Unions & Enumerations

Two handy tools: union and enum

This lesson covers two smaller but useful C features: the union, which lets several members share one memory space, and the enum, which gives readable names to numbers. Both are simple once you see them in action.

What is a union?

A union looks like a structure, but with one big difference: all its members share the same memory. Only one member holds a real value at a time — writing to one overwrites the others.

#include <stdio.h>
union Value {
    int i;
    float f;
};
int main() {
    union Value v;
    v.i = 10;
    printf("i = %d\n", v.i);
    v.f = 3.14;              // overwrites the same memory
    printf("f = %.2f\n", v.f);
    return 0;
}

How a union shares memory

Because members overlap, a union is only as big as its largest member — not the sum.

union Value { int i; double d; char c; };
printf("%lu\n", sizeof(union Value));   // size of the largest (double)

The union takes 8 bytes — the size of the double — even though it has three members, because they all live in the same space.

When unions are useful

• Storing a value that could be one of several types, but only one at a time.
• Saving memory in tight, embedded or low-level programs.
• Interpreting the same bytes in two ways (advanced, low-level use).

Enumerations (enum)

An enum gives names to a list of related integer constants, so your code reads like words instead of numbers.

#include <stdio.h>
enum Day { MON, TUE, WED, THU, FRI };   // 0,1,2,3,4
int main() {
    enum Day today = WED;
    printf("Day number: %d\n", today);   // 2
    if (today == WED) printf("Midweek!\n");
    return 0;
}

By default the first name is 0 and each next one is one higher. You can also set values explicitly: enum Status { OK = 1, FAIL = 5 };.

Why enum beats plain numbers

Common mistakes

• Reading a union member you did not last write — you get garbage.
• Expecting a union to hold all members at once (that is a structure).
• Assuming enum values always start at 1 — they start at 0 by default.
• Forgetting that an enum is really just an int underneath.

Summary

• A union's members share one memory space; only one is valid at a time.
• A union's size equals its largest member.
• Use unions when a value is one of several types, one at a time.
• An enum gives readable names to integer constants, starting at 0.
• enum makes code clearer than bare numbers; see structure vs union for the full struct/union comparison.