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[TOC]

Object-Oriented Programming Features

Object-oriented programming (OOP) is a way of modeling programs. Objects as a programmatic concept were introduced in the programming language Simula in the 1960s. Those objects influenced Alan Kay’s programming architecture in which objects pass messages to each other. To describe this architecture, he coined the term object-oriented programming in 1967. Many competing definitions describe what OOP is, and by some of these definitions Rust is object oriented but by others it is not. In this chapter, we’ll explore certain characteristics that are commonly considered object oriented and how those characteristics translate to idiomatic Rust. We’ll then show you how to implement an object-oriented design pattern in Rust and discuss the trade-offs of doing so versus implementing a solution using some of Rust’s strengths instead.

Characteristics of Object-Oriented Languages

There is no consensus in the programming community about what features a language must have to be considered object oriented. Rust is influenced by many programming paradigms, including OOP; for example, we explored the features that came from functional programming in Chapter 13. Arguably, OOP languages share certain common characteristics, namely objects, encapsulation, and inheritance. Let’s look at what each of those characteristics means and whether Rust supports it.

Objects Contain Data and Behavior

The book Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison-Wesley, 1994), colloquially referred to as The Gang of Four book, is a catalog of object-oriented design patterns. It defines OOP in this way:

Object-oriented programs are made up of objects. An object packages both data and the procedures that operate on that data. The procedures are typically called methods or operations.

Using this definition, Rust is object oriented: structs and enums have data, and impl blocks provide methods on structs and enums. Even though structs and enums with methods aren’t called objects, they provide the same functionality, according to the Gang of Four’s definition of objects.

Encapsulation That Hides Implementation Details

Another aspect commonly associated with OOP is the idea of encapsulation, which means that the implementation details of an object aren’t accessible to code using that object. Therefore, the only way to interact with an object is through its public API; code using the object shouldn’t be able to reach into the object’s internals and change data or behavior directly. This enables the programmer to change and refactor an object’s internals without needing to change the code that uses the object.

We discussed how to control encapsulation in Chapter 7: we can use the pub keyword to decide which modules, types, functions, and methods in our code should be public, and by default everything else is private. For example, we can define a struct AveragedCollection that has a field containing a vector of i32 values. The struct can also have a field that contains the average of the values in the vector, meaning the average doesn’t have to be computed on demand whenever anyone needs it. In other words, AveragedCollection will cache the calculated average for us. Listing 17-1 has the definition of the AveragedCollection struct.

Filename: src/lib.rs

pub struct AveragedCollection {
    list: Vec<i32>,
    average: f64,
}

Listing 17-1: An AveragedCollection struct that maintains a list of integers and the average of the items in the collection

The struct is marked pub so that other code can use it, but the fields within the struct remain private. This is important in this case because we want to ensure that whenever a value is added or removed from the list, the average is also updated. We do this by implementing add, remove, and average methods on the struct, as shown in Listing 17-2.

Filename: src/lib.rs

impl AveragedCollection {
    pub fn add(&mut self, value: i32) {
        self.list.push(value);
        self.update_average();
    }

    pub fn remove(&mut self) -> Option<i32> {
        let result = self.list.pop();
        match result {
            Some(value) => {
                self.update_average();
                Some(value)
            }
            None => None,
        }
    }

    pub fn average(&self) -> f64 {
        self.average
    }

    fn update_average(&mut self) {
        let total: i32 = self.list.iter().sum();
        self.average = total as f64 / self.list.len() as f64;
    }
}

Listing 17-2: Implementations of the public methods add, remove, and average on AveragedCollection

The public methods add, remove, and average are the only ways to access or modify data in an instance of AveragedCollection. When an item is added to list using the add method or removed using the remove method, the implementations of each call the private update_average method that handles updating the average field as well.

We leave the list and average fields private so there is no way for external code to add or remove items to or from the list field directly; otherwise, the average field might become out of sync when the list changes. The average method returns the value in the average field, allowing external code to read the average but not modify it.

