In file included from main.cpp:16:0:
set.h: In member function 'Set Set::Difference(const Set&)':
set.h:58:1: error: 'res' was not declared in this scope
 res.push_back(s.S[i]);
 ^~~
set.h: In member function 'Set Set::SymmetricDifference(const Set&)':
set.h:71:17: error: 'const class Set' has no member named 'set'; did you mean 'Set'?
 if (std::find(s.set.begin(), s.set.end(), this->S[i]) != s.S.end()) {
                 ^~~
                 Set
set.h:71:32: error: 'const class Set' has no member named 'set'; did you mean 'Set'?
 if (std::find(s.set.begin(), s.set.end(), this->S[i]) != s.S.end()) {
                                ^~~
                                Set
set.h:73:1: error: 'res' was not declared in this scope
 res.push_back(this->S[i]);
 ^~~