enum.md

Enumerated types

r[type.enum]

r[type.enum.intro] An enumerated type is a nominal, heterogeneous disjoint union type, denoted by the name of an enum item. ¹

r[type.enum.declaration] An enum item declares both the type and a number of variants, each of which is independently named and has the syntax of a struct, tuple struct or unit-like struct.

r[type.enum.constructor] New instances of an enum can be constructed with a struct expression.

r[type.enum.name] Enum types cannot be denoted structurally as types, but must be denoted by named reference to an enum item.

Enum values and representation

r[type.enum.value]

r[type.enum.value.intro] An enum value corresponds to exactly one variant of the enum, and consists of the fields of that variant

Note

An enum with no variants therefore has no values.

r[type.enum.value.value-padding] A byte is a [padding][type.union.value.padding] byte of an enum if that byte is not part of the representation of the discriminant of the enum, and in each variant it either:

• Does not overlap with a field of the variant, or
• Overlaps with a padding byte in a field of that variant.

r[type.enum.value.repr] The representation of a value of an enum type includes the representation of each field of the variant at the appropriate offsets. When encoding a value of an enum type, each byte which is not use d to store a field of the variant or the discriminant is . In the case of a [repr(C)][layout.repr.c.adt] or a [primitive-repr][layout.repr.primitive.adt] enum, the discriminant of the variant is represented as though by the appropriate integer type stored at offset 0.

Note

Most repr(Rust) enums will also store a discriminant in the representation of the enum, but the exact placement or type of the discriminant is unspecified, as is the value that represents each variant.

Footnotes

1. The enum type is analogous to a data constructor declaration in Haskell, or a pick ADT in Limbo. ↩