20.9 Serialization and Trait Objects

Serializing (saving) and deserializing (loading) data structures is a common requirement. However, directly serializing Rust trait objects (e.g., Box<dyn MyTrait>) using popular libraries like Serde is generally not straightforward or directly supported.

The fundamental issue is that a trait object is inherently tied to runtime information (the vtable pointer) which identifies the concrete type’s method implementations. This runtime information cannot be reliably serialized and deserialized. When deserializing raw data, there’s no inherent information to reconstruct the correct vtable pointer or even determine which concrete type the data represents.

Common strategies to handle serialization with polymorphic types include:

  1. Using Enums: If working with a closed set of types, define an enum where each variant wraps one of the possible concrete types. Enums can typically be serialized easily with Serde, assuming the contained types are serializable. This is often the simplest solution when applicable.
  2. Type Tagging and Manual Dispatch: Store an explicit type identifier (e.g., a string name or an enum discriminant) alongside the serialized data for the object. During deserialization, read the identifier first, then use it to determine which concrete type to deserialize the remaining data into. Libraries like typetag can help automate this process for types implementing a specific trait.
  3. Avoiding Trait Objects at Serialization Boundaries: Convert trait objects into a serializable representation (perhaps a concrete enum or a struct with a type tag) before serialization. Upon deserialization, reconstruct the trait objects if needed for runtime logic.

There is no built-in, transparent mechanism in Rust to serialize and deserialize arbitrary Box<dyn Trait> instances directly. Careful design is required at the serialization layer.