Using a mixed C/C++ header to use C++ object in C - c++

Question

I have been following this guide on how to call a member function of a C++ object from C. As I've understood it, the C code should interpret the class as a struct of the same name, and whenever it wants to call a function through an object of this class it should use an intermediate callback function. The header looks like this:
// CInterface.h
#ifdef __cplusplus
...
class CInterface
{
public:
...
void OnMessage(U8* bytes); // I want to call this function from C.
private:
...
};
#else
typedef
struct CInterface
CInterface;
#endif
#ifdef __cplusplus
extern "C" {
#endif
#if defined(__STDC__) || defined(__cplusplus)
//extern void c_function(CInterface*); /* ANSI C prototypes (shouldn't be needed) */
extern CInterface* cpp_callback_function(CInterface* self, unsigned char * bytes);
#else
//extern void c_function(); /* K&R style (shouldn't be needed) */
extern CInterface* cpp_callback_function(unsigned char * bytes);
#endif
#ifdef __cplusplus
}
#endif
The C code that fails right now looks like this:
// main.c
#include "CInterface.h"
int main(int argc, char* argv[])
{
void* ptr;
int *i = ptr; // Code that only compiles with a C compiler
CInterface cinterface; // This should declare a struct
}
The error is: error C2079: 'cinterface' uses undefined struct 'CInterface'.
It sounds like the header is being read as c++ code as the struct is not defined, but main.c is being compiled by C according to Visual Studio (I also double checked this by adding some C-specific code). However, if I add parentheses like this:
CInterface cinterface();
the code compiles which makes no sense to me as it now is an object which shouldn't work in C.
The callback function is implemented in a third file, CInterface.cpp, which acts as the "intermediate".
So the question is how I solve this error message, or if I got the entire approach wrong. It's the first time I mix C/C++ code and I'm relatively new to both languages.

Answer 1

In your example CInterface is only defined for C++. If you take a closer look at the example you linked you'll notice that this is also the case for the Fred class.
From C you can only pass around pointers to CInterface and you have to rely on C++ functions defined with C linkage to actually manipulate CInterface instances.
Otherwise you could define a struct as a means to pass around data between C and C++. Just ensure its definition is declared as extern "C" when used from C++:
#ifdef __cplusplus
extern "C" {
#endif
struct CandCPlusPlus {
// ...
};
#ifdef __cplusplus
}
#endif