Trait Objects for Using Values of Different Types - Trait Objects for Using Values of Different Types - The Rust Programming Language

## Using Trait Objects that Allow for Values of Different Types

In Chapter 8, we mentioned that one limitation of vectors is that they can only store elements of one type. We created a workaround in Listing 8-10 where we defined a `SpreadsheetCell` enum that had variants to hold integers, floats, and text. This meant we could store different types of data in each cell and still have a vector that represented a row of cells. This is a perfectly good solution when our interchangeable items are a fixed set of types that we know when our code gets compiled.

Sometimes, however, we want the user of our library to be able to extend the set of types that are valid in a particular situation. To show how we might achieve this, we’ll create an example Graphical User Interface tool that iterates through a list of items, calling a `draw` method on each one to drawn it to the screen; a common technique for GUI tools. We’re going to create a library crate containing the structure of a GUI library called `rust_gui`. This crate might include some types for people to use, such as `Button` or `TextField`. On top of these, users of `rust_gui` will want to create their own types that can be drawn on the screen: for instance, one programmer might add an `Image`, another might add a `SelectBox`.

We won’t implement a fully-fledged GUI library for this example, but will show how the pieces would fit together. At the time of writing the library, we can’t know and define all the types other programmers will want to create. What we do know is that `rust_gui` needs to keep track of a bunch of values that are of different types, and it needs to be able to call a `draw` method on each of these differently-typed values. It doesn’t need to know exactly what will happen when we call the `draw` method, just that the value will have that method available for us to call.

To do this in a language with inheritance, we might define a class named `Component` that has a method named `draw` on it. The other classes like `Button`, `Image`, and `SelectBox` would inherit from `Component` and thus inherit the `draw` method. They could each override the `draw` method to define their custom behavior, but the framework could treat all of the types as if they were `Component` instances and call `draw` on them. But Rust doesn’t have inheritance, so we need another way.

### Defining a Trait for Common Behavior

To implement the behavior we want `rust_gui` to have, we’ll define a trait named `Draw` that will have one method named `draw`. Then we can define a vector that takes a trait object. A trait object points to an instance of a type that implements the trait we specify. We create a trait object by specifying some sort of pointer, such as a `&` reference or a `Box<T>` smart pointer, and then specifying the relevant trait (we’ll talk about the reason trait objects have to use a pointer in Chapter 19 in the section on Dynamically Sized Types). We can use trait objects in place of a generic or concrete type. Wherever we use a trait object, Rust’s type system will ensure at compile-time that any value used in that context will implement the trait object’s trait. This way we don’t need to know all the possible types at compile time.

We’ve mentioned that in Rust we refrain from calling structs and enums “objects” to distinguish them from other languages’ objects. In a struct or enum, the data in the struct fields and the behavior in `impl` blocks is separated, whereas in other languages the data and behavior combined into one concept is often labeled an object. Trait objects, though, are more like objects in other languages, in the sense that they combine both data and behavior. However, trait objects differ from traditional objects in that we can’t add data to a trait object. Trait objects aren’t as generally useful as objects in other languages: their specific purpose is to allow abstraction across common behavior.

Listing 17-3 shows how to define a trait named `Draw` with one method named `draw`:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    pub trait Draw {
        fn draw(&self);
    }
    #}

Listing 17-3: Definition of the `Draw` trait

This should look familiar from our discussions on how to define traits in Chapter 10. Next comes something new: Listing 17-4 defines a struct named `Screen` that holds a vector named `components`. This vector is of type `Box<Draw>`, which is a trait object: it’s a stand-in for any type inside a `Box` that implements the `Draw` trait.

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub trait Draw {
    #     fn draw(&self);
    # }
    #
    pub struct Screen {
        pub components: Vec<Box<Draw>>,
    }
    #}

Listing 17-4: Definition of the `Screen` struct with a `components` field holding a vector of trait objects that implement the `Draw` trait

On the `Screen` struct, we’ll define a method named `run` that will call the `draw` method on each of its `components`, as shown in Listing 17-5:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub trait Draw {
    #     fn draw(&self);
    # }
    #
    # pub struct Screen {
    #     pub components: Vec<Box<Draw>>,
    # }
    #
    impl Screen {
        pub fn run(&self) {
            for component in self.components.iter() {
                component.draw();
            }
        }
    }
    #}

