此源代码在 C 中打开一个字符串.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

我正在阅读一些模拟器代码，我反驳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?我以为你只能 switch 整数类型.是否有一些宏诡计正在发生?

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

推荐答案

(只有你可以回答宏诡计"部分 - 除非你粘贴更多代码.但是这里没有太多宏可以工作 - 正式你是不允许重新定义关键字；这样做的行为是未定义的.)

(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' 不是字符串，而是多字符常量.非常小心地注意 eax 周围的单引号字符.在您的情况下，它很可能会为您提供一个 int ，这是该字符组合所独有的.(通常每个字符在 32 位 int 中占据 8 位).每个人都知道你可以在 int 上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:

C99 标准说:

6.4.4.4p10:一个整数字符常量的值包含多个字符(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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