Because we’ve encapsulated the implementation details of the struct AveragedCollection, we can easily change aspects, such as the data structure, in the future. For instance, we could use a HashSet<i32> instead of a Vec<i32> for the list field. As long as the signatures of the add, remove, and average public methods stayed the same, code using AveragedCollection wouldn’t need to change. If we made list public instead, this wouldn’t necessarily be the case: HashSet<i32> and Vec<i32> have different methods for adding and removing items, so the external code would likely have to change if it were modifying list directly.

If encapsulation is a required aspect for a language to be considered object oriented, then Rust meets that requirement. The option to use pub or not for different parts of code enables encapsulation of implementation details.

Inheritance as a Type System and as Code Sharing

Inheritance is a mechanism whereby an object can inherit elements from another object’s definition, thus gaining the parent object’s data and behavior without you having to define them again.

If a language must have inheritance to be object oriented, then Rust is not such a language. There is no way to define a struct that inherits the parent struct’s fields and method implementations without using a macro.

However, if you’re used to having inheritance in your programming toolbox, you can use other solutions in Rust, depending on your reason for reaching for inheritance in the first place.

You would choose inheritance for two main reasons. One is for reuse of code: you can implement particular behavior for one type, and inheritance enables you to reuse that implementation for a different type. You can do this in a limited way in Rust code using default trait method implementations, which you saw in Listing 10-14 when we added a default implementation of the summarize method on the Summary trait. Any type implementing the Summary trait would have the summarize method available on it without any further code. This is similar to a parent class having an implementation of a method and an inheriting child class also having the implementation of the method. We can also override the default implementation of the summarize method when we implement the Summary trait, which is similar to a child class overriding the implementation of a method inherited from a parent class.

The other reason to use inheritance relates to the type system: to enable a child type to be used in the same places as the parent type. This is also called polymorphism, which means that you can substitute multiple objects for each other at runtime if they share certain characteristics.

Polymorphism

To many people, polymorphism is synonymous with inheritance. But it’s actually a more general concept that refers to code that can work with data of multiple types. For inheritance, those types are generally subclasses.

Rust instead uses generics to abstract over different possible types and trait bounds to impose constraints on what those types must provide. This is sometimes called bounded parametric polymorphism.

Inheritance has recently fallen out of favor as a programming design solution in many programming languages because it’s often at risk of sharing more code than necessary. Subclasses shouldn’t always share all characteristics of their parent class but will do so with inheritance. This can make a program’s design less flexible. It also introduces the possibility of calling methods on subclasses that don’t make sense or that cause errors because the methods don’t apply to the subclass. In addition, some languages will only allow single inheritance (meaning a subclass can only inherit from one class), further restricting the flexibility of a program’s design.

For these reasons, Rust takes the different approach of using trait objects instead of inheritance. Let’s look at how trait objects enable polymorphism in Rust.

Using Trait Objects That Allow for Values of Different Types

In Chapter 8, we mentioned that one limitation of vectors is that they can store elements of only one type. We created a workaround in Listing 8-9 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 is compiled.

However, sometimes we want our library user 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 (GUI) tool that iterates through a list of items, calling a draw method on each one to draw it to the screen—a common technique for GUI tools. We’ll create a library crate called gui that contains the structure of a GUI library. This crate might include some types for people to use, such as Button or TextField. In addition, gui users will want to create their own types that can be drawn: for instance, one programmer might add an Image and another might add a SelectBox.

We won’t implement a full-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 might want to create. But we do know that gui needs to keep track of many values of different types, and it needs 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, such as 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 because Rust doesn’t have inheritance, we need another way to structure the gui library to allow users to extend it with new types.

