1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html><meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 3<title>ASIO/Networking TS: Boost >= 1.70 - Boost.Outcome documentation</title> 4<link rel="stylesheet" href="../css/boost.css" type="text/css"> 5<meta name="generator" content="Hugo 0.52 with Boostdoc theme"> 6<meta name="viewport" content="width=device-width,initial-scale=1.0"/> 7 8<link rel="icon" href="../images/favicon.ico" type="image/ico"/> 9<body><div class="spirit-nav"> 10<a accesskey="p" href="../recipes/asio-integration.html"><img src="../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../recipes.html"><img src="../images/up.png" alt="Up"></a> 12 <a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="../recipes/foreign-try.html"><img src="../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">ASIO/Networking TS: Boost >= 1.70</h1></div></div></div> 14 <div class="toc"><dl class="toc"> 15<dt> 16<dd><dl> 17<dt> 18<dd><dl> 19<dt><a href="#compatibility-note">Compatibility note</a></dt> 20<dt><a href="#use-case">Use case</a></dt> 21<dt><a href="#implementation">Implementation</a></dt> 22</dl></dd></dt> 23</dl></dd></dt> 24</dl> 25 </div> 26 27 28<p><em>Thanks to <a href="https://github.com/cstratopoulos">Christos Stratopoulos</a> for this Outcome recipe.</em></p> 29 30<hr /> 31 32<h3 id="compatibility-note">Compatibility note</h3> 33 34<p>This recipe targets Boost versions including and after 1.70, where coroutine support is 35based around the <code>asio::use_awaitable</code> completion token. For integration with Boost versions 36before 1.70, see <a href="asio-integration">this recipe</a>.</p> 37 38<hr /> 39 40<h3 id="use-case">Use case</h3> 41 42<p><a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio.html">Boost.ASIO</a> 43and <a href="https://think-async.com/Asio/">standalone ASIO</a> provide the 44<a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio/reference/async_result.html"><code>async_result</code></a> 45customisation point for adapting arbitrary third party libraries, such as Outcome, into ASIO.</p> 46 47<p>Historically in ASIO you need to pass completion handler instances 48to the ASIO asynchronous i/o initiation functions. These get executed when the i/o 49completes.</p> 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</span><span class="c1"></span> <span class="c1">// so it can be attached to the completion handler, hence the unique_ptr. 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"><</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="n">byte</span><span class="o">>></span><span class="p">(</span><span class="mi">1024</span><span class="p">);</span> 54 55 <span class="c1">// Begin an asynchronous socket read, upon completion invoke 56</span><span class="c1"></span> <span class="c1">// the lambda function specified 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">-></span><span class="n">data</span><span class="p">(),</span> <span class="n">buffer</span><span class="o">-></span><span class="n">size</span><span class="p">()),</span> 58 59 <span class="c1">// Retain lifetime of the i/o buffer until completion 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">&</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 <span class="c1">// Handle the buffer read 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 <span class="p">{</span> 64 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Buffer read failed with "</span> <span class="o"><<</span> <span class="n">ec</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 65 <span class="k">return</span><span class="p">;</span> 66 <span class="p">}</span> 67 <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="s">"Read "</span> <span class="o"><<</span> <span class="n">bytes</span> <span class="o"><<</span> <span class="s">" bytes into buffer"</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 68 69 <span class="c1">// buffer will be dynamically freed now 70</span><span class="c1"></span> <span class="p">});</span> 71</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration_1_70.cpp#L48" class="code-snippet-url" target="_blank">View this code on Github</a></div> 72 73 74<p>One of the big value adds of the Coroutines TS is the ability to not have to write 75so much boilerplate if you have a Coroutines supporting compiler:</p> 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</span><span class="c1"></span> <span class="c1">// operation executes, we can use stack allocation instead of dynamic 79</span><span class="c1"></span> <span class="c1">// allocation 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 82 <span class="c1">// Asynchronously read data, suspending this coroutine until completion, 83</span><span class="c1"></span> <span class="c1">// returning the bytes of the data read into the result. 84</span><span class="c1"></span> <span class="k">try</span> 85 <span class="p">{</span> 86 <span class="c1">// The use_awaitable completion token represents the current coroutine 87</span><span class="c1"></span> <span class="c1">// (requires Coroutines TS) 88</span><span class="c1"></span> <span class="n">size_t</span> <span class="n">bytesread</span> <span class="o">=</span> <span class="c1">// 89</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">asio</span><span class="o">::</span><span class="n">use_awaitable</span><span class="p">);</span> 90 <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="s">"Read "</span> <span class="o"><<</span> <span class="n">bytesread</span> <span class="o"><<</span> <span class="s">" bytes into buffer"</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 91 <span class="p">}</span> 92 <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">&</span><span class="n">e</span><span class="p">)</span> 93 <span class="p">{</span> 94 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Buffer read failed with "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 95 <span class="p">}</span> 96</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration_1_70.cpp#L74" class="code-snippet-url" target="_blank">View this code on Github</a></div> 97 98 99<p>The default ASIO implementation always throws exceptions on failure through 100its 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> 101token transformation recovers the option to use the <code>error_code</code> interface, 102but it suffers from the <a href="../motivation/error_codes.html">same drawbacks</a> 103that make pure error codes unappealing in the synchronous case.