I have the following code on my main.cpp:

Code:
#ifdef _WIN64
#define hOS 1
#elif _WIN32
#define hOS 1
#elif __APPLE__
#define hOS 2
#elif __linux
#define hOS 3
#else
#define hOS 0
#endif

#include "functions.h"
functions.h:
Code:
#ifndef _FunctionsFile
#define _FunctionsFile

#include <string.h>
#include <iostream>

using namespace std;

void someFunction(bool someParameter);

#endif
functions.cpp:
Code:
#include "functions.h"
void someFunction(bool someParameter= false)
{
  if (hOS==1)
  {
    std::cout<<"Windows";
  }
}
When trying to complile I get this error in functions.cpp:
C:\Dev\CPP\functions.cpp `hOS' undeclared (first use this function)