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<title>Keeping state - Boost.Outcome documentation</title>
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<h1 class="heading-title">
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<span class="heading-boost">Boost</span>
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<p class="heading-quote">
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<q>...one of the most highly
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regarded and expertly designed C++ library projects in the
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world.</q> <span class="heading-attribution">— <a href=
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">Keeping state</h2></div></div></div>
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<p>The first thing we are going to need is somewhere to store the stack backtrace.
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We could take the easier route and simply store it into an allocated block and
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keep the pointer as a custom payload in a <code>result<T, std::pair<error_code, std::unique_ptr<stack_backtrace>>></code>
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(see previous section on <a href="../../payload">Custom payloads</a>). But let us assume that we care so deeply about bounded execution times
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that ever calling <code>malloc</code> is unacceptable.</p>
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<p>We therefore are going to need some completely static and trivially typed storage
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perhaps kept per-thread to avoid the need to keep mutexes.</p>
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<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
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</span><span class="cm">to ensure the hooks apply only to the types declared in this namespace only
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</span><span class="cm">*/</span>
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<span class="k">namespace</span> <span class="n">error_code_extended</span>
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<span class="p">{</span>
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<span class="c1">// The extra error information we will keep
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</span><span class="c1"></span> <span class="k">struct</span> <span class="n">extended_error_info</span>
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<span class="p">{</span>
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<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
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</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
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</span><span class="c1"></span> <span class="p">};</span>
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<span class="k">struct</span> <span class="n">mythreadlocaldata_t</span>
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<span class="p">{</span>
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<span class="c1">// Keep 16 slots of extended error info as a ringbuffer
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</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>
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<span class="c1">// The current oldest slot
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</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>
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<span class="c1">// Return the oldest slot
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</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>
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<span class="c1">// Retrieve a previously stored slot, detecting if it is stale
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</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>
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<span class="p">{</span>
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<span class="c1">// If the idx is stale, return not found
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</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>
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<span class="p">{</span>
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<span class="k">return</span> <span class="k">nullptr</span><span class="p">;</span>
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<span class="p">}</span>
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<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>
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<span class="p">}</span>
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<span class="p">};</span>
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<span class="c1">// Meyers' singleton returning a thread local data structure for this thread
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</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>
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<span class="p">{</span>
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<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>
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<span class="k">return</span> <span class="n">v</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/error_code_extended.cpp#L52" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>The extended error info is kept in a sixteen item long ring buffer. We continuously
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increment the current index pointer which is a 16 bit value which will wrap after
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65,535. This lets us detect an attempt to access recycled storage, and thus return
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item-not-found instead of the wrong extended error info.</p>
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</div><p><small>Last revised: December 05, 2018 at 14:18:52 UTC</small></p>
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