我们可以有一个可变长度数组类型的struct元素吗? [英] Can we have a struct element of type Variable length array?

问题描述

我们可以声明一个可变长度的结构元素吗?

Can we declare a structure element of variable length?

条件如下:

typedef struct
{
   uint8_t No_Of_Employees;
   uint8_t Employee_Names[No_Of_Employees][15];
}st_employees;

推荐答案

如果使用 C99 或C11，则可能要使用灵活的数组成员(您没有给出明确的尺寸，但是您应该在运行时对此有一个约定).

If coding in C99 or C11, you might want to use flexible array members (you don't give an explicit dimension, but you should have a convention about it at runtime in your head).

 typedef struct {
    unsigned No_Of_Employees;
    char* Employee_Names[]; // conventionally with No_of_Employees slots
 }st_employees;

对于任何阵列，柔性阵列成员的每个插槽都具有固定的大小.我使用的是指针(例如，在Linux/x86-64计算机上为8个字节).

As for any array, each slot of a flexible array member has a fixed size. I'm using a pointer (e.g. 8 bytes on my Linux/x86-64 machine).

^{(在C99标准之前的旧编译器中，即使违反标准，您也可以尝试提供0尺寸，例如char* Employee_Names[0];)}

^{(In old compilers before the C99 standards, you might try give a 0 dimension like char* Employee_Names[0]; even if it is against the standard)}

然后，您将使用例如

 st_employees* make_employees(unsigned n) {
    st_employees* s = malloc(sizeof(s_employees)+n*sizeof(char*));
    if (!s) { perror("malloc make_employees"); exit(EXIT_FAILURE); };
    s->No_of_Employees = n;
    for (unsigned i=0; i<n; i++) s->Employe_Names[i] = NULL;
    return s;
 }

，您可能会使用(与 strdup(3)一起使用在堆中复制一个字符串)

and you might use (with strdup(3) duplicating a string in the heap) it like

 st_employees* p = make_employees(3);
 p->Employee_Names[0] = strdup("John");
 p->Employee_Names[1] = strdup("Elizabeth");
 p->Employee_Names[2] = strdup("Brian Kernighan");

您将需要一个void destroy_employee(st_employee*e)函数(作为练习供读者阅读).它可能应该在每个e->Employee_Names[i]上依次在i上循环到free，然后在free(e); ...

You'll need a void destroy_employee(st_employee*e) function (left as an exercise to the reader). It probably should loop on i to free every e->Employee_Names[i], then free(e);...

请不要忘记记录有关内存使用的约定(谁负责调用malloc和free).阅读有关 C动态内存分配的更多信息(不要害怕未定义的行为).

Don't forget to document the conventions about memory usage (who is in charge of calling malloc and free). Read more about C dynamic memory allocation (and be scared of memory fragmentation and buffer overflows and any other undefined behavior).

如果使用的 GCC 早于

If using a GCC older than GCC 5 be sure to compile with gcc -std=c99 -Wall since the default standard for old GCC 4 compilers is C89. For newer compilers, ask for all warnings and more of them, e.g. gcc -Wall -Wextra...

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