此源代码在C中打开字符串.它是如何做到的? [英] This source code is switching on a string in C. How does it do that?

问题描述

我正在阅读一些仿真器代码，并且已经发现了一些真正奇怪的东西:

I'm reading through some emulator code and I've countered something truly odd:

switch (reg){
    case 'eax':
    /* and so on*/
}

这怎么可能?我以为您只能在整数类型上使用switch.发生了一些大骗局吗?

How is this possible? I thought you could only switch on integral types. Is there some macro trickery going on?

推荐答案

(除非您粘贴更多的代码，否则您只能回答宏骗术"部分.但是这里没有太多要处理的宏-正式而言，您可以不允许重新定义关键字；这样做的行为是不确定的.)

(Only you can answer the "macro trickery" part - unless you paste up more code. But there's not much here for macros to work on - formally you are not allowed to redefine keywords; the behaviour on doing that is undefined.)

为了实现程序的可读性，机智的开发人员正在利用实现定义的行为. 'eax'是不是字符串，而是多字符常量.请非常仔细地注意eax周围的单引号字符.在您的情况下，很可能会给您提供int，该字符对于该字符组合是唯一的. (通常每个字符在32位int中占据8位).大家都知道您可以在int上使用switch！

In order to achieve program readability, the witty developer is exploiting implementation defined behaviour. 'eax' is not a string, but a multi-character constant. Note very carefully the single quotation characters around eax. Most likely it is giving you an int in your case that's unique to that combination of characters. (Quite often each character occupies 8 bits in a 32 bit int). And everyone knows you can switch on an int!

最后，一个标准参考文献:

Finally, a standard reference:

C99标准说:

6.4.4.4p10:包含多个字符(例如'ab')或包含一个字符或 不映射到单字节执行的转义序列 字符，是实现定义的."

6.4.4.4p10: "The value of an integer character constant containing more than one character (e.g., 'ab'), or containing a character or escape sequence that does not map to a single-byte execution character, is implementation-defined."

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