Output needs some explanation

Can any one give any valid reason why the output of the following program in VC++ 6.0 Compiler is

Code:

#include <iostream>
using namespace std;

class base
{
public:
  int a;
  base(){ a=0;}
};

class der:public base
{
public:
  int b;
  der(){ b=1;}
};
void F(base *ptr,int n)
{
  for(int i=0; i<n; i++,ptr++)
    cout<<ptr->a;
  cout<<endl;
}

int main()
{
  base Base1[5];
  F(Base1,5);
  der Der1[5];
  F(Der1,5);
  return 0;
}

Output
00000
01010