static pointer data member

I have switched from c to c++ recently for my project. I am facing difficulty in initializing static pointer.
Here is some what similar code in my project

file1 static.h

Code:

#include <iostream>
#include<string.h>
using namespace std;
class shared {
	static int *ptr;
	static int a;
	int b;
	public:
	void set(int i) {a=i;}
	void show();
} ;

file2 static.cc

Code:

#include "static.h"
int shared::a = 2; // define a
int shared::*ptr = NULL;
void shared::show()
{
	memcpy(&ptr, &a, sizeof(int));
	cout << "This is static a: " << a;
	cout << "\nThis is ptr =  " <<*ptr;
	cout << "\n";
}
int main()
{
	shared x;
	x.set(1); // set a to 1
	x.show();
	return 0;
}

when i compile the piece of code i get following error
g++ static.cc
/tmp/cc7dngkw.o: In function `shared::show()':
static.cc:(.text+0x1a): undefined reference to `shared::ptr'
static.cc:(.text+0x4a): undefined reference to `shared::ptr'
collect2: ld returned 1 exit status

Any help would be appreciated. thanks in advance.

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I cant say i can spot the problem here, might be something very obvious,,,but hard to see with your non formatted post! One thing to note is that it is best practice to refer to static members by their 'full' names like:

Code:

shared::a += 100;

for example