================== Advice on signed types ================
Avoid unnecessary complexity by minimizing your use of unsigned types.Specifically, dong‘t use an unsigned type to represent a quantity just because it will never be negative (e.g., "age" or "national_debt").
Use a signed type like int and you won‘t have to worry about boundary cases in the detailed rules for promoting mixed types.
Only use unsigned types for bit fields or binary masks.
Use casts in expressions, to make all the operands signed or unsigned, so the compiler does not have to choose the result type.
原文：http://www.cnblogs.com/likeatree/p/4293734.html