[scorchin]

    template<typename _RandomAccessIterator>
    inline void
    nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth,
                _RandomAccessIterator __last)
    {
      typedef typename iterator_traits<_RandomAccessIterator>::value_type _ValueType;
 
      // concept requirements
      __glibcxx_function_requires(_Mutable_RandomAccessIteratorConcept<_RandomAccessIterator>)
      __glibcxx_function_requires(_LessThanComparableConcept<_ValueType>)
      __glibcxx_requires_valid_range(__first, __nth);
      __glibcxx_requires_valid_range(__nth, __last);

      if (__first == __last || __nth == __last)
        return;

      std::__introselect(__first, __nth, __last,
                         std::__lg(__last - __first) * 2);
    }

[veyron]

you should look at how std::__introselect is defined (hint: its in /usr/include/c++/4.2.1/bits/stl_algo.h )