C++ Headers & Undefined Reference - c++

Question

So I'm just getting to grips with C++ and progressing onto using header files. Thing is, I'm totally confused. I've read a documentation but none to make me understand it well enough.
I'm just making a silly 'game' which is interactive, it will probably be discarded and thought I could practice use on header files. This is my file structure:
terminal_test
├── core.cpp
└── game
├── game.cpp
└── game.h
Now, here is my core.cpp:
#include <iostream>
#include <stdio.h>
#include <unistd.h>
#include "game/game.h"
using namespace std;
void mainMenu();
void rootInterface() {
cout << "root#system:~# ";
}
int main() {
system("clear");
usleep(2000);
mainMenu();
return 0;
}
void mainMenu() {
int menuChoice = 0;
cout << "[1] - Start Game";
cout << "[2] - How To Play";
cout << endl;
rootInterface();
cin >> menuChoice;
if ( menuChoice == 1 ) {
startGame();
} else if ( menuChoice == 2 ) {
cout << "This worked.";
}
}
Everything else works fine but startGame(); under my menu choice. When I compile using g++ core.cpp game/game.cpp it bounces back with this error: undefined reference to startGame();. I firstly did some troubleshooting to see if it was properly finding game.h by changing the #include "game/game.h" to something like #include "game.h" without the directory listed inside and it gave me a game.h could not be found so I know it's recognising it, just not compiling at all.
Here is my game.h:
#ifndef GAME_H // Making sure not to include the header multiple times
#define GAME_H
#include "game.h"
void startGame();
#endif
game.cpp:
#include <iostream>
#include <stdio.h>
#include "game.h"
int main(int argc, char const *argv[]) {
void startGame() {
cout << "It worked.";
}
return 0;
}
My file structure isn't named properly either, I just threw it in because it was something to just get to grips with header files in C++.
So, here are my questions:
1) - What is this error specifically saying and what should I do to fix it?
2) - How do header files communicate and work with other files and is there clear documentation/guides out there that can help?

Answer 1

Local function definitions are not what you want here:
#include <iostream>
#include <stdio.h>
#include "game.h"
int main(int argc, char const *argv[]) {
// an attempt to define startGame() inside of main()
void startGame() {
cout << "It worked.";
}
return 0;
}
main is not needed in your game.cpp file. You should define startGame() outside of main, like this:
#include <iostream>
#include <stdio.h>
#include "game.h"
// definition of startGame
void startGame() {
cout << "It worked.";
}