Rust 1.94.0
4a4ef493e

# Trait Objects

There is a new edition of the book and this is an old link.

> Trait objects combine the data made up of the pointer to a concrete object with the behavior of the methods defined in the trait. A trait defines behavior that we need in a given situation. We can then use a trait as a trait object in places where we would use a concrete type or a generic type.

ⓘ

    pub struct InputBox {
        pub label: String,
    }

    impl Draw for InputBox {
        fn draw(&self) {
            // Code to actually draw an input box
        }
    }

    pub struct Button {
        pub label: String,
    }

    impl Draw for Button {
        fn draw(&self) {
            // Code to actually draw a button
        }
    }

    pub struct Screen<T: Draw> {
        pub components: Vec<T>,
    }

    impl<T> Screen<T>
        where T: Draw {
        pub fn run(&self) {
            for component in self.components.iter() {
                component.draw();
            }
        }
    }

    fn main() {
        let screen = Screen {
            components: vec![
                Box::new(InputBox {
                    label: String::from("OK"),
                }),
                Box::new(Button {
                    label: String::from("OK"),
                }),
            ],
        };

        screen.run();
    }

Here are the relevant sections in the new and old books:

- in the current edition: Ch 18.02 — Trait Objects
- In the first edition: Ch 3.22 — Trait Objects