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<title>Completion Token Adapters</title>
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../../asio.png"></td></tr></table>
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<a accesskey="p" href="compose.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../composition.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="type_erasure.html"><img src="../../../next.png" alt="Next"></a>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="asio.overview.composition.token_adapters"></a><a class="link" href="token_adapters.html" title="Completion Token Adapters">Completion
Token Adapters</a>
</h4></div></div></div>
<p>
A <span class="emphasis"><em>completion token adapter</em></span> is a utility that can be
generically applied to a <a class="link" href="../model/completion_tokens.html" title="Completion Tokens">completion
token</a>, to produce a new completion token with modified behaviour.
Common uses of completion token adapters include:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Automatically wrapping the completion handler to add <a class="link" href="../model/associators.html" title="Associated Characteristics and Associators">associated
characteristics</a>.
</li>
<li class="listitem">
Transforming the completion signature and arguments passed to the completion
handler.
</li>
</ul></div>
<p>
Asio includes a number of completion token adapters as described below.
</p>
<h6>
<a name="asio.overview.composition.token_adapters.h0"></a>
<span><a name="asio.overview.composition.token_adapters.bind_executor__bind_allocator__and_bind_cancellation_slot"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.bind_executor__bind_allocator__and_bind_cancellation_slot">bind_executor,
bind_allocator, and bind_cancellation_slot</a>
</h6>
<p>
The <a class="link" href="../../reference/bind_executor.html" title="bind_executor"><code class="computeroutput"><span class="identifier">bind_executor</span></code></a>
function adapts a completion token to imbue the completion handler with
an <a class="link" href="../model/executors.html" title="Executors">associated executor</a>.
</p>
<p>
This example shows the <code class="computeroutput"><span class="identifier">bind_executor</span></code>
adapter applied to a lambda, to specify that the handler should execute
in the specified strand. The arguments to the completion handler are passed
through as-is.
</p>
<pre class="programlisting"><span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</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">bind_executor</span><span class="special">(</span><span class="identifier">my_strand</span><span class="special">,</span>
<span class="special">[](</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">error</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}));</span>
</pre>
<p>
When applied to completion tokens that cause the initiating function to
produce a result, such as <a class="link" href="../../reference/use_awaitable.html" title="use_awaitable"><code class="computeroutput"><span class="identifier">use_awaitable</span></code></a>, the result is returned
unmodified.
</p>
<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">awaitable</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="identifier">my_coroutine</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span> <span class="special">=</span>
<span class="identifier">co_await</span> <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</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">bind_executor</span><span class="special">(</span><span class="identifier">my_strand</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_awaitable</span><span class="special">));</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<p>
The <a class="link" href="../../reference/bind_allocator.html" title="bind_allocator"><code class="computeroutput"><span class="identifier">bind_allocator</span></code></a>
and <a class="link" href="../../reference/bind_cancellation_slot.html" title="bind_cancellation_slot"><code class="computeroutput"><span class="identifier">bind_cancellation_slot</span></code></a> adapters
work similarly, to imbue the completion handler with an <a class="link" href="../model/allocators.html" title="Allocators">associated
allocator</a> or <a class="link" href="../model/cancellation.html" title="Cancellation">associated
cancellation slot</a> respectively.
</p>
<h6>
<a name="asio.overview.composition.token_adapters.h1"></a>
<span><a name="asio.overview.composition.token_adapters.redirect_error"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.redirect_error">redirect_error</a>
</h6>
<p>
The <a class="link" href="../../reference/redirect_error.html" title="redirect_error"><code class="computeroutput"><span class="identifier">redirect_error</span></code></a>
function adapts a completion token to capture the <code class="computeroutput"><span class="identifier">error_code</span></code>
produced by an operation into a specified variable. In doing so, it modifies
the completion signature to remove the initial <code class="computeroutput"><span class="identifier">error_code</span></code>
parameter.
</p>
<p>
This example shows the <code class="computeroutput"><span class="identifier">redirect_error</span></code>
adapter applied to a lambda, to specify that the error should be captured
into <code class="computeroutput"><span class="identifier">my_error</span></code>. The <code class="computeroutput"><span class="identifier">error_code</span></code> is no longer passed to the
completion handler, but the remaining arguments are passed through as-is.
