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  3. <title>Hook outcome - Boost.Outcome documentation</title>
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  10. <a accesskey="p" href="../../../tutorial/advanced/hooks/poke_exception.html"><img src="../../../images/prev.png" alt="Prev"></a>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Hook outcome</h1></div></div></div>
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  14.   <p>The final step is to add ADL discovered event hooks for the very specific case of
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  15. when our localised <code>outcome</code> is copy or move constructed from our localised <code>result</code>.</p>
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  16. 

  17. <p>You ought to be very careful that the <code>noexcept</code>-ness of these matches the <code>noexcept</code>-ness
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  18. of the types in the <code>outcome</code>. You may have noticed that <code>poke_exception()</code> creates
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  19. a <code>std::string</code> and appends to it. This can throw an exception. If the copy and/or
    20. 

  20. move constructors of <code>T</code>, <code>EC</code> and <code>EP</code> are <code>noexcept</code>, then so will be <code>outcome</code>&rsquo;s
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  21. copy and/or move constructor. Thus if <code>poke_exception()</code> throws, instant program
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  22. termination would occur, which is bad.</p>
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  23. 

  24. <p>We avoid that problem in this case by wrapping <code>poke_exception()</code> in a <code>try...catch</code>
    25. 

  25. which throws away any exceptions thrown.</p>
    26. 

  26. 

  27. <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>
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  28. <span class="p">{</span>
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  29.   <span class="c1">// Specialise the outcome copy and move conversion hook for when our localised result
    30. 

  30. </span><span class="c1"></span>  <span class="c1">// is used as the source for copy construction our localised outcome
    31. 

  31. </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_outcome_copy_construction</span><span class="p">(</span><span class="n">outcome</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="k">const</span> <span class="n">result</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&gt;</span> <span class="o">&amp;</span> <span class="cm">/*unused*/</span><span class="p">)</span> <span class="k">noexcept</span>
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  32.   <span class="p">{</span>
    33. 

  33.     <span class="k">try</span>
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  34.     <span class="p">{</span>
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  35.       <span class="c1">// when copy constructing from a result&lt;T&gt;, poke in an exception
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  36. </span><span class="c1"></span>      <span class="n">poke_exception</span><span class="p">(</span><span class="n">res</span><span class="p">);</span>
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  37.     <span class="p">}</span>
    38. 

  38.     <span class="k">catch</span><span class="p">(...)</span>
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  39.     <span class="p">{</span>
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  40.       <span class="c1">// Do nothing
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  41. </span><span class="c1"></span>    <span class="p">}</span>
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  42.   <span class="p">}</span>
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  43.   <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_outcome_move_construction</span><span class="p">(</span><span class="n">outcome</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">result</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&gt;</span> <span class="o">&amp;&amp;</span> <span class="cm">/*unused*/</span><span class="p">)</span> <span class="k">noexcept</span>
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  44.   <span class="p">{</span>
    45. 

  45.     <span class="k">try</span>
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  46.     <span class="p">{</span>
    47. 

  47.       <span class="c1">// when move constructing from a result&lt;T&gt;, poke in an exception
    48. 

  48. </span><span class="c1"></span>      <span class="n">poke_exception</span><span class="p">(</span><span class="n">res</span><span class="p">);</span>
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  49.     <span class="p">}</span>
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  50.     <span class="k">catch</span><span class="p">(...)</span>
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  51.     <span class="p">{</span>
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  52.       <span class="c1">// Do nothing
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  53. </span><span class="c1"></span>    <span class="p">}</span>
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  54.   <span class="p">}</span>
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  55. <span class="p">}</span>
    56. 

  56. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/error_code_extended.cpp#L182" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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  57. 

  58. 

  59. 

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