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| <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"><span class="keyword">class</span> <span class="identifier">coroutine</span> |
| </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"><span class="identifier">coroutine</span></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"><span class="keyword">class</span> <span class="identifier">session</span> <span class="special">:</span> <span class="identifier">coroutine</span> |
| <span class="special">{</span> |
| <span class="special">...</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| or as a data member: |
| </p> |
| <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">session</span> |
| <span class="special">{</span> |
| <span class="special">...</span> |
| <span class="identifier">coroutine</span> <span class="identifier">coro_</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| or even bound in as a function argument using lambdas or <code class="computeroutput"><span class="identifier">bind</span><span class="special">()</span></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"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">asio</span><span class="special">/</span><span class="identifier">yield</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <p> |
| and may conversely be undefined as follows: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">asio</span><span class="special">/</span><span class="identifier">unyield</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <p> |
| <span class="bold"><strong>reenter</strong></span> |
| </p> |
| <p> |
| The <code class="computeroutput"><span class="identifier">reenter</span></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"><span class="identifier">reenter</span> <span class="special">(</span><span class="keyword">this</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="special">...</span> <span class="identifier">coroutine</span> <span class="identifier">body</span> <span class="special">...</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| and if a data member or other variable you can write: |
| </p> |
| <pre class="programlisting"><span class="identifier">reenter</span> <span class="special">(</span><span class="identifier">coro_</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="special">...</span> <span class="identifier">coroutine</span> <span class="identifier">body</span> <span class="special">...</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| When <code class="computeroutput"><span class="identifier">reenter</span></code> is executed |
| at runtime, control jumps to the location of the last <code class="computeroutput"><span class="identifier">yield</span></code> |
| or <code class="computeroutput"><span class="identifier">fork</span></code>. |
| </p> |
| <p> |
| The coroutine body may also be a single statement, such as: |
| </p> |
| <pre class="programlisting"><span class="identifier">reenter</span> <span class="special">(</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">for</span> <span class="special">(;;)</span> |
| <span class="special">{</span> |
| <span class="special">...</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>Limitation:</strong></span> The <code class="computeroutput"><span class="identifier">reenter</span></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"><span class="identifier">yield</span></code> keyword |
| is often used with asynchronous operations: |
| </p> |
| <pre class="programlisting"><span class="identifier">yield</span> <span class="identifier">socket_</span><span class="special">-></span><span class="identifier">async_read_some</span><span class="special">(</span><span class="identifier">buffer</span><span class="special">(*</span><span class="identifier">buffer_</span><span class="special">),</span> <span class="special">*</span><span class="keyword">this</span><span class="special">);</span> |
| </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"><span class="identifier">yield</span></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"><span class="identifier">reenter</span></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"><span class="identifier">yield</span> |
| <span class="special">{</span> |
| <span class="identifier">mutable_buffers_1</span> <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">buffer</span><span class="special">(*</span><span class="identifier">buffer_</span><span class="special">);</span> |
| <span class="identifier">socket_</span><span class="special">-></span><span class="identifier">async_read_some</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="special">*</span><span class="keyword">this</span><span class="special">);</span> |
| <span class="special">}</span> |
| </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"><span class="identifier">yield</span></code> is often |
| used in generators or coroutine-based parsers. For example, the function |
| object: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">interleave</span> <span class="special">:</span> <span class="identifier">coroutine</span> |
| <span class="special">{</span> |
| <span class="identifier">istream</span><span class="special">&</span> <span class="identifier">is1</span><span class="special">;</span> |
| <span class="identifier">istream</span><span class="special">&</span> <span class="identifier">is2</span><span class="special">;</span> |
| <span class="keyword">char</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">reenter</span> <span class="special">(</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">for</span> <span class="special">(;;)</span> |
| <span class="special">{</span> |
| <span class="identifier">yield</span> <span class="keyword">return</span> <span class="identifier">is1</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span> |
