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82 lines
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<title>Limitations - Boost.Outcome documentation</title>
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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><p>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>.</p></li>
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<li><p>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.</p></li>
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<li><p>You cannot use overloading in your <code>extern "C"</code> functions.</p></li>
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<li><p>You may only use types in your C++ function declaration for which these traits are both true:</p>
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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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</ul>
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<p>(Note that <code>std::is_trivially_copyable_v<T></code> requires trivial destruction,
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but NOT trivial construction. This means that C++ can do non-trivial construction
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of otherwise trivial types)</p></li>
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</ol>
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<hr />
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<p>The above is what the standard officially requires for <em>well defined</em> C and C++ interop.
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However, all of the three major compilers MSVC, GCC and clang are considerably more relaxed.
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In those three major compilers, “almost-standard-layout” C++ types work fine in C.</p>
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<p>“Almost-standard-layout” C++ types have these requirements:</p>
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<ol>
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<li>No virtual functions or virtual base classes i.e.
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<a href="http://en.cppreference.com/w/cpp/types/is_polymorphic"><code>std::is_polymorphic_v<T></code></a>
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must be false. This is because the vptrs offset the proper front of the data layout
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in an unknowable way to C.</li>
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<li>Non-static data members of reference type appear to C as pointers. You
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must never supply from C to C++ a non-null pointer which is seen as a reference in C++.</li>
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<li>C++ inheritance is seen in C data layout as if the most derived class has nested
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variables of the inherited types at the top, in order of inheritance.</li>
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<li>Types with non-trivial destructors work fine so long as at least move construction
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and assignment is the same as
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copying bits like <code>memcpy()</code>. You just need to make sure instances of the type return
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to C++, and don’t get orphaned in C. This was referred to in previous pages in this
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section as “move relocating” types.</li>
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</ol>
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<p>Experimental Outcome’s support for being used from C does not meet the current strict
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requirements, and thus relies on the (very common) implementation defined behaviour just
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described (it is hoped that future C++ standards can relax the requirements to those
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just described).</p>
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<p>Specifically, proposed <code>status_code</code> is an almost-standard-layout type,
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and thus while it can’t be returned from <code>extern "C"</code> functions as the compiler
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will complain, it is perfectly safe to return from C++ functions to C code on the
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three major compilers, as it is an “almost-standard-layout” C++ type if <code>T</code> is
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an “almost-standard-layout” C++ type.</p>
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</div><p><small>Last revised: February 05, 2019 at 17:14:18 UTC</small></p>
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