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  <div class="titlepage"><div><div><h1 style="clear: both">The constructor</h1></div></div></div>
  <p>Code domains are 100% constexpr to construct and destruct, as are status codes.
This enables the compiler to 100% instantiate both only in its mind, and to emit
zero code and thus zero overhead.</p>

<p>Unfortunately it also means that it must be possible for each domain to be instantiated an infinite
number of times, and being 100% in constexpr, any instances never have a unique
address in memory either. Thus we cannot compare domains for equivalence using
their address in memory, as 
<a href="https://en.cppreference.com/w/cpp/error/error_category" class="api-reference" target="_blank"><i class="fa fa-book" aria-hidden="true"></i> <code>std::error_category</code></a>
 does.</p>

<p>We solve this by using a <em>very</em> random 64 bit number taken from a hard random
number source. The website <a href="https://www.random.org/cgi-bin/randbyte?nbytes=8&amp;format=h">https://www.random.org/cgi-bin/randbyte?nbytes=8&amp;format=h</a>
is strongly suggested as the source for this number.</p>

<p>(In case you are wondering about the chance of collision for a 64 bit integer, SG14 estimated that
approximately 190,000 separate domains would need to exist in a single process
for there to be a 0.00000001% probability of collision if the random number
source is very random)</p>

<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++">  <span class="c1">// unique id must be from a hard random number source
</span><span class="c1"></span>  <span class="c1">// Use https://www.random.org/cgi-bin/randbyte?nbytes=8&amp;format=h to get a hard random 64 bit id.
</span><span class="c1"></span>  <span class="c1">// Do NOT make up your own value. Do NOT use zero.
</span><span class="c1"></span>  <span class="k">constexpr</span> <span class="k">explicit</span> <span class="nf">_file_io_error_domain</span><span class="p">(</span><span class="k">typename</span> <span class="n">_base</span><span class="o">::</span><span class="n">unique_id_type</span> <span class="n">id</span> <span class="o">=</span> <span class="mh">0x230f170194fcc6c7</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">:</span> <span class="n">_base</span><span class="p">(</span><span class="n">id</span><span class="p">)</span> <span class="p">{}</span>
  <span class="k">static</span> <span class="kr">inline</span> <span class="k">constexpr</span> <span class="k">const</span> <span class="n">_file_io_error_domain</span> <span class="o">&amp;</span><span class="n">get</span><span class="p">();</span>
</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/experimental_status_code.cpp#L96" class="code-snippet-url" target="_blank">View this code on Github</a></div>


<p>A nice side effect of this approach is that custom error domains in header-only
libraries are safe, unlike custom <code>&lt;system_error&gt;</code> error categories. Boost.System&rsquo;s
error categories can now opt into this same safe mechanism in order to also be
safe in header only library use cases.</p>


        </div><p><small>Last revised: January 26, 2019 at 23:38:56 UTC</small></p>
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