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<title>coroutine</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.coroutine"></a><a class="link" href="coroutine.html" title="coroutine">coroutine</a>
</h3></div></div></div>
<p>
Provides support for implementing stackless coroutines.
</p>
<pre class="programlisting">class coroutine
</pre>
<h5>
<a name="boost_asio.reference.coroutine.h0"></a>
<span class="phrase"><a name="boost_asio.reference.coroutine.member_functions"></a></span><a class="link" href="coroutine.html#boost_asio.reference.coroutine.member_functions">Member
Functions</a>
</h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a class="link" href="coroutine/coroutine.html" title="coroutine::coroutine"><span class="bold"><strong>coroutine</strong></span></a>
</p>
</td>
<td>
<p>
Constructs a coroutine in its initial state.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="coroutine/is_child.html" title="coroutine::is_child"><span class="bold"><strong>is_child</strong></span></a>
</p>
</td>
<td>
<p>
Returns true if the coroutine is the child of a fork.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="coroutine/is_complete.html" title="coroutine::is_complete"><span class="bold"><strong>is_complete</strong></span></a>
</p>
</td>
<td>
<p>
Returns true if the coroutine has reached its terminal state.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="coroutine/is_parent.html" title="coroutine::is_parent"><span class="bold"><strong>is_parent</strong></span></a>
</p>
</td>
<td>
<p>
Returns true if the coroutine is the parent of a fork.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
The <code class="computeroutput">coroutine</code> class may be used to implement stackless coroutines.
The class itself is used to store the current state of the coroutine.
</p>
<p>
Coroutines are copy-constructible and assignable, and the space overhead
is a single int. They can be used as a base class:
</p>
<pre class="programlisting">class session : coroutine
{
...
};
</pre>
<p>
or as a data member:
</p>
<pre class="programlisting">class session
{
...
coroutine coro_;
};
</pre>
<p>
or even bound in as a function argument using lambdas or <code class="computeroutput">bind()</code>.
The important thing is that as the application maintains a copy of the object
for as long as the coroutine must be kept alive.
</p>
<h5>
<a name="boost_asio.reference.coroutine.h1"></a>
<span class="phrase"><a name="boost_asio.reference.coroutine.pseudo_keywords"></a></span><a class="link" href="coroutine.html#boost_asio.reference.coroutine.pseudo_keywords">Pseudo-keywords</a>
</h5>
<p>
A coroutine is used in conjunction with certain "pseudo-keywords",
which are implemented as macros. These macros are defined by a header file:
</p>
<pre class="programlisting">#include <boost/asio/yield.hpp>
</pre>
<p>
and may conversely be undefined as follows:
</p>
<pre class="programlisting">#include <boost/asio/unyield.hpp>
</pre>
<p>
<span class="bold"><strong>reenter</strong></span>
</p>
<p>
The <code class="computeroutput">reenter</code> macro is used to define the body of a coroutine.
It takes a single argument: a pointer or reference to a coroutine object.
For example, if the base class is a coroutine object you may write:
</p>
<pre class="programlisting">reenter (this)
{
... coroutine body ...
}
</pre>
<p>
and if a data member or other variable you can write:
</p>
<pre class="programlisting">reenter (coro_)
{
... coroutine body ...
}
</pre>
<p>
When <code class="computeroutput">reenter</code> is executed at runtime, control jumps to the location
of the last <code class="computeroutput">yield</code> or <code class="computeroutput">fork</code>.
</p>
<p>
The coroutine body may also be a single statement, such as:
</p>
<pre class="programlisting">reenter (this) for (;;)
{
...
}
</pre>
<p>
<span class="bold"><strong>Limitation:</strong></span> The <code class="computeroutput">reenter</code> macro
is implemented using a switch. This means that you must take care when using
local variables within the coroutine body. The local variable is not allowed
in a position where reentering the coroutine could bypass the variable definition.
</p>
<p>
<span class="bold"><strong>yield <span class="emphasis"><em>statement</em></span></strong></span>
</p>
<p>
This form of the <code class="computeroutput">yield</code> keyword is often used with asynchronous
operations:
</p>
<pre class="programlisting">yield socket_->async_read_some(buffer(*buffer_), *this);
</pre>
<p>
This divides into four logical steps:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput">yield</code> saves the current state of the coroutine.
</li>
<li class="listitem">
The statement initiates the asynchronous operation.
</li>
<li class="listitem">
The resume point is defined immediately following the statement.
</li>
<li class="listitem">
Control is transferred to the end of the coroutine body.
</li>
</ul></div>
<p>
When the asynchronous operation completes, the function object is invoked
and <code class="computeroutput">reenter</code> causes control to transfer to the resume point.
It is important to remember to carry the coroutine state forward with the
asynchronous operation. In the above snippet, the current class is a function
object object with a coroutine object as base class or data member.