</p>
<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">my_error</span><span class="special">;</span> <span class="comment">// N.B. must be valid until operation completes</span>
<span class="comment">// ...</span>
<span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</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">redirect_error</span><span class="special">(</span>
<span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">},</span> <span class="identifier">my_error</span><span class="special">));</span>
</pre>
<p>
When applied to completion tokens that cause the initiating function to
produce a result, such as <a class="link" href="../../reference/use_awaitable.html" title="use_awaitable"><code class="computeroutput"><span class="identifier">use_awaitable</span></code></a>, the result is returned
unmodified. However, if the operation fails, the <code class="computeroutput"><span class="identifier">co_await</span></code>
expression will no longer throw an exception on resumption.
</p>
<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">awaitable</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="identifier">my_coroutine</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">my_error</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span> <span class="special">=</span>
<span class="identifier">co_await</span> <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</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">redirect_error</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_awaitable</span><span class="special">,</span> <span class="identifier">my_error</span><span class="special">));</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<h6>
<a name="asio.overview.composition.token_adapters.h2"></a>
<span><a name="asio.overview.composition.token_adapters.as_tuple"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.as_tuple">as_tuple</a>
</h6>
<p>
The <a class="link" href="../../reference/as_tuple.html" title="as_tuple"><code class="computeroutput"><span class="identifier">as_tuple</span></code></a>
adapter can be used to specify that the completion handler arguments should
be combined into a single tuple argument.
</p>
<p>
For example, the <code class="computeroutput"><span class="identifier">as_tuple</span></code>
adapter may be used in conjunction with <a class="link" href="../../reference/use_awaitable.html" title="use_awaitable"><code class="computeroutput"><span class="identifier">use_awaitable</span></code></a> and structured bindings
as follows:
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="special">[</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">n</span><span class="special">]</span> <span class="special">=</span>
<span class="identifier">co_await</span> <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</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">as_tuple</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_awaitable</span><span class="special">));</span>
</pre>
<p>
This adapter may also be used as a default completion token:
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">default_token</span> <span class="special">=</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">as_tuple_t</span><span class="special">&lt;</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_awaitable_t</span><span class="special">&lt;&gt;&gt;;</span>
<span class="keyword">using</span> <span class="identifier">tcp_socket</span> <span class="special">=</span> <span class="identifier">default_token</span><span class="special">::</span><span class="identifier">as_default_on_t</span><span class="special">&lt;</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span class="special">&gt;;</span>
<span class="comment">// ...</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">awaitable</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="identifier">do_read</span><span class="special">(</span><span class="identifier">tcp_socket</span> <span class="identifier">my_socket</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="keyword">auto</span> <span class="special">[</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">n</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">co_await</span> <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span><span class="identifier">my_buffer</span><span class="special">);</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<h6>
<a name="asio.overview.composition.token_adapters.h3"></a>
<span><a name="asio.overview.composition.token_adapters.as_single"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.as_single">as_single</a>
</h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
This is an experimental feature.
</p></td></tr>
</table></div>
<p>
The <a class="link" href="../../reference/experimental__as_single.html" title="experimental::as_single"><code class="computeroutput"><span class="identifier">experimental</span><span class="special">::</span><span class="identifier">as_single</span></code></a> adapter can be used
to specify that the completion handler arguments should be combined into
a single argument. For completion signatures with a single parameter, the
argument is passed through as-is. For signatures with two or more parameters,
the arguments are combined into a tuple.
</p>
<p>
For example, when applied to a timer wait operation, the single <code class="computeroutput"><span class="identifier">error_code</span></code> argument is passed directly
to the completion handler:
</p>
<pre class="programlisting"><span class="identifier">my_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">as_single</span><span class="special">(</span>
<span class="special">[](</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">error</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}));</span>
</pre>
<p>
When applied to a socket read operation, where the completion signature
specifies two parameters, the handler is passed the result as a tuple:
</p>
<pre class="programlisting"><span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</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">as_single</span><span class="special">,</span>
<span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;</span> <span class="identifier">result</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}));</span>
</pre>
<h6>
<a name="asio.overview.composition.token_adapters.h4"></a>
<span><a name="asio.overview.composition.token_adapters.append"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.append">append</a>
</h6>
<p>
The <a class="link" href="../../reference/append.html" title="append"><code class="computeroutput"><span class="identifier">append</span></code></a>
completion token adapter can be used to pass additional completion handler
arguments at the end of the completion signature.
