How does "extern C++" work?

Question

I jumped into winnt.h and I found out the code as following:

extern "C++" // templates cannot be declared to have 'C' linkage
template <typename T, size_t N>
char (*RtlpNumberOf( UNALIGNED T (&)[N] ))[N];

I'd like to ask questions as following:

1. how does extern "C++" work?
2. is this portable among GCC, and Clang?
3. can all templates be exported with this syntax?

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With question 3, I mean that can I separate declearation and definition of the templates, and then generate a dynamic link for the template without actually give the implementation by using this trick?

Answer 1

Well, extern "C++" won't work in C, of course (though some compilers might support it as an extension). So it only makes sense to use it in C++.

That's because in the case of multiple nested extern linkage specifiers, the innermost one takes effect. So if you have a header file surrounded with extern "C", you can use extern "C++" to temporarily break out of it and declare something with C++ linkage.

It makes the most sense when you want to provide a generally C interface for a C++ library, but you also want to provide C++ helper bits for people actually using it in C++. So you'd put #ifdef __cplusplus \ extern "C" { \ #endif around the header as a whole, and then you ifdef-in those bits with extern "C++" to revert to C++ linkage.

Answer 2

1. It works by forcing the compiler to use C++ linkage when the surrounding code uses C linkage by default (e.g., you include winnt.h in a C program).
2. Yes, it should be portable.
3. Yes they can. There is not much use for "extern "C++"" in C++ programs because the linkage is "C++" anyway. It makes sense to use "extern "C++"" only if there is a good chance that your C++ code will be included into a C code.