Main.cc:8:10: error: ‘sgh’ was not declared in this scope bool cmp(sgh x, sgh y) { ^~~ Main.cc:8:17: error: ‘sgh’ was not declared in this scope bool cmp(sgh x, sgh y) { ^~~ Main.cc:8:22: error: expression list treated as compound expression in initializer [-fpermissive] bool cmp(sgh x, sgh y) { ^ In file included from /usr/include/c++/7/bits/stl_algobase.h:71:0, from /usr/include/c++/7/bits/char_traits.h:39, from /usr/include/c++/7/ios:40, from /usr/include/c++/7/istream:38, from /usr/include/c++/7/sstream:38, from /usr/include/c++/7/complex:45, from /usr/include/c++/7/ccomplex:39, from /usr/include/x86_64-linux-gnu/c++/7/bits/stdc++.h:52, from Main.cc:1: /usr/include/c++/7/bits/predefined_ops.h: In instantiation of ‘bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = shg*; _Iterator2 = shg*; _Compare = bool]’: /usr/include/c++/7/bits/stl_algo.h:1847:14: required from ‘void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1885:25: required from ‘void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1971:31: required from ‘void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:4868:18: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = shg*; _Compare = bool]’ Main.cc:18:23: required from here /usr/include/c++/7/bits/predefined_ops.h:143:18: error: expression cannot be used as a function { return bool(_M_comp(*__it1, *__it2)); } ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /usr/include/c++/7/bits/predefined_ops.h: In instantiation of ‘bool __gnu_cxx::__ops::_Val_comp_iter<_Compare>::operator()(_Value&, _Iterator) [with _Value = shg; _Iterator = shg*; _Compare = bool]’: /usr/include/c++/7/bits/stl_algo.h:1828:20: required from ‘void std::__unguarded_linear_insert(_RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Val_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1855:36: required from ‘void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1885:25: required from ‘void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1971:31: required from ‘void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:4868:18: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = shg*; _Compare = bool]’ Main.cc:18:23: required from here /usr/include/c++/7/bits/predefined_ops.h:215:11: error: expression cannot be used as a function { return bool(_M_comp(__val, *__it)); } ^~~~~~~~~~~~~~~~~~~~~~~~~~~ /usr/include/c++/7/bits/predefined_ops.h: In instantiation of ‘bool __gnu_cxx::__ops::_Iter_comp_val<_Compare>::operator()(_Iterator, _Value&) [with _Iterator = shg*; _Value = shg; _Compare = bool]’: /usr/include/c++/7/bits/stl_heap.h:133:48: required from ‘void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare&) [with _RandomAccessIterator = shg*; _Distance = long int; _Tp = shg; _Compare = __gnu_cxx::__ops::_Iter_comp_val]’ /usr/include/c++/7/bits/stl_heap.h:237:23: required from ‘void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = shg*; _Distance = long int; _Tp = shg; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_heap.h:342:22: required from ‘void std::__make_heap(_RandomAccessIterator, _RandomAccessIterator, _Compare&) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1672:23: required from ‘void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1933:25: required from ‘void std::__partial_sort(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1948:27: required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = shg*; _Size = long int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:1968:25: required from ‘void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = shg*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter]’ /usr/include/c++/7/bits/stl_algo.h:4868:18: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = shg*; _Compare = bool]’ Main.cc:18:23: required from here /usr/include/c++/7/bits/predefined_ops.h:177:11: error: expression cannot be used as a function { return bool(_M_comp(*__it, __val)); } ^~~~~~~~~~~~~~~~~~~~~~~~~~~