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2023-10-24 21:59:30 -04:00
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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.overview.composition.promises"></a><a class="link" href="promises.html" title="Promises">Promises</a>
</h4></div></div></div>
<div class="note"><table border="0" summary="Note">
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../note.png"></td>
<th align="left">Note</th>
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<tr><td align="left" valign="top"><p>
This is an experimental feature.
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<p>
The <a class="link" href="../../reference/experimental__promise.html" title="experimental::promise"><code class="computeroutput"><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">promise</span></code></a> type and <a class="link" href="../../reference/experimental__use_promise.html" title="experimental::use_promise"><code class="computeroutput"><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span></code></a> completion token allow
eager execution and synchronisation of asynchronous operations. For example:
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">promise</span> <span class="special">=</span> <span class="identifier">async_read</span><span class="special">(</span>
<span class="identifier">stream</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">my_buffer</span><span class="special">),</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span><span class="special">);</span>
<span class="special">...</span> <span class="keyword">do</span> <span class="identifier">other</span> <span class="identifier">stuff</span> <span class="keyword">while</span> <span class="identifier">the</span> <span class="identifier">read</span> <span class="identifier">is</span> <span class="identifier">going</span> <span class="identifier">on</span> <span class="special">...</span>
<span class="identifier">promise</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span> <span class="comment">// completion the operation</span>
<span class="special">[](</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_read</span><span class="special">)</span>
<span class="special">{</span>
<span class="special">...</span>
<span class="special">});</span>
</pre>
<p>
Promises can be safely disregarded if the result is no longer required.
Different operations can be combined to either wait for all to complete
or for one to complete (and cancel the rest). For example, to wait for
one to complete:
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">timeout_promise</span> <span class="special">=</span>
<span class="identifier">timer</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">read_promise</span> <span class="special">=</span> <span class="identifier">async_read</span><span class="special">(</span>
<span class="identifier">stream</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">my_buffer</span><span class="special">),</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">promise</span> <span class="special">=</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">make_parallel_group</span><span class="special">(</span>
<span class="identifier">timeout_promise</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">),</span>
<span class="identifier">read_promise</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">)</span>
<span class="special">).</span><span class="identifier">async_wait</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">wait_for_one</span><span class="special">(),</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span><span class="special">);</span>
<span class="identifier">promise</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span>
<span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">array</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span> <span class="identifier">order</span><span class="special">,</span>
<span class="identifier">error_code</span> <span class="identifier">timer_ec</span><span class="special">,</span>
<span class="identifier">error_code</span> <span class="identifier">read_ec</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">read_size</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">order</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// timed out</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="comment">// completed in time</span>
<span class="special">}</span>
<span class="special">});</span>
</pre>
<p>
or to wait for all to complete:
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">write_promise</span> <span class="special">=</span> <span class="identifier">async_write</span><span class="special">(</span>
<span class="identifier">stream</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">my_write_buffer</span><span class="special">),</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">read_promise</span> <span class="special">=</span> <span class="identifier">async_read</span><span class="special">(</span>
<span class="identifier">stream</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">my_buffer</span><span class="special">),</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">promise</span> <span class="special">=</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">make_parallel_group</span><span class="special">(</span>
<span class="identifier">timeout_promise</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">),</span>
<span class="identifier">read_promise</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">)</span>
<span class="special">).</span><span class="identifier">async_wait</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">wait_for_all</span><span class="special">(),</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">use_promise</span><span class="special">);</span>
<span class="identifier">promise</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span>
<span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">array</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span> <span class="identifier">order</span><span class="special">,</span>
<span class="identifier">error_code</span> <span class="identifier">timer_ec</span><span class="special">,</span>
<span class="identifier">error_code</span> <span class="identifier">read_ec</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">read_size</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">});</span>
</pre>
<h6>
<a name="asio.overview.composition.promises.h0"></a>
<span><a name="asio.overview.composition.promises.see_also"></a></span><a class="link" href="promises.html#asio.overview.composition.promises.see_also">See
Also</a>
</h6>
<p>
<a class="link" href="../../reference/experimental__promise.html" title="experimental::promise">promise</a>, <a class="link" href="../../reference/experimental__use_promise.html" title="experimental::use_promise">use_promise</a>.
</p>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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