CODEWITHSUNDEEP
cpointersstructpointer-to-pointer
NUO
NUO

Reputation: 267

nested struct with nested pointers

I am using a data structure to implement a spellchecking. I had two struct, node and table, which are defined in the following:

#include <stdlib.h>
typedef struct node *tree_ptr;
typedef struct table * Table;
struct node
{
    char* element;
    tree_ptr left, right;
};

typedef struct table
{
    tree_ptr head;
    int tree_h;
}table;

int main() {
    Table t = malloc(sizeof(table));
    t->head = NULL;
    tree_ptr ptr = t->head;
    ptr = malloc(sizeof(tree_ptr));
    ptr->element = "one";
    ptr->left = NULL;
    ptr->right = NULL;
    printf("%s\n",t->head->element);
   return 0;
}

This programme has bug in the last line of print function, since t->head is pointing to NULL.

As I know, when changing a pointer's content value, the variable which the pointer points to is automatically changed.

Since t->head and ptr are both pointers, and ptr points to the t->head, that's, they are pointing to the same object.

Then when I change the ptr's value, why t->head doesn't change in the same way?? What should I do to achieve that t->head changes as ptr changes??

Upvotes: 0

Views: 932

Answers (1)

Rabbid76
Rabbid76

Reputation: 210928

You have to assign ptr back to t->head. Apart from this you have to allocate sizeof(struct node) for one node:

int main() {
    Table t = malloc(sizeof(table));
    t->head = NULL;

    tree_ptr ptr = malloc( sizeof(struct node) );
                              //         ^^^^      
    ptr->element = "one";
    ptr->left = NULL;
    ptr->right = NULL;

    t->head = ptr; // <-------

    printf("%s\n",t->head->element);
   return 0;
}

Note ptr = t->head only assigns the value of t->head to ptr. ptr = malloc(....) allocates dynamic memory and assigns the address of the memory to ptr and overwrite the value of t->head which was there before. But the address of the memory is never assigned to t->head. There is no magical linkage between ptr and t->head.

What you tried to do is somthing like this:

tree_ptr *ptr = &(t->head);
*ptr = malloc( sizeof(struct node) );
(*ptr)->element = "one";
(*ptr)->left = NULL;
(*ptr)->right = NULL

In this case ptris a pointer to t->head and *ptr = malloc( sizeof(struct node) ) assigns the address of the allocated memory where ptr refers to and that is t->head.

Upvotes: 2

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