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<div class="titlepage"><div><div><h1 style="clear: both">Example C++ function</h1></div></div></div>
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<p>Let us start with a simple C++ function which we wish to make available to C code:</p>
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<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="c1">// Fill the supplied buffer with the integer v converted to a string, returning
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</span><span class="c1">// length of string minus null terminator
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</span><span class="c1"></span><span class="k">extern</span> <span class="s">"C"</span> <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">size_t</span><span class="o">></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>
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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="c1">// Could throw an exception!
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</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>
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<span class="c1">// Will this string exceed the supplied buffer?
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</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">></span> <span class="n">bufferlen</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">std</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>
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<span class="c1">// Copy the string into the supplied buffer, and return length of string
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</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>
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<span class="k">return</span> <span class="n">temp</span><span class="p">.</span><span class="n">size</span><span class="p">();</span>
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<span class="p">}</span>
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<span class="k">catch</span><span class="p">(...)</span>
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<span class="p">{</span>
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<span class="c1">// This utility function rethrows the C++ exception, matching it
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</span><span class="c1"></span> <span class="c1">// against every standard library exception and generating an
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</span><span class="c1"></span> <span class="c1">// error code exactly matching it if possible. So, if the
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</span><span class="c1"></span> <span class="c1">// string creation threw std::bad_alloc, that would be converted
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</span><span class="c1"></span> <span class="c1">// into make_error_code(std::errc::not_enough_memory).
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</span><span class="c1"></span> <span class="k">return</span> <span class="n">outcome</span><span class="o">::</span><span class="n">error_from_exception</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><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/cpp_api.cpp.disabled#L31" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>A surprise to some may be that one can return an <code>outcome::result<size_t></code>
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from a C function! This is because if for some <code>result<T, EC></code> where both <code>T</code> and <code>EC</code> have
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standard layout and are trivially copyable, then Outcome guarantees so will
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<code>result<T, EC></code>. Thus <code>outcome::result<size_t></code> is a perfectly legal C type,
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and can be returned directly to C code.</p>
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</div><p><small>Last revised: October 20, 2017 at 18:57:40 +0100</small></p>
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