演员的两个含义 [英] two meanings of a cast
问题描述
多年来我一直没有考虑过这个问题，但是最近我偶然发现了''cast''实际上正在做（至少）两个不同的事实。 >
东西：
一方面''cast''意味着：''将某些东西变成某种东西
else''。示例：double d = 9.99; long l =（double）d;
另一方面''cast''表示：''将某事物视为某物
其他没有改变'' 。例如，在投射函数指针时。
你同意吗？
IMO转换和处理之间的区别对于C新人而言，并不是很可能已经指出了足够多的b $ b可能是造成危险的一个原因。
Felix
Felix Kater写道：
我没想过它已经好几年了，但最近我偶然发现
''cast''实际上正在做（至少）两种不同的b
事物：
一方面''cast''意味着：''将某些东西变成某种东西
else''。示例：double d = 9.99; long l =（double）d;
另一方面''cast''表示：''将某事物视为某物
其他没有改变'' 。例如，在投射函数指针时。
你同意吗？
No.
在演员表达式`（T）E`中，我们要求的值是表达
`E`使用适当的转换机制转换为类型`T`。
各地都是如此。
有时，适当的转换机制使基础
位模式保持不变，导致这种错觉，即演员意味着将某些东西视为没有变化的东西。但这是一个意外的实施。
[我们在过去看到过从`char *`转换为`int *`和<在某些实现中，
返回需要有点代码。]
IMO转换和处理之间的这种区别是可能
足以指出C新人，可能是造成危险的原因之一。
我不认为这么简单。我怀疑危险来自
指针的投射以及如果`p`指向某处理智的信念那么
指针`（T *）p`指向同样理智的地方并且是dereferencable
来获取类型`T`的值。
-
尼特采摘最好在朋友之间进行。
Hewlett-Packard Limited Cain Road，Bracknell，注册号：
注册办事处：Berks RG12 1HN 690597 England
>>>>" FK" == Felix Kater< fk **** @ googlemail.comwrites：
FKOn一方面''cast''意味着：''将某些内容改为
FKsomthing''。示例：double d = 9.99; long l =（double）d;
FKOn另一方面''cast''意味着：''处理某事物为
FKs其他没有变化'' 。例如，当你施放
FKfunction指针时。
FK你同意吗？
否;在你的第一个案例中，你也将某事物视为某种东西而不是改变。当你把它变成双倍时，d的值不会改变（完全没必要，因为那就是它）和
将它分配给l。 br $>
Charlton
-
Charlton Wilbur
cw ***** @ chromatico.net
4月27日下午1:23，Chris Dollin< ; chris.dol ... @ hp.comwrote：
< snip>
有时，适当转换机械使基础
位模式保持不变，导致这种错觉，即演员意味着将某些东西视为没有变化的东西。但这是实施
的意外。
< snip>
你在这里说有一个实现会
改变对象的位模式，因为它是[保持
原始大小不变] - 或者位的变化可能是
结果改变原件尺寸？
I haven''t been thinking about it for years but recently I''ve stumbled on
the fact that ''casting'' is actually doing (at least) two different
things:
On the one hand ''casting'' means: ''Change something into somthing
else''. Example: double d=9.99; long l=(double)d;
On the other hand ''casting'' means: ''Treating something as something
else without change''. For instance when casting function pointers.
Would You agree?
IMO this difference between ''conversion'' and ''treatment'' isn''t probably
pointed out enough to C newcomers and may be one reason for the danger
of casts.
Felix
Felix Kater wrote:
I haven''t been thinking about it for years but recently I''ve stumbled on
the fact that ''casting'' is actually doing (at least) two different
things:
On the one hand ''casting'' means: ''Change something into somthing
else''. Example: double d=9.99; long l=(double)d;
On the other hand ''casting'' means: ''Treating something as something
else without change''. For instance when casting function pointers.
Would You agree?No.
In a cast expression `(T) E`, we''re requiring the value of the expression
`E` to be converted to type `T` using the appropriate conversion machinery.
That''s true everywhere.
Sometimes, the "appropriate conversion machinery" leaves the underlying
bit-pattern unchanged, leading to the illusion that the cast means "Treat
something as something else without change". But this is an accident of
implementation.
[We have seen in the past that conversion from say `char*` to `int*` and
back requires bit-fiddling code in some implementations.]
IMO this difference between ''conversion'' and ''treatment'' isn''t probably
pointed out enough to C newcomers and may be one reason for the danger
of casts.I don''t think it''s as simple as that. I suspect the danger comes from
casts of pointers and a belief that if `p` points somewhere sane then
the pointer `(T*) p` points somewhere equally sane and is dereferencable
to get values of type `T`.
--
Nit-picking is best done among friends.
Hewlett-Packard Limited Cain Road, Bracknell, registered no:
registered office: Berks RG12 1HN 690597 England
>>>>"FK" == Felix Kater <fk****@googlemail.comwrites:
FKOn the one hand ''casting'' means: ''Change something into
FKsomthing else''. Example: double d=9.99; long l=(double)d;
FKOn the other hand ''casting'' means: ''Treating something as
FKsomething else without change''. For instance when casting
FKfunction pointers.
FKWould You agree?
No; in your first case, you''re also "treating something as something
else without change." The value of d does not change when you cast it
to a double (completely unnecessary, since that''s what it is) and
assign it to l.
Charlton
--
Charlton Wilbur
cw*****@chromatico.net
On Apr 27, 1:23 pm, Chris Dollin <chris.dol...@hp.comwrote:
<snip>
Sometimes, the "appropriate conversion machinery" leaves the underlying
bit-pattern unchanged, leading to the illusion that the cast means "Treat
something as something else without change". But this is an accident of
implementation.<snip>
Are you saying here that there exists an implementation that would
alter the bit pattern of an object as it is cast [keeping the
original''s size constant] - or that the change in bits would likely be
the result of changing the size of the original?
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