Redefinition of struct error, I only defined it once - c++

Question

I really don't understand how to fix this redefinition error.
COMPILE+ERRORS
g++ main.cpp list.cpp line.cpp
In file included from list.cpp:5:0:
line.h:2:8: error: redefinition of âstruct Lineâ
line.h:2:8: error: previous definition of âstruct Lineâ
main.cpp
#include <iostream>
using namespace std;
#include "list.h"
int main() {
int no;
// List list;
cout << "List Processor\n==============" << endl;
cout << "Enter number of items : ";
cin >> no;
// list.set(no);
// list.display();
}
list.h
#include "line.h"
#define MAX_LINES 10
using namespace std;
struct List{
private:
struct Line line[MAX_LINES];
public:
void set(int no);
void display() const;
};
line.h
#define MAX_CHARS 10
struct Line {
private:
int num;
char numOfItem[MAX_CHARS + 1]; // the one is null byte
public:
bool set(int n, const char* str);
void display() const;
};
list.cpp
#include <iostream>
#include <cstring>
using namespace std;
#include "list.h"
#include "line.h"
void List::set(int no) {}
void List::display() const {}
line.cpp
#include <iostream>
#include <cstring>
using namespace std;
#include "line.h"
bool Line::set(int n, const char* str) {}
void Line::display() const {}

Answer 1

You need to put include guards in your headers.
#ifndef LIST_H_
#define LIST_H_
// List.h code
#endif

Answer 2

In list.cpp, you are including both "line.h" and "list.h". But "list.h" already includes "line.h" so "list.h" is actually included twice in your code. (the preprocessor is not smart enough to not include something it already has).
There are two solutions:
Do not include "list.h" directly in your list.cpp file, but it is a practice that does not scale: you have to remember what every of your header file includes and that will be too much very quickly.
use include guards, as explained by #juanchopanza

Answer 3

You are including "line.h" twice, and you don't have include guards in your header files.
If you add something like:
#ifndef LINE_H
#define LINE_H
... rest of header file goes here ...
#endif
to your header files, it will all work out fine.