the above posted should work. that's weird. the "string6.h" is a different header file, most likely for a class.
for the "but why? am i not supposed to ever use namespace?", you could do this either of three ways...
using namespace std;
OR
using std::cout;
using std::endl;
//continue with more if necessary
OR
std::cout << std::endl;
//like in previous examples