Defining a Trait for Common Behavior

To implement the behavior we want 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 both an instance of a type implementing our specified trait and a table used to look up trait methods on that type at runtime. We create a trait object by specifying some sort of pointer, such as a & reference or a Box<T> smart pointer, then the dyn keyword, and then specifying the relevant trait. (We’ll talk about the reason trait objects must use a pointer in “Dynamically Sized Types and the Sized Trait” on page XX.) 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. Consequently, 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 are separated, whereas in other languages, the data and behavior combined into one concept is often labeled an object. However, trait objects are more like objects in other languages in the sense that they combine data and behavior. But 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

pub trait Draw {
    fn draw(&self);
}

Listing 17-3: Definition of the Draw trait

This syntax should look familiar from our discussions on how to define traits in Chapter 10. Next comes some new syntax: Listing 17-4 defines a struct named Screen that holds a vector named components. This vector is of type Box<dyn 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

pub struct Screen {
    pub components: Vec<Box<dyn 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

impl Screen {
    pub fn run(&self) {
        for component in self.components.iter() {
            component.draw();
        }
    }
}

Listing 17-5: A run method on Screen that calls the draw method on each component

This works differently from 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, whereas 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

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 because the definitions will be monomorphized at compile time to use the concrete types.

On the other hand, with the method using trait objects, one Screen instance can hold a Vec<T> that contains a Box<Button> as well as a Box<TextField>. Let’s look at how this works, and then we’ll talk about the runtime performance implications.

Implementing the Trait

Now we’ll add some types that implement the Draw trait. We’ll provide the Button type. Again, actually implementing a GUI library is beyond the 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

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; for example, a TextField type might have those same fields plus a placeholder field. Each of the types we want to draw on the screen will implement the Draw trait but will use different code in the draw method to define how to draw that particular type, as Button has here (without the actual GUI code, as mentioned). The Button type, for instance, might have an additional impl block containing methods related to what happens when a user clicks the button. These kinds of methods won’t apply to types like TextField.

If someone using our library decides to implement a SelectBox struct that has width, height, and options fields, they would implement the Draw trait on the SelectBox type as well, as shown in Listing 17-8.

Filename: src/main.rs

use 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 gui and implementing the Draw trait on a SelectBox struct

Our library’s user can now write their main function to create a Screen instance. To the Screen instance, 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 gui::{Button, Screen};

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 might add the SelectBox type, but our Screen implementation was able to operate on the new type and draw it because SelectBox implements the Draw trait, 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 the concept of 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 whether a component is an instance of a Button or a SelectBox, it just calls the draw method on the component. By specifying Box<dyn 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 write code similar to code using duck typing is that we never have to check whether 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

use gui::Screen;

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 `String: Draw` is not satisfied
 --> src/main.rs:5:26
  |
5 |         components: vec![Box::new(String::from("Hi"))],
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Draw` is
not implemented for `String`
  |
  = note: required for the cast to the object type `dyn Draw`

This error lets us know that either we’re passing something to Screen that we didn’t mean to pass and so 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 “Performance of Code Using Generics” on page XX our discussion on the monomorphization process performed by the compiler when we use trait bounds on generics: the compiler generates nongeneric 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, which is when the compiler knows what method you’re calling at compile time. This is opposed to dynamic dispatch, which is when the compiler can’t tell at compile time which method you’re calling. In dynamic dispatch cases, the compiler emits code that at runtime will figure out which method to call.

When we use trait objects, Rust must use dynamic dispatch. The compiler doesn’t know all the types that might be used with the code that’s using trait objects, so it doesn’t know which method implemented on which type to call. Instead, at runtime, Rust uses the pointers inside the trait object to know which method to call. This lookup incurs a runtime cost that doesn’t occur with static dispatch. Dynamic dispatch also prevents the compiler from choosing to inline a method’s code, which in turn prevents some optimizations. However, we did get extra flexibility in the code that we wrote in Listing 17-5 and were able to support in Listing 17-9, so it’s a trade-off to consider.

Implementing an Object-Oriented Design Pattern

The state pattern is an object-oriented design pattern. The crux of the pattern is that we define a set of states a value can have internally. The states are represented by a set of state objects, and the value’s behavior changes based on its state. We’re going to work through an example of a blog post struct that has a field to hold its state, which will be a state object from the set “draft,” “review,” or “published.”

The state objects share functionality: in Rust, of course, we use structs and traits rather than objects and inheritance. Each state object is responsible for its own behavior and for governing when it should change into another state. The value that holds a state object knows nothing about the different behavior of the states or when to transition between states.

