STL - Bitoperator functions

Im pretty new on STL-programming. For the moment Im playing around with bit-operators.
The problem is that I'm trying to use a function called to_string() that should return a 'bitpattern' in format of string, but it isn;t workinnggg.

Code:

#include <iostream.h>
#include <bitset>
#include <string>

int main()
{
int value = 2:
bitset <4> b = value;
string bitpattern = b.to_string():
cout << "BitString: " << bitpattern << endl:
}

When I try compile this code a get an error like "no matching function for call to bitset<4, long unsigned int>::tostring()"

Thx in advance

here's some info on syntax:
http://www.josuttis.com/libbook/cont/bitset2.cpp.html