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Owned Trait Objects

We previously saw how trait objects can be used with references, e.g &dyn Pet. However, we can also use trait objects with smart pointers like Box to create an owned trait object: Box<dyn Pet>.

// Copyright 2024 Google LLC
// SPDX-License-Identifier: Apache-2.0

struct Dog {
    name: String,
    age: i8,
}
struct Cat {
    lives: i8,
}

trait Pet {
    fn talk(&self) -> String;
}

impl Pet for Dog {
    fn talk(&self) -> String {
        format!("Woof, my name is {}!", self.name)
    }
}

impl Pet for Cat {
    fn talk(&self) -> String {
        String::from("Miau!")
    }
}

fn main() {
    let pets: Vec<Box<dyn Pet>> = vec![
        Box::new(Cat { lives: 9 }),
        Box::new(Dog { name: String::from("Fido"), age: 5 }),
    ];
    for pet in pets {
        println!("Hello, who are you? {}", pet.talk());
    }
}

Memory layout after allocating pets:

<Dog as Pet>::talk<Cat as Pet>::talkStackHeapFidoptrlives9len2capacity2data:name,4,4age5vtablevtablepets: Vec<Box<dyn Pet>>data: CatDogProgram text
This slide should take about 10 minutes.
  • Types that implement a given trait may be of different sizes. This makes it impossible to have things like Vec<dyn Pet> in the example above.
  • dyn Pet is a way to tell the compiler about a dynamically sized type that implements Pet.
  • In the example, pets is allocated on the stack and the vector data is on the heap. The two vector elements are fat pointers:
    • A fat pointer is a double-width pointer. It has two components: a pointer to the actual object and a pointer to the virtual method table (vtable) for the Pet implementation of that particular object.
    • The data for the Dog named Fido is the name and age fields. The Cat has a lives field.
  • Compare these outputs in the above example: 
    // Copyright 2024 Google LLC
// SPDX-License-Identifier: Apache-2.0

println!("{} {}", std::mem::size_of::<Dog>(), std::mem::size_of::<Cat>());
println!("{} {}", std::mem::size_of::<&Dog>(), std::mem::size_of::<&Cat>());
println!("{}", std::mem::size_of::<&dyn Pet>());
println!("{}", std::mem::size_of::<Box<dyn Pet>>());