The advantage of using the state pattern is that, when the business requirements of the program change, we won’t need to change the code of the value holding the state or the code that uses the value. We’ll only need to update the code inside one of the state objects to change its rules or perhaps add more state objects.

First we’re going to implement the state pattern in a more traditional object-oriented way, then we’ll use an approach that’s a bit more natural in Rust. Let’s dig in to incrementally implement a blog post workflow using the state pattern.

The final functionality will look like this:

  1. A blog post starts as an empty draft.
  2. When the draft is done, a review of the post is requested.
  3. When the post is approved, it gets published.
  4. Only published blog posts return content to print, so unapproved posts can’t accidentally be published.

Any other changes attempted on a post should have no effect. For example, if we try to approve a draft blog post before we’ve requested a review, the post should remain an unpublished draft.

Listing 17-11 shows this workflow in code form: this is an example usage of the API we’ll implement in a library crate named blog. This won’t compile yet because we haven’t implemented the blog crate.

Filename: src/main.rs

use blog::Post;

fn main() {
  1 let mut post = Post::new();

  2 post.add_text("I ate a salad for lunch today");
  3 assert_eq!("", post.content());

  4 post.request_review();
  5 assert_eq!("", post.content());

  6 post.approve();
  7 assert_eq!("I ate a salad for lunch today", post.content());
}

Listing 17-11: Code that demonstrates the desired behavior we want our blog crate to have

We want to allow the user to create a new draft blog post with Post::new [1]. We want to allow text to be added to the blog post [2]. If we try to get the post’s content immediately, before approval, we shouldn’t get any text because the post is still a draft. We’ve added assert_eq! in the code for demonstration purposes [3]. An excellent unit test for this would be to assert that a draft blog post returns an empty string from the content method, but we’re not going to write tests for this example.

Next, we want to enable a request for a review of the post [4], and we want content to return an empty string while waiting for the review [5]. When the post receives approval [6], it should get published, meaning the text of the post will be returned when content is called [7].

Notice that the only type we’re interacting with from the crate is the Post type. This type will use the state pattern and will hold a value that will be one of three state objects representing the various states a post can be in—draft, review, or published. Changing from one state to another will be managed internally within the Post type. The states change in response to the methods called by our library’s users on the Post instance, but they don’t have to manage the state changes directly. Also, users can’t make a mistake with the states, such as publishing a post before it’s reviewed.

Defining Post and Creating a New Instance in the Draft State

Let’s get started on the implementation of the library! We know we need a public Post struct that holds some content, so we’ll start with the definition of the struct and an associated public new function to create an instance of Post, as shown in Listing 17-12. We’ll also make a private State trait that will define the behavior that all state objects for a Post must have.

Then Post will hold a trait object of Box<dyn State> inside an Option<T> in a private field named state to hold the state object. You’ll see why the Option<T> is necessary in a bit.

Filename: src/lib.rs

pub struct Post {
    state: Option<Box<dyn State>>,
    content: String,
}

impl Post {
    pub fn new() -> Post {
        Post {
          1 state: Some(Box::new(Draft {})),
          2 content: String::new(),
        }
    }
}

trait State {}

struct Draft {}

impl State for Draft {}

Listing 17-12: Definition of a Post struct and a new function that creates a new Post instance, a State trait, and a Draft struct

The State trait defines the behavior shared by different post states. The state objects are Draft, PendingReview, and Published, and they will all implement the State trait. For now, the trait doesn’t have any methods, and we’ll start by defining just the Draft state because that is the state we want a post to start in.

When we create a new Post, we set its state field to a Some value that holds a Box [1]. This Box points to a new instance of the Draft struct. This ensures that whenever we create a new instance of Post, it will start out as a draft. Because the state field of Post is private, there is no way to create a Post in any other state! In the Post::new function, we set the content field to a new, empty String [2].

Storing the Text of the Post Content

We saw in Listing 17-11 that we want to be able to call a method named add_text and pass it a &str that is then added as the text content of the blog post. We implement this as a method, rather than exposing the content field as pub, so that later we can implement a method that will control how the content field’s data is read. The add_text method is pretty straightforward, so let’s add the implementation in Listing 17-13 to the impl Post block.

