Thread: access C++ objects within C. but linking error.

Hello everybody,
My goal is accessing c++ objects within c by using wrapper and externals. To get the pointer to the c++ object I use a type "void *".
But i get an error while linking: undefined reference to "create_mycpp".

Does anybody has an idea what the problem is? Or should I take an other way to access c++ objects?

Code:

//-------------------------------------
//mycpp.cpp
#include "mycpp.h"
Mycpp::void func(int i)
{
    i += 1;
}

//-------------------------------------
//mycpp.h
#ifndef MYCPP_H
#define MYCPP_H
#endif
class Mycpp {
    public:
        void func(int);
}

//-------------------------------------
//mywrapper.h
#ifndef MYWRAPPER_H
#define MYWRAPPER_H
#endif
typedef void * tMYCPP;
tMYCPP create_mycpp();
void mycpp_func(tMYCPP, int);

//-------------------------------------
//mywrapper.cpp
#include "mywrapper.h"
#include "mycpp.h"
#ifdef __cplusplus
extern "C" {
    void * create_mycpp() {
        return (tMYCPP)new Mycpp();
    }
    void mycpp_func(void * _mycpp, int _value) {
        ((Mycpp*) _mycpp)->func(_value);
    }
}
#endif

//-------------------------------------
//main.c
#include "mywrapper.h"
tMYCPP aCpp = create_mycpp();
mycpp_func(aCpp, 2);

//-------------------------------------

Originally Posted by Salem

Your mywrapper.h file needs the same extern "C" scope that your mywrapper.cpp file has.

Hello Salem,
have still same error : undefined reference to `create_mycpp' with:

Code:

#ifndef MYWRAPPER_H
#define MYWRAPPER_H
#endif

#ifdef __cplusplus
extern "C" {
    typedef void * tMYCPP;
    tMYCPP create_mycpp();
    void mycpp_func(tMYCPP, int);
}
#endif

shouldn't this :

Code:

Mycpp::void func(int i)

be this:

Code:

void Mycpp::func(int i)

and here is usual convention for an include file that is used in both C and cpp files

Code:

#ifdef __cplusplus
extern "C" {
#endif
typedef void * tMYCPP;
tMYCPP create_mycpp();
void mycpp_func(tMYCPP, int);
#ifdef __cplusplus
}
#endif

Thank you very much dmh2000 and Salem now it works fine. The problem was also that i called gcc as compiler. i thought gcc can manage c++ files too?

gcc -Wall foo.c mycpp.cpp mywrapper.cpp -> gives errors.

Originally Posted by Salem

*shrug*
When I fixed some syntax errors, it seems fine here.

Code:

// -------------------------------
// mycpp.cpp
// -------------------------------
//-------------------------------------
//mycpp.cpp
#include "mycpp.h"
void Mycpp::func(int i)
{
    i += 1;
}
// -------------------------------
// mycpp.h
// -------------------------------
#ifndef MYCPP_H
#define MYCPP_H
class Mycpp {
    public:
        void func(int);
};
#endif
// -------------------------------
// mywrapper.cpp
// -------------------------------
//-------------------------------------
//mywrapper.cpp
#include "mywrapper.h"
#include "mycpp.h"
#ifdef __cplusplus
extern "C" {
    void * create_mycpp() {
        return (tMYCPP)new Mycpp();
    }
    void mycpp_func(void * _mycpp, int _value) {
        ((Mycpp*) _mycpp)->func(_value);
    }
}
#endif
// -------------------------------
// mywrapper.h
// -------------------------------
//-------------------------------------
//mywrapper.h
#ifndef MYWRAPPER_H
#define MYWRAPPER_H
extern "C" {
  typedef void * tMYCPP;
  tMYCPP create_mycpp();
  void mycpp_func(tMYCPP, int);
}
#endif
// -------------------------------
// foo.cpp
// -------------------------------
#include "mywrapper.h"
int main ( ) {
  tMYCPP aCpp = create_mycpp();
  mycpp_func(aCpp, 2);
}
$ 
$ g++ -Wall foo.cpp mycpp.cpp mywrapper.cpp
$

Thank you very much dmh2000 and Salem now it works fine. The problem was also that i called gcc as compiler. i thought gcc can manage c++ files too?

gcc -Wall foo.c mycpp.cpp mywrapper.cpp -> gives errors.