Compilation Error

In file included from /usr/include/c++/4.8/algorithm:62:0,
                 from Test.cpp:3:
/usr/include/c++/4.8/bits/stl_algo.h: In instantiation of ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&) [with _RandomAccessIterator = Entree*; _Tp = Entree]’:
/usr/include/c++/4.8/bits/stl_algo.h:2283:70:   required from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator) [with _RandomAccessIterator = Entree*]’
/usr/include/c++/4.8/bits/stl_algo.h:2315:54:   required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size) [with _RandomAccessIterator = Entree*; _Size = long int]’
/usr/include/c++/4.8/bits/stl_algo.h:5452:36:   required from ‘void std::sort(_RAIter, _RAIter) [with _RAIter = Entree*]’
Test.cpp:66:35:   required from here
/usr/include/c++/4.8/bits/stl_algo.h:2245:19: error: passing ‘const Entree’ as ‘this’ argument of ‘bool Entree::operator<(Entree)’ discards qualifiers [-fpermissive]
    while (__pivot < *__last)
                   ^
Test.cpp: In function ‘int main()’:
Test.cpp:37:39: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result]
  freopen("measurement.in", "r", stdin);

                                       ^
Test.cpp:38:41: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result]
  freopen("measurement.out", "w", stdout);

                                         ^