Filename: src/lib.rs

impl Post {
    --snip--
    pub fn add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }
}

Listing 17-13: Implementing the add_text method to add text to a post’s content

The add_text method takes a mutable reference to self because we’re changing the Post instance that we’re calling add_text on. We then call push_str on the String in content and pass the text argument to add to the saved content. This behavior doesn’t depend on the state the post is in, so it’s not part of the state pattern. The add_text method doesn’t interact with the state field at all, but it is part of the behavior we want to support.

Ensuring the Content of a Draft Post Is Empty

Even after we’ve called add_text and added some content to our post, we still want the content method to return an empty string slice because the post is still in the draft state, as shown at [3] in Listing 17-11. For now, let’s implement the content method with the simplest thing that will fulfill this requirement: always returning an empty string slice. We’ll change this later once we implement the ability to change a post’s state so it can be published. So far, posts can only be in the draft state, so the post content should always be empty. Listing 17-14 shows this placeholder implementation.

Filename: src/lib.rs

impl Post {
    --snip--
    pub fn content(&self) -> &str {
        ""
    }
}

Listing 17-14: Adding a placeholder implementation for the content method on Post that always returns an empty string slice

With this added content method, everything in Listing 17-11 up to the line at [3] works as intended.

Requesting a Review Changes the Post’s State

Next, we need to add functionality to request a review of a post, which should change its state from Draft to PendingReview. Listing 17-15 shows this code.

Filename: src/lib.rs

impl Post {
    --snip--
  1 pub fn request_review(&mut self) {
      2 if let Some(s) = self.state.take() {
          3 self.state = Some(s.request_review())
        }
    }
}

trait State {
  4 fn request_review(self: Box<Self>) -> Box<dyn State>;
}

struct Draft {}

impl State for Draft {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
      5 Box::new(PendingReview {})
    }
}

struct PendingReview {}

impl State for PendingReview {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
      6 self
    }
}

Listing 17-15: Implementing request_review methods on Post and the State trait

We give Post a public method named request_review that will take a mutable reference to self [1]. Then we call an internal request_review method on the current state of Post [3], and this second request_review method consumes the current state and returns a new state.

We add the request_review method to the State trait [4]; all types that implement the trait will now need to implement the request_review method. Note that rather than having self, &self, or &mut self as the first parameter of the method, we have self: Box<Self>. This syntax means the method is only valid when called on a Box holding the type. This syntax takes ownership of Box<Self>, invalidating the old state so the state value of the Post can transform into a new state.

To consume the old state, the request_review method needs to take ownership of the state value. This is where the Option in the state field of Post comes in: we call the take method to take the Some value out of the state field and leave a None in its place because Rust doesn’t let us have unpopulated fields in structs [2]. This lets us move the state value out of Post rather than borrowing it. Then we’ll set the post’s state value to the result of this operation.

We need to set state to None temporarily rather than setting it directly with code like self.state = self.state.request_review(); to get ownership of the state value. This ensures Post can’t use the old state value after we’ve transformed it into a new state.

The request_review method on Draft returns a new, boxed instance of a new PendingReview struct [5], which represents the state when a post is waiting for a review. The PendingReview struct also implements the request_review method but doesn’t do any transformations. Rather, it returns itself [6] because when we request a review on a post already in the PendingReview state, it should stay in the PendingReview state.

Now we can start seeing the advantages of the state pattern: the request_review method on Post is the same no matter its state value. Each state is responsible for its own rules.

We’ll leave the content method on Post as is, returning an empty string slice. We can now have a Post in the PendingReview state as well as in the Draft state, but we want the same behavior in the PendingReview state. Listing 17-11 now works up to the line at [5]!

Adding approve to Change the Behavior of content

The approve method will be similar to the request_review method: it will set state to the value that the current state says it should have when that state is approved, as shown in Listing 17-16.

