Simple C++ class with C interface for Luajit ffi bindings

gistfile1.cpp

// g++ simple.cpp -shared -o libsimple.dylib
#include <stdio.h>

class Simple {
    int id_;
public:
    Simple(int id);
    ~Simple();
    int id();
};

Simple::Simple(int id) : id_(id) {
    printf("[%p:%i] Simple()\n", this, id_);
}

Simple::~Simple() {
    printf("[%p:%i] ~Simple()\n", this, id_);
}

int Simple::id() {
    return id_;
}

extern "C" {
    Simple *Simple_Simple(int id) {
        return new Simple(id);
    }

    void Simple__gc(Simple *this_) {
        delete this_;
    }

    int Simple_id(Simple *this_) {
        return this_->id();
    }
}

gistfile2.c

/* This is C code */
#include "Fred.h"

void c_function(Fred* fred)
{
    cplusplus_callback_function(fred);
}

gistfile2.cpp

// This is C++ code
#include "Fred.h"

Fred::Fred() : a_(0) {}
void Fred::wilma(int a) {}

Fred* cplusplus_callback_function(Fred* fred)
{
    fred->wilma(123);
    return fred;
}

gistfile2.h

/* This header can be read by both C and C++ compilers */
#ifndef FRED_H
#define FRED_H
#ifdef __cplusplus
    class Fred {
        public:
            Fred();
            void wilma(int);
        private:
            int a_;
    };
#else
    typedef struct Fred Fred;
#endif
#ifdef __cplusplus
extern "C" {
#endif
#if defined(__STDC__) || defined(__cplusplus)
    extern void c_function(Fred*);   /* ANSI C prototypes */
    extern Fred* cplusplus_callback_function(Fred*);
#else
    extern void c_function();        /* K&R style */
    extern Fred* cplusplus_callback_function();
#endif
#ifdef __cplusplus
}
#endif
#endif /*FRED_H*/

gistfile2b.cpp

// This is C++ code
#include "Fred.h"

int main()
{
    Fred fred;
    c_function(&fred);
    // ...
}