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asio-integration.html

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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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  3. <title>ASIO/Networking TS : Boost &lt; 1.70 - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">ASIO/Networking TS : Boost &lt; 1.70</h1></div></div></div>
    14. 

  14.   <div class="toc"><dl class="toc">
    15. 

  15. <dt>
    16. 

  16. <dd><dl>
    17. 

  17. <dt>
    18. 

  18. <dd><dl>
    19. 

  19. <dt><a href="#compatibility-note">Compatibility note</a></dt>
    20. 

  20. <dt><a href="#use-case">Use case</a></dt>
    21. 

  21. <dt><a href="#implementation">Implementation</a></dt>
    22. 

  22. </dl></dd></dt>
    23. 

  23. </dl></dd></dt>
    24. 

  24. </dl>
    25. 

  25.   </div>
    26. 

  26.   
    27. 

  27. 

  28. <p><em>Thanks to <a href="https://github.com/cstratopoulos">Christos Stratopoulos</a> for this Outcome recipe.</em></p>
    29. 

  29. 

  30. <hr />
    31. 

  31. 

  32. <h3 id="compatibility-note">Compatibility note</h3>
    33. 

  33. 

  34. <p>This recipe targets Boost versions before 1.70, where coroutine support is based around
    35. 

  35. the <code>asio::experimental::this_coro::token</code> completion token. For integration with Boost
    36. 

  36. versions 1.70 and onward, see <a href="asio-integration-1-70">this recipe</a>.</p>
    37. 

  37. 

  38. <hr />
    39. 

  39. 

  40. <h3 id="use-case">Use case</h3>
    41. 

  41. 

  42. <p><a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio.html">Boost.ASIO</a>
    43. 

  43. and <a href="https://think-async.com/Asio/">standalone ASIO</a> provide the
    44. 

  44. <a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio/reference/async_result.html"><code>async_result</code></a>
    45. 

  45. customisation point for adapting arbitrary third party libraries, such as Outcome, into ASIO.</p>
    46. 

  46. 

  47. <p>Historically in ASIO you need to pass completion handler instances
    48. 

  48. to the ASIO asynchronous i/o initiation functions. These get executed when the i/o
    49. 

  49. completes.</p>
    50. 

  50. 

  51. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++">  <span class="c1">// Dynamically allocate a buffer to read into. This must be move-only
    52. 

  52. </span><span class="c1"></span>  <span class="c1">// so it can be attached to the completion handler, hence the unique_ptr.
    53. 

  53. </span><span class="c1"></span>  <span class="k">auto</span> <span class="n">buffer</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">make_unique</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">byte</span><span class="o">&gt;&gt;</span><span class="p">(</span><span class="mi">1024</span><span class="p">);</span>
    54. 

  54. 

  55.   <span class="c1">// Begin an asynchronous socket read, upon completion invoke
    56. 

  56. </span><span class="c1"></span>  <span class="c1">// the lambda function specified
    57. 

  57. </span><span class="c1"></span>  <span class="n">skt</span><span class="p">.</span><span class="n">async_read_some</span><span class="p">(</span><span class="n">asio</span><span class="o">::</span><span class="n">buffer</span><span class="p">(</span><span class="n">buffer</span><span class="o">-&gt;</span><span class="n">data</span><span class="p">(),</span> <span class="n">buffer</span><span class="o">-&gt;</span><span class="n">size</span><span class="p">()),</span>
    58. 

  58. 

  59.                       <span class="c1">// Retain lifetime of the i/o buffer until completion
    60. 

  60. </span><span class="c1"></span>                      <span class="p">[</span><span class="n">buffer</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">buffer</span><span class="p">)](</span><span class="k">const</span> <span class="n">error_code</span> <span class="o">&amp;</span><span class="n">ec</span><span class="p">,</span> <span class="n">size_t</span> <span class="n">bytes</span><span class="p">)</span> <span class="p">{</span>
    61. 

  61.                         <span class="c1">// Handle the buffer read
    62. 

  62. </span><span class="c1"></span>                        <span class="k">if</span><span class="p">(</span><span class="n">ec</span><span class="p">)</span>
    63. 

  63.                         <span class="p">{</span>
    64. 

