Structs and Unions

After this lesson, you will be able to: Define structs, use typedef, nested structs, unions for memory-saving variants.

Structs let you compose multiple values into one unit. The closest thing C has to objects.

struct + typedef

Group related fields.

tsx

struct Point {
    double x;
    double y;
};

// Use
struct Point p1 = {1.0, 2.0};
struct Point p2 = { .x = 3.0, .y = 4.0 };   // designated initializer (C99+)
p1.x = 10.0;

// typedef saves the `struct` prefix
typedef struct {
    double x, y;
} Point;
Point p3 = { .x = 1.0, .y = 2.0 };

// Pointer to struct
Point *pp = &p3;
printf("%f\n", pp->x);    // shortcut for (*pp).x

Functions that take struct pointers

More efficient than passing by value for large structs.

tsx

typedef struct {
    char name[64];
    int age;
    double balance;
} User;

void promote(User *u) {
    u->balance *= 1.10;     // 10% bonus
}

// const for read-only access (best practice)
void print(const User *u) {
    printf("%s (%d): $%.2f\n", u->name, u->age, u->balance);
}

Unions

Variant types sharing memory.

tsx

// Tagged union, common pattern for sum types
typedef enum { TAG_INT, TAG_FLOAT, TAG_STR } ValueTag;

typedef struct {
    ValueTag tag;
    union {
        int i;
        double f;
        char *s;
    } data;
} Value;

Value v = { .tag = TAG_INT, .data.i = 42 };
switch (v.tag) {
    case TAG_INT:   printf("%d\n", v.data.i); break;
    case TAG_FLOAT: printf("%f\n", v.data.f); break;
    case TAG_STR:   printf("%s\n", v.data.s); break;
}

// Memory: union is sized to fit the LARGEST member.
// Reading a member you didn't write is type-punning, implementation-defined.

Common mistakes only experienced C devs avoid

Forgetting structs are passed by VALUE. Use pointers for big structs. Not initializing all fields. Garbage values in uninitialized fields. Use designated initialisers + `{0}` for the rest. Reading the wrong union member. Tagged unions are the safe pattern. Struct padding, `sizeof(struct)` may be larger than sum of fields due to alignment.

Quick Check

Why pass `const Point *p` instead of `Point p`?

Pick the senior answer.

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