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Name mangling rules are part of the ABI.

That's because your mental model differs from the language's view: as far as C++ is concerned, there is no such thing as a "struct". There's just classes introduced with the struct keyword.

Note, however, that while GCC is perfectly compliant here, Visual Studio is not! In particular, VS mangles classes and structs in incompatible ways. That is, if you have one file:

class A;
void...