  64.                           <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Buffer read failed with &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">ec</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    65. 

  65.                           <span class="k">return</span><span class="p">;</span>
    66. 

  66.                         <span class="p">}</span>
    67. 

  67.                         <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Read &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">bytes</span> <span class="o">&lt;&lt;</span> <span class="s">&#34; bytes into buffer&#34;</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    68. 

  68. 

  69.                         <span class="c1">// buffer will be dynamically freed now
    70. 

  70. </span><span class="c1"></span>                      <span class="p">});</span>
    71. 

  71. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration.cpp#L47" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    72. 

  72. 

  73. 

  74. <p>One of the big value adds of the Coroutines TS is the ability to not have to write
    75. 

  75. so much boilerplate if you have a Coroutines supporting compiler:</p>
    76. 

  76. 

  77. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++">  <span class="c1">// As coroutines suspend the calling thread whilst an asynchronous
    78. 

  78. </span><span class="c1"></span>  <span class="c1">// operation executes, we can use stack allocation instead of dynamic
    79. 

  79. </span><span class="c1"></span>  <span class="c1">// allocation
    80. 

  80. </span><span class="c1"></span>  <span class="kt">char</span> <span class="n">buffer</span><span class="p">[</span><span class="mi">1024</span><span class="p">];</span>
    81. 

  81. 

  82.   <span class="c1">// Get an ASIO completion token for the current coroutine (requires
    83. 

  83. </span><span class="c1"></span>  <span class="c1">// Coroutines TS)
    84. 

  84. </span><span class="c1"></span>  <span class="n">asio</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">await_token</span> <span class="n">token</span> <span class="o">=</span>  <span class="c1">//
    85. 

  85. </span><span class="c1"></span>  <span class="n">co_await</span> <span class="n">asio</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">this_coro</span><span class="o">::</span><span class="n">token</span><span class="p">();</span>
    86. 

  86. 

  87.   <span class="c1">// Asynchronously read data, suspending this coroutine until completion,
    88. 

  88. </span><span class="c1"></span>  <span class="c1">// returning the bytes of the data read into the result.
    89. 

  89. </span><span class="c1"></span>  <span class="k">try</span>
    90. 

  90.   <span class="p">{</span>
    91. 

  91.     <span class="n">size_t</span> <span class="n">bytesread</span> <span class="o">=</span>  <span class="c1">//
    92. 

  92. </span><span class="c1"></span>    <span class="n">co_await</span> <span class="n">skt</span><span class="p">.</span><span class="n">async_read_some</span><span class="p">(</span><span class="n">asio</span><span class="o">::</span><span class="n">buffer</span><span class="p">(</span><span class="n">buffer</span><span class="p">),</span> <span class="n">token</span><span class="p">);</span>
    93. 

  93.     <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Read &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">bytesread</span> <span class="o">&lt;&lt;</span> <span class="s">&#34; bytes into buffer&#34;</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    94. 

  94.   <span class="p">}</span>
    95. 

  95.   <span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">system_error</span> <span class="o">&amp;</span><span class="n">e</span><span class="p">)</span>
    96. 

  96.   <span class="p">{</span>
    97. 

  97.     <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Buffer read failed with &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    98. 

  98.   <span class="p">}</span>
    99. 

  99. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration.cpp#L73" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    100. 

  100. 

  101. 

  102. <p>The default ASIO implementation always throws exceptions on failure through
    103. 

  103. its coroutine token transformation. The <a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio/reference/experimental__redirect_error.html"><code>redirect_error</code></a>
    104. 

  104. token transformation recovers the option to use the <code>error_code</code> interface,
    105. 

  105. but it suffers from the <a href="../motivation/error_codes.html">same drawbacks</a>
    106. 

  106. that make pure error codes unappealing in the synchronous case.</p>
    107. 

  107. 

  108. <p>This recipe fixes that by making it possible for coroutinised
    109. 

  109. i/o in ASIO to return a <code>result&lt;T&gt;</code>:</p>
    110. 

  110. 

  111. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++">  <span class="c1">// Asynchronously read data, suspending this coroutine until completion,
    112. 

