i>&gt;;
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code><i>inner_it_</i>.valueless_by_exception()</code> is <code>false</code>.</ins>
<p/>
-16- <i>Effects</i>: Equivalent to: [&hellip;]
</p>
</blockquote>
</blockquote>
</li>

</ol>





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