请问VC下类静态成员编译出错的有关问题

请教VC下类静态成员编译出错的问题
程序如下，请大家给指点一下
两个文件log.h和main.cpp
vc2005下编译出错
main.obj : error LNK2001: 无法解析的外部符号 "public: static class CLog * CLog::_instance" (?_instance@CLog@@2PAV1@A)

//log.h
 

#ifndef _CLOG_H
#define _CLOG_H
#pragma once
 
#include <fstream>
#include <ios>
using namespace std;

class CLog
{
  ofstream *pf;

  public:
static CLog* _instance;

CLog()
{  
pf= new ofstream ("default.log", ios::app);
}

~CLog()
{
_instance=0;  
pf->close();
}

static CLog* GetPtr()
  {
  if(!_instance)
  _instance = new CLog;
  return(_instance);
  }
};
#endif
//end of file log.h


#include <stdio.h>
#include "log.h"

int main(int argc, char *argv[])
{
CLog loglog;
  return 0;
}


[解决办法]
需要在类外定义一下
CLog* CLog::_instance = NULL;