Static variable in header file

[imgravity]

I have program like this

//header.hh
#include <iostream>
using std::ios;
using std::endl;
class A : public B
{
  public: 
    static bool abool;
    static std::ostream xyz;
}

//class.cc
#include <iostream>
#include "header.hh"

using std::ios;
using std::endl;

// i have to define those static variables first to use it 
bool A::abool;
std::ostream A::xyz;

Here, while compilng i get error saying that
undefined reference to A::xyz
but there is no error with A::abool ??

Can anyone explain why this error is there and how to remove it ??
Thanks.

[writetonsharma]

you need to declare all your static variables at the top of .cpp files where you use them.

[imgravity]

can u explain it a bit..
i have declared the static variables abool and xyz in cpp

[helios]

std::ostream doesn't have a constructor that takes no parameters.

http://www.cplusplus.com/reference/iostream/ostream/ostream/

[writetonsharma]

see if you have a static variable in a class lets say

//some.h
class some
{

public:
static int m_i;

//some more code

};

then you need to declare that static variable in the .cpp files. like

//some.cpp

static some::m_i;//you need to do this or else the compiler give error

//here goes the class implementation