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<title>Hooking events - Boost.Outcome documentation</title>
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  <div class="titlepage"><div><div><h1 style="clear: both">Hooking events</h1></div></div></div>
<p>Outcome provides multiple methods for user code to intercept various events which occur.
The deepest method is simply to inherit from <code>basic_result</code> or <code>basic_outcome</code>, and override member functions,
for which you will need to study the source code as that form of customisation is out of scope for this tutorial.</p>

<p>Another option is to supply a custom <code>NoValuePolicy</code> which can get you surprisingly
far into customisation (<a href="../../tutorial/essential/no-value/custom.html">see preceding section</a>).</p>

<p>The final option, which this section covers, is to use the ADL discovered event hooks
which tell you when a namespace-localised <code>basic_outcome</code> or <code>basic_result</code> has been:</p>

<ul>
<li>Constructed

<ul>
<li><a href="../../reference/functions/hooks/hook_result_construction.html" class="api-reference"><code>void hook_result_construction(T *, U &amp;&amp;) noexcept</code></a>
</li>
<li><a href="../../reference/functions/hooks/hook_outcome_construction.html" class="api-reference"><code>void hook_outcome_construction(T *, U &amp;&amp;) noexcept</code></a>
</li>
</ul></li>
<li>In-place constructed

<ul>
<li><a href="../../reference/functions/hooks/hook_result_in_place_construction.html" class="api-reference"><code>void hook_result_in_place_construction(T *, in_place_type_t&lt;U&gt;, Args &amp;&amp;...) noexcept</code></a>
</li>
<li><a href="../../reference/functions/hooks/hook_outcome_in_place_construction.html" class="api-reference"><code>void hook_outcome_in_place_construction(T *, in_place_type_t&lt;U&gt;, Args &amp;&amp;...) noexcept</code></a>
</li>
</ul></li>
<li>Copied

<ul>
<li><a href="../../reference/functions/hooks/hook_result_copy_construction.html" class="api-reference"><code>void hook_result_copy_construction(T *, U &amp;&amp;) noexcept</code></a>
</li>
<li><a href="../../reference/functions/hooks/hook_outcome_copy_construction.html" class="api-reference"><code>void hook_outcome_copy_construction(T *, U &amp;&amp;) noexcept</code></a>
</li>
</ul></li>
<li>Moved

<ul>
<li><a href="../../reference/functions/hooks/hook_result_move_construction.html" class="api-reference"><code>void hook_result_move_construction(T *, U &amp;&amp;) noexcept</code></a>
</li>
<li><a href="../../reference/functions/hooks/hook_outcome_move_construction.html" class="api-reference"><code>void hook_outcome_move_construction(T *, U &amp;&amp;) noexcept</code></a>
</li>
</ul></li>
</ul>

<p>One criticism often levelled against library-based exception throw alternatives is that they do
not provide as rich a set of facilities as C++ exception throws. This section shows
you how to configure Outcome, using the ADL event hooks, to take a stack backtrace on
construction of an errored <code>result&lt;T, error_code&gt;</code>,
and if that <code>result&lt;T, error_code&gt;</code> should ever be converted into an <code>outcome&lt;T, error_code, std::exception_ptr&gt;</code>,
a custom <code>std::exception_ptr</code> will be just-in-time synthesised consisting of the <code>std::system_error</code>
for the error code, plus an expanded message string containing the stack backtrace of where
the error originally occurred.</p>

<p>One can see the use case for such a configuration where low-level, deterministic,
fixed latency code is built with <code>result</code>, and it dovetails into higher-level
application code built with <code>outcome</code> where execution time guarantees are not
important, and thus where a <code>malloc</code> is okay. One effectively has constructed a
&ldquo;lazy indeterminism&rdquo;, or &ldquo;just-in-time indeterminism&rdquo; mechanism for handling
failure, but with all the rich information of throwing C++ exceptions.</p>


        </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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