| r[type.union] |
| # Union types |
| |
| r[type.union.intro] |
| A *union type* is a nominal, heterogeneous C-like union, denoted by the name of a [`union` item][item]. |
| |
| r[type.union.access] |
| Unions have no notion of an "active field". Instead, every union access transmutes parts of the content of the union to the type of the accessed field. |
| |
| r[type.union.safety] |
| Since transmutes can cause unexpected or undefined behaviour, `unsafe` is required to read from a union field. |
| |
| r[type.union.constraint] |
| Union field types are also restricted to a subset of types which ensures that they never need dropping. See the [item] documentation for further details. |
| |
| r[type.union.layout] |
| The memory layout of a `union` is undefined by default (in particular, fields do *not* have to be at offset 0), but the `#[repr(...)]` attribute can be used to fix a layout. |
| |
| [`Copy`]: ../special-types-and-traits.md#copy |
| [item]: ../items/unions.md |
