How to declare a pointer to a nested C++ class in C

Question

I have a nested class in C++ that I want C code to be able to use. Because it is nested I cannot forward declare it to C, so instead we have code like this in a shared header file:

#ifdef __cplusplus
class Mgr {
public:
    class Obj {
        public:
        int x;
    };
};

typedef Mgr::Obj * PObj;
#else

typedef void* PObj;

#endif

This results in C and C++ each seeing a different definitions of PObj which I fear violates the one definition rule. However it is "just" a pointer, so I'm not sure if this can go wrong or not.

C does nothing with the pointer besides pass it along to C++ functions which are wrappers for methods. For instance we'll have this in the combined header:

#ifdef __cplusplus
extern "C" {
#endif
int obj_GetX(PObj pObj);

#ifdef __cplusplus
}
#endif /* __cplusplus */

With this as the implementation in the C++ file:

int obj_GetX(PObj pObj) {
    return pObj->x;
}

So:

1. Is this really a problem?
2. If so, what is the best way to share this pointer between C and C++

Answer 1

It's not a problem per-se, but you might violate the strict aliasing rule if and when you cast that void * back to Mgr::Obj * in your C++ code.

There's no good solution to this - it is (to me) a glaring omission in the standard. The best you can do is to compile any code which does such casts with the -fno-strict-aliasing flag. I do this extensively and have never had a problem performing casts in this manner.