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

## Trait Objects for Using Values of Different Types

In Chapter 8, we said that a limitation of vectors is that vectors can only store elements of one type. We had an example in Listing 8-1 where we defined a `SpreadsheetCell` enum that had variants to hold integers, floats, and text so that we could store different types of data in each cell and still have a vector represent a row of cells. This works for cases in which the kinds of things we want to be able to treat interchangeably are a fixed set of types that we know when our code gets compiled.

Sometimes we want the set of types that we use to be extensible by the programmers who use our library. For example, many Graphical User Interface tools have a concept of a list of items that get drawn on the screen by iterating through the list and calling a `draw` method on each of the items. We're going to create a library crate containing the structure of a GUI library called `rust_gui`. Our GUI library could include some types for people to use, such as `Button` or `TextField`. Programmers that use `rust_gui` will want to create more types that can be drawn on the screen: one programmer might add an `Image`, while another might add a `SelectBox`. We're not going to implement a fully-fledged GUI library in this chapter, but we will show how the pieces would fit together.

When we're writing the `rust_gui` library, we don't know all the types that other programmers will want to create, so we can't define an `enum` containing all the types. What we do know is that `rust_gui` needs to be able to keep track of a bunch of values of all these different types, and it needs to be able to call a `draw` method on each of these values. Our GUI library doesn't need to know what will happen exactly when we call the `draw` method, just that the value will have that method available for us to call.

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.

### Defining a Trait for the Common Behavior

In Rust, though, we can define a trait that we'll name `Draw` and that will have one method named `draw`. Then we can define a vector that takes a trait object, which is a trait behind some sort of pointer, such as a `&` reference or a `Box<T>` smart pointer. We'll talk about the reason trait objects have to be behind a pointer in Chapter 19.

We mentioned that we don't call structs and enums "objects" to distinguish structs and enums from other languages' objects. The data in the struct or enum fields and the behavior in `impl` blocks is separated, as opposed to other languages that have data and behavior combined into one concept called an object. Trait objects are more like objects in other languages, in the sense that they combine the data made up of the pointer to a concrete object with the behavior of the methods defined in the trait. However, trait objects are different from objects in other languages because we can't add data to a trait object. Trait objects aren't as generally useful as objects in other languages: their purpose is to allow abstraction across common behavior.

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. Rust's type system will ensure that any value we substitute in for the trait object will implement the methods of the trait. Then we don't need to know all the possible types at compile time, and we can treat all the instances the same way. 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 since we talked about how to define traits in Chapter 10. Next comes something new: Listing 17-4 has the definition of a struct named `Screen` that holds a vector named `components` that are of type `Box<Draw>`. That `Box<Draw>` 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 that holds a vector of trait objects that implement the `Draw` trait

On the `Screen` struct, we'll define a method named `run`, which 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 is different than 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 only lets us have a `Screen` instance that has a list of components that are 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 definition of `Screen` that holds a component list of trait objects in `Vec<Box<Draw>>` instead, 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.

### Implementations of the Trait from Us or Library Users

Now to add some types that implement the `Draw` trait. We're going to provide the `Button` type, and 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 other components, such as a `TextField` type that might have `width`, `height`, `label`, and `placeholder` fields instead. Each of the types that we want to be able to draw on the screen will implement the `Draw` trait with different code in the `draw` method that defines how to draw that type like `Button` has here (without any actual GUI code that's out of scope of this chapter). In addition to implementing the `Draw` trait, `Button` might also have another `impl` block containing methods having to do with 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 and add a `SelectBox` and a `Button` to the screen 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

Even though we didn't know that someone would add the `SelectBox` type someday, our `Screen` implementation was able to operate on the `SelectBox` and draw it because `SelectBox` implements the `Draw` type, which means it implements the `draw` method.

Only being concerned with the messages a value responds to, rather than the value's concrete type, is similar to a concept called duck typing in dynamically typed languages: 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 that `Screen` needs values that we can call the `draw` method on.

The advantage with using trait objects and Rust's type system to do 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. 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 we didn't mean to pass to `Screen` and we should pass a different type, or we should implement `Draw` on `String` so that `Screen` is able to call `draw` on it.

### Trait Objects Perform Dynamic Dispatch

Recall in Chapter 10 when we discussed the process of monomorphization that the compiler performs 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: when the method is called, the code that goes with that method call has been determined at compile time, and looking up that code is very fast.

When we use trait objects, the compiler can't perform monomorphization because we don't know all the types that might be used with the code. Instead, Rust keeps track of the code that might be used when a method is called and figures out at runtime which code needs to be used for a particular method call. This is known as dynamic dispatch, and there's a runtime cost when this lookup happens. Dynamic dispatch also prevents the compiler from choosing to inline a method's code, which 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

Not all traits can be made into trait objects; only object safe traits can. A trait is object safe as long as both of the following are true:

- The trait does not require `Self` to be `Sized`
- All of the trait's methods are object safe.

`Self` is a keyword that is an alias for the type that we're implementing traits or methods on. `Sized` is a marker trait like the `Send` and `Sync` traits that we talked about in Chapter 16. `Sized` is automatically implemented on types that have a known size at compile time, such as `i32` and references. Types that do not have a known size include slices (`[T]`) and trait objects.

`Sized` is an implicit trait bound on all generic type parameters by default. Most useful operations in Rust require a type to be `Sized`, so making `Sized` a default requirement on trait bounds means we don't have to write `T: Sized` with most every use of generics. If we want to be able to use a trait on slices, however, we need to opt out of the `Sized` trait bound, and we can do that by specifying `T: ?Sized` as a trait bound.

Traits have a default bound of `Self: ?Sized`, which means that they can be implemented on types that may or may not be `Sized`. If we create a trait `Foo` that opts out of the `Self: ?Sized` bound, that would look like the following:

    # #![allow(unused_variables)]
    #fn main() {
    trait Foo: Sized {
        fn some_method(&self);
    }

    #}

The trait `Sized` is now a supertrait of trait `Foo`, which means trait `Foo` requires types that implement `Foo` (that is, `Self`) to be `Sized`. We're going to talk about supertraits in more detail in Chapter 19.

The reason a trait like `Foo` that requires `Self` to be `Sized` is not allowed to be a trait object is that it would be impossible to implement the trait `Foo` for the trait object `Foo`: trait objects aren't sized, but `Foo` requires `Self` to be `Sized`. A type can't be both sized and unsized at the same time!

For the second object safety requirement that says all of a trait's methods must be object safe, a method is object safe if either:

- It requires `Self` to be `Sized` or
- It meets all three of the following:
  - It must not have any generic type parameters
  - Its first argument must be of type `Self` or a type that dereferences to the Self type (that is, it must be a method rather than an associated function and have `self`, `&self`, or `&mut self` as the first argument)
  - It must not use `Self` anywhere else in the signature except for the first argument

Those rules are a bit formal, but think of it this way: if your method requires the concrete `Self` type somewhere in its signature, but an 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.

If we try to implement `Clone` on a trait like the `Draw` trait from Listing 17-3, we wouldn't know whether `Self` would end up being a `Button`, a `SelectBox`, or some other type that will implement the `Draw` trait in the future.

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`