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<div class="titlepage"><div><div><h2 class="title" style="clear: both">Custom payloads</h2></div></div></div>
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<p>So far in this tutorial, type <code>EC</code> in <code>result<T, EC></code> has always been a
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<code>std::error_code</code> (though it can be of any type you wish instead). <code>EC</code> needs
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in fact to merely satisfy
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<a href="reference/success_failure/#standardese-outcome_v2_xxx__trait__has_error_code-T-" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> trait::has_error_code_v<EC></a>
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for <code>EC</code> to be treated as if an <code>std::error_code</code>.</p>
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<p>In turn, <code>trait::has_error_code_v<EC></code> is true if there exists some ADL discovered free
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function:</p>
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<ol>
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<li><code>make_error_code(EC)</code> returning a <code>std::error_code</code>.</li>
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<li><code>make_error_code(get<0>(EC))</code> returning a <code>std::error_code</code> (i.e. EC might be
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<code>pair<std::error_code, U></code> or <code>tuple<std::error_code, ...></code>).</li>
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</ol>
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<p>… or if <code>EC</code> is implicitly convertible into a <code>std::error_code</code>.</p>
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<p>Thus, we can in fact use any custom <code>EC</code> type we like, including one carrying additional
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information, or <em>payload</em>. This payload can carry anything you like, and you can tell
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Outcome to do various things with that payload under various circumstances. For example:</p>
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<ol>
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<li>If the user tries to observe an unsuccessful <code>result</code>, throw a custom exception
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containing the cause of failure with accompanying context from the payload.</li>
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<li>If the user ever constructs an <code>outcome</code> from a payload carrying <code>result</code>,
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set the exception ptr in the constructed <code>outcome</code> to a custom exception
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containing the cause of the failure with accompanying context from the payload.</li>
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<li>Transmit a stack backtrace specifying the exact point at which failure occurred,
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symbolising that backtrace into human readable text at the moment of conversion into human
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readable text.</li>
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<li>Upon a namespace-localised <code>result</code> from library A being copy/moved into a
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namespace-localised <code>result</code> from C bindings library B, set the C <code>errno</code> if
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the error code and category map onto the <code>errno</code> domain.</li>
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</ol>
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<p>There are many, many other options of course. This tutorial can only cover a
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reasonable subset. This section covers Example 1 above, throwing custom exceptions
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with payload upon observation of an unsuccessful <code>result</code>.</p>
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</div><p><small>Last revised: December 05, 2018 at 14:18:52 UTC</small></p>
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