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<title>Issue 1304: Missing preconditions for shared_future</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#Resolved">Resolved</a> status.</em></p>
<h3 id="1304"><a href="lwg-defects.html#1304">1304</a>. Missing preconditions for <code>shared_future</code></h3>
<p><b>Section:</b> 32.10.8 <a href="https://wg21.link/futures.shared.future">[futures.shared.future]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2010-01-23 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.shared.future">issues</a> in [futures.shared.future].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>

<p>
The revised futures package in the current working paper simplified the
<code>is_ready/has_exception/has_value</code> set of APIs, replacing them with a
single 'valid' method.  This method is used in many places to signal pre- and
post- conditions, but that edit is not complete.  Each method on a
<code>shared_future</code> that requires an associated state should have a
pre-condition that <code>valid() == true</code>.
</p>

<p><i>[
2010-01-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>


<p><i>[
2010 Pittsburgh:
]</i></p>


<blockquote><p>
Moved to <del>NAD Editorial</del><ins>Resolved</ins>.  Rationale added below.
</p></blockquote>



<p><b>Rationale:</b></p>
<p>
Solved by <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3058.html">N3058</a>.
</p>


<p id="res-1304"><b>Proposed resolution:</b></p>
<p>
Insert the following extra paragraphs:
</p>

<p>
In  [futures.shared_future]
</p>

<blockquote><pre>
shared_future();
</pre>
<blockquote>
<p>
4 <i>Effects:</i> constructs ...
</p>

<p><ins>
<i>Postcondition:</i> <code>valid() == false</code>.
</ins></p>

<p><ins>
<i>Throws:</i> nothing.
</ins></p>
</blockquote>
</blockquote>

<blockquote><pre>
void wait() const;
</pre>
<blockquote>

<p><ins>
<i>Requires:</i> <code>valid() == true</code>.
</ins></p>

<p>
22 <i>Effects:</i> if the associated ...
</p>
</blockquote>
</blockquote>

<blockquote><pre>
template &lt;class Rep, class Period&gt;
  bool wait_for(const chrono::duration&lt;Rep, Period&gt;&amp; rel_time) const;
</pre>
<blockquote>

<p><ins>
<i>Requires:</i> <code>valid() == true</code>.
</ins></p>

<p>
23 <i>Effects:</i> if the associated ...
</p>
</blockquote>
</blockquote>

<blockquote><pre>
template &lt;class Clock, class Duration&gt;
  bool wait_until(const chrono::time_point&lt;Clock, Duration&gt;&amp; abs_time) const;
</pre>
<blockquote>

<p><ins>
<i>Requires:</i> <code>valid() == true</code>.
</ins></p>

<p>
25 <i>Effects:</i> blocks until ...
</p>
</blockquote>
</blockquote>






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