演员与后缀

本文介绍了演员与后缀的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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

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

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 | | | kenbrody / at\spamcop | www.fptech | #include< std_disclaimer.h> | + ------------------------- + -------------- ------ + ----------------------------- + 不要给我发电子邮件：< mailto：Th ************* @ gmail>

What happens on a system with 16-bit integers with: (long)65537 versus 65537L -- +-------------------------+--------------------+-----------------------------+ | Kenneth J. Brody | www.hvcomputer | | | kenbrody/at\spamcop | www.fptech | #include <std_disclaimer.h> | +-------------------------+--------------------+-----------------------------+ Don''t e-mail me at: <mailto:Th*************@gmail>