  112. </span><span class="c1"></span>  <span class="c1">// returning the bytes of the data read into the result, or any failure.
    113. 

  113. </span><span class="c1"></span>  <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o">&lt;</span><span class="n">size_t</span><span class="p">,</span> <span class="n">error_code</span><span class="o">&gt;</span> <span class="n">bytesread</span> <span class="o">=</span>  <span class="c1">//
    114. 

  114. </span><span class="c1"></span>  <span class="n">co_await</span> <span class="n">skt</span><span class="p">.</span><span class="n">async_read_some</span><span class="p">(</span><span class="n">asio</span><span class="o">::</span><span class="n">buffer</span><span class="p">(</span><span class="n">buffer</span><span class="p">),</span> <span class="n">as_result</span><span class="p">(</span><span class="n">token</span><span class="p">));</span>
    115. 

  115. 

  116.   <span class="c1">// Usage is exactly like ordinary Outcome. Note the lack of exception throw!
    117. 

  117. </span><span class="c1"></span>  <span class="k">if</span><span class="p">(</span><span class="n">bytesread</span><span class="p">.</span><span class="n">has_error</span><span class="p">())</span>
    118. 

  118.   <span class="p">{</span>
    119. 

  119.     <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Buffer read failed with &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">bytesread</span><span class="p">.</span><span class="n">error</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    120. 

  120.     <span class="k">return</span><span class="p">;</span>
    121. 

  121.   <span class="p">}</span>
    122. 

  122.   <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Read &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">bytesread</span><span class="p">.</span><span class="n">value</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="s">&#34; bytes into buffer&#34;</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    123. 

  123. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration.cpp#L200" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    124. 

  124. 

  125. 

  126. <hr />
    127. 

  127. 

  128. <h3 id="implementation">Implementation</h3>
    129. 

  129. 

  130. <div class="notices warning" style="background: url('../images/warning.png') top left no-repeat padding-box padding-box;">
    131. 

  131. <div class="notices heading">warning</div>
    132. 

  132. <div class="notices message"><p>The below involves a lot of ASIO voodoo. <strong>NO SUPPORT WILL BE GIVEN HERE FOR THE ASIO
    133. 

  133. CODE BELOW</strong>. Please raise any questions or problems that you have with how to implement
    134. 

  134. this sort of stuff in ASIO
    135. 

  135. on <a href="https://stackoverflow.com/questions/tagged/boost-asio">Stackoverflow #boost-asio</a>.</p>
    136. 

  136. </div>
    137. 

  137. </div>
    138. 

  138. 

  139. 

  140. <p>The real world, production-level recipe can be found at the bottom of this page.
    141. 

  141. You ought to use that in any real world use case.</p>
    142. 

  142. 

  143. <p>It is however worth providing a walkthrough of a simplified edition of the real world
    144. 

  144. recipe, as a lot of barely documented ASIO voodoo is involved. You should not
    145. 

  145. use the code presented next in your own code, it is too simplified. But it should
    146. 

  146. help you understand how the real implementation works.</p>
    147. 

  147. 

  148. <p>Firstly we need to define some helper type sugar and a factory function for wrapping
    149. 

  149. any arbitrary third party completion token with that type sugar:</p>
    150. 

  150. 

  151. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">namespace</span> <span class="n">detail</span>
    152. 

  152. <span class="p">{</span>
    153. 

  153.   <span class="c1">// Type sugar for wrapping an external completion token
    154. 

  154. </span><span class="c1"></span>  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span><span class="err"> </span><span class="nc">CompletionToken</span><span class="o">&gt;</span> <span class="k">struct</span> <span class="n">as_result_t</span>
    155. 

  155.   <span class="p">{</span>
    156. 

  156.     <span class="n">CompletionToken</span> <span class="n">token</span><span class="p">;</span>
    157. 

  157.   <span class="p">};</span>
    158. 

  158. <span class="p">}</span>  <span class="c1">// namespace detail
    159. 

  159. </span><span class="c1"></span>
    160. 

  160. <span class="c1">// Factory function for wrapping a third party completion token with
    161. 

  161. </span><span class="c1">// our type sugar
    162. 