</p>
<p>
The statement may also be a compound statement, and this permits us to define
local variables with limited scope:
</p>
<pre class="programlisting">yield
{
mutable_buffers_1 b = buffer(*buffer_);
socket_->async_read_some(b, *this);
}
</pre>
<p>
<span class="bold"><strong>yield return <span class="emphasis"><em>expression</em></span> ;</strong></span>
</p>
<p>
This form of <code class="computeroutput">yield</code> is often used in generators or coroutine-based
parsers. For example, the function object:
</p>
<pre class="programlisting">struct interleave : coroutine
{
istream& is1;
istream& is2;
char operator()(char c)
{
reenter (this) for (;;)
{
yield return is1.get();
yield return is2.get();
}
}
};
</pre>
<p>
defines a trivial coroutine that interleaves the characters from two input
streams.
</p>
<p>
This type of <code class="computeroutput">yield</code> divides into three logical steps:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput">yield</code> saves the current state of the coroutine.
</li>
<li class="listitem">
The resume point is defined immediately following the semicolon.
</li>
<li class="listitem">
The value of the expression is returned from the function.
</li>
</ul></div>
<p>
<span class="bold"><strong>yield ;</strong></span>
</p>
<p>
This form of <code class="computeroutput">yield</code> is equivalent to the following steps:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput">yield</code> saves the current state of the coroutine.
</li>
<li class="listitem">
The resume point is defined immediately following the semicolon.
</li>
<li class="listitem">
Control is transferred to the end of the coroutine body.
</li>
</ul></div>
<p>
This form might be applied when coroutines are used for cooperative threading
and scheduling is explicitly managed. For example:
</p>
<pre class="programlisting">struct task : coroutine
{
...
void operator()()
{
reenter (this)
{
while (... not finished ...)
{
... do something ...
yield;
... do some more ...
yield;
}
}
}
...
};
...
task t1, t2;
for (;;)
{
t1();
t2();
}
</pre>
<p>
<span class="bold"><strong>yield break ;</strong></span>
</p>
<p>
The final form of <code class="computeroutput">yield</code> is used to explicitly terminate the
coroutine. This form is comprised of two steps:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput">yield</code> sets the coroutine state to indicate termination.
</li>
<li class="listitem">
Control is transferred to the end of the coroutine body.
</li>
</ul></div>
<p>
Once terminated, calls to <code class="computeroutput">is_complete()</code> return true and the
coroutine cannot be reentered.
</p>
<p>
Note that a coroutine may also be implicitly terminated if the coroutine
body is exited without a yield, e.g. by return, throw or by running to the
end of the body.
</p>
<p>
<span class="bold"><strong>fork <span class="emphasis"><em>statement</em></span></strong></span>
</p>
<p>
The <code class="computeroutput">fork</code> pseudo-keyword is used when "forking" a coroutine,
i.e. splitting it into two (or more) copies. One use of <code class="computeroutput">fork</code>
is in a server, where a new coroutine is created to handle each client connection:
</p>
<pre class="programlisting">reenter (this)
{
do
{
socket_.reset(new tcp::socket(io_context_));
yield acceptor->async_accept(*socket_, *this);
fork server(*this)();
} while (is_parent());
... client-specific handling follows ...
}
</pre>
<p>
The logical steps involved in a <code class="computeroutput">fork</code> are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput">fork</code> saves the current state of the coroutine.
</li>
<li class="listitem">
The statement creates a copy of the coroutine and either executes it
immediately or schedules it for later execution.
</li>
<li class="listitem">
The resume point is defined immediately following the semicolon.
</li>
<li class="listitem">
For the "parent", control immediately continues from the next
line.
</li>
</ul></div>
<p>
The functions <code class="computeroutput">is_parent()</code> and <code class="computeroutput">is_child()</code> can be
used to differentiate between parent and child. You would use these functions
to alter subsequent control flow.
</p>
<p>
Note that <code class="computeroutput">fork</code> doesn't do the actual forking by itself. It is
the application's responsibility to create a clone of the coroutine and call
it. The clone can be called immediately, as above, or scheduled for delayed
execution using something like <code class="computeroutput">io_context::post()</code>.
</p>
<h5>
<a name="boost_asio.reference.coroutine.h2"></a>
<span class="phrase"><a name="boost_asio.reference.coroutine.alternate_macro_names"></a></span><a class="link" href="coroutine.html#boost_asio.reference.coroutine.alternate_macro_names">Alternate
macro names</a>
</h5>
<p>
If preferred, an application can use macro names that follow a more typical
naming convention, rather than the pseudo-keywords. These are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput">BOOST_ASIO_CORO_REENTER</code> instead of <code class="computeroutput">reenter</code>
</li>
<li class="listitem">
<code class="computeroutput">BOOST_ASIO_CORO_YIELD</code> instead of <code class="computeroutput">yield</code>
</li>
<li class="listitem">
<code class="computeroutput">BOOST_ASIO_CORO_FORK</code> instead of <code class="computeroutput">fork</code>
</li>
</ul></div>
<h5>
<a name="boost_asio.reference.coroutine.h3"></a>
<span class="phrase"><a name="boost_asio.reference.coroutine.requirements"></a></span><a class="link" href="coroutine.html#boost_asio.reference.coroutine.requirements">Requirements</a>
</h5>
<p>
<span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/coroutine.hpp</code>
</p>
<p>
<span class="emphasis"><em>Convenience header: </em></span><code class="literal">boost/asio.hpp</code>
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2018 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>)
</p>
</div></td>
</tr></table>
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