演员与后缀 [英] cast vs. suffixes
问题描述
如果你想明确说明一个整数常量的类型，你
基本上有两个选项。您可以使用显式强制转换，例如：
（unsigned int）1234
或您使用UL后缀，如：
1234UL
是否 - 通常 - 使用演员表中的任何语义差异
与后缀相比？
--Ralf
If you want to explicitly state the type of an integer constant, you
basically have two options. Either you use an explicit cast, as in:
(unsigned int) 1234
or you use the UL suffixes, as in:
1234UL
Are there -- in general -- any semantic differences in using casts
versus suffixes?
--Ralf
推荐答案
Ralf写道：
如果你想明确说明一个整数常量的类型，你基本上有两种选择。您可以使用显式强制转换，如下所示：
（unsigned int）1234
这可以在编译器选择时在
运行时。我仍然期望一个好的优化者在
编译时这样做。所以，如果你错误输入
你的常数，你可能会得到令人讨厌的错误。
或你使用UL后缀，如：
1234UL
这总是在编译时完成。在这里，错误的常量
在体外仍被捕获。
是否 - 通常情况下 - 使用强制转换中的语义差异
与后缀相比？
--Ralf
If you want to explicitly state the type of an integer constant, you
basically have two options. Either you use an explicit cast, as in:
(unsigned int) 1234
This, at compiler''s choice, can be left to be evaluated at
run-time. I''d still expect a good optimiser to do it at
compile-time. So, potentially, you get nasty bugs if you mistype
your constants.
or you use the UL suffixes, as in:
1234UL
This is always done at compile time. Here, mistyped constants
are caught while still in vitro.
Are there -- in general -- any semantic differences in using casts
versus suffixes?
--Ralf
目前可能还有其他一些影响我的事情。
干杯
弗拉基米尔
-
我的电子邮件地址是真的，我看了。
There may also be other implications which escape me at the moment.
Cheers
Vladimir
--
My e-mail address is real, and I read it.
Ralf在01/20/06 12:52写道：
Ralf wrote On 01/20/06 12:52,:
如果要显式声明整数的类型不断，你基本上有两个选择。您可以使用显式强制转换，例如：
（unsigned int）1234
或者您使用UL后缀，如：
1234UL
是否 - 通常 - 使用演员表与后缀相比的任何语义差异？
If you want to explicitly state the type of an integer constant, you
basically have two options. Either you use an explicit cast, as in:
(unsigned int) 1234
or you use the UL suffixes, as in:
1234UL
Are there -- in general -- any semantic differences in using casts
versus suffixes?
是的。 `（unsigned int）1234''不是常量：它是
一个表达式，由运算符`（unsigned int）组成''
应用于常量操作数` 1234 ''。
表达式的结果是'unsigned int''，值为
一千二百三十四。
`1234UL''是常数。它的类型是`unsigned long''
，它的值是一千二百三十四。
（请注意，这个常量的类型与
早期表达式的类型。）
一个地方可以产生一个容易观察到的差异
在预处理器中。由于预处理器在编译的早期阶段运行
之前类型存在，
它无法评估演员操作符 - 因为演员表全部是
$关于类型转换的b $ b并且还没有任何类型，
预处理器无法对它们做任何事情。
考虑以下代码：
#include< stdio.h>
int main（无效）{
#if 1234UL> 0
put（＃1：正如预期的那样）;
#else
puts（＃1：难以置信！ ）;
#endif
#if（unsigned int）1234> 0
put（＃2：正如预期的那样）;
#else
puts（＃2：难以置信！ ）;
#endif
返回0;
}
无需编译并运行它，你能预测
输出吗？ （警告：我问了一个技巧问题。）
-
Er ********* @ sun.com
Yes. `(unsigned int)1234'' is not a constant: it is
an expression consisting of the operator `(unsigned int)''
applied to the constant operand `1234''. The type of the
expression''s result is `unsigned int'' and the value is
one thousand two hundred thirty-four.
`1234UL'' is a constant. Its type is `unsigned long''
and its value is one thousand two hundred thirty-four.
(Note that the type of this constant is not the same as
the type of the earlier expression.)
One place this makes an easily-observable difference
is in the preprocessor. Since the preprocessor operates
at an early stage of compilation before types "exist,"
it cannot evaluate cast operators -- since casts are all
about type conversions and there aren''t any types yet,
there''s really nothing the preprocessor can do with them.
Consider the following code:
#include <stdio.h>
int main(void) {
#if 1234UL > 0
puts ("#1: as expected");
#else
puts ("#1: unbelievable!");
#endif
#if (unsigned int)1234 > 0
puts ("#2: as expected");
#else
puts ("#2: unbelievable!");
#endif
return 0;
}
Without compiling and running it, can you predict the
output? (Warning: I''ve asked a trick question.)
--
Er*********@sun.com
Ralf写道：
如果你想明确说明一个整数常量的类型，你基本上有两个选择。您可以使用显式强制转换，例如：
（unsigned int）1234
或者您使用UL后缀，如：
1234UL
是否 - 通常 - 使用强制转换与后缀的任何语义差异？
If you want to explicitly state the type of an integer constant, you
basically have two options. Either you use an explicit cast, as in:
(unsigned int) 1234
or you use the UL suffixes, as in:
1234UL
Are there -- in general -- any semantic differences in using casts
versus suffixes?
具有16位整数的系统会发生什么：
（长）65537
对
65537L
-
+ ------------------------- + ------------------ - + ----------------------------- +
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody / at\spamcop.net | www.fptech.com | #include< std_disclaimer.h> |
+ ------------------------- + -------------- ------ + ----------------------------- +
不要给我发电子邮件：< mailto：Th ************* @ gmail.com>
What happens on a system with 16-bit integers with:
(long)65537
versus
65537L
--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer.h> |
+-------------------------+--------------------+-----------------------------+
Don''t e-mail me at: <mailto:Th*************@gmail.com>
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