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  3. <title>Using Outcome from C code - Boost.Outcome documentation</title>
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  13. 

  14.   <div class="titlepage"><div><div><h1 style="clear: both">Using Outcome from C code</h1></div></div></div>
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  15. <p>A long standing problem for C code (or more usually nowadays, the many other programming
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  16. languages which can speak the C ABI but not the C++ ABI) is how to interpret C++ exception throws. The answer
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  17. is of course that they cannot, thus requiring one to write C shim code on the C++ side
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  18. of things of the form:</p>
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  19. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="c1">// The API we wish to expose to C
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  20. </span><span class="c1"></span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="nf">get_value</span><span class="p">(</span><span class="kt">double</span> <span class="n">v</span><span class="p">);</span>
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  21. 

  22. <span class="c1">// The C shim function for the C++ get_value() function.
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  23. </span><span class="c1"></span><span class="k">extern</span> <span class="s">&#34;C&#34;</span> <span class="kt">int</span> <span class="n">c_get_value</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">**</span><span class="n">ret</span><span class="p">,</span> <span class="kt">double</span> <span class="n">v</span><span class="p">)</span>
    24. 

  24. <span class="p">{</span>
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  25.   <span class="k">try</span>
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  26.   <span class="p">{</span>
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  27.     <span class="o">*</span><span class="n">ret</span> <span class="o">=</span> <span class="n">get_value</span><span class="p">(</span><span class="n">v</span><span class="p">);</span>
    28. 

  28.     <span class="k">return</span> <span class="mi">0</span><span class="p">;</span>  <span class="c1">// success
    29. 

  29. </span><span class="c1"></span>  <span class="p">}</span>
    30. 

  30.   <span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">range_error</span> <span class="o">&amp;</span><span class="p">)</span>
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  31.   <span class="p">{</span>
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  32.     <span class="k">return</span> <span class="n">ERANGE</span><span class="p">;</span>
    33. 

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

  34.   <span class="c1">// More catch clauses may go in here ...
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  35. </span><span class="c1"></span>  <span class="k">catch</span><span class="p">(...)</span>
    36. 

  36.   <span class="p">{</span>
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  37.     <span class="k">return</span> <span class="n">EAGAIN</span><span class="p">;</span>
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  38.   <span class="p">}</span>
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  39. <span class="p">}</span>
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  40. </code></pre></div>
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  41. <p>This is sufficiently painful that most reach for a bindings generator tool like
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  42. <a href="http://www.swig.org/">SWIG</a> to automate this sort of tedious boilerplate generation.
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  43. And this is fine for larger projects, but for smaller projects the cost of
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  44. setting up and configuring SWIG is also non-trivial.</p>
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  45. 

  46. <p>What would be really great is if <code>result&lt;T&gt;</code> returning <code>noexcept</code> C++ functions
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  47. could be used straight from C. And indeed Experimental Outcome provides just that facility
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  48. which this section covers next.</p>
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  49. 

  50. 

  51. 

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