Compilare: user.cpp: In function ‘std::string infToPref(std::string)’: user.cpp:31:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0; i < str.size(); i++) { ^ user.cpp:34:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i < str.size() && !(isOp(str[i]))) { ^ user.cpp: In function ‘short int prcd(char)’: user.cpp:26:1: warning: control reaches end of non-void function [-Wreturn-type] } ^