20.6 When to Use Trait Objects vs. Enums

A common question is whether to use trait objects or enums for handling multiple data types:

  • Trait Objects

    • Open-Ended Sets of Types: If new implementations may appear in the future (or load at runtime), trait objects enable you to extend functionality without modifying existing code.
    • Runtime Polymorphism: When the exact types are not known until runtime, trait objects let you handle them uniformly.
    • Interface-Oriented Design: If your design prioritizes a shared interface (e.g., an Animal trait), dynamic dispatch can be more convenient.

  • Enums

    • Closed Set of Variants: If all variants are known ahead of time, enums are typically more efficient.
    • Compile-Time Guarantees: Enums let you match exhaustively, ensuring you handle every variant.
    • Better Performance: Because the compiler knows all possible variants, it can optimize more aggressively than with dynamic dispatch.

If you know every possible type (e.g., Dog, Cat, Bird, etc.), enums often outperform trait objects. But if your application might add or load new types in the future, trait objects may better fit your needs.