Pointers Inside Structs
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Essential data structures such as stacks, linked lists, and trees can contain references to other objects of the same type to create connections or relationships between elements. Data structures will be covered later in this course.
Elements of such data structures are usually called nodes.
If you try to create a structure containing a field of the same type as the structure itself, you will receive an error:
struct Node {
int data;
struct Node next; // error: `Node` structure contains itself
};
In this case, an infinite recursion occurs when determining the size of such a structure.
This idea can be implemented using a pointer to a structure:
struct Node {
int data;
struct Node *next; // pointer to the `Node` structure
};
This code works because the compiler knows the exact size of a pointer — usually 4 or 8 bytes, depending on the system.
It doesn't try to calculate the size of the entire nested structure; it simply stores a reference (address) to it.
Let's see how this works in practice.
main.c
1234567891011121314151617181920212223242526#include <stdio.h> struct Node { char name[20]; struct Node* next; }; int main() { struct Node Kate = { "Kate" }; struct Node Tom = { "Tom" }; struct Node Bob = { "Bob" }; Kate.next = &Tom; // Kate --> Tom Tom.next = &Bob; // Tom --> Bob // set a pointer to the first structure in the chain struct Node* pointer = &Kate; while (pointer != NULL) { printf("Address: %p | Name: %s-> | Next address: %p\n", pointer, pointer->name, pointer->next); pointer = pointer->next; // go to the next object } return 0; }
struct Node kate = { "Kate" }; - the next field was not initialized explicitly in the struct initializer, it will be automatically initialized to a null pointer (NULL), since this is the standard value for pointers when they do not point to any object.
while (pointer != NULL) - the loop will run until the pointer points to NULL address.
In programming languages such as C and C++, a pointer that is NULL usually means the end of a list (or other data structure). Therefore, this loop will execute until pointer is NULL, which can be interpreted as reaching the end of the list or data structure.
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