</p> 104 105<p>This recipe fixes that by making it possible for coroutinised 106i/o in ASIO to return a <code>result<T></code>:</p> 107 108<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, 109</span><span class="c1"></span> <span class="c1">// returning the bytes of the data read into the result, or any failure. 110</span><span class="c1"></span> <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">size_t</span><span class="p">,</span> <span class="n">error_code</span><span class="o">></span> <span class="n">bytesread</span> <span class="o">=</span> <span class="c1">// 111</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">asio</span><span class="o">::</span><span class="n">use_awaitable</span><span class="p">));</span> 112 113 <span class="c1">// Usage is exactly like ordinary Outcome. Note the lack of exception throw! 114</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> 115 <span class="p">{</span> 116 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Buffer read failed with "</span> <span class="o"><<</span> <span class="n">bytesread</span><span class="p">.</span><span class="n">error</span><span class="p">()</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 117 <span class="k">return</span><span class="p">;</span> 118 <span class="p">}</span> 119 <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="s">"Read "</span> <span class="o"><<</span> <span class="n">bytesread</span><span class="p">.</span><span class="n">value</span><span class="p">()</span> <span class="o"><<</span> <span class="s">" bytes into buffer"</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 120</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration_1_70.cpp#L208" class="code-snippet-url" target="_blank">View this code on Github</a></div> 121 122 123<hr /> 124 125<h3 id="implementation">Implementation</h3> 126 127<div class="notices warning" style="background: url('../images/warning.png') top left no-repeat padding-box padding-box;"> 128<div class="notices heading">warning</div> 129<div class="notices message"><p>The below involves a lot of ASIO voodoo. <strong>NO SUPPORT WILL BE GIVEN HERE FOR THE ASIO 130CODE BELOW</strong>. Please raise any questions or problems that you have with how to implement 131this sort of stuff in ASIO 132on <a href="https://stackoverflow.com/questions/tagged/boost-asio">Stackoverflow #boost-asio</a>.</p> 133</div> 134</div> 135 136 137<p>The real world, production-level recipe can be found at the bottom of this page. 138You ought to use that in any real world use case.</p> 139 140<p>It is however worth providing a walkthrough of a simplified edition of the real world 141recipe, as a lot of barely documented ASIO voodoo is involved. You should not 142use the code presented next in your own code, it is too simplified. But it should 143help you understand how the real implementation works.</p> 144 145<p>Firstly we need to define some helper type sugar and a factory function for wrapping 146any arbitrary third party completion token with that type sugar:</p> 147 148<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> 149<span class="p">{</span> 150 <span class="c1">// Type sugar for wrapping an external completion token 151</span><span class="c1"></span> <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">CompletionToken</span><span class="o">></span> <span class="k">struct</span> <span class="n">as_result_t</span> 152 <span class="p">{</span> 153 <span class="n">CompletionToken</span> <span class="n">token</span><span class="p">;</span> 154 <span class="p">};</span> 155<span class="p">}</span> <span class="c1">// namespace detail 156</span><span class="c1"></span> 157<span class="c1">// Factory function for wrapping a third party completion token with 158</span><span class="c1">// our type sugar 159</span><span class="c1"></span><span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">CompletionToken</span><span class="o">></span> <span class="c1">// 160</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">&&</span><span class="n">token</span><span class="p">)</span> 161<span class="p">{</span> 162 <span class="k">return</span> <span class="n">detail</span><span class="o">::</span><span class="n">as_result_t</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o"><</span><span class="n">CompletionToken</span><span class="o">>></span><span class="p">{</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">CompletionToken</span><span class="o">></span><span class="p">(</span><span class="n">token</span><span class="p">)};</span> 163<span class="p">};</span> 164</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration_1_70.cpp#L97" class="code-snippet-url" target="_blank">View this code on Github</a></div> 165 166 167<p>Next we tell ASIO about a new completion token it ought to recognise by