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Sometimes a value could be an integer or a float, but only one at a time. A union stores exactly one of several possible types in a single slot.

You write union and list named fields, each with its own type. The value holds just one of those fields at any moment.

Unlike a struct, a union does not keep every field at once. It stores a single active field, and the others are not present.

You initialize a union by choosing one field with the dot syntax. That field becomes the active one for the value.

You read a union through the field you set. Access n.int and you get the integer you stored, just like reaching into a struct.

A union reserves room for its largest field and reuses that space. This makes it a compact way to hold one of several variants.

Assigning a new field makes that field the active one and the old value goes away. The union now represents a different shape.

If you read a field that is not active, that is a bug. In safe builds Zig detects the wrong access and triggers a panic.

A plain union does not remember which field is active. You must track that yourself, which is exactly the problem a tagged union solves.

A struct is a logical AND of its fields, all present together. A union is a logical OR: just one field lives at a time.

Use a union when a value naturally takes one of a few forms, like a token that is a number or a symbol, never both at once.

How many of a union's fields hold a value at the same time?

You met unions: one slot that holds a single field from several typed options. They share memory, so set and read only the active field. 🔀