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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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  3. <title>Example C&#43;&#43; function - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Example C++ function</h1></div></div></div>
    14. 

  14.   <p>Let us start with a simple C++ function which we wish to make available to C code:</p>
    15. 

  15. 

  16. <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">outcome_e</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_V2_NAMESPACE</span><span class="o">::</span><span class="n">experimental</span><span class="p">;</span>
    17. 

  17. 

  18. <span class="c1">// Fill the supplied buffer with the integer v converted to a string,
    19. 

  19. </span><span class="c1">// returning length of string minus null terminator
    20. 

  20. </span><span class="c1"></span><span class="n">outcome_e</span><span class="o">::</span><span class="n">status_result</span><span class="o">&lt;</span><span class="n">size_t</span><span class="o">&gt;</span> <span class="n">to_string</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">buffer</span><span class="p">,</span> <span class="n">size_t</span> <span class="n">bufferlen</span><span class="p">,</span> <span class="kt">int</span> <span class="n">v</span><span class="p">)</span> <span class="k">noexcept</span>
    21. 

  21. <span class="p">{</span>
    22. 

  22.   <span class="k">try</span>
    23. 

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

  24.     <span class="c1">// Could throw an exception!
    25. 

  25. </span><span class="c1"></span>    <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">temp</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">to_string</span><span class="p">(</span><span class="n">v</span><span class="p">));</span>
    26. 

  26. 

  27.     <span class="c1">// Will this string exceed the supplied buffer?
    28. 

  28. </span><span class="c1"></span>    <span class="k">if</span><span class="p">(</span><span class="n">temp</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">+</span> <span class="mi">1</span> <span class="o">&gt;</span> <span class="n">bufferlen</span><span class="p">)</span>
    29. 

  29.       <span class="k">return</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">errc</span><span class="o">::</span><span class="n">no_buffer_space</span><span class="p">;</span>
    30. 

  30. 

  31.     <span class="c1">// Copy the string into the supplied buffer, and return length of string
    32. 

  32. </span><span class="c1"></span>    <span class="n">memcpy</span><span class="p">(</span><span class="n">buffer</span><span class="p">,</span> <span class="n">temp</span><span class="p">.</span><span class="n">data</span><span class="p">(),</span> <span class="n">temp</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">+</span> <span class="mi">1</span><span class="p">);</span>
    33. 

  33.     <span class="k">return</span> <span class="n">temp</span><span class="p">.</span><span class="n">size</span><span class="p">();</span>
    34. 

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

  35.   <span class="k">catch</span><span class="p">(...)</span>
    36. 

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

  37.     <span class="c1">// This is the &lt;system_error2&gt; analogue of Standard Outcome&#39;s
    38. 

  38. </span><span class="c1"></span>    <span class="c1">// error_from_exception() utility function. It consumes an exception
    39. 

  39. </span><span class="c1"></span>    <span class="c1">// ptr (defaulted to current exception), and tries to match it to a
    40. 

  40. </span><span class="c1"></span>    <span class="c1">// standard C++ library exception type, returning a system_code
    41. 

  41. </span><span class="c1"></span>    <span class="c1">// with an appropriate code domain (generic_code, posix_code,
    42. 

  42. </span><span class="c1"></span>    <span class="c1">// win32_code).
    43. 

  43. </span><span class="c1"></span>    <span class="c1">//
    44. 

  44. </span><span class="c1"></span>    <span class="c1">// Note that using this function requires including
    45. 

  45. </span><span class="c1"></span>    <span class="c1">// &lt;boost/outcome/experimental/status-code/system_code_from_exception.hpp&gt;
    46. 

  46. </span><span class="c1"></span>    <span class="c1">// It is NOT included by Experimental Outcome by default.
    47. 

  47. </span><span class="c1"></span>    <span class="k">return</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">system_code_from_exception</span><span class="p">();</span>
    48. 

  48.   <span class="p">}</span>
    49. 

  49. <span class="p">}</span>
    50. 

  50. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/cpp_api.cpp#L37" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    51. 

  51. 

  52. 

  53. <p>As the alias <code>status_result&lt;size_t&gt;</code> defaults the erased type to the alias <code>system_code</code>,
    54. 

  54. the <code>to_string()</code> function returns (in concrete types) <code>basic_result&lt;size_t, status_code&lt;erased&lt;intptr_t&gt;&gt;&gt;</code>.</p>
    55. 

  55. 

  56. <p>The standard Outcome function referenced is documented at
    57. 

  57. 

  58. <a href="https://en.cppreference.com/w/cpp/error/error_code" class="api-reference" target="_blank"><i class="fa fa-book" aria-hidden="true"></i> <code>std::error_code error_from_exception(std::exception_ptr &amp;&amp;ep = std::current_exception(), std::error_code not_matched = std::make_error_code(std::errc::resource_unavailable_try_again)) noexcept</code></a>
    59. 

  59. .
    60. 

  60. The proposed <code>&lt;system_error2&gt;</code> reference library implementation provides an identically named
    61. 

  61. function taking similar parameters, but it returns a <code>outcome_e::system_code</code> (<code>status_code&lt;erased&lt;intptr_t&gt;&gt;</code>) instead of a <code>std::error_code</code>.</p>
    62. 

  62. 

  63. 

  64.         </div><p><small>Last revised: February 05, 2019 at 17:14:18 UTC</small></p>
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