specialising 168<a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio/reference/async_result.html"><code>async_result</code></a>:</p> 169 170<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 171</span><span class="c1">// from our as_result() factory function. This implementation is 172</span><span class="c1">// for functions with handlers void(error_code, T) only. 173</span><span class="c1"></span><span class="k">template</span> <span class="o"><</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">></span> <span class="c1">// 174</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"><</span><span class="n">detail</span><span class="o">::</span><span class="n">as_result_t</span><span class="o"><</span><span class="n">CompletionToken</span><span class="o">></span><span class="p">,</span> <span class="c1">// 175</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">></span> <span class="c1">// 176</span><span class="c1"></span> 177<span class="p">{</span> 178 <span class="c1">// The result type we shall return 179</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"><</span><span class="n">T</span><span class="p">,</span> <span class="n">error_code</span><span class="o">></span><span class="p">;</span> 180 <span class="c1">// The awaitable type to be returned by the initiating function, 181</span><span class="c1"></span> <span class="c1">// the co_await of which will yield a result_type 182</span><span class="c1"></span> <span class="k">using</span> <span class="n">return_type</span> <span class="o">=</span> <span class="c1">// 183</span><span class="c1"></span> <span class="k">typename</span> <span class="n">asio</span><span class="o">::</span><span class="n">async_result</span><span class="o"><</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">></span> <span class="c1">// 184</span><span class="c1"></span> <span class="o">::</span><span class="n">return_type</span><span class="p">;</span> 185</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration_1_70.cpp#L116" class="code-snippet-url" target="_blank">View this code on Github</a></div> 186 187 188<p>There are a couple tricky parts to understand. First of all, we want our 189<code>async_result</code> specialization to work, in particular, with the <code>async_result</code> for 190ASIO’s 191<a href="https://www.boost.org/doc/libs/develop/doc/html/boost_asio/reference/use_awaitable_t.html"><code>use_awaitable_t</code> completion token</a>. 192With this token, the <code>async_result</code> specialization takes the form with a static 193<code>initiate</code> method which defers initiation of the asynchronous operation until, 194for example, 195<code>co_await</code> is called on the returned <code>awaitable</code>. Thus, our <code>async_result</code> 196specialization will take the same form. With this in mind, we need only 197understand how our specialization will implement its <code>initiate</code> method. The trick 198is that it will pass the initiation work off to an <code>async_result</code> for the 199supplied completion token type with a completion handler which consumes <code>result<T></code>. 200Our <code>async_result</code> is thus just a simple wrapper over this underlying 201<code>async_result</code>, but we inject a completion handler with the 202<code>void(error_code, size_t)</code> signature which constructs from that a <code>result</code>:</p> 203 204<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"> <span class="c1">// Wrap a completion handler with void(error_code, T) converting 205</span><span class="c1"></span> <span class="c1">// handler 206</span><span class="c1"></span> <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">Handler</span><span class="o">></span> 207 <span class="k">struct</span> <span class="n">completion_handler</span> <span class="p">{</span> 208 <span class="c1">// Our completion handler spec 209</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> 210 <span class="p">{</span> 211 <span class="c1">// Call the underlying completion handler, whose 212</span><span class="c1"></span> <span class="c1">// completion function is void(result_type) 213</span><span class="c1"></span> <span class="k">if</span><span class="p">(</span><span class="n">ec</span><span class="p">)</span> 214 <span class="p">{</span> 215 <span class="c1">// Complete with a failed result 216</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> 217 <span class="k">return</span><span class="p">;</span> 218 <span class="p">}</span> 219 <span class="c1">// Complete with a successful result 220</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> 221 <span class="p">}</span> 222 223 <span class="n">Handler</span> <span class="n">_handler</span><span class="p">;</span> 224 <span class="p">};</span> 225 226 <span class="c1">// NOTE the initiate member function initiates the async operation, 227</span><span class="c1"></span> <span class="c1">// and we want to defer to what would be the initiation of the 228</span><span class="c1"></span> <span class="c1">// async_result whose handler signature is void(result_type). 229</span><span class="c1"></span> <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">Initiation</span><span class="p">,</span> <span class="k">class</span><span class="err">... </span><span class="nc">Args</span><span class="o">></span> 230 <span class="k">static</span> <span class="n">return_type</span> 231 <span class="n">initiate</span><span class="p">(</span> 232 <span class="n">Initiation</span><span class="o">&&</span> <span class="n">init</span><span class="p">,</span> 233 <span class="n">detail</span><span class="o">::</span><span class="n">as_result_t</span><span class="o"><</span><span class="n">CompletionToken</span><span class="o">>&&</span> <span class="n">token</span><span class="p">,</span> 234 <span class="n">Args</span><span class="o">&&</span><span class="p">...