[中文版本](/posts/c教程/zh-CN/04-数据结构/031-Structures and Nested Structures)

Real-world programs often deal with groups of related data, not just single variables. For example, a person has a name, an age, and an address. Instead of juggling separate variables, you can combine them into a single structure usingstruct.

struct is one of the most powerful features in C, it lets you define your own data types that group information logically and efficiently.

What Is a Structure?

A structure is a user-defined type that holds variables of different kinds under one name.

struct Person {
    char name[50];
    int age;
    float height;
};

This declares a template for aPerson object. It doesn’t create actual data yet, just the blueprint.

Declaring and Using Structures

You can now create variables of this new type:

#include <stdio.h>
struct Person {
    char name[50];
    int age;
    float height;
};
int main(void) {
    struct Person p1 = {"Alice", 25, 1.65f};
    printf("Name: %s\n", p1.name);
    printf("Age: %d\n", p1.age);
    printf("Height: %.2f m\n", p1.height);
    return 0;
}

Output:

Name: Alice
Age: 25
Height: 1.65 m

Accessing Members

Use the dot operator. to access fields of a structure variable:

p1.age = 26;
printf("Updated age: %d\n", p1.age);

If you have a pointer to a structure, use the arrow operator->:

struct Person *ptr = &p1;
printf("Pointer access: %s is %d years old.\n", ptr->name, ptr->age);

Output:

Pointer access: Alice is 26 years old.

Initializing and Copying Structures

You can initialize a struct directly:

struct Person p2 = {.name = "Bob", .age = 30, .height = 1.75f};

Structures can be assigned and copied by value:

struct Person copy = p2;
printf("Copy: %s (%d)\n", copy.name, copy.age);

This performs a shallow copy, all fields are copied, but if any contain pointers, they’ll still refer to the same memory (you’ll learn how to make deep copies later).

Nested Structures

Structures can contain other structures. This helps you organize complex data clearly.

Example:

#include <stdio.h>
struct Date {
    int day;
    int month;
    int year;
};
struct Student {
    char name[50];
    int id;
    struct Date birthdate; // nested structure
};
int main(void) {
    struct Student s = {
        .name = "Carol",
        .id = 1234,
        .birthdate = {15, 8, 2003}
    };
    printf("%s (ID %d) was born on %02d/%02d/%04d\n",
           s.name, s.id,
           s.birthdate.day, s.birthdate.month, s.birthdate.year);
    return 0;
}

Output:

Carol (ID 1234) was born on 15/08/2003

You access nested fields with the dot operator:

s.birthdate.year = 2004;

Structures and Functions

You can pass structs to functions by value or by pointer:

void print_person(struct Person p);
void update_age(struct Person *p, int new_age);

Example:

void update_age(struct Person *p, int new_age) {
    p->age = new_age;
}

Passing a pointer is more efficient, especially for large structures.

Tiny Code

Here’s a full example combining everything above:

#include <stdio.h>
struct Date {
    int day;
    int month;
    int year;
};
struct Person {
    char name[50];
    int age;
    float height;
    struct Date birthdate;
};
void print_person(const struct Person *p) {
    printf("%s, %d years old, born on %02d/%02d/%04d, height %.2fm\n",
           p->name, p->age,
           p->birthdate.day, p->birthdate.month, p->birthdate.year,
           p->height);
}
int main(void) {
    struct Person person = {"Alice", 25, 1.68f, {1, 2, 1999}};
    print_person(&person);
    person.age++;
    person.birthdate.year++;
    printf("After update:\n");
    print_person(&person);
    return 0;
}

Output:

Alice, 25 years old, born on 01/02/1999, height 1.68m
After update:
Alice, 26 years old, born on 01/02/2000, height 1.68m

Why It Matters

Structures let you:

• Combine related data into logical units.
• Model real-world entities directly in code.
• Pass data efficiently between functions.
• Build higher-level data abstractions like lists, trees, or objects.

They’re the foundation of all complex C systems, files, network packets, kernel data, even database rows are built on top ofstruct.

Try It Yourself

1. Create aBook structure withtitle,author, andyear.
2. Write a function to print all details of aBook.
3. Create a nested structureLibrary that contains multiple books.
4. Access a nested field (e.g., the title of the first book).
5. Modify theLibrary through a pointer using the-> operator.

Structures are how C lets you model the world, compact, explicit, and fast. Next, you’ll learn about unions and how C lets different data types share the same memory space efficiently.