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<title>Issue 3712: chunk_view and slide_view should not be default_initializable</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="3712"><a href="lwg-defects.html#3712">3712</a>. <code>chunk_view</code> and <code>slide_view</code> should not be <code>default_initializable</code></h3>
<p><b>Section:</b> 25.7.29.2 <a href="https://wg21.link/range.chunk.view.input">[range.chunk.view.input]</a>, 25.7.29.6 <a href="https://wg21.link/range.chunk.view.fwd">[range.chunk.view.fwd]</a>, 25.7.30.2 <a href="https://wg21.link/range.slide.view">[range.slide.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-06-10 <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.chunk.view.input">issues</a> in [range.chunk.view.input].</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>
Both <code>chunk_view</code> and <code>slide_view</code> have a precondition that <code>N</code> must be positive, 
but they are still <code>default_initializable</code> when the underlying range is <code>default_initializable</code>, 
which makes the member variable <code><i>n_</i></code> initialized with an invalid value <code>0</code> when they 
are default-constructed, which produces the following unexpected result:
</p>
<blockquote><pre>
#include &lt;ranges&gt;

using V = std::ranges::iota_view&lt;int, int&gt;;
static_assert(std::ranges::slide_view&lt;V&gt;().empty()); // <span style="color:red;font-weight:bolder">fails</span>
static_assert(std::ranges::chunk_view&lt;V&gt;().empty()); // <span style="color:red;font-weight:bolder">division by zero is not a constant expression</span>
</pre></blockquote>
<p>
Although we could provide a default positive value for <code><i>n_</i></code>, I think a more appropriate solution 
would be to not provide the default constructor, since default-constructed values for integer types will 
never be valid.
</p>

<p><i>[2022-06-21; Reflector poll]</i></p>

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

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3712"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 25.7.29.2 <a href="https://wg21.link/range.chunk.view.input">[range.chunk.view.input]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
 
  template&lt;view V&gt;
    requires input_range&lt;V&gt;
  class chunk_view : public view_interface&lt;chunk_view&lt;V&gt;&gt; {
    V <i>base_</i> <del>= V()</del>;                                        <i>// exposition only</i>
    range_difference_t&lt;V&gt; <i>n_</i> <del>= 0</del>;                         <i>// exposition only</i>
    range_difference_t&lt;V&gt; <i>remainder_</i> = 0;                 <i>// exposition only</i>
    [&hellip;]
  public:
    <del>chunk_view() requires default_initializable&lt;V&gt; = default;</del>
    constexpr explicit chunk_view(V base, range_difference_t&lt;V&gt; n);
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.29.6 <a href="https://wg21.link/range.chunk.view.fwd">[range.chunk.view.fwd]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;view V&gt;
    requires forward_range&lt;V&gt;
  class chunk_view&lt;V&gt; : public view_interface&lt;chunk_view&lt;V&gt;&gt; {
    V <i>base_</i> <del>= V()</del>;                   <i>// exposition only</i>
    range_difference_t&lt;V&gt; <i>n_</i> <del>= 0</del>;    <i>// exposition only</i>
    [&hellip;]
  public:
    <del>chunk_view() requires default_initializable&lt;V&gt; = default;</del>
    constexpr explicit chunk_view(V base, range_difference_t&lt;V&gt; n);

    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.30.2 <a href="https://wg21.link/range.slide.view">[range.slide.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]

  template&lt;forward_range V&gt;
    requires view&lt;V&gt;
  class slide_view : public view_interface&lt;slide_view&lt;V&gt;&gt; {
    V <i>base_</i> <del>= V()</del>;                      <i>// exposition only</i>
    range_difference_t&lt;V&gt; <i>n_</i> <del>= 0</del>;       <i>// exposition only</i>
    [&hellip;]
  public:
    <del>slide_view() requires default_initializable&lt;V&gt; = default;</del>
    constexpr explicit slide_view(V base, range_difference_t&lt;V&gt; n);

    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>





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