</span> <span class="n">args</span><span class="p">)</span> 235 <span class="p">{</span> 236 <span class="c1">// The async_initiate<CompletionToken, void(result_type)> helper 237</span><span class="c1"></span> <span class="c1">// function will invoke the async initiation method of the 238</span><span class="c1"></span> <span class="c1">// async_result<CompletionToken, void(result_type)>, as desired. 239</span><span class="c1"></span> <span class="c1">// Instead of CompletionToken and void(result_type) we start with 240</span><span class="c1"></span> <span class="c1">// detail::as_result_t<CompletionToken> and void(ec, T), so 241</span><span class="c1"></span> <span class="c1">// the inputs need to be massaged then passed along. 242</span><span class="c1"></span> <span class="k">return</span> <span class="n">asio</span><span class="o">::</span><span class="n">async_initiate</span><span class="o"><</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">></span><span class="p">(</span> 243 <span class="c1">// create a new initiation which wraps the provided init 244</span><span class="c1"></span> <span class="p">[</span><span class="n">init</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">Initiation</span><span class="o">></span><span class="p">(</span><span class="n">init</span><span class="p">)](</span> 245 <span class="k">auto</span><span class="o">&&</span> <span class="n">handler</span><span class="p">,</span> <span class="k">auto</span><span class="o">&&</span><span class="p">...</span> <span class="n">initArgs</span><span class="p">)</span> <span class="k">mutable</span> <span class="p">{</span> 246 <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">init</span><span class="p">)(</span> 247 <span class="c1">// we wrap the handler in the converting completion_handler from 248</span><span class="c1"></span> <span class="c1">// above, and pass along the args 249</span><span class="c1"></span> <span class="n">completion_handler</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span><span class="o">>></span><span class="p">{</span> 250 <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span><span class="o">></span><span class="p">(</span><span class="n">handler</span><span class="p">)},</span> 251 <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">initArgs</span><span class="p">)</span><span class="o">></span><span class="p">(</span><span class="n">initArgs</span><span class="p">)...);</span> 252 <span class="p">},</span> 253 <span class="c1">// the new initiation is called with the handler unwrapped from 254</span><span class="c1"></span> <span class="c1">// the token, and the original initiation arguments. 255</span><span class="c1"></span> <span class="n">token</span><span class="p">.</span><span class="n">token</span><span class="p">,</span> 256 <span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)...);</span> 257 <span class="p">}</span> 258<span class="p">};</span> 259</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/boost-only/asio_integration_1_70.cpp#L134" class="code-snippet-url" target="_blank">View this code on Github</a></div> 260 261 262<p>To use, simply wrap the third party completion token with <code>as_result</code> to cause 263ASIO to return from <code>co_await</code> a <code>result</code> instead of throwing exceptions on 264failure:</p> 265<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> 266 267<span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">size_t</span><span class="p">,</span> <span class="n">error_code</span><span class="o">></span> <span class="n">bytesread</span> <span class="o">=</span> 268 <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">asio</span><span class="o">::</span><span class="n">use_awaitable</span><span class="p">));</span> 269</code></pre></div> 270<p>The real world production-level implementation below is a lot more complex than the 271above which has been deliberately simplified to aid exposition. The above 272should help you get up and running with the below, eventually.</p> 273 274<p>One again I would like to remind you that Outcome is not the appropriate place 275to seek help with ASIO voodoo. Please ask on 276<a href="https://stackoverflow.com/questions/tagged/boost-asio">Stackoverflow #boost-asio</a>.</p> 277 278<hr /> 279 280<p>Here follows the real world, production-level adapation of Outcome into 281ASIO, written and maintained by <a href="https://github.com/cstratopoulos">Christos Stratopoulos</a>. 282If the following does not load due to Javascript being disabled, you can visit the gist at 283<a href="https://gist.github.com/cstratopoulos/901b5cdd41d07c6ce6d83798b09ecf9b/863c1dbf3b063a5ff9ff2bdd834242ead556e74e">https://gist.github.com/cstratopoulos/901b5cdd41d07c6ce6d83798b09ecf9b/863c1dbf3b063a5ff9ff2bdd834242ead556e74e</a>.</p> 284 285<script type="application/javascript" src="//gist.github.com/cstratopoulos/901b5cdd41d07c6ce6d83798b09ecf9b/863c1dbf3b063a5ff9ff2bdd834242ead556e74e.js"></script> 286 287 288 </div><p><small>Last revised: August 06, 2019 at 10:42:06 +0100</small></p> 289<hr> 290<div class="spirit-nav"> 291<a accesskey="p" href="../recipes/asio-integration.html"><img src="../images/prev.png" alt="Prev"></a> 292 <a accesskey="u" href="../recipes.html"><img src="../images/up.png" alt="Up"></a> 293 <a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="../recipes/foreign-try.html"><img src="../images/next.png" alt="Next"></a></div></body> 294</html> 295
