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  3. <title>Hook result - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Hook result</h1></div></div></div>
    14. 

  14.   <p>We now tell Outcome that for every instance of our localised <code>result&lt;T&gt;</code>, that
    15. 

  15. on failure construction only, we want custom code to be run which increments the current
    16. 

  16. slot in TLS storage and writes the current stack backtrace into it.</p>
    17. 

  17. 

  18. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">namespace</span> <span class="n">error_code_extended</span>
    19. 

  19. <span class="p">{</span>
    20. 

  20.   <span class="c1">// Specialise the result construction hook for our localised result
    21. 

  21. </span><span class="c1"></span>  <span class="c1">// We hook any non-copy, non-move, non-inplace construction, capturing a stack backtrace
    22. 

  22. </span><span class="c1"></span>  <span class="c1">// if the result is errored.
    23. 

  23. </span><span class="c1"></span>  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span><span class="err"> </span><span class="nc">T</span><span class="p">,</span> <span class="k">class</span><span class="err"> </span><span class="nc">U</span><span class="o">&gt;</span> <span class="kr">inline</span> <span class="kt">void</span> <span class="n">hook_result_construction</span><span class="p">(</span><span class="n">result</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="o">*</span><span class="n">res</span><span class="p">,</span> <span class="n">U</span> <span class="o">&amp;&amp;</span> <span class="cm">/*unused*/</span><span class="p">)</span> <span class="k">noexcept</span>
    24. 

  24.   <span class="p">{</span>
    25. 

  25.     <span class="k">if</span><span class="p">(</span><span class="n">res</span><span class="o">-&gt;</span><span class="n">has_error</span><span class="p">())</span>
    26. 

  26.     <span class="p">{</span>
    27. 

  27.       <span class="c1">// Grab the next extended info slot in the TLS
    28. 

  28. </span><span class="c1"></span>      <span class="n">extended_error_info</span> <span class="o">&amp;</span><span class="n">eei</span> <span class="o">=</span> <span class="n">mythreadlocaldata</span><span class="p">().</span><span class="n">next</span><span class="p">();</span>
    29. 

  29. 

  30.       <span class="c1">// Write the index just grabbed into the spare uint16_t
    31. 

  31. </span><span class="c1"></span>      <span class="n">BOOST_OUTCOME_V2_NAMESPACE</span><span class="o">::</span><span class="n">hooks</span><span class="o">::</span><span class="n">set_spare_storage</span><span class="p">(</span><span class="n">res</span><span class="p">,</span> <span class="n">mythreadlocaldata</span><span class="p">().</span><span class="n">current</span> <span class="o">-</span> <span class="mi">1</span><span class="p">);</span>
    32. 

  32. 

  33.       <span class="c1">// Capture a backtrace into my claimed extended info slot in the TLS
    34. 

  34. </span><span class="c1"></span>      <span class="n">eei</span><span class="p">.</span><span class="n">items</span> <span class="o">=</span> <span class="o">::</span><span class="n">backtrace</span><span class="p">(</span><span class="n">eei</span><span class="p">.</span><span class="n">backtrace</span><span class="p">.</span><span class="n">data</span><span class="p">(),</span> <span class="n">eei</span><span class="p">.</span><span class="n">backtrace</span><span class="p">.</span><span class="n">size</span><span class="p">());</span>
    35. 

  35.     <span class="p">}</span>
    36. 

  36.   <span class="p">}</span>
    37. 

  37. <span class="p">}</span>
    38. 

  38. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/error_code_extended.cpp#L119" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    39. 

  39. 

  40. 

  41. <p>The only non-obvious part above is the call to <a href="../../../reference/functions/hooks/set_spare_storage.html" class="api-reference"><code>void set_spare_storage(basic_result|basic_outcome *, uint16_t) noexcept</code></a>
    42. 

  42. .</p>
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  43. 

  44. <p>Both <code>result</code> and <code>outcome</code> keep their internal state metadata in a <code>uint32_t</code>,
    45. 

  45. half of which is not used by Outcome. As it can be very useful to keep a small
    46. 

  46. unique number attached to any particular <code>result</code> or <code>outcome</code> instance, we
    47. 

  47. permit user code to set those sixteen bits to anything they feel like.
    48. 

  48. The corresponding function to retrieve those sixteen bits is <a href="../../../reference/functions/hooks/spare_storage.html" class="api-reference"><code>uint16_t spare_storage(const basic_result|basic_outcome *) noexcept</code></a>
    49. 

  49. .</p>
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  50. 

  51. <p>The state of the sixteen bits of spare storage are ignored during comparison operations.</p>
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  52. 

  53. <p>The sixteen bits of spare storage propagate during the following operations:</p>
    54. 

  54. 

  55. <ol>
    56. 

  56. <li>Copy and move construction between <code>result</code>&rsquo;s.</li>
    57. 

  57. <li>Copy and move construction between <code>outcome</code>&rsquo;s.</li>
    58. 

  58. <li>Copy and move construction from a <code>result</code> to an <code>outcome</code>.</li>
    59. 

  59. <li>Converting copy and move constructions for all the above.</li>
    60. 

  60. <li>Assignment for all of the above.</li>
    61. 

  61. </ol>
    62. 

  62. 

  63. <p>They are NOT propagated in these operations:</p>
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  64. 

  65. <ol>
    66. 

  66. <li>Any conversion or translation which goes through a <code>failure_type</code> or <code>success_type</code>.</li>
    67. 

  67. <li>Any conversion or translation which goes through a <code>ValueOrError</code> concept match.</li>
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  68. <li>Any unpacking or repacking of value/error/exception e.g. a manual repack of an
    69. 

  69. <code>outcome</code> into a <code>result</code>.</li>
    70. 

  70. </ol>
    71. 

  71. 

  72. 

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