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copied!<p>This is an interesting question. As you say, compiling the code as C code rightly produces no error. And only MSVC seems to have trouble with it when compiled as C++ code.</p> <p>Since other C++ compilers don't have a problem with the code, this might be a bug in MSVC, but I can see how MSVC might have a rationale for this error. When the C++ compiler hits the line:</p> <pre><code>struct Test; </code></pre> <p>That's an incomplete declaration of <code>struct Test</code> - the compiler doesn't know if the complete definition of <code>struct Test</code> will contain C++ specific items (virtual functions, inheritance, etc). Note that types in an <code>extern "C"</code> block can still use all C++ facilities; the <code>extern "C"</code> language linkage specification applies only to "function types of all function declarators, function names, and variable names introduced by the declaration(s)" (7.5/4 "Linkage specifications").</p> <p>So I could see how when MSVC's C++ compiler comes across an <code>extern "C"</code> function that's returning an incomplete type, it might decide that it needs to return an error at that point in case the type turns out to not be a plain C-style POD type.</p> <p>The C++ standard does say (7.5/9 "Linkage specifications"):</p> <blockquote> <p>Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved.</p> </blockquote> <p>So MSVC might have some leeway (standards-wise) if it has a reason to not permit <code>extern "C"</code> functions from returning non-POD objects, though I'm not sure why MSVC would have a problem when other Windows compilers don't. If anyone knows details (or if they know I'm just plain off-base here), I'd appreciate a note.</p> <p>Not that any of this this helps you - it's just my guess at a rationale.</p> <p>Without knowing more about your codegen process and how you might be able to influence it, I'm not sure what decent options you might have - maybe a post-processing of the generated files to split out the stuff that <em>needs</em> to be compiled as C (or rearranges the declarations). But I can imagine that that might be a nightmare to get working and especially to maintain.</p>

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