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<title>Issue 3063: Parallel algorithms in &lt;memory&gt; are underspecified</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#New">New</a> status.</em></p>
<h3 id="3063"><a href="lwg-active.html#3063">3063</a>. Parallel algorithms in <code>&lt;memory&gt;</code> are underspecified</h3>
<p><b>Section:</b> 26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2018-02-12 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#specialized.algorithms">active issues</a> in [specialized.algorithms].</p>
<p><b>View all other</b> <a href="lwg-index.html#specialized.algorithms">issues</a> in [specialized.algorithms].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The parallel forms of the uninitialized memory algorithms in <code>&lt;memory&gt;</code> are
underspecified in two ways. First, they missed the change that all parallel
algorithms require at least Forward Iterators, even for input ranges. See
<a href="https://wg21.link/p0467r2">P0467R2</a> for more details.
<p/>
The second problem is that they do not have a separate specification to
the serial forms. This is a problem in two ways. First, there is no more
blanket wording saying a parallel policy algorithm has the same semantics
as the serial form unless otherwise specified, so in principle these algorithms
are totally unspecified. However, assuming that intent, all of the existing
specifications use an as-if formulation with code that is explicitly serial in
nature, so need a new specification that talks about the effects on each
element without including the iteration order.
</p>

<p><i>[2018-02-20, Priority set to 3 after mailing list discussion]</i></p>



<p id="res-3063"><b>Proposed resolution:</b></p>





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