When I try to use std::distance with a custom iterator under gcc 4.7, it complains about not finding the difference_type. I have sadly no idea why it fails.

#include <iterator>

class nit {
public:
    typedef int difference_type;
};

int main() {
  const nit test1;
  std::distance( test1, test1 );
  return 0;
}

gives the error:

/usr/include/c++/4.7/bits/stl_iterator_base_funcs.h:114:5: error: no type named ‘difference_type’ in ‘struct std::iterator_traits<nit>’

c++ gcc gcc4.7
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