Compilare: user.cpp: In function ‘int eval()’: user.cpp:17:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i<s.size() && (s[i]=='+' || s[i]=='-')) ^ user.cpp: In function ‘int prod()’: user.cpp:28:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i<s.size() && (s[i]=='*' || s[i]=='/')) ^ user.cpp: At global scope: user.cpp:47:6: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type] main() ^