Link error in C++ by g++

Question

Please take a look at the program below. Why am I getting an error?

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


using namespace std;

class serverData
{
public:
    static int serverTemp;
    static int server;
};
int main(int argc, char** argv)
{
    string s = "sajad bahmani";
    serverData::server = 90 ;

    const char * a = s.data();
    cout << a[0] << endl;

    return (EXIT_SUCCESS);
}

In conjunction, I get this error when trying to link:

build/Debug/GNU-Linux-x86/main.o: In function `main':
/home/sb23/pr/main.cpp:14: undefined reference to `serverData::server'
collect2: ld returned 1 exit status

Answer 1

Static member variables must have storage allocated in one of your .CPP files:

/* static */
int serverData::serverTemp;
int serverData::server;

Answer 2

You have just declared your static members inside the class but haven't defined them yet. You need to define them outside the class.

//definition
int serverData::serverTemp; //implicitly initialized to 0
int serverData::server = 5; // initialized to 5