## 预处理器运算符的应用和需要考虑的问题是什么? [英] What are the applications of the ## preprocessor operator and gotchas to consider?

问题描述

正如我之前的许多问题中所提到的，我正在使用 K&R，目前正在使用预处理器.更有趣的事情之一—从我之前尝试学习 C 的任何尝试中我从未知道的东西 -mdash;是 ## 预处理器运算符.根据 K&R:

As mentioned in many of my previous questions, I'm working through K&R, and am currently into the preprocessor. One of the more interesting things — something I never knew before from any of my prior attempts to learn C — is the ## preprocessor operator. According to K&R:

预处理操作符##提供了一种连接实际的方法宏扩展期间的参数.如果一个替换文本中的参数是与 ## 相邻，参数为替换为实际参数，## 和周围的空白是删除，并重新扫描结果.例如，宏 paste连接它的两个参数:

The preprocessor operator ## provides a way to concatenate actual arguments during macro expansion. If a parameter in the replacement text is adjacent to a ##, the parameter is replaced by the actual argument, the ## and surrounding white space are removed, and the result is re-scanned. For example, the macro paste concatenates its two arguments:

#define paste(front, back) front ## back

so paste(name, 1) 创建令牌name1.

有人会如何以及为什么会在现实世界中使用它?它的使用有哪些实际例子，是否有需要考虑的问题?

How and why would someone use this in the real world? What are practical examples of its use, and are there gotchas to consider?

推荐答案

CrashRpt:使用##将宏多字节字符串转换为Unicode

CrashRpt(崩溃报告库)中一个有趣的用法如下:

An interesting usage in CrashRpt (crash reporting library) is the following:

#define WIDEN2(x) L ## x
#define WIDEN(x) WIDEN2(x)
//Note you need a WIDEN2 so that __DATE__ will evaluate first.

这里他们想使用一个两字节的字符串，而不是一个每字符一个字节的字符串.这可能看起来真的毫无意义，但他们这样做是有充分理由的.

Here they want to use a two-byte string instead of a one-byte-per-char string. This probably looks like it is really pointless, but they do it for a good reason.

 std::wstring BuildDate = std::wstring(WIDEN(__DATE__)) + L" " + WIDEN(__TIME__);

他们将它与另一个返回带有日期和时间的字符串的宏一起使用.

They use it with another macro that returns a string with the date and time.

将 L 放在 __ DATE __ 旁边会给你一个编译错误.

Putting L next to a __ DATE __ would give you a compiling error.

Windows:对通用 Unicode 或多字节字符串使用 ##

Windows 使用类似以下内容:

Windows uses something like the following:

#ifdef  _UNICODE
    #define _T(x)      L ## x
#else
    #define _T(x) x
#endif

并且_T在代码中随处可见

各种库，用于干净的访问器和修饰符名称:

我还看到它在代码中用于定义访问器和修饰符:

I've also seen it used in code to define accessors and modifiers:

#define MYLIB_ACCESSOR(name) (Get##name)
#define MYLIB_MODIFIER(name) (Set##name)

同样，您可以使用相同的方法创建任何其他类型的巧妙名称.

Likewise you can use this same method for any other types of clever name creation.

各种库，使用它一次进行多个变量声明:

#define CREATE_3_VARS(name) name##1, name##2, name##3
int CREATE_3_VARS(myInts);
myInts1 = 13;
myInts2 = 19;
myInts3 = 77;

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