# Unions & Enums — C Programming

Source: https://www.geekswithgeeks.com/en/c/c-unions-enums

> A union shares memory; an enum names a set of integer constants.

## A union: one slot, many types

A `union`'s members all **share the same memory** — its size equals its largest member, and writing one member overwrites the others. Structs, by contrast, give each member its own space.

## Union in action

Only the most recently written member holds a valid value.

```c
union Data {
    int i;
    float f;
};

union Data d;
d.i = 10;
printf("%d\n", d.i);   // 10
d.f = 3.14f;
printf("%d\n", d.i);   // garbage now -- i was overwritten
```

Output:

```
10
```

## Enums name constants

An `enum` lists related integer constants, starting at `0` by default, making code more readable than raw numbers.

```c
enum Day { MON, TUE, WED, THU, FRI, SAT, SUN };

enum Day today = WED;
printf("%d\n", today);  // 2
```

Output:

```
2
```