// foo.h
#ifndef FOO_H
#define FOO_H

#ifdef __cplusplus
#include <iostream>
extern "C" {
#endif

typedef struct foo {
    float x;
    float y;
    float z;

#ifdef __cplusplus
    float w;

    inline foo operator+(const foo& addend) {
        foo result = {
            x + addend.x,
            y + addend.y,
            z + addend.z,
            w + addend.w
        };
        return result;
    }
#endif

}foo;

#ifdef __cplusplus

} // End C linkage

inline std::ostream& operator<<(std::ostream &os, const foo &inst) {
    os << inst.x << ' ' << inst.y << ' ' << inst.z << ' ' << inst.w << '\n';
    return os;
}

#endif

#endif // FOO_H

//cfoo.c
#include "foo.h"
extern foo c_foo;
foo c_foo = { 3.0f, 3.0f, 3.0f };

// main.cpp
#include "foo.h"
#include <iostream>

extern "C" foo c_foo;
static foo cpp_foo = { 1.0f, 2.0f, 3.0f, 4.0f };
int main(){
    foo a = cpp_foo + c_foo;
    std::cout << c_foo;
    std::cout << cpp_foo;
    std::cout << a;

    float w = a.w;
    std::cout << w << std::endl;

    std::getchar();
    return 0;
}