Compilare: user.cpp: In function ‘int prec(char)’: user.cpp:61:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (pos == string::npos) ^ user.cpp: In function ‘int main()’: user.cpp:77:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] FOR(i, 0, expr.size()) { ^ user.cpp:33:42: note: in definition of macro ‘FOR’ #define FOR(i, a, b) for (int i = a; i < b; ++i) ^