C/C++编程：变长数组

雨点打透心脏的1/2处 2023-01-03 01:29 183阅读 0赞

我们知道，[零长度数组][Link 1]可以声明一个长度可变的结构体

#include <stdio.h>
    #include <stdlib.h>
    
    struct line {
        
    
        int length;
        char contents[0];   // 零长度数组，只能是结构体的最后一个成员
    } ;
    
    
    int main(void)
    {
        
        int i, count = 9;
        char letter = 'A';
    
        struct line *thisline = (struct line *)malloc(sizeof(struct line) + count);
    
        thisline->length = count;
        for (i = 0; i < count; i++)
            thisline->contents[i] = letter++;
    
        printf("sizeof(struct line) = %d\n", sizeof(struct line));
    
        for (i = 0; i < thisline->length; i++)
            printf("%c ", thisline->contents[i]);
        printf("\n");
    
    
        return 0;
    }

![在这里插入图片描述][20210106164012575.png]  
在ISO C99中，使用变长数组也可以实现同样的功能（比如上面，优点在于大小可以在初始化的时候指定）:

#include <stdio.h>
    #include <stdlib.h>
    
    struct line {
        
        int length;
        char contents[];
    };
    
    struct line thisline = {
         5, {
        '1', '2', '3', '4', '5' } };
    
    int main(void)
    {
        
        int i;
    
        printf("sizeof(struct line) = %d\n", sizeof(struct line));
        printf("sizeof(thisline) = %d\n", sizeof(thisline));
    
        for (i = 0; i < thisline.length; i++)
            printf("%c ", thisline.contents[i]);
        printf("\n");
    
        return 0;
    }

![在这里插入图片描述][202101061642094.png]  
注意，此结构体的变量必须在函数外定义和初始化，否则会报错：

#include <stdio.h>
    #include <stdlib.h>
    
    struct line {
        
        int length;
        char contents[];
    };
    
    
    
    int main(void)
    {
        
        int i;
        struct line thisline = {
         5, {
        '1', '2', '3', '4', '5' } };
        printf("sizeof(struct line) = %d\n", sizeof(struct line));
        printf("sizeof(thisline) = %d\n", sizeof(thisline));
    
        for (i = 0; i < thisline.length; i++)
            printf("%c ", thisline.contents[i]);
        printf("\n");
    
        return 0;
    }
    
    
    //----------------------
    #include <stdio.h>
    #include <stdlib.h>
    
    
    int main(void)
    {
        
        struct line {
        
            int length;
            char contents[];
        };
        int i;
        struct line thisline = {
         5, {
        '1', '2', '3', '4', '5' } };
        printf("sizeof(struct line) = %d\n", sizeof(struct line));
        printf("sizeof(thisline) = %d\n", sizeof(thisline));
    
        for (i = 0; i < thisline.length; i++)
            printf("%c ", thisline.contents[i]);
        printf("\n");
    
        return 0;
    }

![在这里插入图片描述][2021010616445849.png]

注意，不能是这样的结构

struct mystruct {
        
        int arr[];
    };

![在这里插入图片描述][20210106164808411.png]

[C语言变长数组讲解][C]

[Link 1]: https://blog.csdn.net/zhizhengguan/article/details/111472884
[20210106164012575.png]: /images/20221119/413517b1343b401c95951fb310876312.png
[202101061642094.png]: /images/20221119/dab6885e043c4efe936fd4de131b1541.png
[2021010616445849.png]: /images/20221119/12b5a091da2d4d93b782743c4f5408f0.png
[20210106164808411.png]: /images/20221119/3d9dc6ea279646a690759d49ffe300d8.png
[C]: https://blog.csdn.net/houzijushi/article/details/80245894