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<title>Issue 620: valid uses of empty valarrays</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="620"><a href="lwg-defects.html#620">620</a>. valid uses of empty valarrays</h3>
<p><b>Section:</b> 29.6.2.2 <a href="https://wg21.link/valarray.cons">[valarray.cons]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2007-01-20 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#valarray.cons">issues</a> in [valarray.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
        <p>

The <i>Effects</i>  clause for the  default <code>valarray</code> ctor
suggests  that  it  is possible  to  increase  the  size of  an  empty
<code>valarray</code>  object   by  calling  other   non-const  member
functions of the class besides <code>resize()</code>. However, such an
interpretation would  be contradicted by  the requirement on  the copy
assignment  operator  (and  apparently   also  that  on  the  computed
assignments)  that the  assigned arrays  be  the same  size.  See  the
reflector discussion starting with c++std-lib-17871.

        </p>
        <p>

In  addition,  <i>Footnote</i> 280  uses  some questionable  normative
language.

        </p>


<p id="res-620"><b>Proposed resolution:</b></p>
        <p>

Reword the <i>Effects</i> clause and <i>Footnote 280</i> as follows (29.6.2.2 <a href="https://wg21.link/valarray.cons">[valarray.cons]</a>):

        </p>
        <blockquote>
            <p>

<code>valarray();</code>

            </p>
            <p>

<i>Effects</i>:      Constructs      an      object      of      class
<code>valarray&lt;T&gt;</code>,<sup>279)</sup>    which    has    zero
length<del> until it is passed into a library function as a modifiable
lvalue or through a non-constant this pointer</del>.<sup>280)</sup>

            </p>
            <p>

<ins><i>Postcondition</i>: <code>size() == 0</code>.</ins>

            </p>
            <p>

<i>Footnote  280</i>:  This default  constructor  is essential,  since
arrays  of  <code>valarray</code>  <del>are  likely to  prove  useful.
There  shall also  be  a way  to change  the  size of  an array  after
initialization;  this  is  supplied  by the  semantics</del>  <ins>may be
useful.   The  length  of  an  empty  array  can  be  increased  after
initialization  by  means</ins>  of the  <code>resize()</code>  member
function.

            </p>
        </blockquote>





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