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<q>...one of the most highly
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regarded and expertly designed C++ library projects in the
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world.</q> <span class="heading-attribution">— <a href=
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<div class="titlepage"><div><div><h1 style="clear: both">Limitations</h1></div></div></div>
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<p>C++ has excellent two-way compatibility with the C ABI, but there are some
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limitations you must observe to write C++ code which C code can call without
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marshalling at the ABI boundary:</p>
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<ol>
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<li>A C++ function may not throw exceptions if it is safe to call from C, and
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so should always be marked <code>noexcept</code>.</li>
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<li>A C++ function should be annotated with <code>extern "C"</code> to prevent its symbol
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being mangled, and thus give it the C rather than C++ ABI.</li>
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<li>You cannot use overloading in your <code>extern "C"</code> functions.</li>
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<li>You may only use types in your C++ function for which these traits are both true:
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<ul>
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<li><a href="http://en.cppreference.com/w/cpp/types/is_standard_layout"><code>std::is_standard_layout_v<T></code></a></li>
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<li><a href="http://en.cppreference.com/w/cpp/types/is_trivially_copyable"><code>std::is_trivially_copyable_v<T></code></a></li>
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<li>Note that <code>std::is_trivially_copyable_v<T></code> also requires trivial destruction, but NOT trivial construction.
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This means that C++ can do non-trivial construction of otherwise trivial types.</li>
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</ul></li>
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<li>Your C++ function should return either a <code>result<T, E></code> or a <code>result<T></code> i.e. with the <code>EC</code> defaulted
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to <code>std::error_code</code> or something with equal layout. Note that <code>std::error_code</code>
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has standard layout, and is trivially copyable, and thus is a legal type in C.</li>
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</ol>
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<p>These type limitations might seem to be showstoppers to C++ programmers, but with a bit of
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care during library design, you can actually express a lot of
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complex C++ and still meet these requirements. For example, more than 80%
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of the APIs in my C++ 17 <a href="https://ned14.github.io/afio/">AFIO</a> library (which I hope to propose
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in 2018 to become the File I/O TS for C++ 24) meets the above restrictions,
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and thus can be called directly from C code or any programming language which
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speaks the C ABI.</p>
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<p>A useful tip is to sprinkle <code>static_assert()</code> testing the two traits above around your header files for any types which
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can traverse the C ABI boundary, and add <code>extern "C"</code> free function thunks for your class
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member functions<sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup>. You should be ready to go from the C++ side at least.</p>
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<div class="footnotes">
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<hr />
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<ol>
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<li id="fn:1">Or simply write a script to do this for you. A starter script can be found at <a href="https://github.com/ned14/afio/blob/master/scripts/make_free_functions.py">https://github.com/ned14/afio/blob/master/scripts/make_free_functions.py</a>.
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<a class="footnote-return" href="#fnref:1"><sup>[return]</sup></a></li>
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</ol>
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</div>
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</div><p><small>Last revised: December 14, 2017 at 18:57:08 UTC</small></p>
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