Nested structures in C refer to placing one structure inside another, allowing you to group complex and related information in a hierarchical way. This is useful for representing entities that have multiple levels of attributes.

Definition of Nested Structures

A nested structure is defined by including one structure as a member of another structure.

For example, you may have an Address structure and a Person structure. The Address structure can be nested inside the Person structure:

struct Address {
    char street[50];
    char city[50];
    int zipCode;
};

struct Person {
    char name[50];
    int age;
    struct Address address;  // Nested structure
};

In this example:

• Address is a structure that has street, city, and zipCode as members.
• Person is another structure that has name, age, and an address member of type struct Address. The address is a nested structure.

Declaring Variables of Nested Structures

You can declare a variable of the Person structure type and initialize it like so:

struct Person person1;

strcpy(person1.name, "Alice");
person1.age = 25;
strcpy(person1.address.street, "123 Main St");
strcpy(person1.address.city, "New York");
person1.address.zipCode = 10001;

Accessing Members of a Nested Structure

To access members of a nested structure, you use the dot (.) operator multiple times:

printf("Name: %s\n", person1.name);           // Accessing 'name' of 'person1'
printf("Age: %d\n", person1.age);             // Accessing 'age' of 'person1'
printf("Street: %s\n", person1.address.street);  // Accessing 'street' of 'address'
printf("City: %s\n", person1.address.city);      // Accessing 'city' of 'address'
printf("Zip Code: %d\n", person1.address.zipCode); // Accessing 'zipCode' of 'address'

Example Program with Nested Structures

Here is a complete example that defines, initializes, and accesses members of nested structures:

#include <stdio.h>
#include <string.h>

struct Address {
    char street[50];
    char city[50];
    int zipCode;
};

struct Person {
    char name[50];
    int age;
    struct Address address;  // Nested structure
};

int main() {
    struct Person person1;

    // Assigning values to members
    strcpy(person1.name, "Alice");
    person1.age = 25;
    strcpy(person1.address.street, "123 Main St");
    strcpy(person1.address.city, "New York");
    person1.address.zipCode = 10001;

    // Accessing and printing nested structure members
    printf("Name: %s\n", person1.name);
    printf("Age: %d\n", person1.age);
    printf("Street: %s\n", person1.address.street);
    printf("City: %s\n", person1.address.city);
    printf("Zip Code: %d\n", person1.address.zipCode);

    return 0;
}

Memory Layout of Nested Structures

When you define a nested structure, memory is allocated for all the members, including the nested structure. The Person structure in the above example contains:

• name (50 bytes),
• age (4 bytes), and
• an address (the size of the Address structure).

The compiler will allocate memory for all these members and may add padding for alignment purposes.

Nested Structure Initialization

You can also initialize nested structures at the time of declaration:

struct Person person1 = {
    "Alice", 
    25, 
    {"123 Main St", "New York", 10001}
};

This approach directly initializes all the members, including those of the nested structure.

Passing Nested Structures to Functions

You can pass nested structures to functions either by value or by reference.

Passing by Value

void displayPerson(struct Person p) {
    printf("Name: %s, City: %s\n", p.name, p.address.city);
}

int main() {
    struct Person person1 = {"Alice", 25, {"123 Main St", "New York", 10001}};
    displayPerson(person1);
    return 0;
}

Passing by Reference

Passing by reference is more efficient because the structure is not copied:

void updateZipCode(struct Person *p, int newZip) {
    p->address.zipCode = newZip;  // Using arrow operator to access nested structure member
}

int main() {
    struct Person person1 = {"Alice", 25, {"123 Main St", "New York", 10001}};
    updateZipCode(&person1, 10118);
    printf("Updated Zip Code: %d\n", person1.address.zipCode);
    return 0;
}

Summary

• Nested structures are structures that contain other structures as members, allowing you to represent hierarchical data.
• Use the dot operator (.) to access members of a nested structure.
• Nested structures help organize data logically and are particularly useful for representing complex entities that have multiple attributes or sub-components.
• When working with nested structures, consider passing them by reference to functions for better performance, especially if they are large.

Nested structures are a powerful feature that helps in modeling real-world entities that involve different levels of information.