Unions

Unions

A union looks like a struct, but all its members share the same memory. The size of a union equals the size of its largest member. Only one member holds a valid value at any time.

Syntax

union Number {
    int i;
    char c;
};

Both i and c occupy the same bytes in memory. Writing to one overwrites the other.

Unions vs Structs

In a struct, each member has its own storage. In a union, all members overlap:

struct S { int a; int b; };  // size = 8 (two ints)
union  U { int a; int b; };  // size = 4 (one int, shared)

Basic Usage

union Number n;
n.i = 42;
printf("%d\n", n.i);  // 42

n.c = 'A';
// n.i is now invalid -- only n.c is meaningful
printf("%c\n", n.c);  // A

Tagged Unions

The most important union pattern is the tagged union (also called a discriminated union or variant). You pair a union with an enum tag that tracks which member is active:

enum ValueType { TYPE_INT, TYPE_CHAR };

struct TaggedValue {
    enum ValueType type;
    union {
        int i;
        char c;
    } data;
};

This pattern is the foundation of variant types in many languages. You check the tag before accessing the union:

void print_value(struct TaggedValue v) {
    if (v.type == TYPE_INT) {
        printf("%d\n", v.data.i);
    } else {
        printf("%c\n", v.data.c);
    }
}

Your Task

Define an enum ShapeType with values CIRCLE and RECTANGLE. Define a struct Shape that contains a type field (enum ShapeType) and a union data with int radius for circles and a nested struct with int width and int height for rectangles. Write int compute_area(struct Shape s) that returns 3 * radius * radius for circles (using 3 as an approximation for pi) and width * height for rectangles. Print the area of a circle with radius 5 and a rectangle 4x6.