Filename: src/lib.rs

impl Post {
    --snip--
    pub fn approve(&mut self) {
        if let Some(s) = self.state.take() {
            self.state = Some(s.approve())
        }
    }
}

trait State {
    fn request_review(self: Box<Self>) -> Box<dyn State>;
    fn approve(self: Box<Self>) -> Box<dyn State>;
}

struct Draft {}

impl State for Draft {
    --snip--
    fn approve(self: Box<Self>) -> Box<dyn State> {
      1 self
    }
}

struct PendingReview {}

impl State for PendingReview {
    --snip--
    fn approve(self: Box<Self>) -> Box<dyn State> {
      2 Box::new(Published {})
    }
}

struct Published {}

impl State for Published {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        self
    }

    fn approve(self: Box<Self>) -> Box<dyn State> {
        self
    }
}

Listing 17-16: Implementing the approve method on Post and the State trait

We add the approve method to the State trait and add a new struct that implements State, the Published state.

Similar to the way request_review on PendingReview works, if we call the approve method on a Draft, it will have no effect because approve will return self [1]. When we call approve on PendingReview, it returns a new, boxed instance of the Published struct [2]. The Published struct implements the State trait, and for both the request_review method and the approve method, it returns itself because the post should stay in the Published state in those cases.

Now we need to update the content method on Post. We want the value returned from content to depend on the current state of the Post, so we’re going to have the Post delegate to a content method defined on its state, as shown in Listing 17-17.

Filename: src/lib.rs

impl Post {
    --snip--
    pub fn content(&self) -> &str {
        self.state.as_ref().unwrap().content(self)
    }
    --snip--
}

Listing 17-17: Updating the content method on Post to delegate to a content method on State

Because the goal is to keep all of these rules inside the structs that implement State, we call a content method on the value in state and pass the post instance (that is, self) as an argument. Then we return the value that’s returned from using the content method on the state value.

We call the as_ref method on the Option because we want a reference to the value inside the Option rather than ownership of the value. Because state is an Option<Box<dyn State>>, when we call as_ref, an Option<&Box<dyn State>> is returned. If we didn’t call as_ref, we would get an error because we can’t move state out of the borrowed &self of the function parameter.

We then call the unwrap method, which we know will never panic because we know the methods on Post ensure that state will always contain a Some value when those methods are done. This is one of the cases we talked about in “Cases in Which You Have More Information Than the Compiler” on page XX when we know that a None value is never possible, even though the compiler isn’t able to understand that.

At this point, when we call content on the &Box<dyn State>, deref coercion will take effect on the & and the Box so the content method will ultimately be called on the type that implements the State trait. That means we need to add content to the State trait definition, and that is where we’ll put the logic for what content to return depending on which state we have, as shown in Listing 17-18.

Filename: src/lib.rs

trait State {
    --snip--
    fn content<'a>(&self, post: &'a Post) -> &'a str {
      1 ""
    }
}

--snip--
struct Published {}

impl State for Published {
    --snip--
    fn content<'a>(&self, post: &'a Post) -> &'a str {
      2 &post.content
    }
}

Listing 17-18: Adding the content method to the State trait

We add a default implementation for the content method that returns an empty string slice [1]. That means we don’t need to implement content on the Draft and PendingReview structs. The Published struct will override the content method and return the value in post.content [2].

Note that we need lifetime annotations on this method, as we discussed in Chapter 10. We’re taking a reference to a post as an argument and returning a reference to part of that post, so the lifetime of the returned reference is related to the lifetime of the post argument.

And we’re done—all of Listing 17-11 now works! We’ve implemented the state pattern with the rules of the blog post workflow. The logic related to the rules lives in the state objects rather than being scattered throughout Post.

Why Not An Enum?

You may have been wondering why we didn’t use an enum with the different possible post states as variants. That’s certainly a possible solution; try it and compare the end results to see which you prefer! One disadvantage of using an enum is that every place that checks the value of the enum will need a match expression or similar to handle every possible variant. This could get more repetitive than this trait object solution.

