unable to link between c and c++

[jeevs]

I have a class whose member I need to access in c prog.
I have defined the class object as extern.

when I tried to access the class member in c it says undefine reference.

where as I'm able to access the same class members in different places.
please help me

[Banfa]

You will not be able to access any class members from C because a class is a C++ construct that C is unaware of.

What you can do is declare functions with C naming convention in a C++ file

Header

#ifdef __cplusplus
extern "C" {
#endif
 
extern void MyFunction(void);
 
#ifdef __cplusplus
}
#endif
 

cpp code

#include "header"
 
void MyFunction(void)
{
    // Code here that accesses the class
}
 

c code

#include "header"
 
void SomeFunction(void)
{
    MyFunction();
}
 

This enables C code to call into C++ modules, you then put the C++ code (code accessing classes for instance) inside the function with the C naming convention.

[weaknessforcats]

Continuing Banfa's reply, you can write a function (extern "C") in C++ that calls the member function of the class and then you can call this function from C.

These are called buck-passer functions and are quite common.