c - 简单和双指针公式

Question

我制作了 delete fct 二叉树 header ，它成功了!但我没有做任何努力，我依赖于调试器。

你能解释一下简单指针、双指针和值之间的关系吗？

Tree **P2,*P1,P; //(P consider it for explanation only)
P1=&P2;

//what type are these
(*P2)->data;
&(*P2)->data;
P1->data;
*P1->data;

Answer 1

上图显示了指向指针的指针的内存表示。第一个指针 ptr1(P2 在您的例子中)存储第二个指针 ptr2 的地址，第二个指针 ptr2(P1 在您的例子中)存储变量的地址。

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

typedef struct _s {
    int data;
} Tree;

int main(int argc, char *args[])
{

    /*
     * Arrow operator example
     * foo->bar is equivalent to (*foo).bar
     * it gets the member called 'bar' from the struct that 'foo' points to.
     */

    Tree **P2,*P1;
    P1 = (Tree*)malloc(sizeof(Tree));
    P1->data = 789;
    //P1=&P2;               // It's wrong, Incompatible pointer types Tree* and Tree***
    P2 = &P1;               // It's right, p2 points to address of p1
    printf("%d\n", (*P2)->data);     // same as (**P2).data
    printf("%p\n", (void*)  &(*P2)->data);    // same as &(**P2).data,  (void* casting to print address)
    printf("%d\n", P1->data);        // same as (*P1).data
    //printf("%d",*P1->data);        // same as *(P1->data), it's already dereferenced type, you're trying to dereference again??
    free(P1);


    return 0;
}