C++ types are structs and unions if you get down to the bottom of things. This is possibly not guaranteed by the standard, but AFAIK, extern "C" doesn’t ever af

C++ types are structs and unions if you get down to the bottom of things. This is possibly not guaranteed by the standard, but AFAIK, extern "C" doesn’t ever affect the calling convention or memory layout for the parameters, only the name mangling for the linker. So in theory, if you manage to construct a valid std::string in C code (which is hard, given its invariants, and would be absolutely nonportable), then you could pass it to an extern "C" function that’s written in C++.