C++ Headers & Undefined Reference

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?

Show how you compile and link. The problem is probably there. — Jabberwocky, Mar 09 '17 at 15:16
in game.cpp `startGame` is nested inside `main`. 1. `startGame` must be at file scope. 2. remove `main` from game.cpp You already have it in `core.cpp`. — bolov, Mar 09 '17 at 15:19
you need `g++ core.cpp game/game.cpp`. Although there are other errors in the program(or you do not post real code). — apple apple, Mar 09 '17 at 15:29
Yeah, sorry I have been doing that and Chad fixed my compile errors instantly. — Jordan Savell, Mar 09 '17 at 15:34

score 3 · Accepted Answer · answered Mar 09 '17 at 15:18

3

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.";

}

answered Mar 09 '17 at 15:18

Chad

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It's compiling now, thank you. But now that I have it outside of main, how am I supposed to call it after the user prompts 1 to start the game? – Jordan Savell Mar 09 '17 at 15:30
2

@JordanSavell You really should read a basic tutorial. You simply call it by startGame();. It being "inside of main" makes not much sense, except if we talk about inner functions (which we are not). Look at your mainMenu function - it is basically the same here. Your header declares startGame just like you declared mainMenu before the main function, your source implements startGame, just like you implemented mainMenu after the main function. By the way, start to work with classes as soon as possible. game.h looks like something that probably should become the header of a class Game. – Aziuth Mar 09 '17 at 15:37
Ohh I understand. It's not that I didn't understand, I was confused since I had to take it out of main but I was thinking about the wrong file. What you just said makes sense. I can see how these work a lot easier. – Jordan Savell Mar 09 '17 at 15:39
Also, [nested functions like that aren't actually part of the C++ standard, although GCC supports them as an extension](http://stackoverflow.com/questions/4324763/c-can-we-have-functions-inside-functions). Lambdas come close, but they're a type of object, not actual functions, and have their own syntax. – Justin Time - Reinstate Monica Mar 09 '17 at 15:44
Understood. That's really useful, actually. Cheers. – Jordan Savell Mar 09 '17 at 15:51

C++ Headers & Undefined Reference

1 Answers1