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<div class="titlepage"><div><div><h1 style="clear: both">Using result<T> from C code</h1></div></div></div>
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<p>A long standing problem for C code (or more usually nowadays, the many other programming
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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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is of course that they cannot, thus requiring one to write C shim code on the C++ side
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of things of the form:</p>
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<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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</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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<span class="c1">// The C shim function for the C++ get_value() function.
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</span><span class="c1"></span><span class="k">extern</span> <span class="s">"C"</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>
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<span class="p">{</span>
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<span class="k">try</span>
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<span class="p">{</span>
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<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>
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<span class="k">return</span> <span class="mi">0</span><span class="p">;</span> <span class="c1">// success
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</span><span class="c1"></span> <span class="p">}</span>
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<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">&</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="k">return</span> <span class="n">ERANGE</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="c1">// More catch clauses may go in here ...
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</span><span class="c1"></span> <span class="k">catch</span><span class="p">(...)</span>
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<span class="p">{</span>
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<span class="k">return</span> <span class="n">EAGAIN</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</code></pre></div>
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<p>This is sufficiently painful that most reach for a bindings generator tool like
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<a href="http://www.swig.org/">SWIG</a> to automate this sort of tedious boilerplate generation.
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And this is fine for larger projects, but for smaller projects the cost of
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setting up and configuring SWIG is also non-trivial.</p>
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<p>What would be really great is if <code>result<T></code> returning <code>noexcept</code> C++ functions
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could be used straight from C. And indeed Outcome provides just that facility
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which this section covers next.</p>
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</div><p><small>Last revised: October 20, 2017 at 02:06:07 +0100</small></p>
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