Rust for C-Programmers

20.6 When to Use Trait Objects vs. Enums

A common question arises when deciding between using trait objects or using enums to handle multiple data types. Here are some guidelines:

• Trait Objects

  • Open-Ended Sets of Types: When you anticipate adding new implementations in the future or have a wide variety of unknown types, trait objects allow you to extend functionality without modifying existing code.
  • Runtime Polymorphism: If you need the flexibility to handle objects whose specific types aren’t known until runtime, trait objects can help.
  • Interface-Oriented Design: If you want to work with different structs through a consistent interface (e.g., all implement a certain trait), trait objects offer a dynamic solution.

• Enums

  • Closed Set of Variants: If you know all the possible data types in advance and don’t expect to add more, enums are efficient. Each variant is well-defined at compile time.
  • Compile-Time Guarantees: Enums make it explicit that only certain variants are valid, and pattern matching can exhaustively check them.
  • Better Performance: Enums are resolved at compile time and often compile into a straightforward data structure without vtable overhead.

In practice, if you can define all the types upfront (like Dog, Cat, Bird, etc.), prefer an enum. But if you expect your program to accept user-defined Animal types—or load additional implementations from external libraries—trait objects might be more flexible.