从C代码调用C ++方法

Question

I've read the following article here that explain how to call c++ based functions from pure C code.

The method is best explained with the following example :

// C++ code:
class C {
    // ...
    virtual double f(int);
};
extern "C" double call_C_f(C* p, int i) // wrapper function
{
    return p->f(i);
}

---

/* C code: */
double call_C_f(struct C* p, int i);
void ccc(struct C* p, int i)
{
    double d = call_C_f(p,i);
    /* ... */
}

---

However, I fail to understand if where do i define an instance that is pointed by 'p'. notice that in the C code, it is delivered as pointer to opaque struct, and in the C++ code it's assumed to be pointer to valid instance of class C , but i didn't see where this pointer is being initialized.

ps Of course that each code block that relate to different language is compiled with an appropriate compiler, and different object file is created.

Answer 1

You need similar wrapper methods to create and destroy the C++ object.

C++ code:

extern "C" C * create_c()
{
    return new C;
}

extern "C" void destroy_c(C *p)
{
    delete p;
}

---

C code:

// forward declarations (better to put those in a header file)
struct C;
struct C * create_c();
double call_C_f(struct C* p, int i);
void destroy_c(struct C *p);

struct C * p = create_c();
double result = call_C_f(p, i);
destroy_c(p);
p = NULL;

Answer 2

In the case of your example, p is an object of your class C . You would have to create it in C++ code, and than pass it to C.

You should also declare the pointers to your C++ class as void * in C. The type is unknown, and struct C implies it's a known struct, which is wrong.

Example:

extern "C" {
struct C;
struct C *create_c(void)
{
     return new C;
}

void destroy_c(struct C *ptr)
{
    delete ((C*)ptr);
}
}

Answer 3

You'd create it in C++, most probably with new C() . There is no way of portably creating it in C, so in case the C code must be able to construct new instances, you'd have to write an extern "C" C* construct_C(); factory function for it; likewise for a destructor.