extern C can not be used at class level?

Question

Just want to confirm in Windows environment, VSTS 2008 + C++ project, we could only apply extern C to function level, not be able to apply to class level (so that all member functions from the class use C language name mangling)? I have tried several ways, but always compile error.

thanks in advance, George

Answer 1

You can sort of apply extern "C" to a member function via a very convoluted (but entirely legal) hack:

extern "C" typedef int bar_t(int x);

struct foo {
     bar_t bar; // yes, this declares a nonstatic member function!
};

int foo::bar(int x) { return x; } // definition

This is possible according to ISO C++03 9.3[class.mfct]/9:

a member function can be declared (but not defined) using a typedef for a function type. The resulting member function has exactly the same type as it would have if the function declarator were provided explicitly, see 8.3.5.

However, this doesn't really buy you anything, because of ISO C++03 7.5[dcl.link]/4:

A C language linkage is ignored for the names of class members and the member function type of class member functions.

Answer 2

extern "c" uses c-style linking; that is, the raw function name is what exposed from the library. Because it is just a raw function name, none of the C++-only features will work with it, including methods or extern data members in namespaces, classes, structs or unions.

Clarifying: Structs and unions are in C, but have no member functions, so their member functions in C++ cannot be exported in a c-style (and the struct and union definitions need not be exported, since it is already in the header)