C中的隐式整数类型转换

Many operators that expect operands of arithmetic type cause conversions and yield result
types in a similar way. The purpose is to determine a common real type for the operands
and result. For the specified operands,each operand is converted,without change of type
domain,to a type whose corresponding real type is the common real type. Unless
explicitly stated otherwise,the common real type is also the corresponding real type of
the result,whose type domain is the type domain of the operands if they are the same,
and complex otherwise. This pattern is called the usual arithmetic conversions:

• First,if the corresponding real type of either operand is long double,the other
  operand is converted,without change of type domain,to a type whose
  corresponding real type is long double.
• Otherwise,if the corresponding real type of either operand is double,to a type whose
  corresponding real type is double.
• Otherwise,if the corresponding real type of either operand is float,to a type whose
  corresponding real type is float. 51)
• Otherwise,the integer promotions are performed on both operands. Then the
  following rules are applied to the promoted operands:
  • If both operands have the same type,then no further conversion is needed.
  • Otherwise,if both operands have signed integer types or both have unsigned
    integer types,the operand with the type of lesser integer conversion rank is
    converted to the type of the operand with greater rank.
  • Otherwise,if the operand that has unsigned integer type has rank greater or
    equal to the rank of the type of the other operand,then the operand with
    signed integer type is converted to the type of the operand with unsigned
    integer type.
  • Otherwise,if the type of the operand with signed integer type can represent
    all of the values of the type of the operand with unsigned integer type,then
    the operand with unsigned integer type is converted to the type of the
    operand with signed integer type.
  • Otherwise,both operands are converted to the unsigned integer type
    corresponding to the type of the operand with signed integer type.

添加执行通常的算术转换,因此,在添加unsigned char和signed int时,要么：

>首先将unsigned char提升为int,然后两个类型相同,因此结果的类型为int,或>(uncommon)int不能表示所有可能的unsigned char值.在这种情况下,unsigned char被提升为unsigned int,并且第三个子项应用：unsigned int具有与int相等的等级,因此int操作数被转换为unsigned int,结果的类型为unsigned int.