How to use c++ objects in c?

Question

I have 2 projects decoder and dec in my visual studio. One has C code and other has C++ code using stl respectively.How do I instantiate the c++ classes in my c code inside

Answer 1

Give the C++ module a C interface:

magic.hpp:

struct Magic
{
    Magic(char const *, int);
    double work(int, int);
};

magic.cpp: (Implement Magic.)

magic_interface.h:

struct Magic;

#ifdef __cplusplus
extern "C" {
#endif

typedef Magic * MHandle;
MHandle create_magic(char const *, int);
void    free_magic(MHandle);
double  work_magic(MHandle, int, int);

#ifdef __cplusplus
}
#endif

magic_interface.cpp:

#include "magic_interface.h"
#include "magic.hpp"

extern "C"
{
    MHandle create_magic(char const * s, int n) { return new Magic(s, n); }
    void    free_magic(MHandle p) { delete p; }
    double  work_magic(MHandle p, int a, int b) { return p->work(a, b); }
}

Now a C program can #include "magic_interface.h" and use the code:

MHandle h = create_magic("Hello", 5);
double d = work_magic(h, 17, 29);
free_magic(h);

(You might even want to define MHandle as void * and add casts everywhere so as to avoid declaring struct Magic in the C header at all.)

Answer 2

Make wrapper for instantiating C++ objects using C++ exported functions.And then call these functions from C code to generate objects.

Since one is function oriented and other is object oriented, you can use a few ideas in your wrapper:-

1. In order to copy class member, pass an equivalent prespecified struct from C code to corresponding C++ function in order to fetch the data.
2. Try using function pointers, as it will cut the cost, but be careful they can be exploited as well.

A few other ways.

Answer 3

you would need to write a wrapper in C.

something like this:

in class.h:

struct A{
   void f();
}

in class.cpp:

void A::f(){
}

the wrapper.cpp:

#include "wrapper.h"
void fWrapper(struct A *a){a->f();};
struct A *createA(){
   A *tmp=new A();
   return tmp;
}

void deleteA(struct A *a){
 delete a;
}

the wrapper.h for C:

struct A;
void fWrapper(struct A *a);
A *createA();

the C program:

#include "wrapper.h"
int main(){
    A *a;
    a=createA();
    fWrapper(a);
    deleteA(a);
}