Listing 17-5: Implementing a `run` method on `Screen` that calls the `draw` method on each component

This works differently to defining a struct that uses a generic type parameter with trait bounds. A generic type parameter can only be substituted with one concrete type at a time, while trait objects allow for multiple concrete types to fill in for the trait object at runtime. For example, we could have defined the `Screen` struct using a generic type and a trait bound as in Listing 17-6:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub trait Draw {
    #     fn draw(&self);
    # }
    #
    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();
            }
        }
    }
    #}

Listing 17-6: An alternate implementation of the `Screen` struct and its `run` method using generics and trait bounds

This restricts us to a `Screen` instance that has a list of components all of type `Button` or all of type `TextField`. If you’ll only ever have homogeneous collections, using generics and trait bounds is preferable since the definitions will be monomorphized at compile time to use the concrete types.

With the the method using trait objects, on the other hand, one `Screen` instance can hold a `Vec` that contains a `Box<Button>` as well as a `Box<TextField>`. Let’s see how that works, and then talk about the runtime performance implications.

### Implementing the Trait

Now we’ll add some types that implement the `Draw` trait. We’re going to provide the `Button` type. Again, actually implementing a GUI library is out of scope of this book, so the `draw` method won’t have any useful implementation in its body. To imagine what the implementation might look like, a `Button` struct might have fields for `width`, `height`, and `label`, as shown in Listing 17-7:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub trait Draw {
    #     fn draw(&self);
    # }
    #
    pub struct Button {
        pub width: u32,
        pub height: u32,
        pub label: String,
    }

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

Listing 17-7: A `Button` struct that implements the `Draw` trait

The `width`, `height`, and `label` fields on `Button` will differ from the fields on other components, such as a `TextField` type that might have those plus a `placeholder` field instead. Each of the types we want to draw on the screen will implement the `Draw` trait, with different code in the `draw` method to define how to draw that particular type, like `Button` has here (without the actual GUI code that’s out of scope of this chapter). `Button`, for instance, might have an additional `impl` block containing methods related to what happens if the button is clicked. These kinds of methods won’t apply to types like `TextField`.

Someone using our library has decided to implement a `SelectBox` struct that has `width`, `height`, and `options` fields. They implement the `Draw` trait on the `SelectBox` type as well, as shown in Listing 17-8:

Filename: src/main.rs

    extern crate rust_gui;
    use rust_gui::Draw;

    struct SelectBox {
        width: u32,
        height: u32,
        options: Vec<String>,
    }

    impl Draw for SelectBox {
        fn draw(&self) {
            // Code to actually draw a select box
        }
    }

Listing 17-8: Another crate using `rust_gui` and implementing the `Draw` trait on a `SelectBox` struct

The user of our library can now write their `main` function to create a `Screen` instance. To this they can add a `SelectBox` and a `Button` by putting each in a `Box<T>` to become a trait object. They can then call the `run` method on the `Screen` instance, which will call `draw` on each of the components. Listing 17-9 shows this implementation:

Filename: src/main.rs

    use rust_gui::{Screen, Button};

    fn main() {
        let screen = Screen {
            components: vec![
                Box::new(SelectBox {
                    width: 75,
                    height: 10,
                    options: vec![
                        String::from("Yes"),
                        String::from("Maybe"),
                        String::from("No")
                    ],
                }),
                Box::new(Button {
                    width: 50,
                    height: 10,
                    label: String::from("OK"),
                }),
            ],
        };

        screen.run();
    }

Listing 17-9: Using trait objects to store values of different types that implement the same trait

When we wrote the library, we didn’t know that someone would add the `SelectBox` type someday, but our `Screen` implementation was able to operate on the new type and draw it because `SelectBox` implements the `Draw` type, which means it implements the `draw` method.

This concept---of being concerned only with the messages a value responds to, rather than the value’s concrete type---is similar to a concept in dynamically typed languages called duck typing: if it walks like a duck, and quacks like a duck, then it must be a duck! In the implementation of `run` on `Screen` in Listing 17-5, `run` doesn’t need to know what the concrete type of each component is. It doesn’t check to see if a component is an instance of a `Button` or a `SelectBox`, it just calls the `draw` method on the component. By specifying `Box<Draw>` as the type of the values in the `components` vector, we’ve defined `Screen` to need values that we can call the `draw` method on.

