空字符串

超、凢脫俗 2022-11-20 13:00 188阅读 0赞

#  空字符串 #

## 1. 空字符串 ##

字符串常量的书写方式是用一对双引号包围一串字符，如下所示：

“Hello”
    “\aWarning!\a”
    “Line 1\nLine2”
    “”

**字符串常量 (不像字符常量) 可以是空的。即使是空字符串，依然存在作为终止符的 `NUL` 宇节。**

literal [ˈlɪtərəl]：adj. 文字的，逐字的，无夸张的
    string [strɪŋ]：n. 线，弦，细绳，一串，一行

## 2. `'\0'` ##

//============================================================================
    // Name        : '\0'
    // Author      : Yongqiang Cheng
    // Version     : Version 1.0.0
    // Copyright   : Copyright (c) 2019 Yongqiang Cheng
    // Description : Hello World in C++, Ansi-style
    //============================================================================
    
    #include <stdio.h>
    #include <string.h>
    
    int main(int argc, char **argv)
    {
    	char *str_1 = "yong";
    	char *str_2 = "qiang";
    	char *str_3 = "";
    
    	int len_str1 = 0;
    	int len_str2 = 0;
    	int len_str3 = 0;
    
    	len_str1 = strlen(str_1);
    	len_str2 = strlen(str_2);
    	len_str3 = strlen(str_3);
    
    	printf("len_str1 = %d\n", len_str1);
    	printf("len_str2 = %d\n", len_str2);
    	printf("len_str3 = %d\n", len_str3);
    
    	printf("%s\n", str_1);
    	printf("%s\n", str_2);
    	printf("%s\n", str_3);
    
    	if ('\0' == *str_3)
    	{
    		printf("yongqiang!\n");
    	}
    
    	return 0;
    }

len_str1 = 4
    len_str2 = 5
    len_str3 = 0
    yong
    qiang
    
    yongqiang!

## 3. `#include <string.h>` ##

//============================================================================
    // Name        : '\0'
    // Author      : Yongqiang Cheng
    // Version     : Version 1.0.0
    // Copyright   : Copyright (c) 2019 Yongqiang Cheng
    // Description : Hello World in C++, Ansi-style
    //============================================================================
    
    #include <stdio.h>
    
    int main(int argc, char **argv)
    {
    	char *str_1 = "yong";
    	char *str_2 = "qiang";
    	char *str_3 = "";
    
    	int len_str1 = 0;
    	int len_str2 = 0;
    	int len_str3 = 0;
    
    	len_str1 = strlen(str_1);
    	len_str2 = strlen(str_2);
    	len_str3 = strlen(str_3);
    
    	printf("len_str1 = %d\n", len_str1);
    	printf("len_str2 = %d\n", len_str2);
    	printf("len_str3 = %d\n", len_str3);
    
    	printf("%s\n", str_1);
    	printf("%s\n", str_2);
    	printf("%s\n", str_3);
    
    	if ('\0' == *str_3)
    	{
    		printf("yongqiang!\n");
    	}
    
    	return 0;
    }

09:02:07 **** Build of configuration Debug for project yongqiang_example ****
    make all 
    Building file: ../src/yongqiang.c
    Invoking: GCC C Compiler
    gcc -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/yongqiang.d" -MT"src/yongqiang.o" -o "src/yongqiang.o" "../src/yongqiang.c"
    ../src/yongqiang.c: In function ‘main’:
    ../src/yongqiang.c:21:13: warning: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration]
      len_str1 = strlen(str_1);
                 ^
    ../src/yongqiang.c:21:13: warning: incompatible implicit declaration of built-in function ‘strlen’
    ../src/yongqiang.c:21:13: note: include ‘<string.h>’ or provide a declaration of ‘strlen’
    Finished building: ../src/yongqiang.c
     
    Building target: yongqiang_example
    Invoking: GCC C Linker
    gcc  -o "yongqiang_example"  ./src/yongqiang.o   
    Finished building target: yongqiang_example
     
    
    09:02:08 Build Finished (took 515ms)

len_str1 = 4
    len_str2 = 5
    len_str3 = 0
    yong
    qiang
    
    yongqiang!