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<title>Issue 2556: Wide contract for future::share()</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++17">C++17</a> status.</em></p>
<h3 id="2556"><a href="lwg-defects.html#2556">2556</a>. Wide contract for <code>future::share()</code></h3>
<p><b>Section:</b> 32.10.7 <a href="https://wg21.link/futures.unique.future">[futures.unique.future]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Agust&iacute;n K-ballo Berg&eacute; <b>Opened:</b> 2015-11-05 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.unique.future">issues</a> in [futures.unique.future].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>future::share()</code> is not <code>noexcept</code>, it has a narrow contact requiring <code>valid()</code> as per the blanket
wording in  [futures.unique_future] p3. Its effects, however, are <code>return shared_future&lt;R&gt;(std::move(*this))</code>,
which is <code>noexcept</code> as it has a wide contract. If the source <code>future</code> isn't valid then the target
<code>shared_future</code> simply won't be valid either. There appears to be no technical reason preventing <code>future::share()</code>
from having a wide contract, and thus being <code>noexcept</code>.
</p>

<p><i>[2016-08-03 Chicago]</i></p>

<p>Fri AM: Moved to Tentatively Ready</p>


<p id="res-2556"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4567.</p>

<ol>
<li><p>Change  [futures.unique_future] as indicated:</p>

<blockquote>
<p>
-3- The effect of calling any member function other than the destructor, the move-assignment operator, <ins><code>share</code>,</ins>
or <code>valid</code> on a <code>future</code> object for which <code>valid() == false</code> is undefined. [<i>Note</i>: Implementations
are encouraged to detect this case and throw an object of type <code>future_error</code> with an error condition of
<code>future_errc::no_state</code>. &mdash; <i>end note</i>]
</p>
<blockquote><pre>
namespace std {
  template &lt;class R&gt;
  class future {
  public:
    [&hellip;]
    shared_future&lt;R&gt; share() <ins>noexcept</ins>;
    [&hellip;]
  };
}
</pre></blockquote>
<p>
[&hellip;]
</p>
<pre>
shared_future&lt;R&gt; share() <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
-12- <i>Returns</i>: <code>shared_future&lt;R&gt;(std::move(*this))</code>.
<p/>
-13- <i>Postcondition</i>: <code>valid() == false</code>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
</blockquote>
</li>
</ol>





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