Undefined reference Error

X is a base class in which 'data' is being set. while executing the code I am getting this error: In function `main':
undefined reference to `X::data;

Can anyone please help me resolve this error.
Any help will be appreciated.

#include <iostream>
using namespace std;
struct ICP{ int id; };
struct Q752{ int id; };
class X
{
protected:
enum{ DT =0, Q7};
static void *data;
public:
  X();
  ~X();
  static void *get() { return data; }
   static void set(int type) 
   {
     cout << "X::set()" <<endl;
     
     switch(type)
     {
       case DT:
               {
                 cout << "case: ICP" <<endl;
                 ICP* dt= new ICP;
                  //cout << (int)dt << endl;
                 dt->id = 10; data = dt;
               }
               break;
       case Q7:
               {
                 
                 cout << "case: Q752" <<endl;
                 Q752* dt1 = new Q752;
                 dt1->id = 20;// data = (Q752*)dt1;
               } 
               break;
       default: 
               cout << "Invlaid type";
     }
   }
  static void delX(int type) { if(data)
                        switch(type){
                          case DT:
                              delete (ICP*)data;
                          case Q7:
                              delete (Q752*)data;
                          default:
                               cout<< "DEL ERROR" << endl;
                        } 
                     }

};

class A{
friend class X;
public:
A();
~A() { X::delX(0); }
static void set(int type) {X::set(type);}
static ICP *get() { return (ICP *)X::get();}
};

class B{
friend class X;
public:
B();
~B(){ X::delX(1); }
static void set(int type) {X::set(type);}
static Q752*get() { return (Q752*)X::get();}
};

int main()
{
cout << "Set() is Start!!" <<endl;
A::set(0);
cout << "Set() is Done!!" <<endl;

return 0;
}

Tags: C++