Trade-offs of the State Pattern

We’ve shown that Rust is capable of implementing the object-oriented state pattern to encapsulate the different kinds of behavior a post should have in each state. The methods on Post know nothing about the various behaviors. The way we organized the code, we have to look in only one place to know the different ways a published post can behave: the implementation of the State trait on the Published struct.

If we were to create an alternative implementation that didn’t use the state pattern, we might instead use match expressions in the methods on Post or even in the main code that checks the state of the post and changes behavior in those places. That would mean we would have to look in several places to understand all the implications of a post being in the published state! This would only increase the more states we added: each of those match expressions would need another arm.

With the state pattern, the Post methods and the places we use Post don’t need match expressions, and to add a new state, we would only need to add a new struct and implement the trait methods on that one struct.

The implementation using the state pattern is easy to extend to add more functionality. To see the simplicity of maintaining code that uses the state pattern, try a few of these suggestions:

  • Add a reject method that changes the post’s state from PendingReview back to Draft.
  • Require two calls to approve before the state can be changed to Published.
  • Allow users to add text content only when a post is in the Draft state. Hint: have the state object responsible for what might change about the content but not responsible for modifying the Post.

One downside of the state pattern is that, because the states implement the transitions between states, some of the states are coupled to each other. If we add another state between PendingReview and Published, such as Scheduled, we would have to change the code in PendingReview to transition to Scheduled instead. It would be less work if PendingReview didn’t need to change with the addition of a new state, but that would mean switching to another design pattern.

Another downside is that we’ve duplicated some logic. To eliminate some of the duplication, we might try to make default implementations for the request_review and approve methods on the State trait that return self. However, this wouldn’t work: when using State as a trait object, the trait doesn’t know what the concrete self will be exactly, so the return type isn’t known at compile time.

Other duplication includes the similar implementations of the request_review and approve methods on Post. Both methods delegate to the implementation of the same method on the value in the state field of Option and set the new value of the state field to the result. If we had a lot of methods on Post that followed this pattern, we might consider defining a macro to eliminate the repetition (see “Macros” on page XX).

By implementing the state pattern exactly as it’s defined for object-oriented languages, we’re not taking as full advantage of Rust’s strengths as we could. Let’s look at some changes we can make to the blog crate that can make invalid states and transitions into compile-time errors.

Encoding States and Behavior as Types

We’ll show you how to rethink the state pattern to get a different set of trade-offs. Rather than encapsulating the states and transitions completely so outside code has no knowledge of them, we’ll encode the states into different types. Consequently, Rust’s type checking system will prevent attempts to use draft posts where only published posts are allowed by issuing a compiler error.

Let’s consider the first part of main in Listing 17-11:

Filename: src/main.rs

fn main() {
    let mut post = Post::new();

    post.add_text("I ate a salad for lunch today");
    assert_eq!("", post.content());
}

We still enable the creation of new posts in the draft state using Post::new and the ability to add text to the post’s content. But instead of having a content method on a draft post that returns an empty string, we’ll make it so draft posts don’t have the content method at all. That way, if we try to get a draft post’s content, we’ll get a compiler error telling us the method doesn’t exist. As a result, it will be impossible for us to accidentally display draft post content in production because that code won’t even compile. Listing 17-19 shows the definition of a Post struct and a DraftPost struct, as well as methods on each.

Filename: src/lib.rs

pub struct Post {
    content: String,
}

pub struct DraftPost {
    content: String,
}

impl Post {
  1 pub fn new() -> DraftPost {
        DraftPost {
            content: String::new(),
        }
    }

  2 pub fn content(&self) -> &str {
        &self.content
    }
}

impl DraftPost {
  3 pub fn add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }
}

Listing 17-19: A Post with a content method and a DraftPost without a content method

Both the Post and DraftPost structs have a private content field that stores the blog post text. The structs no longer have the state field because we’re moving the encoding of the state to the types of the structs. The Post struct will represent a published post, and it has a content method that returns the content [2].

We still have a Post::new function, but instead of returning an instance of Post, it returns an instance of DraftPost [1]. Because content is private and there aren’t any functions that return Post, it’s not possible to create an instance of Post right now.

