在struct中的struct内部结构

hello lets say i have this code

你好,我说我有这个代码

typedef struct entry {
    union {
        struct A {
            char  *c;
        } *A;
        struct B {
            char *c;
        } *B;
    } value;
} *TableEntry;

i m doing a malloc for entry and now i want to copy a string to c from struct A . do i have to allocate memory for struct A and then for c or the first malloc for table entry allocates for all of them ? thank you in advance

我正在做一个malloc用于输入,现在我想从struct A复制一个字符串到c。我是否必须为结构A分配内存,然后为c或第一个malloc分配表的所有内容?先感谢您

3 个解决方案

#1

you have to allocate memory for both of them

你必须为它们分配内存

#2

When you allocate the TableEntry - you allocate the memory for the whole union. Pointers in it are allocated, but what they point to - are not. So you should assign values that you allocate to c members of the struct and A/B members of the union.

分配TableEntry时 - 为整个联合分配内存。它中的指针是分配的,但它们指的是 - 不是。因此,您应该将分配的值分配给结构的c成员和union的A / B成员。

Note that the A and B share the same space.

请注意,A和B共享相同的空间。

#3

To clarify, three allocs are needed, e.g.:

澄清一下,需要三个分配,例如:

TableEntry *t = malloc(sizeof *t);
t->A = malloc(sizeof *t->A);
t->A->c = malloc(50);

This design is questionable though, as there is no way of telling which is currently active out of A or B. You will have to have another index or something which keeps track of whether this entry is an A or a B.

但是这个设计值得怀疑,因为没有办法告诉A或B当前有哪些活动。你必须有另一个索引或者某个东西来跟踪这个条目是A还是B.

#1