在c中u'123'或U'\ n \ t'或L'abc'整数常数怎么样？(How are u'123' or U'\n\t' or L'abc' integer constants in c? [duplicate])

 
 
 这个问题在这里已有答案： 
 
 
 
  
 C ++多字符文字 5个答案 
 
 
 
 
 在为C语言编写代码时，我一直在寻找它的语法。 我偶然发现了ANSI C Grammar（Lex） 。 在那里我发现了以下正则表达式（RE）： 
 
{CP}?"'"([^'\\\n]|{ES})+"'"     { return I_CONSTANT; }
 
 其中CP和ES如下 
 
CP  (u|U|L)
ES  (\\(['"\?\\abfnrtv]|[0-7]{1,3}|x[a-fA-F0-9]+))
 
 据我所知，ES是逃脱序列的RE。 
 
 如果我正确理解了正则表达式，那么u'123'或U'\n\t'或L'abc'是有效的I_CONSTANT s。 
 
 我编写了以下小程序来查看它们代表的常量值。 
 
#include <stdio.h>

int main(void) {
    printf("%d %d %d\n", u'123', U'\n\t', L'abc');
    return 0;
}
 
 这给出了以下输出。 
 
 51 9 99 
 
 我破译他们代表单引号内最右边字符的ASCII值。 但是，我无法理解的是这种整数常量的用法和重要性。 

 
 
This question already has an answer here:
 
 
 
  
 C++ multicharacter literal  5 answers  
 
 
 
 
While making a compiler for C language, I was looking for its grammar. I stumbled upon ANSI C Grammar (Lex). There I found the following regular expression (RE):
 
{CP}?"'"([^'\\\n]|{ES})+"'"     { return I_CONSTANT; }
 
where CP and ES are as follows
 
CP  (u|U|L)
ES  (\\(['"\?\\abfnrtv]|[0-7]{1,3}|x[a-fA-F0-9]+))
 
I understand that ES is RE for escape sequence.
 
If I understand the regular expression correctly then, u'123' or U'\n\t' or L'abc' are valid I_CONSTANTs.
 
I wrote the following small program to see what constant values they represent. 
 
#include <stdio.h>

int main(void) {
    printf("%d %d %d\n", u'123', U'\n\t', L'abc');
    return 0;
}
 
This gave the following output.
 
51 9 99
 
I deciphered that they represent the ASCII value of the right-most character inside single quotes. However, what I fail to understand is the use and importance of this kind of integer constant.

原文：https://stackoverflow.com/questions/47960999