The advantage of using trait objects and Rust’s type system to do something similar to duck typing is that we never have to check that a value implements a particular method at runtime or worry about getting errors if a value doesn’t implement a method but we call it anyway. Rust won’t compile our code if the values don’t implement the traits that the trait objects need.

For example, Listing 17-10 shows what happens if we try to create a `Screen` with a `String` as a component:

Filename: src/main.rs

    extern crate rust_gui;
    use rust_gui::Draw;

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

        screen.run();
    }

Listing 17-10: Attempting to use a type that doesn’t implement the trait object’s trait

We’ll get this error because `String` doesn’t implement the `Draw` trait:

    error[E0277]: the trait bound `std::string::String: Draw` is not satisfied
      -->
       |
     4 |             Box::new(String::from("Hi")),
       |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Draw` is not
       implemented for `std::string::String`
       |
       = note: required for the cast to the object type `Draw`

This lets us know that either we’re passing something to `Screen` we didn’t mean to pass, and we should pass a different type, or implement `Draw` on `String` so that `Screen` is able to call `draw` on it.

### Trait Objects Perform Dynamic Dispatch

Recall from Chapter 10 our discussion on the monomorphization process performed by the compiler when we use trait bounds on generics: the compiler generates non-generic implementations of functions and methods for each concrete type that we use in place of a generic type parameter. The code that results from monomorphization is doing static dispatch. Static dispatch is when the compiler knows what method you’re calling at compile time. This is opposed to dynamic dispatch, when the compiler can’t tell at compile time which method you’re calling. In these cases, the compiler emits code that will figure out at runtime which method to call.

When we use trait objects, Rust has to use dynamic dispatch. The compiler doesn’t know all the types that might be used with the code using trait objects, so it doesn’t know which method implemented on which type to call. Instead, Rust uses the pointers inside of the trait object at runtime to know which specific method to call. There’s a runtime cost when this lookup happens, compared to static dispatch. Dynamic dispatch also prevents the compiler from choosing to inline a method’s code which in turn prevents some optimizations. We did get extra flexibility in the code that we wrote and were able to support, though, so it’s a tradeoff to consider.

### Object Safety is Required for Trait Objects

Only object safe traits can be made into trait objects. There are some complex rules around all the properties that make a trait object safe, but in practice, there are only two rules that are relevant. A trait is object safe if all of the methods defined in the trait have the following properties:

- The return type isn’t `Self`
- There aren’t any generic type parameters

The `Self` keyword is an alias for the type we’re implementing traits or methods on. Object safety is required for trait objects because once you have a trait object, you no longer know what the concrete type implementing that trait is. If a trait method returns the concrete `Self` type, but a trait object forgets the exact type that it is, there’s no way that the method can use the original concrete type that it’s forgotten. Same with generic type parameters that are filled in with concrete type parameters when the trait is used: the concrete types become part of the type that implements the trait. When the type is erased by the use of a trait object, there’s no way to know what types to fill in the generic type parameters with.

An example of a trait whose methods are not object safe is the standard library’s `Clone` trait. The signature for the `clone` method in the `Clone` trait looks like this:

    # #![allow(unused_variables)]
    #fn main() {
    pub trait Clone {
        fn clone(&self) -> Self;
    }
    #}

`String` implements the `Clone` trait, and when we call the `clone` method on an instance of `String` we get back an instance of `String`. Similarly, if we call `clone` on an instance of `Vec`, we get back an instance of `Vec`. The signature of `clone` needs to know what type will stand in for `Self`, since that’s the return type.

The compiler will tell you if you’re trying to do something that violates the rules of object safety in regards to trait objects. For example, if we had tried to implement the `Screen` struct in Listing 17-4 to hold types that implement the `Clone` trait instead of the `Draw` trait, like this:

    pub struct Screen {
        pub components: Vec<Box<Clone>>,
    }

We’ll get this error:

    error[E0038]: the trait `std::clone::Clone` cannot be made into an object
     -->
      |
    2 |     pub components: Vec<Box<Clone>>,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::clone::Clone` cannot be
      made into an object
      |
      = note: the trait cannot require that `Self : Sized`

This means you can’t use this trait as a trait object in this way. If you’re interested in more details on object safety, see Rust RFC 255.