C
Nested structs
Learn nested structs and composition.
Nested structs
A struct can contain another struct type. This is called a nested struct. It's the natural way to break data into meaningful parts.
Example: struct Date { int day, month, year; }; and struct Event { char title[64]; struct Date when; };. Here the when field is a struct Date. Access chains with dots: e.when.day. The inner struct must be defined first; that's how the compiler knows its size. This pattern provides composition when modeling data.
Nested struct example
Nested structs are useful when modeling real-world data. For example, a person's address can be a separate struct.
Example: struct Address { char city[32]; int zip; }; and struct Person { char name[32]; struct Address addr; };. City access: p.addr.city. This structure embeds the address inside the person; it groups it in one struct. If the address is used elsewhere, the same struct Address definition can be reused. Modularity improves and code duplication drops.
Exercise Create an event with a date
Define struct Date with int day, month, year;. Define struct Event with char title[64]; and struct Date when;. In main, write struct Event e;; assign e.when.day = 15;, e.when.month = 3;, e.when.year = 2025;. Print with printf("%d.%d.%d\n", e.when.day, e.when.month, e.when.year);.
Show example solution
#include <stdio.h>
struct Date {
int day, month, year;
};
struct Event {
char title[64];
struct Date when;
};
int main(void) {
struct Event e;
e.when.day = 15;
e.when.month = 3;
e.when.year = 2025;
printf("%d.%d.%d\n", e.when.day, e.when.month, e.when.year);
return 0;
}
Test yourself
-
How to access day inside when inside e?
- e.when->day
- e->when.day
- e.when.day
- when.e.day
Show answer
e.when.day — Nested struct access chains with dots.
-
Match the access forms
- e.when.day
- Nested field access
- e.when
- Inner struct variable
- struct Date
- Inner struct type
- e.title
- Outer struct field
-
Assign a nested field
struct Event e;
e____.day = 15;
Show answer
struct Event e;
e.when.day = 15;
Next lesson: Array of structs
