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140 lines
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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>
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<p>Outcome provides multiple methods for user code to intercept various lifecycle events which occur.
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The deepest method is simply to inherit from <code>basic_result</code> or <code>basic_outcome</code>, and override member functions,
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for which you will need to study the source code as that form of customisation is out of scope for this tutorial.</p>
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<p>Another option is to supply a custom <code>NoValuePolicy</code>
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(<a href="../../tutorial/essential/no-value/custom.html">see preceding section</a>).
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From Outcome v2.2 onwards, intercepting construction, copies and moves <em>requires</em>
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a custom <code>NoValuePolicy</code>.</p>
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<p>Before Outcome v2.2, there was an ADL discovered event hook mechanism for intercepting
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construction, copies and moves (it was found to be brittle, error prone and surprising
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in empirical use, which is why it was replaced). The ADL discovered event hooks still
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function in Outcome v2.2 and later if <a href="../../reference/macros/enable_legacy_support_for.html" class="api-reference"><code>BOOST_OUTCOME_ENABLE_LEGACY_SUPPORT_FOR</code></a>
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is set to less than <code>220</code> to enable emulation.</p>
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<p>You will note that the naming is simply one of <code>hook_*</code> => <code>on_*</code>, the parameters remain
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identical. This eases porting code from Outcome v2.1 to v2.2, it’s usually just a case
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of copy-pasting the ADL hook code into a custom <code>NoValuePolicy</code>.</p>
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<hr />
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<p><strong>Policy set event hooks (Outcome v2.2 onwards):</strong></p>
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<ul>
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<li>Constructed
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<ul>
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<li><a href="../../reference/policies/base/on_result_construction.html" class="api-reference"><code>void on_result_construction(T *, U &&) noexcept</code></a>
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</li>
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<li><a href="../../reference/policies/base/on_outcome_construction.html" class="api-reference"><code>void on_outcome_construction(T *, U &&) noexcept</code></a>
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</li>
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</ul></li>
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<li>In-place constructed
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<ul>
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<li><a href="../../reference/policies/base/on_result_in_place_construction.html" class="api-reference"><code>void on_result_in_place_construction(T *, in_place_type_t<U>, Args &&...) noexcept</code></a>
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</li>
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<li><a href="../../reference/policies/base/on_outcome_in_place_construction.html" class="api-reference"><code>void on_outcome_in_place_construction(T *, in_place_type_t<U>, Args &&...) noexcept</code></a>
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</li>
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</ul></li>
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<li>Copied
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<ul>
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<li><a href="../../reference/policies/base/on_result_copy_construction.html" class="api-reference"><code>void on_result_copy_construction(T *, U &&) noexcept</code></a>
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</li>
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<li><a href="../../reference/policies/base/on_outcome_copy_construction.html" class="api-reference"><code>void on_outcome_copy_construction(T *, U &&) noexcept</code></a>
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</li>
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</ul></li>
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<li>Moved
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<ul>
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<li><a href="../../reference/policies/base/on_result_move_construction.html" class="api-reference"><code>void on_result_move_construction(T *, U &&) noexcept</code></a>
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</li>
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<li><a href="../../reference/policies/base/on_outcome_move_construction.html" class="api-reference"><code>void on_outcome_move_construction(T *, U &&) noexcept</code></a>
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</li>
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</ul></li>
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</ul>
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<hr />
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<p><strong>ADL discovered event hooks (before Outcome v2.2):</strong></p>
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<ul>
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<li>Constructed
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<ul>
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<li><a href="../../reference/functions/hooks/hook_result_construction.html" class="api-reference"><code>void hook_result_construction(T *, U &&) noexcept</code></a>
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</li>
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<li><a href="../../reference/functions/hooks/hook_outcome_construction.html" class="api-reference"><code>void hook_outcome_construction(T *, U &&) noexcept</code></a>
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</li>
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</ul></li>
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<li>In-place constructed
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<ul>
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<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<U>, Args &&...) noexcept</code></a>
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</li>
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<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<U>, Args &&...) noexcept</code></a>
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</li>
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</ul></li>
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<li>Copied
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<ul>
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<li><a href="../../reference/functions/hooks/hook_result_copy_construction.html" class="api-reference"><code>void hook_result_copy_construction(T *, U &&) noexcept</code></a>
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</li>
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<li><a href="../../reference/functions/hooks/hook_outcome_copy_construction.html" class="api-reference"><code>void hook_outcome_copy_construction(T *, U &&) noexcept</code></a>
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</li>
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</ul></li>
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<li>Moved
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<ul>
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<li><a href="../../reference/functions/hooks/hook_result_move_construction.html" class="api-reference"><code>void hook_result_move_construction(T *, U &&) noexcept</code></a>
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</li>
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<li><a href="../../reference/functions/hooks/hook_outcome_move_construction.html" class="api-reference"><code>void hook_outcome_move_construction(T *, U &&) noexcept</code></a>
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</li>
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</ul></li>
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</ul>
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<hr />
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<p>One criticism often levelled against library-based exception throw alternatives is that they do
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not provide as rich a set of facilities as C++ exception throws. This section shows
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you how to configure Outcome, using the event hooks, to take a stack backtrace on
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construction of an errored <code>result<T, error_code></code>,
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and if that <code>result<T, error_code></code> should ever be converted into an <code>outcome<T, error_code, std::exception_ptr></code>,
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a custom <code>std::exception_ptr</code> will be just-in-time synthesised consisting of the <code>std::system_error</code>
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for the error code, plus an expanded message string containing the stack backtrace of where
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the error originally occurred.</p>
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<p>One can see the use case for such a configuration where low-level, deterministic,
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fixed latency code is built with <code>result</code>, and it dovetails into higher-level
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application code built with <code>outcome</code> where execution time guarantees are not
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important, and thus where a <code>malloc</code> is okay. One effectively has constructed a
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“lazy indeterminism”, or “just-in-time indeterminism” mechanism for handling
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failure, but with all the rich information of throwing C++ exceptions.</p>
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</div><p><small>Last revised: September 16, 2020 at 11:58:04 +0100</small></p>
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