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object safty
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src/ch17-02-trait-objects.md
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@ -258,28 +258,16 @@ objects. Clone is an example of one. You'll get errors that will let you know
if a trait can't be a trait object, look up object safety if you're interested
in the details"? Thanks! /Carol -->
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:
不是所有的trait都可以被放进trait对象中; 只有*对象安全的*trait才可以这样做. 一个trait只有同时满足如下两点时才被认为是对象安全的:
* The trait does not require `Self` to be `Sized`
* All of the trait's methods are object safe.
* 该trait要求`Self`不是`Sized`;
* 该trait的所有方法都是对象安全的;
`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.
`Self`是一个类型的别名关键字，它表示当前正被实现的trait类型或者是方法所属的类型. `Sized`是一个像在第16章中介绍的`Send`和`Sync`那样的标记trait, 在编译时它会自动被放进大小确定的类型里，比如`i32`和引用. 大小不确定的类型有切片(`[T]`)和trait对象.
`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.
`Sized`是一个默认会被绑定到所有常规类型参数的内隐trait. Rust中要求一个类型是`Sized`的最具可用性的用法是让`Sized`成为一个默认的trait绑定，这样我们就可以在大多数的常规的用法中不去写`T: Sized`了. 如果我们想在切片(slice)中使用一个trait, 我们需要取消对`Sized`的trait绑定, 我们只需制定`T: ?Sized`作为trait绑定.
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:
默认绑定到`Self: ?Sized`的trait可以被实现到是`Sized`或非`Sized`的类型上. 如果我们创建一个不绑定`Self: ?Sized`的trait`Foo`，它看上去应该像这样:
```rust
trait Foo: Sized {
@ -287,40 +275,21 @@ trait Foo: Sized {
}
```
The trait `Sized` is now a *super trait* of trait `Foo`, which means trait
`Foo` requires types that implement `Foo` (that is, `Self`) to be `Sized`.
We're going to talk about super traits in more detail in Chapter 19.
Trait`Sized`现在就是trait`Foo`的一个*超级trait*, 也就是说trait`Foo`需要实现了`Foo`的类型(即`Self`)是`Sized`. 我们将在第19章中更详细的介绍超trait(supertrait).
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!
像`Foo`那样要求`Self`是`Sized`的trait不允许成为trait对象的原因是不可能为trait对象`Foo`实现trait`Foo`: trait对象是无确定大小的，但是`Foo`要求`Self`是`Sized`. 一个类型不可能同时既是有大小的又是无确定大小的.
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:
第二点说对象安全要求一个trait的所有方法必须是对象安全的. 一个对象安全的方法满足下列条件:
* 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
* 它要求`Self`是`Sized`或者
* 它符合下面全部三点:
* 它不包含任意类型的常规参数
* 它的第一个参数必须是类型`Self`或一个引用到`Self`的类型(也就是说它必须是一个方法而非关联函数并且以`self`、`&self`或`&mut self`作为第一个参数)
* 除了第一个参数外它不能在其它地方用`Self`作为方法的参数签名
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.
虽然这些规则有一点形式化, 但是换个角度想一下: 如果你的方法在它的参数签名的其它地方也需要具体的`Self`类型参数, 但是一个对象又忘记了它的具体类型是什么, 这时该方法就无法使用被它忘记的原先的具体类型. 当该trait被使用时, 被具体类型参数填充的常规类型参数也是如此: 这个具体的类型就成了实现该trait的类型的某一部分, 如果使用一个trait对象时这个类型被抹掉了, 就没有办法知道该用什么类型来填充这个常规类型参数.
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:
一个trait的方法不是对象安全的一个例子是标准库中的`Clone`trait. `Clone`trait的`clone`方法的参数签名是这样的:
```rust
pub trait Clone {
@ -328,21 +297,11 @@ pub trait Clone {
}
```
`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.
`String`实现了`Clone` trait, 当我们在一个`String实例上调用`clone`方法时, 我们会得到一个`String`实例. 同样地, 如果我们在一个`Vec`实例上调用`clone`方法, 我们会得到一个`Vec`实例. `clone`的参数签名需要知道`Self`是什么类型, 因为它需要返回这个类型.
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.
如果我们想在像17-3中列出的`Draw`trait那样的trait上实现`Clone`, 我们就不知道`Self`将会是一个`Button`, 一个`SelectBox`, 或者是其它的在将来要实现`Draw`trait的类型.