</p>
<p>
For example:
</p>
<pre class="programlisting"><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">append</span><span class="special">(</span>
<span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">},</span>
<span class="number">42</span>
<span class="special">)</span>
<span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">f</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">append</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span><span class="special">,</span>
<span class="number">42</span>
<span class="special">)</span>
<span class="special">);</span>
</pre>
<h6>
<a name="asio.overview.composition.token_adapters.h5"></a>
<span><a name="asio.overview.composition.token_adapters.prepend"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.prepend">prepend</a>
</h6>
<p>
The <a class="link" href="../../reference/prepend.html" title="prepend"><code class="computeroutput"><span class="identifier">prepend</span></code></a>
completion token adapter can be used to pass additional completion handler
arguments before the existing completion handler arguments.
</p>
<p>
For example:
</p>
<pre class="programlisting"><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">prepend</span><span class="special">(</span>
<span class="special">[](</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">},</span>
<span class="number">42</span>
<span class="special">)</span>
<span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&gt;&gt;</span> <span class="identifier">f</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">prepend</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span><span class="special">,</span>
<span class="number">42</span>
<span class="special">)</span>
<span class="special">);</span>
</pre>
<h6>
<a name="asio.overview.composition.token_adapters.h6"></a>
<span><a name="asio.overview.composition.token_adapters.consign"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.consign">consign</a>
</h6>
<p>
The <a class="link" href="../../reference/consign.html" title="consign"><code class="computeroutput"><span class="identifier">consign</span></code></a>
completion token adapter can be used to attach additional values to a completion
handler. This is typically used to keep at least one copy of an object,
such as a smart pointer, alive until the completion handler is called.
</p>
<p>
For example:
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">timer1</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special">&lt;</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span><span class="special">&gt;(</span><span class="identifier">my_io_context</span><span class="special">);</span>
<span class="identifier">timer1</span><span class="special">-&gt;</span><span class="identifier">expires_after</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
<span class="identifier">timer1</span><span class="special">-&gt;</span><span class="identifier">async_wait</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">consign</span><span class="special">(</span>
<span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">},</span>
<span class="identifier">timer1</span>
<span class="special">)</span>
<span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">timer2</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special">&lt;</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span><span class="special">&gt;(</span><span class="identifier">my_io_context</span><span class="special">);</span>
<span class="identifier">timer2</span><span class="special">-&gt;</span><span class="identifier">expires_after</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">30</span><span class="special">));</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="identifier">f</span> <span class="special">=</span>
<span class="identifier">timer2</span><span class="special">-&gt;</span><span class="identifier">async_wait</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">consign</span><span class="special">(</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span><span class="special">,</span>
<span class="identifier">timer2</span>
<span class="special">)</span>
<span class="special">);</span>
</pre>
<h6>
<a name="asio.overview.composition.token_adapters.h7"></a>
<span><a name="asio.overview.composition.token_adapters.see_also"></a></span><a class="link" href="token_adapters.html#asio.overview.composition.token_adapters.see_also">See
Also</a>
</h6>
<p>
<a class="link" href="../../reference/bind_executor.html" title="bind_executor">bind_executor</a>, <a class="link" href="../../reference/bind_allocator.html" title="bind_allocator">bind_allocator</a>, <a class="link" href="../../reference/bind_cancellation_slot.html" title="bind_cancellation_slot">bind_cancellation_slot</a>,
<a class="link" href="../../reference/redirect_error.html" title="redirect_error">redirect_error</a>, <a class="link" href="../../reference/as_tuple.html" title="as_tuple">as_tuple</a>, <a class="link" href="../../reference/experimental__as_single.html" title="experimental::as_single">experimental::as_single</a>,
<a class="link" href="../../reference/append.html" title="append">append</a>, <a class="link" href="../../reference/prepend.html" title="prepend">prepend</a>.
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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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