Addition of two signed ints results in a signed int.
两个有符号整数相加的结果是一个有符号的整数。
If one of the operands is unsigned and the other is a signed int, the expression becomes an unsigned.
如果一个操作数是无符号整数，另外一个操作数是有符号整数，那么表达式的结果就是无符号整数。
If one of the operands is unsigned and the other is a signed int, the expression becomes an unsigned.
如果一个操作数是无符号整数，另外一个操作数是有符号整数，那么表达式的结果就是无符号整数。
应用推荐