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:
如果你做了违反trait对象的对象安全性规则的事情, 编译器将会告诉你. 比如, 如果你实现在17-4中列出的`Screen`结构, 你想让该结构像这样持有实现了`Clone`trait的类型而不是`Draw`trait:
```rust,ignore
pub struct Screen {
@ -350,7 +309,7 @@ pub struct Screen {
}
```
We'll get this error:
我们将会得到下面的错误:
```text
error[E0038]: the trait `std::clone::Clone` cannot be made into an object

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## 面向对象设计模式的实现
让我们看一下状态设计模式和怎样在Rust中来使用它的例子. *状态模式*就是当一个值有多个内部状态时，值的行为改变基于内部状态. Let's look at an example of the state design pattern and how to use it in Rust.
The *state pattern* is when a value has some internal state, and the value's
behavior changes based on the internal state. The internal state is represented
by a set of objects that inherit shared functionality (we'll use structs and
traits since Rust doesn't have objects and inheritance). Each state object is
responsible for its own behavior and the rules for when it should change into
another state. The value that holds one of these state objects doesn't know
anything about the different behavior of the states or when to transition
between states. In the future when requirements 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.
In order to explore this idea, we're going to implement a blog post workflow in
an incremental way. The workflow that we want our blog posts to follow, once
we're done with the implementation, is:
1. A blog post starts as an empty draft.
2. Once the draft is done, we request a review of the post.
3. Once the post is approved, it gets published.
4. Only published blog posts return content to print so that we can't
accidentally print the text of a post that hasn't been approved.
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 stay an unpublished draft.
Listing 17-11 shows this workflow in code form. This is an example usage of the
API we're going to implement in a library crate named `blog`:
<span class="filename">Filename: src/main.rs</span>
```rust,ignore
extern crate blog;
use blog::Post;
fn main() {
let mut post = Post::new();
post.add_text("I ate a salad for lunch today");
assert_eq!("", post.content());
post.request_review();
assert_eq!("", post.content());
post.approve();
assert_eq!("I ate a salad for lunch today", post.content());
}
```
<span class="caption">Listing 17-11: Code that demonstrates the desired
behavior we want our `blog` crate to have</span>
We want to be able to create a new draft blog post with `Post::new`. Then, we
want to add some text to the blog post while we're in the draft state. If we
try to print out the post's content immediately, though, we shouldn't get any
text, since the post is still a draft. We've added an `assert_eq!` here for
demonstration purposes. Asserting that a draft blog post returns an empty
string from the `content` method would make an excellent unit test in our
library, but we're not going to write tests for this example.
Next, we want to be able to request a review of our post, and `content` should
still return an empty string while waiting for a review. Lastly, when we
approve the blog post, it should get published, which means the text we added
will be returned when we call `content`.
Notice that the only type we're interacting with from the crate is the `Post`
type. The various states a post can be in (draft, waiting for review,
published) are managed internally to the `Post` type. The states change due to
the methods we call on the `Post` instance, but we don't have to manage the
state changes directly. This also means we won't make a mistake with the
states, like forgetting to request a review before publishing.
### Defining `Post` and Creating a New Instance in the Draft State
Let's get started on the implementation of the library! We know we want to have
a public `Post` struct that holds some content, so let's 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're also going to have a
private trait `State`. `Post` will hold a trait object of `Box<State>` inside
an `Option` in a private field named `state`. We'll see why the `Option` is
necessary in a bit. The `State` trait defines all the behavior different post
states share, and the `Draft`, `PendingReview`, and `Published` states will all
implement the `State` trait. For now, the trait does not have any methods, and
we're going to start by defining just the `Draft` state since that's the state
we want to start in:
<span class="filename">Filename: src/lib.rs</span>
```rust
pub struct Post {
state: Option<Box<State>>,
content: String,
}
impl Post {
pub fn new() -> Post {
Post {
state: Some(Box::new(Draft {})),
content: String::new(),
}
}
}
trait State {}
struct Draft {}
impl State for Draft {}
```
<span class="caption">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 that implements `State`</span>
When we create a new `Post`, we set its `state` field to a `Some` value holding
a `Box` pointing to a new instance of the `Draft` struct. This ensures whenever
we create a new instance of `Post`, it'll start out as a draft. Because the
`state` field of `Post` is private, there's no way to create a `Post` in any
other state!
