Compilare: user.cpp: In function ‘int eval_T(const string&)’: user.cpp:19:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(lookahead >= s.size()) return 0; ^ user.cpp: In function ‘int eval_F(const string&)’: user.cpp:38:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(lookahead >= s.size()) return 0; ^ user.cpp: In function ‘int match_number(const string&)’: user.cpp:60:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(lookahead < s.size() && '0' <= s[lookahead] && s[lookahead] <= '9') { ^ user.cpp: In function ‘int eval_A(const string&)’: user.cpp:69:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(lookahead >= s.size()) return 0; ^