docs/guide/rust/polymorphism.md

title: Trait Object Serialization sidebar_position: 6 id: polymorphism license: | Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

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Apache Fory™ supports polymorphic serialization through trait objects, enabling dynamic dispatch and type flexibility.

Supported Trait Object Types

Box<dyn Trait> - Owned trait objects
Rc<dyn Trait> - Reference-counted trait objects
Arc<dyn Trait> - Thread-safe reference-counted trait objects
Vec<Box<dyn Trait>>, HashMap<K, Box<dyn Trait>> - Collections of trait objects

Basic Trait Object Serialization

use fory::{Fory, register_trait_type};
use fory::Serializer;
use fory::ForyObject;

trait Animal: Serializer {
    fn speak(&self) -> String;
    fn name(&self) -> &str;
}

#[derive(ForyObject)]
struct Dog { name: String, breed: String }

impl Animal for Dog {
    fn speak(&self) -> String { "Woof!".to_string() }
    fn name(&self) -> &str { &self.name }
}

#[derive(ForyObject)]
struct Cat { name: String, color: String }

impl Animal for Cat {
    fn speak(&self) -> String { "Meow!".to_string() }
    fn name(&self) -> &str { &self.name }
}

// Register trait implementations
register_trait_type!(Animal, Dog, Cat);

#[derive(ForyObject)]
struct Zoo {
    star_animal: Box<dyn Animal>,
}

let mut fory = Fory::default().compatible(true);
fory.register::<Dog>(100);
fory.register::<Cat>(101);
fory.register::<Zoo>(102);

let zoo = Zoo {
    star_animal: Box::new(Dog {
        name: "Buddy".to_string(),
        breed: "Labrador".to_string(),
    }),
};

let bytes = fory.serialize(&zoo);
let decoded: Zoo = fory.deserialize(&bytes)?;

assert_eq!(decoded.star_animal.name(), "Buddy");
assert_eq!(decoded.star_animal.speak(), "Woof!");

Serializing dyn Any Trait Objects

Apache Fory™ supports serializing Rc<dyn Any> and Arc<dyn Any> for runtime type dispatch:

Key points:

Works with any type that implements Serializer
Requires downcasting after deserialization to access the concrete type
Type information is preserved during serialization
Useful for plugin systems and dynamic type handling

use std::rc::Rc;
use std::any::Any;

let dog_rc: Rc<dyn Animal> = Rc::new(Dog {
    name: "Rex".to_string(),
    breed: "Golden".to_string()
});

// Convert to Rc<dyn Any> for serialization
let dog_any: Rc<dyn Any> = dog_rc.clone();

// Serialize the Any wrapper
let bytes = fory.serialize(&dog_any);
let decoded: Rc<dyn Any> = fory.deserialize(&bytes)?;

// Downcast back to the concrete type
let unwrapped = decoded.downcast_ref::<Dog>().unwrap();
assert_eq!(unwrapped.name, "Rex");

For thread-safe scenarios, use Arc<dyn Any>:

use std::sync::Arc;
use std::any::Any;

let dog_arc: Arc<dyn Animal> = Arc::new(Dog {
    name: "Buddy".to_string(),
    breed: "Labrador".to_string()
});

// Convert to Arc<dyn Any>
let dog_any: Arc<dyn Any> = dog_arc.clone();

let bytes = fory.serialize(&dog_any);
let decoded: Arc<dyn Any> = fory.deserialize(&bytes)?;

// Downcast to concrete type
let unwrapped = decoded.downcast_ref::<Dog>().unwrap();
assert_eq!(unwrapped.name, "Buddy");

Rc/Arc-Based Trait Objects in Structs

For fields with Rc<dyn Trait> or Arc<dyn Trait>, Fory automatically handles the conversion:

use std::sync::Arc;
use std::rc::Rc;
use std::collections::HashMap;

#[derive(ForyObject)]
struct AnimalShelter {
    animals_rc: Vec<Rc<dyn Animal>>,
    animals_arc: Vec<Arc<dyn Animal>>,
    registry: HashMap<String, Arc<dyn Animal>>,
}

let mut fory = Fory::default().compatible(true);
fory.register::<Dog>(100);
fory.register::<Cat>(101);
fory.register::<AnimalShelter>(102);

let shelter = AnimalShelter {
    animals_rc: vec![
        Rc::new(Dog { name: "Rex".to_string(), breed: "Golden".to_string() }),
        Rc::new(Cat { name: "Mittens".to_string(), color: "Gray".to_string() }),
    ],
    animals_arc: vec![
        Arc::new(Dog { name: "Buddy".to_string(), breed: "Labrador".to_string() }),
    ],
    registry: HashMap::from([
        ("pet1".to_string(), Arc::new(Dog {
            name: "Max".to_string(),
            breed: "Shepherd".to_string()
        }) as Arc<dyn Animal>),
    ]),
};

let bytes = fory.serialize(&shelter);
let decoded: AnimalShelter = fory.deserialize(&bytes)?;

assert_eq!(decoded.animals_rc[0].name(), "Rex");
assert_eq!(decoded.animals_arc[0].speak(), "Woof!");

Standalone Trait Object Serialization

Due to Rust's orphan rule, Rc<dyn Trait> and Arc<dyn Trait> cannot implement Serializer directly. For standalone serialization (not inside struct fields), the register_trait_type! macro generates wrapper types.

Note: If you don't want to use wrapper types, you can serialize as Rc<dyn Any> or Arc<dyn Any> instead (see the dyn Any section above).

The register_trait_type! macro generates AnimalRc and AnimalArc wrapper types:

// For Rc<dyn Trait>
let dog_rc: Rc<dyn Animal> = Rc::new(Dog {
    name: "Rex".to_string(),
    breed: "Golden".to_string()
});
let wrapper = AnimalRc::from(dog_rc);

let bytes = fory.serialize(&wrapper);
let decoded: AnimalRc = fory.deserialize(&bytes)?;

// Unwrap back to Rc<dyn Animal>
let unwrapped: Rc<dyn Animal> = decoded.unwrap();
assert_eq!(unwrapped.name(), "Rex");

// For Arc<dyn Trait>
let dog_arc: Arc<dyn Animal> = Arc::new(Dog {
    name: "Buddy".to_string(),
    breed: "Labrador".to_string()
});
let wrapper = AnimalArc::from(dog_arc);

let bytes = fory.serialize(&wrapper);
let decoded: AnimalArc = fory.deserialize(&bytes)?;

let unwrapped: Arc<dyn Animal> = decoded.unwrap();
assert_eq!(unwrapped.name(), "Buddy");

Best Practices