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  <div class="titlepage"><div><div><h1 style="clear: both">Worked example: Custom domain</h1></div></div></div>


<p>Here follows a worked example of use of Experimental Outcome. It presents
the same sample program I sent to the San Diego 2018 WG21 standards meeting
after I was asked by the committee to demonstrate how P1095 implements P0709
in a working code example they could study and discuss.</p>

<p>We will walk through this worked example, step by step, explaining how each
part works in detail. This will help you implement your own code based on
Experimental Outcome.</p>

<p>You may find it useful to open now in a separate browser tab the reference API
documentation for proposed <code>&lt;system_error2&gt;</code> at <a href="https://ned14.github.io/status-code/">https://ned14.github.io/status-code/</a>
(scroll half way down). The references in the comments to P1028 are to
<a href="http://wg21.link/P1028">P1028 <em>SG14 status_code and standard error object for P0709 Zero-overhead
deterministic exceptions</em></a>, which is the WG21 proposal
paper for potential <code>&lt;system_error2&gt;</code>.</p>

<h3 id="goal-of-this-section">Goal of this section</h3>

<p>We are going to define a simple custom code domain which defines that
the status code&rsquo;s payload will consist of a POSIX error code, and the
<code>__FILE__</code> and <code>__LINE__</code> where the failure occurred. This custom status
code will have an implicit conversion to type erased <code>error</code> defined, which dynamically
allocates memory for the original status code, and outputs an <code>error</code>
which manages that dynamic allocation, indirecting all queries etc
to the erased custom status code type such that the <code>error</code> instance
quacks as if just like the original. This demonstrates that <code>error</code> could
just as equally convey a <code>std::exception_ptr</code>, for example, or indeed
manage the lifetime of any pointer.</p>


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