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<title>Issue 3760: cartesian_product_view::iterator's parent_ is never valid</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#C++23">C++23</a> status.</em></p>
<h3 id="3760"><a href="lwg-defects.html#3760">3760</a>. <code>cartesian_product_view::<i>iterator</i></code>'s <code><i>parent_</i></code> is never valid</h3>
<p><b>Section:</b> 25.7.33 <a href="https://wg21.link/range.cartesian">[range.cartesian]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-08-27 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>cartesian_product_view::<i>iterator</i></code> has a pointer member <code><i>parent_</i></code> that points 
to <code>cartesian_product_view</code>, but its constructor only accepts a <code>tuple</code> of iterators, which 
makes <code><i>parent_</i></code> always default-initialized to <code>nullptr</code>.
<p/>
The proposed resolution is to add an aliased <code><i>Parent</i></code> parameter to the constructor and 
initialize <code><i>parent_</i></code> with <code>addressof</code>, as we usually do.
</p>

<p><i>[2022-09-23; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after eight votes in favour during reflector poll.
</p>

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3760"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 25.7.33.2 <a href="https://wg21.link/range.cartesian.view">[range.cartesian.view]</a> as indicated:</p>

<blockquote>
<pre>
constexpr <i>iterator</i>&lt;false&gt; begin()
  requires (!<i>simple-view</i>&lt;First&gt; || ... || !<i>simple-view</i>&lt;Vs&gt;);
</pre>
<blockquote>
<p>
-2- <i>Effects</i>: Equivalent to: <code>return <i>iterator</i>&lt;false&gt;(<ins>*this,</ins> <i>tuple-transform</i>(ranges::begin, 
<i>bases_</i>));</code>
</p>
</blockquote>  
<pre>
constexpr <i>iterator</i>&lt;true&gt; begin() const
  requires (range&lt;const First&gt; &amp;&amp; ... &amp;&amp; range&lt;const Vs&gt;);
</pre>
<blockquote>
<p>
-3- <i>Effects</i>: Equivalent to: <code>return <i>iterator</i>&lt;true&gt;(<ins>*this,</ins> <i>tuple-transform</i>(ranges::begin, 
<i>bases_</i>));</code>
</p>
</blockquote>
<pre>
constexpr <i>iterator</i>&lt;false&gt; end()
  requires ((!<i>simple-view</i>&lt;First&gt; || ... || !<i>simple-view</i>&lt;Vs&gt;)
    &amp;&amp; <i>cartesian-product-is-common</i>&lt;First, Vs...&gt;);
constexpr <i>iterator</i>&lt;true&gt; end() const
  requires <i>cartesian-product-is-common</i>&lt;const First, const Vs...&gt;;
</pre>
<blockquote>
<p>
-4- Let:
</p>
<ol style="list-style-type: none">
<li><p>(4.1) &mdash; <code><i>is-const</i></code> be <code>true</code> for the const-qualified overload, and <code>false</code> otherwise;</p></li>
<li><p>(4.2) &mdash; <code><i>is-empty</i></code> be <code>true</code> if the expression <code>ranges::empty(rng)</code> is <code>true</code> 
for any <code>rng</code> among the underlying ranges except the first one and <code>false</code> otherwise; and</p></li>
<li><p>(4.3) &mdash; <code><i>begin-or-first-end</i>(rng)</code> be expression-equivalent to 
<code><i>is-empty</i> ? ranges::begin(rng) : <i>cartesian-common-arg-end</i>(rng)</code> if <code>rng</code> is the first underlying range 
and <code>ranges::begin(rng)</code> otherwise.</p></li>
</ol>
<p>
-5- <i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
<i>iterator</i>&lt;<i>is-const</i>&gt; it(<ins>*this,</ins> <i>tuple-transform</i>(
  [](auto&amp; rng){ return <i>begin-or-first-end</i>(rng); }, <i>bases_</i>));
return it;
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify  [ranges.cartesian.iterator] as indicated:</p>
<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range First, forward_range... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; view&lt;Vs&gt;)
  template&lt;bool Const&gt;
  class cartesian_product_view&lt;First, Vs...&gt;::<i>iterator</i> {
  public:
    [&hellip;]

  private:
    <ins>using <i>Parent</i> = <i>maybe-const</i>&lt;Const, cartesian_product_view&gt;;           <i>// exposition only</i></ins>
    <ins><i>Parent</i></ins><del><i>maybe-const</i>&lt;Const, cartesian_product_view&gt;</del>* <i>parent_</i> = nullptr; <i>// exposition only</i>
    tuple&lt;iterator_t&lt;<i>maybe-const</i>&lt;Const, First&gt;&gt;,
      iterator_t&lt;<i>maybe-const</i>&lt;Const, Vs&gt;&gt;...&gt; <i>current_</i>;                   <i>// exposition only</i>
    
    template&lt;size_t N = sizeof...(Vs)&gt;
      constexpr void <i>next</i>();                                             <i>// exposition only</i>
    
    template&lt;size_t N = sizeof...(Vs)&gt;
      constexpr void <i>prev</i>();                                             <i>// exposition only</i>
    
    template&lt;class Tuple>
      constexpr difference_type <i>distance-from</i>(Tuple t);                  <i>// exposition only</i>
    
    constexpr <del>explicit</del> <i>iterator</i>(<ins><i>Parent</i>&amp; parent,</ins> tuple&lt;iterator_t&lt;<i>maybe-const</i>&lt;Const, First&gt;&gt;,
      iterator_t&lt;<i>maybe-const</i>&lt;Const, Vs&gt;&gt;...&gt; current);                   <i>// exposition only</i>
  };
}
</pre>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
constexpr <del>explicit</del> <i>iterator</i>(<ins><i>Parent</i>&amp; parent,</ins> tuple&lt;iterator_t&lt;<i>maybe-const</i>&lt;Const, First&gt;&gt;,
  iterator_t&lt;<i>maybe-const</i>&lt;Const, Vs&gt;&gt;...&gt; current);
</pre>
<blockquote>
<p>
-10- <i>Effects</i>:  Initializes <ins><code><i>parent_</i></code> with <code>addressof(parent)</code> and</ins> 
<code><i>current_</i></code> with <code>std::move(current)</code>.
</p>
</blockquote>
<pre>
constexpr <i>iterator</i>(<i>iterator</i>&lt;!Const&gt; i) requires Const &amp;&amp;
  (convertible_to&lt;iterator_t&lt;First&gt;, iterator_t&lt;const First&gt;&gt; &amp;&amp;
    ... &amp;&amp; convertible_to&lt;iterator_t&lt;Vs&gt;, iterator_t&lt;const Vs&gt;&gt;);
</pre>
<blockquote>
<p>
-11- <i>Effects</i>: Initializes <ins><code><i>parent_</i></code> with <code>i.<i>parent_</i></code> and</ins> 
<code><i>current_</i></code> with <code>std::move(i.<i>current_</i>)</code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>





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