### Storing the Text of the Post Content
In the `Post::new` function, we set the `content` field to a new, empty
`String`. In Listing 17-11, we showed that we want to be able to call a method
named `add_text` and pass a `&str` to it to add that text to the content of the
blog post. We're choosing to implement this as a method rather than exposing
the `content` field as `pub` because we want to be able to control how the
`content` field's data is read by implementing a method later. The `add_text`
method is pretty straightforward though, let's add the implementation in
Listing 17-13 to the `impl Post` block:
<span class="filename">Filename: src/lib.rs</span>
```rust
# pub struct Post {
# content: String,
# }
#
impl Post {
// ...snip...
pub fn add_text(&mut self, text: &str) {
self.content.push_str(text);
}
}
```
<span class="caption">Listing 17-13: Implementing the `add_text` method to add
text to a post's `content`</span>
`add_text` takes a mutable reference to `self`, since 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 isn't part of the state pattern since its behavior doesn't
depend on the state that the post is in. The `add_text` method doesn't interact
with the `state` field at all, but it is part of the behavior we want to
support.
### Content of a Draft Post is Empty
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 since the post is still in
the draft state, as shown on line 8 of 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're going to change this later once
we implement the ability to change a post's state to be published. With what we
have so far, though, posts can only be in the draft state, which means the post
content should always be empty. Listing 17-14 shows this placeholder
implementation:
<span class="filename">Filename: src/lib.rs</span>
```rust
# pub struct Post {
# content: String,
# }
#
impl Post {
// ...snip...
pub fn content(&self) -> &str {
""
}
}
```
<span class="caption">Listing 17-14: Adding a placeholder implementation for
the `content` method on `Post` that always returns an empty string slice</span>
With this added `content` method, everything in Listing 17-11 up to line 8
works as we intend.
### Requesting a Review of the Post Changes its State
Next up is requesting a review of a post, which should change its state from
`Draft` to `PendingReview`. We want `post` to have a public method named
`request_review` that will take a mutable reference to `self`. Then we're going
to call an internal `request_review` method on the state that we're holding, and
this second `request_review` method will consume the current state and return a
new state. In order to be able to consume the old state, the second `request_review`
method needs to take ownership of the state value. This is where the `Option` comes
in: we're going to `take` the `Some` value out of the `state` field and leave a
`None` in its place since Rust doesn't let us have unpopulated fields in
structs. Then we'll set the post's `state` value to the result of this
operation. Listing 17-15 shows this code:
<span class="filename">Filename: src/lib.rs</span>
```rust
# pub struct Post {
# state: Option<Box<State>>,
# content: String,
# }
#
impl Post {
// ...snip...
pub fn request_review(&mut self) {
if let Some(s) = self.state.take() {
self.state = Some(s.request_review())
}
}
}
trait State {
fn request_review(self: Box<Self>) -> Box<State>;
}
struct Draft {}
impl State for Draft {
fn request_review(self: Box<Self>) -> Box<State> {
Box::new(PendingReview {})
}
}
struct PendingReview {}
impl State for PendingReview {
fn request_review(self: Box<Self>) -> Box<State> {
self
}
}
```
<span class="caption">Listing 17-15: Implementing `request_review` methods on
`Post` and the `State` trait</span>
We've added the `request_review` method to the `State` trait; 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>`, which is what we want because we're transforming the
old state into a new state, and we want the old state to no longer be valid.
The implementation for the `request_review` method on `Draft` is to return a
new, boxed instance of the `PendingReview` struct, which is a new type we've
introduced that represents the state when a post is waiting for a review. The
`PendingReview` struct also implements the `request_review` method, but it
doesn't do any transformations. It returns itself since requesting a review on
a post that's already in the `PendingReview` state 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 what its `state` value
is. Each state is responsible for its own rules.
We're going to leave the `content` method on `Post` as it is, returning an
empty string slice. We can now have a `Post` in the `PendingReview` state, not
just the `Draft` state, but we want the same behavior in the `PendingReview`
state. Listing 17-11 now works up until line 11!
