C99标准中Usual arithmetic conversions的一个问题

baozhao

今天翻阅C99标准,条款列在最下面.有一个疑惑,C99里面Usual arithmetic conversions里面关于浮点数的,没有提到两个操作数不同才conversion,

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.

另一个是不是long double全然没有提及.

整型就有

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.

哪位能解释一下,谢谢!

完整的内容见下

6.3.1.8 Usual arithmetic conversions
1 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, the other
operand is converted, without change of type domain, to a type whose
corresponding real type is double.
Otherwise, if the corresponding real type of either operand is float, the other
operand is converted, without change of type domain, 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.
2 The values of floating operands and of the results of floating expressions may be
represented in greater precision and range than that required by the type; the types are not
changed thereby.52)
51)

folklore

楼主读死书， 死读书啊，类型相同如何转换？
再者， 人家说了：First,．．．　Otherwise, ．．．　就是这个是个需要特别的情况，这个情况优先。

baozhao

回复 2# folklore


    你的说法我无法赞同.之所以问,我主要是怀疑标准是不是在别的地方提到了?
   

bruceteen

撇开C标准不谈
在通常的计算机中，浮点数是80387数字协处理器运行的吧
无论是 float 和 float 运算，还是 double 和 double 运算，都在80387中转成了 long double

bruceteen

我明白楼主的意思了，楼主是想说C为什么不说明“假如另一个为long double，就不要转化了”？

老鸨说：“姑娘们，贵客就要来了，大家给我更骚点儿。”
翠花说：“我已经骚到极致了，所以你应该说‘姑娘们，贵客就要来了，除了翠花之外的人给我更骚点儿’，这样表达才精确。”

baozhao

回复 4# bruceteen

你这个就是答非所问了. 你谈论的就是一个实现问题(实际标准里面有所提到),我问的是标准为什么这样写?
   

liuiang

两个大神都hold不住了哇~~~~~~，专门来顶两个大神的~~~~~~~