  162. </span><span class="c1"></span><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span><span class="err"> </span><span class="nc">CompletionToken</span><span class="o">&gt;</span>  <span class="c1">//
    163. 

  163. </span><span class="c1"></span><span class="kr">inline</span> <span class="k">auto</span> <span class="n">as_result</span><span class="p">(</span><span class="n">CompletionToken</span> <span class="o">&amp;&amp;</span><span class="n">token</span><span class="p">)</span>
    164. 

  164. <span class="p">{</span>
    165. 

  165.   <span class="k">return</span> <span class="n">detail</span><span class="o">::</span><span class="n">as_result_t</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">CompletionToken</span><span class="o">&gt;&gt;</span><span class="p">{</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">CompletionToken</span><span class="o">&gt;</span><span class="p">(</span><span class="n">token</span><span class="p">)};</span>
    166. 

  166. <span class="p">};</span>
    167. 

  167. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration.cpp#L99" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    168. 

  168. 

  169. 

  170. <p>Next we tell ASIO about a new completion token it ought to recognise by specialising
    171. 

  171. <a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio/reference/async_result.html"><code>async_result</code></a>:</p>
    172. 

  172. 

  173. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="c1">// Tell ASIO about a new kind of completion token, the kind returned
    174. 

  174. </span><span class="c1">// from our as_result() factory function. This implementation is
    175. 

  175. </span><span class="c1">// for functions with handlers void(error_code, T) only.
    176. 

  176. </span><span class="c1"></span><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span><span class="err"> </span><span class="nc">CompletionToken</span><span class="p">,</span> <span class="k">class</span><span class="err"> </span><span class="nc">T</span><span class="o">&gt;</span>                        <span class="c1">//
    177. 

  177. </span><span class="c1"></span><span class="k">struct</span> <span class="n">asio</span><span class="o">::</span><span class="n">async_result</span><span class="o">&lt;</span><span class="n">detail</span><span class="o">::</span><span class="n">as_result_t</span><span class="o">&lt;</span><span class="n">CompletionToken</span><span class="o">&gt;</span><span class="p">,</span>  <span class="c1">//
    178. 

  178. </span><span class="c1"></span>                          <span class="kt">void</span><span class="p">(</span><span class="n">error_code</span><span class="p">,</span> <span class="n">T</span><span class="p">)</span><span class="o">&gt;</span>                   <span class="c1">//
    179. 

  179. </span><span class="c1"></span>
    180. 

  180.     <span class="c1">// NOTE we subclass for an async result taking an outcome::result
    181. 

  181. </span><span class="c1"></span>    <span class="c1">// as its completion handler. We will mangle the void(error_code, T)
    182. 

  182. </span><span class="c1"></span>    <span class="c1">// completion handler into this completion handler below.
    183. 

  183. </span><span class="c1"></span>    <span class="o">:</span> <span class="k">public</span> <span class="n">asio</span><span class="o">::</span><span class="n">async_result</span><span class="o">&lt;</span><span class="n">CompletionToken</span><span class="p">,</span> <span class="kt">void</span><span class="p">(</span><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">error_code</span><span class="o">&gt;</span><span class="p">)</span><span class="o">&gt;</span>
    184. 

  184. <span class="p">{</span>
    185. 

  185.   <span class="c1">// The result type we shall return
    186. 

  186. </span><span class="c1"></span>  <span class="k">using</span> <span class="n">result_type</span> <span class="o">=</span> <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">error_code</span><span class="o">&gt;</span><span class="p">;</span>
    187. 

  187.   <span class="k">using</span> <span class="n">_base</span> <span class="o">=</span> <span class="n">asio</span><span class="o">::</span><span class="n">async_result</span><span class="o">&lt;</span><span class="n">CompletionToken</span><span class="p">,</span> <span class="kt">void</span><span class="p">(</span><span class="n">result_type</span><span class="p">)</span><span class="o">&gt;</span><span class="p">;</span>
    188. 

  188.   <span class="c1">// The awaitable type to be returned by the initiating function,
    189. 

  189. </span><span class="c1"></span>  <span class="c1">// the co_await of which will yield a result_type
    190. 