The DraftPost struct has an add_text method, so we can add text to content as before [3], but note that DraftPost does not have a content method defined! So now the program ensures all posts start as draft posts, and draft posts don’t have their content available for display. Any attempt to get around these constraints will result in a compiler error.

Implementing Transitions as Transformations into Different Types

So how do we get a published post? We want to enforce the rule that a draft post has to be reviewed and approved before it can be published. A post in the pending review state should still not display any content. Let’s implement these constraints by adding another struct, PendingReviewPost, defining the request_review method on DraftPost to return a PendingReviewPost and defining an approve method on PendingReviewPost to return a Post, as shown in Listing 17-20.

Filename: src/lib.rs

impl DraftPost {
    --snip--
    pub fn request_review(self) -> PendingReviewPost {
        PendingReviewPost {
            content: self.content,
        }
    }
}

pub struct PendingReviewPost {
    content: String,
}

impl PendingReviewPost {
    pub fn approve(self) -> Post {
        Post {
            content: self.content,
        }
    }
}

Listing 17-20: A PendingReviewPost that gets created by calling request_review on DraftPost and an approve method that turns a PendingReviewPost into a published Post

The request_review and approve methods take ownership of self, thus consuming the DraftPost and PendingReviewPost instances and transforming them into a PendingReviewPost and a published Post, respectively. This way, we won’t have any lingering DraftPost instances after we’ve called request_review on them, and so forth. The PendingReviewPost struct doesn’t have a content method defined on it, so attempting to read its content results in a compiler error, as with DraftPost. Because the only way to get a published Post instance that does have a content method defined is to call the approve method on a PendingReviewPost, and the only way to get a PendingReviewPost is to call the request_review method on a DraftPost, we’ve now encoded the blog post workflow into the type system.

But we also have to make some small changes to main. The request_review and approve methods return new instances rather than modifying the struct they’re called on, so we need to add more let post = shadowing assignments to save the returned instances. We also can’t have the assertions about the draft and pending review posts’ contents be empty strings, nor do we need them: we can’t compile code that tries to use the content of posts in those states any longer. The updated code in main is shown in Listing 17-21.

Filename: src/main.rs

use blog::Post;

fn main() {
    let mut post = Post::new();

    post.add_text("I ate a salad for lunch today");

    let post = post.request_review();

    let post = post.approve();

    assert_eq!("I ate a salad for lunch today", post.content());
}

Listing 17-21: Modifications to main to use the new implementation of the blog post workflow

The changes we needed to make to main to reassign post mean that this implementation doesn’t quite follow the object-oriented state pattern anymore: the transformations between the states are no longer encapsulated entirely within the Post implementation. However, our gain is that invalid states are now impossible because of the type system and the type checking that happens at compile time! This ensures that certain bugs, such as display of the content of an unpublished post, will be discovered before they make it to production.

Try the tasks suggested at the start of this section on the blog crate as it is after Listing 17-21 to see what you think about the design of this version of the code. Note that some of the tasks might be completed already in this design.

We’ve seen that even though Rust is capable of implementing object-oriented design patterns, other patterns, such as encoding state into the type system, are also available in Rust. These patterns have different trade-offs. Although you might be very familiar with object-oriented patterns, rethinking the problem to take advantage of Rust’s features can provide benefits, such as preventing some bugs at compile time. Object-oriented patterns won’t always be the best solution in Rust due to certain features, like ownership, that object-oriented languages don’t have.

Summary

Regardless of whether you think Rust is an object-oriented language after reading this chapter, you now know that you can use trait objects to get some object-oriented features in Rust. Dynamic dispatch can give your code some flexibility in exchange for a bit of runtime performance. You can use this flexibility to implement object-oriented patterns that can help your code’s maintainability. Rust also has other features, like ownership, that object-oriented languages don’t have. An object-oriented pattern won’t always be the best way to take advantage of Rust’s strengths, but it is an available option.

Next, we’ll look at patterns, which are another of Rust’s features that enable lots of flexibility. We’ve looked at them briefly throughout the book but haven’t seen their full capability yet. Let’s go!