Static int inside struct is undefined !!

[gurpartap]

Why static int is not defined inside struct abc..

  #include <iostream>
using namespace std;
struct abc
{
static int x;
};
//int abc::x;
int main()
{
abc obj1;
obj1.x = 10;
cout << "Value of 10 = " << obj1.x << endl; 
return  0;
}
This code produces error.
undefined reference to X.
But If I uncomment line above int main() then this code works.

[helios]

Static members have to be defined outside the class somewhere, like you're doing on line 7.