Linker Error when separating code into .h and .cpp files - c++

Question

I have an implementation for printing out enum values in c++
If I put all the code in a .h file, everything works nicely. If I separate out the function implementation into .cpp files, I get a linker error.
Here is my main file
#include <iostream>
#include <vector>
#include "Day.h"
using namespace std;
int main(){
initializeDayNames();
Day a = Clubs;
cout << a;
}
Here is the .h file
#ifndef __Day__
#define __Day__
#include <iostream>
#include <vector>
#include <string>
using namespace std;
enum Day {Clubs, Hearts, Diamonds, Spades} ;
vector<string> DayNames = vector<string>();
ostream & operator<<(ostream & out, Day cs);
void initializeDayNames();
#endif
and the .cpp file
#include <iostream>
#include "Day.h"
#include<string>
#include<vector>
using namespace std;
void initializeDayNames(){
DayNames.push_back("Clubs");
DayNames.push_back("Hearts");
DayNames.push_back("Diamonds");
DayNames.push_back("Spades");
}
ostream & operator<<(ostream & out, Day cs){
out << DayNames[cs];
return out;
}
What am I doing wrong here
The specific error is
Error 1 error LNK2005: "class std::vector<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::allocator<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > DayNames" (?DayNames##3V?$vector#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##V?$allocator#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std###2##std##A) already defined in Day.obj main.obj
and
Error 2 fatal error LNK1169: one or more multiply defined symbols found

Answer 1

You are initializing two copies of
vector<string> DayNames = vector<string>();
because you included the header twice.
You should replace it with
extern vector<string> DayNames;
in the h file and
vector<string> DayNames = vector<string>();
in the cpp file.
Also you seem to have two copies of
ostream & operator<<(ostream & out, Day cs);
The reason your header guards isn't helping this case is that when you include the headers, you basically duplicate the definitions in all your files that you have included the header.
In C, when you declare a variable, you basically instantiate it/allocate static space for it. When you put the variable declaration in the header, what you are effectively doing is allocating static storage space for the variable in two different objects, giving you your error.

Answer 2

You're missing extern on the DayNames declaration.