Compilare: user.cpp: In function ‘bool exp()’: user.cpp:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0; i<s.length(); ++i){ ^ user.cpp: In function ‘int main()’: user.cpp:79:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0; i<s.length(); ++i) ^ user.cpp:87:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0; i<s.length(); ++i) ^ user.cpp: In function ‘bool add(char, bool, bool)’: user.cpp:16:1: warning: control reaches end of non-void function [-Wreturn-type] } ^