Global Variables Across Multiple Classes Without Getting Redefinition errors? - c++

Question

Extreme c++ beginner here. I have a global variable i've tried placing into a separate header file, and also into the specific classes themselves. Throughout, i'm receiving a "redefinition" error.
Here are the global variables:
enum GRADE {F, D, CMINUS, C, CPLUS, BMINUS, B, BPLUS, AMINUS, A};
const int BAR = 60;
const int MAXSTUDENT = 100;
I have them being used and called upon in two separate classes, called student.cpp and course.cpp. They are contained in "global.cpp"
here is the header in student.cpp
#include "student.hpp"
#include "global.cpp"
#include <iostream>
using namespace std;
Here is the header in course.cpp
#include "course.hpp"
#include "student.cpp"
#include <iostream>
using namespace std;
No matter what combination of headers, or attempts to split the global variables into the classes they are being used by, i am getting a "redefinition" error. If i uninclude "#student.cpp" in the course.cpp section, i won't get the error, but i need the student.cpp for the course.cpp to work.
I'm way out of my element, would appreciate any help.

Answer 1

Create a file global.h. In that file you can declare your global variables. enum GRADE is not a variable but a type. It is important to remember that you can declare types and variables multiple times, but you can define them only once. So do not define them in the header, just declare them. If you want to use the same global variable across multiple cpp files you must declare them with external linkage.
/* File global.h */
#pragma once
extern const int BAR;
extern const int MAXSTUDENT;
This is an example student.h:
/* File student.h */
#pragma once
enum GRADE {F, D, CMINUS, C, CPLUS, BMINUS, B, BPLUS, AMINUS, A};
class Student
{
public:
Student();
virtual ~Student();
/* put more members here */
};
Here is an example course.h:
/* File course.h */
#pragma once
#include "student.h"
class Course
{
public:
Course();
virtual ~Course();
void addStudent(Student& student);
private:
std:vector<Student*> students;
}
Then you can choose one *.cpp file to define the globals.
For example you put them in student.cpp. Then you can already use them in student.cpp:
/* File student.cpp */
#include "student.h"
const int BAR = 60;
const int MAXSTUDENT = 100;
/* definition of student methods ... for example */
Student::Student()
{ }
Student::~Student()
{ }
In that case you do not include the global.h in the student.cpp file.
Also please do not #include any *.cpp files.
Put the declaration of your student class into student.h, put the definition of methods of the student class into student.cpp.
In the course.cpp just include global.h, course.h. But the student.h is already included via course.h. Then you can also use the global variable in this cpp.
/* file course.cpp */
#include "course.h"
#include "global.h"
/* definition of course methods ... for example */
Course::Course()
{ }
Course::~Course()
{ }
Course::addStudent(Student& student)
{
if(students.size() < MAXSTUDENT) {
// ......
}
}
Hope that helps you.