c++ - 你能解释一下指针和递归结构吗

Question

你能解释一下结构体中的指针的含义吗？递归结构有何用处？您能为我解释一下这段代码吗？它在内存中的表现如何？ 这是我的 C 代码:

struct State { 
unsigned long Out; 
unsigned long Time; //ms 
const struct State *Next[4];}; 

Answer 1

在这种情况下，Next可以在只读地址(4个不可修改的引用)中保存4个指向相同类型(struct State)对象的指针。

一个例子:

#include <stdio.h>
#include <stdlib.h>

struct State { 
    unsigned long Out; 
    unsigned long Time; //ms 
    const struct State *Next[4];
}; 

void fn(struct State *data)
{
    /* data->Next[0]->Out = 1; error: assignment of member ‘Out’ in read-only object */
    for (int i = 0; i < 4; i++) {
        printf("%ld %ld\n", data->Next[i]->Out, data->Next[i]->Time);
        free((struct State *)data->Next[i]); /* cast to non const */
    }
}

int main(void)
{
    struct State data;
    struct State *next;

    for (int i = 0; i < 4; i++) {
        next = malloc(sizeof(*next));
        next->Out = i;
        next->Time = i * 10;
        data.Next[i] = next;
    }
    fn(&data);
    return 0;
}