ceph/src/boost/libs/fiber/doc/html/fiber/callbacks/overview.html

   1 <html>
   2 <head>
   3 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
   4 <title>Overview</title>
   5 <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
   6 <meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
   7 <link rel="home" href="../../index.html" title="Chapter&#160;1.&#160;Fiber">
   8 <link rel="up" href="../callbacks.html" title="Integrating Fibers with Asynchronous Callbacks">
   9 <link rel="prev" href="../callbacks.html" title="Integrating Fibers with Asynchronous Callbacks">
  10 <link rel="next" href="return_errorcode.html" title="Return Errorcode">
  11 </head>
  12 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
  13 <table cellpadding="2" width="100%"><tr>
  14 <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
  15 <td align="center"><a href="../../../../../../index.html">Home</a></td>
  16 <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
  17 <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
  18 <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
  19 <td align="center"><a href="../../../../../../more/index.htm">More</a></td>
  20 </tr></table>
  21 <hr>
  22 <div class="spirit-nav">
  23 <a accesskey="p" href="../callbacks.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../callbacks.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="return_errorcode.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
  24 </div>
  25 <div class="section">
  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="fiber.callbacks.overview"></a><a class="link" href="overview.html" title="Overview">Overview</a>
  28 </h3></div></div></div>
  29 <p>
  30         One of the primary benefits of <span class="bold"><strong>Boost.Fiber</strong></span>
  31         is the ability to use asynchronous operations for efficiency, while at the
  32         same time structuring the calling code <span class="emphasis"><em>as if</em></span> the operations
  33         were synchronous. Asynchronous operations provide completion notification
  34         in a variety of ways, but most involve a callback function of some kind.
  35         This section discusses tactics for interfacing <span class="bold"><strong>Boost.Fiber</strong></span>
  36         with an arbitrary async operation.
  37       </p>
  38 <p>
  39         For purposes of illustration, consider the following hypothetical API:
  40       </p>
  41 <p>
  42 </p>
  43 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">AsyncAPI</span> <span class="special">{</span>
  44 <span class="keyword">public</span><span class="special">:</span>
  45     <span class="comment">// constructor acquires some resource that can be read and written</span>
  46     <span class="identifier">AsyncAPI</span><span class="special">();</span>
  47
  48     <span class="comment">// callbacks accept an int error code; 0 == success</span>
  49     <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">errorcode</span><span class="special">;</span>
  50
  51     <span class="comment">// write callback only needs to indicate success or failure</span>
  52     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Fn</span> <span class="special">&gt;</span>
  53     <span class="keyword">void</span> <span class="identifier">init_write</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">Fn</span> <span class="special">&amp;&amp;</span> <span class="identifier">callback</span><span class="special">);</span>
  54
  55     <span class="comment">// read callback needs to accept both errorcode and data</span>
  56     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Fn</span> <span class="special">&gt;</span>
  57     <span class="keyword">void</span> <span class="identifier">init_read</span><span class="special">(</span> <span class="identifier">Fn</span> <span class="special">&amp;&amp;</span> <span class="identifier">callback</span><span class="special">);</span>
  58
  59     <span class="comment">// ... other operations ...</span>
  60 <span class="special">};</span>
  61 </pre>
  62 <p>
  63       </p>
  64 <p>
  65         The significant points about each of <code class="computeroutput"><span class="identifier">init_write</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">init_read</span><span class="special">()</span></code> are:
  66       </p>
  67 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
  68 <li class="listitem">
  69             The <code class="computeroutput"><span class="identifier">AsyncAPI</span></code> method only
  70             initiates the operation. It returns immediately, while the requested
  71             operation is still pending.
  72           </li>
  73 <li class="listitem">
  74             The method accepts a callback. When the operation completes, the callback
  75             is called with relevant parameters (error code, data if applicable).
  76           </li>
  77 </ul></div>
  78 <p>
  79         We would like to wrap these asynchronous methods in functions that appear
  80         synchronous by blocking the calling fiber until the operation completes.
  81         This lets us use the wrapper function&#8217;s return value to deliver relevant data.
  82       </p>
  83 <div class="tip"><table border="0" summary="Tip">
  84 <tr>
  85 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
  86 <th align="left">Tip</th>
  87 </tr>
  88 <tr><td align="left" valign="top"><p>
  89           <a class="link" href="../synchronization/futures/promise.html#class_promise"><code class="computeroutput">promise&lt;&gt;</code></a> and <a class="link" href="../synchronization/futures/future.html#class_future"><code class="computeroutput">future&lt;&gt;</code></a> are your friends
  90           here.
  91         </p></td></tr>
  92 </table></div>
  93 </div>
  94 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
  95 <td align="left"></td>
  96 <td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
  97         Distributed under the Boost Software License, Version 1.0. (See accompanying
  98         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>)
  99       </p>
 100 </div></td>
 101 </tr></table>
 102 <hr>
 103 <div class="spirit-nav">
 104 <a accesskey="p" href="../callbacks.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../callbacks.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="return_errorcode.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
 105 </div>
 106 </body>
 107 </html>