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>Keeping state - 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="../../../tutorial/advanced/hooks.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/advanced/hooks.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="../../../tutorial/advanced/hooks/adl_bridging.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Keeping state</h1></div></div></div> 14 <p>The first thing we are going to need is somewhere to store the stack backtrace. 15We could take the easier route and simply store it into an allocated block and 16keep the pointer as a custom payload in a <code>result<T, std::pair<error_code, std::unique_ptr<stack_backtrace>>></code> 17(see previous section on <a href="../../payload">Custom payloads</a>). But let us assume that we care so deeply about bounded execution times 18that ever calling <code>malloc</code> is unacceptable.</p> 19 20<p>We therefore are going to need some completely static and trivially typed storage 21perhaps kept per-thread to avoid the need to keep mutexes.</p> 22 23<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="cm">/* Outcome's hook mechanism works vis ADL, so we will need a custom namespace 24</span><span class="cm">to ensure the hooks apply only to the types declared in this namespace only 25</span><span class="cm">*/</span> 26<span class="k">namespace</span> <span class="n">error_code_extended</span> 27<span class="p">{</span> 28 <span class="c1">// The extra error information we will keep 29</span><span class="c1"></span> <span class="k">struct</span> <span class="n">extended_error_info</span> 30 <span class="p">{</span> 31 <span class="n">std</span><span class="o">::</span><span class="n">array</span><span class="o"><</span><span class="kt">void</span> <span class="o">*</span><span class="p">,</span> <span class="mi">16</span><span class="o">></span> <span class="n">backtrace</span><span class="p">;</span> <span class="c1">// The backtrace 32</span><span class="c1"></span> <span class="n">size_t</span> <span class="n">items</span><span class="p">;</span> <span class="c1">// Items in backtrace array which are valid 33</span><span class="c1"></span> <span class="p">};</span> 34 <span class="k">struct</span> <span class="n">mythreadlocaldata_t</span> 35 <span class="p">{</span> 36 <span class="c1">// Keep 16 slots of extended error info as a ringbuffer 37</span><span class="c1"></span> <span class="n">extended_error_info</span> <span class="n">slots</span><span class="p">[</span><span class="mi">16</span><span class="p">];</span> 38 <span class="c1">// The current oldest slot 39</span><span class="c1"></span> <span class="n">uint16_t</span> <span class="n">current</span><span class="p">{</span><span class="mi">0</span><span class="p">};</span> 40 41 <span class="c1">// Return the oldest slot 42</span><span class="c1"></span> <span class="n">extended_error_info</span> <span class="o">&</span><span class="n">next</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="n">slots</span><span class="p">[(</span><span class="n">current</span><span class="o">++</span><span class="p">)</span> <span class="o">%</span> <span class="mi">16</span><span class="p">];</span> <span class="p">}</span> 43 44 <span class="c1">// Retrieve a previously stored slot, detecting if it is stale 45</span><span class="c1"></span> <span class="n">extended_error_info</span> <span class="o">*</span><span class="n">get</span><span class="p">(</span><span class="n">uint16_t</span> <span class="n">idx</span><span class="p">)</span> 46 <span class="p">{</span> 47 <span class="c1">// If the idx is stale, return not found 48</span><span class="c1"></span> <span class="k">if</span><span class="p">(</span><span class="n">idx</span> <span class="o">-</span> <span class="n">current</span> <span class="o">>=</span> <span class="mi">16</span><span class="p">)</span> 49 <span class="p">{</span> 50 <span class="k">return</span> <span class="k">nullptr</span><span class="p">;</span> 51 <span class="p">}</span> 52 <span class="k">return</span> <span class="n">slots</span> <span class="o">+</span> <span class="p">(</span><span class="n">idx</span> <span class="o">%</span> <span class="mi">16</span><span class="p">);</span> 53 <span class="p">}</span> 54 <span class="p">};</span> 55 56 <span class="c1">// Meyers' singleton returning a thread local data structure for this thread 57</span><span class="c1"></span> <span class="kr">inline</span> <span class="n">mythreadlocaldata_t</span> <span class="o">&</span><span class="n">mythreadlocaldata</span><span class="p">()</span> 58 <span class="p">{</span> 59 <span class="k">static</span> <span class="k">thread_local</span> <span class="n">mythreadlocaldata_t</span> <span class="n">v</span><span class="p">;</span> 60 <span class="k">return</span> <span class="n">v</span><span class="p">;</span> 61 <span class="p">}</span> 62<span class="p">}</span> 63</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/error_code_extended.cpp#L52" class="code-snippet-url" target="_blank">View this code on Github</a></div> 64 65 66<p>The extended error info is kept in a sixteen item long, thread local, ring buffer. We continuously 67increment the current index pointer which is a 16 bit value which will wrap after 6865,535. This lets us detect an attempt to access recycled storage, and thus return 69item-not-found instead of the wrong extended error info.</p> 70 71 72 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 73<hr> 74<div class="spirit-nav"> 75<a accesskey="p" href="../../../tutorial/advanced/hooks.html"><img src="../../../images/prev.png" alt="Prev"></a> 76 <a accesskey="u" href="../../../tutorial/advanced/hooks.html"><img src="../../../images/up.png" alt="Up"></a> 77 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/hooks/adl_bridging.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 78</html> 79
