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intermediate18 min

Structs and typedef

Grouping related fields with struct, and giving struct types a clean name with typedef.

What you'll learn

  • Define a struct type grouping several related fields
  • Use typedef to give a struct type a shorter, reusable name
  • Predict that assigning one struct variable to another copies all its fields by value

Explanation

A struct groups several related fields, possibly of different types, into a single custom type: struct Person { char name[20]; int age; };. You access a field with the dot operator: somePerson.age.

Writing struct Person every time you refer to the type gets verbose, so C offers typedef to give it a shorter alias. Combining a struct definition with typedef, typedef struct { char name[20]; int age; } Person;, lets you write just Person afterward instead of struct Person everywhere -- this is an extremely common pattern in real C codebases.

Like other C values, a struct is copied by value on assignment: Person carol = bob; copies every field of bob into a brand-new, independent carol -- changing carol.age afterward has no effect on bob.age at all. This is the same value semantics you've already seen for plain variables and function parameters, just applied to a type with multiple fields at once. Passing a struct to a function (by value, without a pointer) copies the whole thing the same way, which is worth remembering for larger structs where that copy has a real performance cost.

Guided lab

Predict: A typedef'd struct and value-copy assignment

CNot executed
This lab does not run in your browser or on VisaSparkSchools's servers. Read the code, predict what it does, then reveal the real expected output.

Read this program and predict exactly what it prints.

#include <stdio.h>

typedef struct {
    char name[20];
    int age;
} Person;

void printPerson(Person p) {
    printf("%s is %d years old\n", p.name, p.age);
}

int main(void) {
    Person bob = {"Bob", 40};
    printPerson(bob);

    Person carol = bob;
    carol.age = 41;

    printf("bob age: %d, carol age: %d\n", bob.age, carol.age);

    return 0;
}

Stuck? Get a hint.

Common mistakes

  • Forgetting to use the dot operator to access a struct field, e.g. writing `person.age` incorrectly or omitting it entirely.
  • Assuming `Person carol = bob;` makes carol and bob share the same underlying data, instead of realizing it copies every field independently.
  • Repeating `struct Person` everywhere instead of using typedef to introduce a shorter alias, which is idiomatic in real C code.

Knowledge check

Knowledge check

1. What does `typedef struct { ... } Person;` let you do afterward?
2. After `Person carol = bob;` followed by `carol.age = 41;`, what happens to bob.age?
3. How do you access a field of a struct variable?

Takeaway

typedef gives a struct a shorter, reusable name, and assigning one struct variable to another copies every field independently, just like plain variables.

Summary

struct groups related fields into one type, accessed with the dot operator; typedef gives it a shorter alias; struct assignment and pass-by-value both copy all fields.

References

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