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<title>Issue 543: valarray slice default constructor</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#CD1">CD1</a> status.</em></p>
<h3 id="543"><a href="lwg-defects.html#543">543</a>. valarray slice default constructor</h3>
<p><b>Section:</b> 29.6.4 <a href="https://wg21.link/class.slice">[class.slice]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2005-11-03 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
If one explicitly constructs a slice or glice with the default
constructor, does the standard require this slice to have any usable
state?  It says "creates a slice which specifies no elements", which
could be interpreted two ways:
</p>
<ol>
<li>There are no elements to which the slice refers (i.e. undefined).</li>
<li>The slice specifies an array with no elements in it (i.e. defined).</li>
</ol>
<p>
Here is a bit of code to illustrate:
</p>
<blockquote><pre>
#include &lt;iostream&gt;
#include &lt;valarray&gt;

int main()
{
    std::valarray&lt;int&gt; v(10);
    std::valarray&lt;int&gt; v2 = v[std::slice()];
    std::cout &lt;&lt; "v[slice()].size() = " &lt;&lt; v2.size() &lt;&lt; '\n';
}
</pre></blockquote>

<p>
Is the behavior undefined?  Or should the output be:
</p>

<blockquote><pre>
v[slice()].size() = 0
</pre></blockquote>

<p>
There is a similar question and wording for gslice at 26.3.6.1p1.
</p>


<p id="res-543"><b>Proposed resolution:</b></p>

<p><i>[Martin suggests removing the second sentence in 29.6.4.2 <a href="https://wg21.link/cons.slice">[cons.slice]</a> as well.]</i></p>


<p>
Change 29.6.4.2 <a href="https://wg21.link/cons.slice">[cons.slice]</a>:
</p>

<blockquote><p>
1 - <del>The default constructor for <code>slice</code> creates a <code>slice</code>
which specifies no elements.</del> <ins>The default constructor is equivalent to
<code>slice(0, 0, 0)</code>.</ins> A default constructor is provided only to permit
the declaration of arrays of slices. The constructor with arguments for a slice
takes a start, length, and stride parameter.
</p></blockquote>

<p>
Change 29.6.6.2 <a href="https://wg21.link/gslice.cons">[gslice.cons]</a>:
</p>

<blockquote><p>
1 - <del>The default constructor creates a <code>gslice</code> which specifies no
elements.</del> <ins>The default constructor is equivalent to <code>gslice(0,
valarray&lt;size_t&gt;(), valarray&lt;size_t&gt;())</code>.</ins> The constructor
with arguments builds a <code>gslice</code> based on a specification of start,
lengths, and strides, as explained in the previous section.
</p></blockquote>






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