### Approving a Post Changes the Behavior of `content`
The `approve` method on `Post` will be similar to that of the `request_review`
method: it will set the `state` to the value that the current state says it
should have when that state is approved. We'll need to add the `approve` method
to the `State` trait, and we'll add a new struct that implements `State`, the
`Published` state. Listing 17-16 shows the new code:
<span class="filename">Filename: src/lib.rs</span>
```rust
# pub struct Post {
# state: Option<Box<State>>,
# content: String,
# }
#
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<State>;
fn approve(self: Box<Self>) -> Box<State>;
}
struct Draft {}
impl State for Draft {
# fn request_review(self: Box<Self>) -> Box<State> {
# Box::new(PendingReview {})
# }
#
// ...snip...
fn approve(self: Box<Self>) -> Box<State> {
self
}
}
struct PendingReview {}
impl State for PendingReview {
# fn request_review(self: Box<Self>) -> Box<State> {
# Box::new(PendingReview {})
# }
#
// ...snip...
fn approve(self: Box<Self>) -> Box<State> {
Box::new(Published {})
}
}
struct Published {}
impl State for Published {
fn request_review(self: Box<Self>) -> Box<State> {
self
}
fn approve(self: Box<Self>) -> Box<State> {
self
}
}
```
<span class="caption">Listing 17-16: Implementing the `approve` method on
`Post` and the `State` trait</span>
Similarly to `request_review`, if we call the `approve` method on a `Draft`, it
will have no effect since it will return `self`. When we call `approve` on
`PendingReview`, it returns a new, boxed instance of the `Published` struct.
The `Published` struct implements the `State` trait, and for both the
`request_review` method and the `approve` method, it returns itself since the
post should stay in the `Published` state in those cases.
Now for updating the `content` method on `Post`: we want to return the value in
the post's `content` field if its state is `Published`, otherwise we want to
return an empty string slice. Because the goal is to keep all the rules like
this in the structs that implement `State`, we're going to call a `content`
method on the value in `state` and pass the post instance (that is, `self`) as
an argument. Then we'll return the value returned from the `content` method on
the `state` value as shown in Listing 17-17:
<span class="filename">Filename: src/lib.rs</span>
```rust
# trait State {
# fn content<'a>(&self, post: &'a Post) -> &'a str;
# }
# pub struct Post {
# state: Option<Box<State>>,
# content: String,
# }
#
impl Post {
// ...snip...
pub fn content(&self) -> &str {
self.state.as_ref().unwrap().content(&self)
}
// ...snip...
}
```
<span class="caption">Listing 17-17: Updating the `content` method on `Post` to
delegate to a `content` method on `State`</span>
We're calling the `as_ref` method on the `Option` because we want a reference
to the value inside the `Option`. We're then calling the `unwrap` method, which
we know will never panic because all the methods on `Post` ensure that the
`state` value will have a `Some` value in it when those methods are done. This
is one of the cases we talked about in Chapter 12 where we know that a `None`
value is never possible even though the compiler isn't able to understand that.
The `content` method on the `State` trait is where the logic for what content
to return will be. We're going to add a default implementation for the
`content` method that returns an empty string slice. That lets us not need to
implement `content` on the `Draft` and `PendingReview` structs. The `Published`
struct will override the `content` method and will return the value in
`post.content`, as shown in Listing 17-18:
<span class="filename">Filename: src/lib.rs</span>
```rust
# pub struct Post {
# content: String
# }
trait State {
// ...snip...
fn content<'a>(&self, post: &'a Post) -> &'a str {
""
}
}
// ...snip...
struct Published {}
impl State for Published {
// ...snip...
fn content<'a>(&self, post: &'a Post) -> &'a str {
&post.content
}
}
```
<span class="caption">Listing 17-18: Adding the `content` method to the `State`
trait</span>
Note that we need lifetime annotations on this method, like we discussed in
Chapter 10. We're taking a reference to a `post` as an argument, and we're
returning a reference to a part of that `post`, so the lifetime of the returned
reference is related to the lifetime of the `post` argument.
### Tradeoffs of the State Pattern
We've shown that Rust is capable of implementing the object-oriented state
pattern in order to encapsulate the different kinds of behavior that a post
should have that depends on the state that the post is in. The methods on
`Post` don't know anything about the different kinds of behavior. The way this
code is organized, we have one place to look in order to find out all the
different ways that a published post behaves: the implementation of the `State`
trait on the `Published` struct.
An alternative implementation that didn't use the state pattern might have
`match` statements in the methods on `Post` or even in the code that uses
`Post` (`main` in our case) that checks what the state of the post is and
changes behavior in those places instead. That would mean we'd have a lot of
places to look in order to understand all the implications of a post being in
the published state! This would get worse the more states we added: each of
those `match` statements would need another arm. With the state pattern, the
`Post` methods and the places we use `Post` don't need `match` statements and
adding a new state only involves adding a new `struct` and implementing the
trait methods on that one struct.
