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  3. <title>outcome&lt;&gt; - Boost.Outcome documentation</title>
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  13. 

  14.   <div class="titlepage"><div><div><h1 style="clear: both">outcome&lt;&gt;</h1></div></div></div>
    15. 

  15. 

  16. 

  17. <p>Type <a href="../../reference/aliases/outcome.html" class="api-reference"><code>outcome&lt;T, EC = varies, EP = varies, NoValuePolicy = policy::default_policy&lt;T, EC, EP&gt;&gt;</code></a>
    18. 

  18.  represents either a successfully computed value of type <code>T</code>, or one or two reasons for failure. Failure can be represented by <code>EC</code>, or <code>EP</code>, or both, although usually it will either be an <code>EC</code> or an <code>EP</code>.
    19. 

  19. Similarly to <code>result</code>, <code>EC</code> defaults to <code>std::error_code</code>/<code>boost::system::error_code</code>, and <code>EP</code> defaults to <code>std::exception_ptr</code>/<code>boost::exception_ptr</code>.</p>
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  20. 

  21. <p>The distinction is made into two types, <code>EC</code> and <code>EP</code>:</p>
    22. 

  22. 

  23. <ul>
    24. 

  24. <li><code>EC</code> represents a <em>recoverable</em> failure from a lower-layer function, perhaps which was returned through <a href="../../reference/aliases/result.html" class="api-reference"><code>result&lt;T, E = varies, NoValuePolicy = policy::default_policy&lt;T, E, void&gt;&gt;</code></a>
    25. 

  25. .</li>
    26. 

  26. <li><code>EP</code> represents an <em>unrecoverable</em> failure where a C++ exception would ordinarily have been thrown. This is usually a pointer to an exception throw.</li>
    27. 

  27. </ul>
    28. 

  28. 

  29. <p>It should be emphasised that this distinction is by convention only, but it will be confusing to your
    30. 

  30. users if you deviate significantly from this convention.</p>
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  31. 

  32. <hr>
    33. 

  33. 

  34. <h3 id="legacy-codebases">Legacy codebases</h3>
    35. 

  35. 

  36. <p><code>outcome</code> is useful for transporting exceptions across layers of the program that were never designed with exception safety in mind.</p>
    37. 

  37. 

  38. <p>Consider a program consisting of three layers:</p>
    39. 

  39. 

  40. 

  41. <center><img src="../../tutorial/essential/outcome/layer_chart.gif"></center>
    42. 

  42. 

  43. <p>The highest-level layer, <code>Layer3</code>, uses exceptions for signalling failures. The middle layer, <code>Layer2_old</code>,
    44. 

  44. was not designed with exception safety in mind and functions need to return information about failures in return value.
    45. 

  45. But down in the implementation details, in <code>Layer1</code>, another library is used that again throws exceptions. The goal is
    46. 

  46. to be able to transfer an exception thrown in <code>Layer1</code> through <code>Layer2_old</code>, which is not exception-safe,
    47. 

  47. and be able to rethrow it in <code>Layer3</code>.</p>
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  48. 

  49. <p>In <code>Layer1</code> we have two functions from two libraries: one reports failures by throwing exceptions, the other by returning <code>result&lt;&gt;</code>:</p>
    50. 

  50. 

  51. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">auto</span> <span class="nf">f</span><span class="p">()</span> <span class="o">-&gt;</span> <span class="kt">int</span><span class="p">;</span>  <span class="c1">// throws on failure
    52. 

  52. </span><span class="c1"></span><span class="k">auto</span> <span class="nf">g</span><span class="p">()</span> <span class="k">noexcept</span> <span class="o">-&gt;</span> <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">;</span>
    53. 

  53. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_outcome.cpp#L36" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    54. 

  54. 

  55. 

  56. <p>In <code>Layer2_old</code> we cannot use exceptions, so its function <code>h</code> uses return type <code>outcome&lt;&gt;</code> to report failures. It is using functions <code>f</code> and <code>g</code> and reports their failures inside <code>outcome&lt;&gt;</code>:</p>
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  57. 

  58. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">auto</span> <span class="n">old</span><span class="o">::</span><span class="n">h</span><span class="p">()</span> <span class="k">noexcept</span> <span class="o">-&gt;</span> <span class="n">outcome</span><span class="o">::</span><span class="n">outcome</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span>
    59. 

  59. <span class="p">{</span>
    60. 

  60.   <span class="n">BOOST_OUTCOME_TRY</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="p">(</span><span class="n">g</span><span class="p">()));</span>             <span class="c1">// #1
    61. 

  61. </span><span class="c1"></span>    
    62. 

  62.   <span class="k">try</span> <span class="p">{</span>
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  63.     <span class="k">return</span> <span class="n">i</span> <span class="o">+</span> <span class="n">f</span><span class="p">();</span>
    64. 

  64.   <span class="p">}</span>
    65. 

  65.   <span class="k">catch</span> <span class="p">(...)</span> <span class="p">{</span>
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  66.     <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">current_exception</span><span class="p">();</span> <span class="c1">// #2
    67. 

  67. </span><span class="c1"></span>  <span class="p">}</span>
    68. 

  68. <span class="p">}</span>
    69. 

  69. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_outcome.cpp#L56" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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  70. 

  71. 

  72. <p>#1. Operator <code>TRY</code> can forward failures encoded in <code>result&lt;T, EC&gt;</code> as <code>outcome&lt;T, EC, EP&gt;</code> without any loss in information. You can also use <code>TRY</code> to forward failure from one <code>outcome&lt;&gt;</code> to another.</p>
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  73. 

  74. <p>#2. You can store the current exception through <code>std::exception_ptr</code> inside <code>outcome&lt;T, EC, EP&gt;</code> without any loss in information
    75. 

  75.     (provided that <code>EP</code> is <code>std::exception_ptr</code>).</p>
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  76. 

  77. 

  78. 

  79.         </div><p><small>Last revised: March 19, 2019 at 22:57:48 &#43;0100</small></p>
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