| <span class="identifier">yield</span> <span class="keyword">return</span> <span class="identifier">is2</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| defines a trivial coroutine that interleaves the characters from two input |
| streams. |
| </p> |
| <p> |
| This type of <code class="computeroutput"><span class="identifier">yield</span></code> divides |
| into three logical steps: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">yield</span></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"><span class="identifier">yield</span></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"><span class="identifier">yield</span></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"><span class="keyword">struct</span> <span class="identifier">task</span> <span class="special">:</span> <span class="identifier">coroutine</span> |
| <span class="special">{</span> |
| <span class="special">...</span> |
| <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()()</span> |
| <span class="special">{</span> |
| <span class="identifier">reenter</span> <span class="special">(</span><span class="keyword">this</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">while</span> <span class="special">(...</span> <span class="keyword">not</span> <span class="identifier">finished</span> <span class="special">...)</span> |
| <span class="special">{</span> |
| <span class="special">...</span> <span class="keyword">do</span> <span class="identifier">something</span> <span class="special">...</span> |
| <span class="identifier">yield</span><span class="special">;</span> |
| <span class="special">...</span> <span class="keyword">do</span> <span class="identifier">some</span> <span class="identifier">more</span> <span class="special">...</span> |
| <span class="identifier">yield</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| <span class="special">...</span> |
| <span class="special">};</span> |
| <span class="special">...</span> |
| <span class="identifier">task</span> <span class="identifier">t1</span><span class="special">,</span> <span class="identifier">t2</span><span class="special">;</span> |
| <span class="keyword">for</span> <span class="special">(;;)</span> |
| <span class="special">{</span> |
| <span class="identifier">t1</span><span class="special">();</span> |
| <span class="identifier">t2</span><span class="special">();</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>yield break ;</strong></span> |
| </p> |
| <p> |
| The final form of <code class="computeroutput"><span class="identifier">yield</span></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"><span class="identifier">yield</span></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"><span class="identifier">is_complete</span><span class="special">()</span></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"><span class="identifier">fork</span></code> pseudo-keyword is |
| used when "forking" a coroutine, i.e. splitting it into two (or |
| more) copies. One use of <code class="computeroutput"><span class="identifier">fork</span></code> |
| is in a server, where a new coroutine is created to handle each client connection: |
| </p> |
| <pre class="programlisting"><span class="identifier">reenter</span> <span class="special">(</span><span class="keyword">this</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">do</span> |
| <span class="special">{</span> |
| <span class="identifier">socket_</span><span class="special">.</span><span class="identifier">reset</span><span class="special">(</span><span class="keyword">new</span> <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span class="special">(</span><span class="identifier">io_service_</span><span class="special">));</span> |
| <span class="identifier">yield</span> <span class="identifier">acceptor</span><span class="special">-></span><span class="identifier">async_accept</span><span class="special">(*</span><span class="identifier">socket_</span><span class="special">,</span> <span class="special">*</span><span class="keyword">this</span><span class="special">);</span> |
| <span class="identifier">fork</span> <span class="identifier">server</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)();</span> |
| <span class="special">}</span> <span class="keyword">while</span> <span class="special">(</span><span class="identifier">is_parent</span><span class="special">());</span> |
| <span class="special">...</span> <span class="identifier">client</span><span class="special">-</span><span class="identifier">specific</span> <span class="identifier">handling</span> <span class="identifier">follows</span> <span class="special">...</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| The logical steps involved in a <code class="computeroutput"><span class="identifier">fork</span></code> |
| are: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">fork</span></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"><span class="identifier">is_parent</span><span class="special">()</span></code> |
| and <code class="computeroutput"><span class="identifier">is_child</span><span class="special">()</span></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"><span class="identifier">fork</span></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"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">post</span><span class="special">()</span></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"><span class="identifier">BOOST_ASIO_CORO_REENTER</span></code> |
| instead of <code class="computeroutput"><span class="identifier">reenter</span></code> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">BOOST_ASIO_CORO_YIELD</span></code> |
| instead of <code class="computeroutput"><span class="identifier">yield</span></code> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">BOOST_ASIO_CORO_FORK</span></code> |
| instead of <code class="computeroutput"><span class="identifier">fork</span></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 © 2003-2015 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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