为什么是“\？” C / C ++中的转义序列？ [英] Why is "\?" an escape sequence in C/C++?
问题描述
有四个特殊的非字母字符需要在C / C ++中转义：单引号 \'，双引号 \，反斜杠 \\ 和问号 \？这显然是因为它们有特殊的含义。'单个 char ，用于字符串文字， \ 用于转义序列，但为什么？其中之一？
There are four special non-alphabet characters that need to be escaped in C/C++: the single quote \', the double quote \", the backslash \\ and the question mark \?. It's apparently because they have special meanings. ' for single char, " for string literals, \ for escape sequences, but why is ? one of them?
我今天阅读了一本教科书中的转义序列表，我意识到我已经永远不会转义？之前，从来没有遇到过问题，只是为了确保，我在gcc下测试：
I read the table of escape sequences in a textbook today and I realized that I've never escape ? before and never encountered a problem with it , just to be sure, I tested it under gcc:
#include <stdio.h>
int main(void)
{
printf("question mark ? and escaped \?\n");
return 0;
}
和C ++版本：
#include <iostream>
int main(void)
{
std::cout << "question mark ? and escaped \?" << std::endl;
return 0;
}
两个程序输出：问号？并逃脱？
所以我有两个问题：
- 为什么不转义
？/ code？工作正常，甚至没有警告。
- Why is
\?one of the escape sequence characters ? - Why non-escaping
?works fine, there's not even a warning.
在我要提出这个问题之前，我自己找到了答案，因为我没有在SO中找到一个重复的文件，所以我决定以Q& A的形式发布。
Before I'm about to ask this question, I found the answer myself, since I didn't find a duplicate in SO, I decided to post it in Q&A style.
更有意思的是，转义的 \？可以与其他一些？一样使用语言以及我在Lua / Ruby中测试，即使我没有找到这个记录也是如此。
The more interesting fact is that the escaped \? can be used the same as ? in some other languages as well, I tested in Lua/Ruby, it's also true even though I didn't find this documented.
推荐答案
为什么
\？其中一个转义序列字符？
Why is
\?one of the escape sequence characters ?
因为它是特别的，答案导致 Trigra ph ，C / C ++预处理器将以下三个字符的序列替换为相应的单个字符。 （C11§5.2.1.1和C ++ 11§2.3）
Because it is special, the answer leads to Trigraph, the C/C++ preprocessor replaces following three-character sequence to the corresponding single character. (C11 §5.2.1.1 and C++11 §2.3)
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
现在，Trigraph几乎没用，主要用于混淆目的，一些例子可以在 IOCCC 。
Trigraph is nearly useless now, mainly used for obfuscated purpose, some examples can be seen in IOCCC.
gcc默认情况下不支持三角形，如果代码中有三角形，则会发出警告，除非选项 -trigraphs 3 已启用。在 -trigraphs 选项下，第二个 \？在以下示例中很有用：
gcc doesn't support trigraph by default, and will warn you if there's trigraph in the code, unless the option -trigraphs3 is enabled. Under -trigraphs option, the second \? is useful in the following example:
printf("\?\?!\n");
输出将为 | 如果？不会转义。
Output would be | if ? is not escaped.
有关三位图的更多信息，请参阅加密行??！??！在旧版代码中
For more information on trigraph, see Cryptic line "??!??!" in legacy code
为什么不转义
？工作正常，甚至没有警告。
Why non-escaping
?works fine, there's not even a warning.
因为？ code>（和双引号）可以由标准本身表示：
Because ?(and double quote ") can be represented by themselves by the standard:
C11§6.4.4.4字符常量第4节
双引号
和问号？可以由自己或转义序列\和\？，但单引号'和反斜杠\分别由转义序列\'和\\表示。
C11 §6.4.4.4 Character constants Section 4
The double-quote
"and question-mark?are representable either by themselves or by the escape sequences\"and\?, respectively, but the single-quote'and the backslash\shall be represented, respectively, by the escape sequences\'and\\.
与C ++类似：
C ++ 11§2.13.2字符文字第3节
某些非字符字符，单个te
'，双引号，问号？，并且反斜杠\可以根据表6表示。双引号和问号？可以表示为自己或转义序列\和\ ？，但单引号'和反斜杠\应由转义序列分别为\'和\\。如果反斜杠后面的字符不是指定的那个，则行为是未定义的。转义序列指定单个字符。
C++11 §2.13.2 Character literals Section 3
Certain nongraphic characters, the single quote
’, the double quote", the question mark?, and the backslash\, can be represented according to Table 6. The double quote"and the question mark?, can be represented as themselves or by the escape sequences\"and\?respectively, but the single quote’and the backslash\shall be represented by the escape sequences\’and\\respectively. If the character following a backslash is not one of those specified, the behavior is undefined. An escape sequence specifies a single character.
这篇关于为什么是“\？” C / C ++中的转义序列？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