  190. </span><span class="c1"></span>  <span class="k">using</span> <span class="n">return_type</span> <span class="o">=</span> <span class="k">typename</span> <span class="n">_base</span><span class="o">::</span><span class="n">return_type</span><span class="p">;</span>
    191. 

  191. 

  192.   <span class="c1">// Get what would be the completion handler for the async_result
    193. 

  193. </span><span class="c1"></span>  <span class="c1">// whose completion handler is void(result_type)
    194. 

  194. </span><span class="c1"></span>  <span class="k">using</span> <span class="n">result_type_completion_handler_type</span> <span class="o">=</span>  <span class="c1">//
    195. 

  195. </span><span class="c1"></span>  <span class="k">typename</span> <span class="n">_base</span><span class="o">::</span><span class="n">completion_handler_type</span><span class="p">;</span>
    196. 

  196. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration.cpp#L118" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    197. 

  197. 

  198. 

  199. <p>The tricky part to understand is that our <code>async_result</code> specialisation inherits
    200. 

  200. from an <code>async_result</code> for the supplied completion token type with a completion
    201. 

  201. handler which consumes a <code>result&lt;T&gt;</code>. Our <code>async_result</code> is actually therefore
    202. 

  202. the base <code>async_result</code>, but we layer on top a <code>completion_handler_type</code> with
    203. 

  203. the <code>void(error_code, size_t)</code> signature which constructs from that a <code>result</code>:</p>
    204. 

  204. 

  205. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++">  <span class="c1">// Wrap that completion handler with void(error_code, T) converting
    206. 

  206. </span><span class="c1"></span>  <span class="c1">// handler
    207. 

  207. </span><span class="c1"></span>  <span class="k">struct</span> <span class="n">completion_handler_type</span>
    208. 

  208.   <span class="p">{</span>
    209. 

  209.     <span class="c1">// Pass through unwrapped completion token
    210. 

  210. </span><span class="c1"></span>    <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span><span class="err"> </span><span class="nc">U</span><span class="o">&gt;</span>
    211. 

  211.     <span class="n">completion_handler_type</span><span class="p">(</span><span class="o">::</span><span class="n">detail</span><span class="o">::</span><span class="n">as_result_t</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&gt;</span> <span class="o">&amp;&amp;</span><span class="n">ch</span><span class="p">)</span>
    212. 

  212.         <span class="o">:</span> <span class="n">_handler</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&gt;</span><span class="p">(</span><span class="n">ch</span><span class="p">.</span><span class="n">token</span><span class="p">))</span>
    213. 

  213.     <span class="p">{</span>
    214. 

  214.     <span class="p">}</span>
    215. 

  215. 

  216.     <span class="c1">// Our completion handler spec
    217. 

  217. </span><span class="c1"></span>    <span class="kt">void</span> <span class="k">operator</span><span class="p">()(</span><span class="n">error_code</span> <span class="n">ec</span><span class="p">,</span> <span class="n">T</span> <span class="n">v</span><span class="p">)</span>
    218. 

  218.     <span class="p">{</span>
    219. 

  219.       <span class="c1">// Call the underlying completion handler, whose
    220. 

  220. </span><span class="c1"></span>      <span class="c1">// completion function is void(result_type)
    221. 

  221. </span><span class="c1"></span>      <span class="k">if</span><span class="p">(</span><span class="n">ec</span><span class="p">)</span>
    222. 

  222.       <span class="p">{</span>
    223. 

  223.         <span class="c1">// Complete with a failed result
    224. 

  224. </span><span class="c1"></span>        <span class="n">_handler</span><span class="p">(</span><span class="n">result_type</span><span class="p">(</span><span class="n">outcome</span><span class="o">::</span><span class="n">failure</span><span class="p">(</span><span class="n">ec</span><span class="p">)));</span>
    225. 

  225.         <span class="k">return</span><span class="p">;</span>
    226. 

  226.       <span class="p">}</span>
    227. 

  227.       <span class="c1">// Complete with a successful result
    228. 

  228. </span><span class="c1"></span>      <span class="n">_handler</span><span class="p">(</span><span class="n">result_type</span><span class="p">(</span><span class="n">outcome</span><span class="o">::</span><span class="n">success</span><span class="p">(</span><span class="n">v</span><span class="p">)));</span>
    229. 

