extern C and struct method

Given the following C++ code,

#ifdef __cplusplus
extern "C" {
#endif
        struct foo {
                void getNum() { 
                }
        };
#ifdef __cplusplus
}
#endif

int main (int argc, char * const argv[]) {
        return 0 ;
}

Is开发者_Python百科 it possible to call getNum() from C?

---

No, since getNum is a member function, which C doesn't have.

A possible solution to that problem is to write a C++ function to return a foo instance as a foo* (where foo is changed to be an empty struct) to C (I assume this is binary compiled as C++ to which C is linking), then have a free function in C++ called foo_getNum or something, which takes a foo* (whose definition is modified for the C version to be empty) which calls getNum on it. It wouldn't be type safe though, obviously, (but taking a foo* even when foo is empty would be better than void* - thanks David).

---

The extern "C" has no effect on the member function: getNum() has C++ language linkage.

The C++ Language Standard states (C++03 §7.5/4):

A C language linkage is ignored for the names of class members and the member function type of class member functions.

So, no, you cannot call this function directly from a C program (though, as others have said, you can't compile that code as C anyway because C does not have member functions). It is of course conceivable that some implementations might allow you to call this function from a C program via some implementation-specific method.

---

You can't call that member function from C in a portable way. You need to expose your C++ interface by flattening it.

---

The code you gave will not compile in C mode as C compiler doesn't support functions in struct. However you can create a function in C++ which can call this and link it with C linkage. Create 2 files main.c and abc.cpp

Code for main.c

extern "C" void getNumCaller();
int main ()
{
    getNumCaller();
    return 0;
}

Code for abc.cpp

#include <iostream>

struct foo {
        void getNum() {
            std::cout << "calling getNum" << std::endl;
        }
};

extern "C" void getNumCaller()
{
    struct foo abc;
    abc.getNum();
}

Compile the code :

g++ -o abc abc.cpp main.c

and you will get output:

calling getNum