Accessing Global variable across files - c++

Question

I need to access m[] in c.cpp from d.cpp. The array m[] is a member of CD within a namespace and is declared in c.h within that class. So I declare a static array s[] in the same class CD in the file c.h. Instantiate it in c.cpp and use a function to copy the elements of the original array m[] to static arrays[]. Then, I #include "c.h" in d.cpp where I need to access it.
c.h
namespace k{
class CD {
string m[10];
static string s[10];
}}
c.cpp
#include "c.h"
namespace k{
string CD::s[10] = {"q"}
}
d.cpp
#include "c.h"
void func(){
string n = k::CD::s[0];
}
But I am getting an error saying undefined reference to k::CD::s.
Am I doing something wrong here? Or is there some other way of doing this?

Answer 1

Some notes:
c.h
#ifndef C_H_HEADER_GUARD // add header guard
#define C_H_HEADER_GUARD
#include <string> // you need this header
namespace k {
class CD {
// string is in the "std" namespace, so:
std::string m[10]; // note that this is private!
// "s" can't be private since you want to access it outside the class, in `func()`:
public:
static std::string s[10];
}; // class definitions need to end with ;
} // namespace k
#endif
c.cpp
#include "c.h" // enclose the header with quotation marks
namespace k {
using namespace std; // in here it's ok to use namespace std:
string CD::s[10] = {"q"}; // missing ;
} // namespace k
d.cpp
#include "c.h" // again, quotation marks
void func() {
std::string n = k::CD::s[0]; // missing std::
}
Now, this will compile the two translation units into c.o and d.o that you can use when linking with the TU containing main later.
g++ -c c.cpp d.cpp -Wall -Wextra -pedantic -pedantic-errors
Since d.cpp doesn't have a d.h declaring the function func though, you'll need to declare that where you want to use it. Not nice, but:
void func();