20.6 Choosing Between Trait Objects and Enums

When dealing with a collection of related but distinct types that share common behavior, Rust offers two primary approaches: trait objects and enums.

  • Use Trait Objects (dyn Trait) when:

    • You need an open set of types: New types implementing the trait can be added later, even in downstream crates, without modifying the original code that uses the trait object. This is essential for extensibility (e.g., plugin systems).
    • The exact types involved are determined at runtime.
    • You need to store truly heterogeneous types (that only share the trait) in a collection.

  • Use Enums when:

    • You have a closed set of types: All possible variants are known at compile time and defined within the enum definition. Adding a new type requires modifying the enum definition.
    • You want compile-time exhaustiveness checking: match statements require handling all enum variants, preventing errors from unhandled cases.
    • Performance is a higher priority: Dispatching behavior based on enum variants (often via match) can be more efficient than trait object vtable lookups, potentially allowing the compiler to optimize the dispatch (e.g., using jump tables).
    • You need to access variant-specific data easily within match arms.

Guideline: If you can enumerate all possible types upfront and don’t need external extensibility, an enum is often simpler, safer (due to match exhaustiveness), and potentially faster. If you need the flexibility to add new types later without changing existing code, trait objects are the appropriate tool.