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<title>Issue 3761: cartesian_product_view::iterator::operator- should pass by reference</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="3761"><a href="lwg-defects.html#3761">3761</a>. <code>cartesian_product_view::<i>iterator</i>::operator-</code> should pass by reference</h3>
<p><b>Section:</b> 25.7.33.3 <a href="https://wg21.link/range.cartesian.iterator">[range.cartesian.iterator]</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 other</b> <a href="lwg-index.html#range.cartesian.iterator">issues</a> in [range.cartesian.iterator].</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>
There are two problems with <code>cartesian_product_view::<i>iterator</i>::operator-</code>.
<p/>
First, <code><i>distance-from</i></code> is not <code>const</code>-qualified, which would cause 
the common version of <code>operator-</code> to produce a hard error inside the function body 
since it invokes <code><i>distance-from</i></code> on a const <code><i>iterator</i></code> object.
<p/>
Second, the non-common version of <code>operator-</code> passes <code><i>iterator</i></code> by value,
which unnecessarily prohibits subtracting <code>default_sentinel</code> from an lvalue 
<code><i>iterator</i></code> whose first underlying iterator is non-<code>copyable</code>.
Even if we <code>std::move</code> it into <code>operator-</code>, the other overload will still be ill-formed
since it returns <code>-(i - s)</code> which will calls <code>i</code>'s deleted copy constructor.
<p/>
The proposed resolution is to add <code>const</code> qualification to <code><i>distance-from</i></code> 
and make the non-common version of <code><i>iterator</i>::operator-</code> pass by const reference. 
Although the <code>Tuple</code> parameter of <code><i>distance-from</i></code> is guaranteed to be 
<code>copyable</code>, I think it would be more appropriate to pass it by reference.
</p>

<p><i>[2022-09-08]</i></p>

<p>
As suggested by Tim Song, the originally submitted proposed wording has been refined to use
<code>const Tuple&amp;</code> instead of <code>Tuple&amp;</code>.
</p>

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

<p>
Set status to Tentatively Ready after six 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-3761"><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  [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;]

    friend constexpr difference_type operator-(<ins>const</ins> <i>iterator</i><ins>&amp;</ins> i, default_sentinel_t)
      requires <i>cartesian-is-sized-sentinel</i>&lt;Const, sentinel_t, First, Vs...&gt;;
    friend constexpr difference_type operator-(default_sentinel_t, <ins>const</ins> <i>iterator</i><ins>&amp;</ins> i)
      requires <i>cartesian-is-sized-sentinel</i>&lt;Const, sentinel_t, First, Vs...&gt;;

    [&hellip;]
  private:
    [&hellip;]
    
    template&lt;class Tuple&gt;
      constexpr difference_type <i>distance-from</i>(<ins>const</ins> Tuple<ins>&amp;</ins> t) <ins>const</ins>;            <i>// exposition only</i>
      
    [&hellip;]
  };
}
</pre>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
template&lt;class Tuple&gt;
  constexpr difference_type <i>distance-from</i>(<ins>const</ins> Tuple<ins>&amp;</ins> t) <ins>const</ins>;
</pre>
<blockquote>
<p>
-7- Let:
</p>
<ol style="list-style-type: none">
<li><p>(7.1) &mdash; <code><i>scaled-size</i>(<i>N</i>)</code> be the product of 
<code>static_cast&lt;difference_type&gt;(ranges::size(std::get&lt;<i>N</i>&gt;(<i>parent_</i>-&gt;<i>bases_</i>)))</code> and 
<code><i>scaled-size</i>(<i>N</i> + 1)</code> if <code><i>N</i> &lt; sizeof...(Vs)</code>, otherwise 
<code>static_cast&lt;difference_type&gt;(1)</code>;</p></li>
<li><p>(7.2) &mdash; <code><i>scaled-distance</i>(<i>N</i>)</code> be the product of 
<code>static_cast&lt;difference_type&gt;(std::get&lt;<i>N</i>&gt;(<i>current_</i>) - std::get&lt;<i>N</i>&gt;(t))</code> and 
<code><i>scaled-size</i>(<i>N</i> + 1)</code>; and</p></li>
<li><p>(7.3) &mdash; <code><i>scaled-sum</i></code> be the sum of <code><i>scaled-distance</i>(<i>N</i>)</code> for every integer 
<code>0 &le; <i>N</i> &le; sizeof...(Vs)</code>.</p></li>
</ol>
<p>
-8- <i>Preconditions</i>: <code><i>scaled-sum</i></code> can be represented by <code>difference_type</code>.
</p>
<p>
-9- <i>Returns</i>: <code><i>scaled-sum</i></code>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
friend constexpr difference_type operator-(<ins>const</ins> <i>iterator</i><ins>&amp;</ins> i, default_sentinel_t)
  requires <i>cartesian-is-sized-sentinel</i>&lt;Const, sentinel_t, First, Vs...&gt;;
</pre>
<blockquote>
<p>
-32- Let <code><i>end-tuple</i></code> be an object of a type that is a specialization of <code>tuple</code>, such that:
</p>
<ol style="list-style-type: none">
<li><p>(32.1) &mdash; <code>std::get&lt;0&gt;(<i>end-tuple</i>)</code> has the same value as 
<code>ranges::end(std::get&lt;0&gt;(i.<i>parent_</i>-&gt;<i>bases_</i>))</code>;</p></li>
<li><p>(32.2) &mdash; <code>std::get&lt;<i>N</i>&gt;(<i>end-tuple</i>)</code> has the 
same value as <code>ranges::begin(std::get&lt;<i>N</i>&gt;(i.<i>parent_</i>-&gt;<i>bases_</i>))</code> 
for every integer <code>1 &le; <i>N</i> &le; sizeof...(Vs)</code>.</p></li>
</ol>
<p>
-33- <i>Effects</i>: Equivalent to: <code>return i.<i>distance-from</i>(<i>end-tuple</i>);</code>
</p>
</blockquote>
<pre>
friend constexpr difference_type operator-(default_sentinel_t, <ins>const</ins> <i>iterator</i><ins>&amp;</ins> i)
  requires <i>cartesian-is-sized-sentinel</i>&lt;Const, sentinel_t, First, Vs...&gt;;
</pre>
<blockquote>
<p>
-34- <i>Effects</i>: Equivalent to: <code>return -(i - s);</code>
</p>
</blockquote>
</blockquote>

</li>

</ol>





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