  229.     <span class="p">}</span>
    230. 

  230. 

  231.     <span class="n">result_type_completion_handler_type</span> <span class="n">_handler</span><span class="p">;</span>
    232. 

  232.   <span class="p">};</span>
    233. 

  233. 

  234.   <span class="c1">// Initialise base with the underlying completion handler
    235. 

  235. </span><span class="c1"></span>  <span class="n">async_result</span><span class="p">(</span><span class="n">completion_handler_type</span> <span class="o">&amp;</span><span class="n">h</span><span class="p">)</span>
    236. 

  236.       <span class="o">:</span> <span class="n">_base</span><span class="p">(</span><span class="n">h</span><span class="p">.</span><span class="n">_handler</span><span class="p">)</span>
    237. 

  237.   <span class="p">{</span>
    238. 

  238.   <span class="p">}</span>
    239. 

  239. 

  240.   <span class="k">using</span> <span class="n">_base</span><span class="o">::</span><span class="n">get</span><span class="p">;</span>
    241. 

  241. <span class="p">};</span>
    242. 

  242. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration.cpp#L143" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    243. 

  243. 

  244. 

  245. <p>To use, simply wrap the third party completion token with <code>as_result</code> to cause
    246. 

  246. ASIO to return from <code>co_await</code> a <code>result</code> instead of throwing exceptions on
    247. 

  247. failure:</p>
    248. 

  248. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="kt">char</span> <span class="n">buffer</span><span class="p">[</span><span class="mi">1024</span><span class="p">];</span>
    249. 

  249. <span class="n">asio</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">await_token</span> <span class="n">token</span> <span class="o">=</span>
    250. 

  250.   <span class="n">co_await</span> <span class="n">asio</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">this_coro</span><span class="o">::</span><span class="n">token</span><span class="p">();</span>
    251. 

  251. 

  252. <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o">&lt;</span><span class="n">size_t</span><span class="p">,</span> <span class="n">error_code</span><span class="o">&gt;</span> <span class="n">bytesread</span> <span class="o">=</span>
    253. 

  253.   <span class="n">co_await</span> <span class="n">skt</span><span class="p">.</span><span class="n">async_read_some</span><span class="p">(</span><span class="n">asio</span><span class="o">::</span><span class="n">buffer</span><span class="p">(</span><span class="n">buffer</span><span class="p">),</span> <span class="n">as_result</span><span class="p">(</span><span class="n">token</span><span class="p">));</span>
    254. 

  254. </code></pre></div>
    255. 

  255. <p>The real world production-level implementation below is a lot more complex than the
    256. 

  256. above which has been deliberately simplified to aid exposition. The above
    257. 

  257. should help you get up and running with the below, eventually.</p>
    258. 

  258. 

  259. <p>One again I would like to remind you that Outcome is not the appropriate place
    260. 

  260. to seek help with ASIO voodoo. Please ask on
    261. 

  261. <a href="https://stackoverflow.com/questions/tagged/boost-asio">Stackoverflow #boost-asio</a>.</p>
    262. 

  262. 

  263. <hr />
    264. 

  264. 

  265. <p>Here follows the real world, production-level adapation of Outcome into
    266. 

  266. ASIO, written and maintained by <a href="https://github.com/cstratopoulos">Christos Stratopoulos</a>.
    267. 

  267. If the following does not load due to Javascript being disabled, you can visit the gist at
    268. 

  268. <a href="https://gist.github.com/cstratopoulos/901b5cdd41d07c6ce6d83798b09ecf9b/da584844f58353915dc2600fba959813f793b456">https://gist.github.com/cstratopoulos/901b5cdd41d07c6ce6d83798b09ecf9b/da584844f58353915dc2600fba959813f793b456</a>.</p>
    269. 

  269. 

  270. <script type="application/javascript" src="//gist.github.com/cstratopoulos/901b5cdd41d07c6ce6d83798b09ecf9b/da584844f58353915dc2600fba959813f793b456.js"></script>
    271. 

  271. 

  272. 

  273.         </div><p><small>Last revised: August 06, 2019 at 10:42:06 &#43;0100</small></p>
    274. 

  274. <hr>
    275. 

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