Calling C++ functions from C file - c++

Question

I am quite new to C and C++. But I have some C++ functions which I need to call them from C. I made an example of what I need to do
main.c:
#include "example.h"
#include <stdio.h>
int main(){
helloWorld();
return 0;
}
example.h:
#ifndef HEADER_FILE
#define HEADER_FILE
#ifdef __cplusplus
extern "C" {
#endif
void helloWorld();
#ifdef __cplusplus
}
#endif
#endif
example.cpp:
#include <iostream.h>
void helloWorld(){
printf("hello from CPP");
}
It just doesn't work. I still receive the error of undefined reference to _helloWorld in my main.c. Where is the the problem?

Answer 1

Short answer:
example.cpp should include example.h.
Longer answer:
When you declare a function in C++, it has C++ linkage and calling conventions. (In practice the most important feature of this is name mangling - the process by which a C++ compiler alters the name of a symbol so that you can have functions with the same name that vary in parameter types.) extern "C" (present in your header file) is your way around it - it specifies that this is a C function, callable from C code, eg. not mangled.
You have extern "C" in your header file, which is a good start, but your C++ file is not including it and does not have extern "C" in the declaration, so it doesn't know to compile it as a C function.

Answer 2

the extern "C" tells C++ that the declared function has to use the C ABI (Application Binary interface), hence, whether the language is C or C++, your void HelloWorld() is always seen externally as it is C.
But you implemented it in the cpp file like it is a C++ one, C is not aware of.
You have to make the prototype of HelloWorld coherent for both C and C++, so the cpp file should declare it as extern "C" void Helloworld() { /*your code here*/ }, or simply, #include "example.h" from example.cpp, so that, before implementing it, the compiler already knows it has to follow the C convention.