10.1 Understanding Enums
An enum in Rust defines a type that can hold one of several named variants. This allows you to write clearer, safer code by constraining values to a predefined set. Unlike simple integer constants, Rust enums integrate directly with the type system, enabling structured and type-checked variant handling. They also extend beyond simple enumerations, as variants can store additional data, making Rust enums more expressive than those in many other languages.
10.1.1 Origin of the Term ‘Enum’
Enum is short for enumeration, meaning to list items one by one. In programming, this term describes a type made up of several named values. These named values are called variants, each representing one of the possible states that a variable of that enum type can hold.
10.1.2 Rust’s Enums vs. C’s Enums and Unions
In C, an enum is essentially a named collection of integer constants. While that helps readability, it doesn’t stop you from mixing those integers with other, unrelated values. C’s unions allow different data types to share the same memory space, but the programmer must track which type is currently stored.
Rust merges these ideas. A Rust enum lists its variants, and each variant can optionally hold additional data. This design offers several benefits:
- Type Safety: Rust enums are true types, preventing invalid integer values.
- Pattern Matching: Rust’s
matchand related constructs help you safely handle all variants. - Data Association: Variants can carry data, from basic types to complex structures or even nested enums.