Question

I have a noob question here.
I’m getting my head around the C++ structure and syntax and I’ve hit a bit of a wall.
I know I am missing something from my concept. So first a little code to help describe the situation.

Control.h

#pragma once
#ifndef CONTROL_H
#define CONTROL_H

class Control
{
    public:
        Control();
        ~Control();
    private:
    public:
};

#endif /*CONTROL_H*/

Control.cpp

#include "Control.h"
#include "Hello.h"

Hello helloObj;

Control::Control()
{
}

Control::~Control()
{
}

int main()
{
    int a = helloObj.HelloWorld();
    return 0;
}

Hello.h

#pragma once
#ifndef HELLO_H
#define HELLO_H

class Hello
{
    public:
        Hello();
        ~Hello();
    private:
    public:
         int HelloWorld(void);
};
#endif /*HELLO_H*/

Hello.cpp

#include "Hello.h"

Hello::Hello()
{
}

Hello::~Hello()
{
}

int HelloWorld()
{
    return 5;
}

I try and compile control.cpp with g++ on OSX 10.7 and get

Undefined symbols for architecture x86_64:
      "Hello::Hello()", referenced from:
              __static_initialization_and_destruction_0(int, int)in cccZHWtd.o
      "Hello::~Hello()", referenced from:
              ___tcf_1 in cccZHWtd.o
      "Hello::HelloWorld()", referenced from:
              _main in cccZHWtd.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

Is it the compiler, my code or my concept of whats going on?
Am I not instantiating something correctly?

Any links describing this in more detail would be appreciated.

Ultimately I want to be able to run a function in another class and return the result…normal OO, keeping your program modular stuff….

Answer 1

The errors you are getting are Linking errors not compilation errors.
The linker is not able to find definitions of the said functions & hence it reports the errors. It seems You have not included the Hello.cpp file containing the function definitions in your project.

Make sure Hello.cpp is included in your project and is a part of your project or
If you are using command line for compilation and linking make sure you have specified Hello.cpp in the file names on the command line.