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<div class="titlepage"><div><div><h1 style="clear: both">Custom payloads</h1></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> or <code>boost::system::error_code</code>. With <code>NoValuePolicy</code>
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set to <a href="../reference/aliases/default_policy.html" class="api-reference">default_policy<T, EC, EP></a>, <code>EC</code> needs
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in fact to merely satisfy the trait
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<a href="../reference/traits/is_error_code_available.html" class="api-reference">is_error_code_available<T></a>
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for <code>EC</code> to be treated as if an <code>error_code</code>. Outcome specialises that
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trait for <code>std::error_code</code> and <code>boost::system::error_code</code>,
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hence they “just work”.</p>
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<p>If no specialisation exists, <code>trait::is_error_code_available<EC></code> is true
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if there exists some ADL discovered free function <code>make_error_code(EC)</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: February 06, 2019 at 16:50:01 UTC</small></p>
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