This implementation is easy to extend to add more functionality. Here are some
changes you can try making to the code in this section to see for yourself what
it's like to maintain code using this pattern over time:
- Only allow adding text content when a post is in the `Draft` state
- Add a `reject` method that changes the post's state from `PendingReview` back
to `Draft`
- Require two calls to `approve` before changing the state to `Published`
A downside of the state pattern is that since the states implement the
transitions between the 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 nicer if `PendingReview` wouldn't need to
change because of the addition of a new state, but that would mean switching to
another design pattern.
There are a few bits of duplicated logic that are a downside of this
implementation in Rust. It would be nice if we could make default
implementations for the `request_review` and `approve` methods on the `State`
trait that return `self`, but this would violate object safety since the trait
doesn't know what the concrete `self` will be exactly. We want to be able to
use `State` as a trait object, so we need its methods to be object safe.
The other duplication that would be nice to get rid of is the similar
implementations of the `request_review` and `approve` methods on `Post`. They
both delegate to the implementation of the same method on the value in the
`Option` in the `state` field, 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 Appendix E on
macros).
A downside of implementing this object-oriented pattern exactly as it's defined
for object-oriented languages is that we're not taking advantage of Rust's
strengths as much as we could be. Let's take a look at some changes we can make
to this code that can make invalid states and transitions into compile time
errors.
#### Encoding States and Behavior as Types
We're going to show how to rethink the state pattern a bit in order to get a
different set of tradeoffs. Rather than encapsulating the states and
transitions completely so that outside code has no knowledge of them, we're
going to encode the states into different types. When the states are types,
Rust's type checking will make any attempt to use a draft post where we should
only use published posts into a compiler error.
Let's consider the first part of `main` from Listing 17-11:
<span class="filename">Filename: src/main.rs</span>
```rust,ignore
fn main() {
let mut post = Post::new();
post.add_text("I ate a salad for lunch today");
assert_eq!("", post.content());
}
```
We still want to create a new post in the draft state using `Post::new`, and we
still want to be able 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're going to
make it so that 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 that the
method doesn't exist. This will make it impossible for us to accidentally
display draft post content in production, since that code won't even compile.
Listing 17-19 shows the definition of a `Post` struct, a `DraftPost` struct,
and methods on each:
<span class="filename">Filename: src/lib.rs</span>
```rust
pub struct Post {
content: String,
}
pub struct DraftPost {
content: String,
}
impl Post {
pub fn new() -> DraftPost {
DraftPost {
content: String::new(),
}
}
pub fn content(&self) -> &str {
&self.content
}
}
impl DraftPost {
pub fn add_text(&mut self, text: &str) {
self.content.push_str(text);
}
}
```
<span class="caption">Listing 17-19: A `Post` with a `content` method and a
`DraftPost` without a `content` method</span>
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 since we're moving
the encoding of the state to the types of the structs. `Post` will represent a
published post, and it has a `content` method that returns the `content`.
We still have a `Post::new` function, but instead of returning an instance of
`Post`, it returns an instance of `DraftPost`. It's not possible to create an
instance of `Post` right now since `content` is private and there aren't any
functions that return `Post`. `DraftPost` has an `add_text` method defined on
it so that we can add text to `content` as before, but note that `DraftPost`
does not have a `content` method defined! So we've enforced that 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 be a compiler error.
#### Implementing Transitions as Transformations into Different Types
So how do we get a published post then? The rule we want to enforce is 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:
<span class="filename">Filename: src/lib.rs</span>
```rust
# pub struct Post {
# content: String,
# }
#
# pub struct DraftPost {
# content: String,
# }
#
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,
}
}
}
```
<span class="caption">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`</span>
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 `DraftPost` instances lingering around after we've called
`request_review` on them, and so forth. `PendingReviewPost` doesn't have a
`content` method defined on it, so attempting to read its content is a compiler
error like it is 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.
This does mean we have to make some small changes to `main`. Because
`request_review` and `approve` return new instances rather than modifying the
struct they're called on, 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 post's contents being empty string anymore,
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:
<span class="filename">Filename: src/main.rs</span>
```rust,ignore
extern crate blog;
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());
}
```
<span class="caption">Listing 17-21: Modifications to `main` to use the new
implementation of the blog post workflow</span>
Having to change `main` to reassign `post` is what makes this implementation
not quite following the object-oriented state pattern anymore: the
transformations between the states are no longer encapsulated entirely within
the `Post` implementation. However, we've gained the property of having invalid
states be impossible because of the type system and type checking that happens
at compile time! This ensures that certain bugs, such as displaying the content
of an unpublished post, will be discovered before they make it to production.
Try the tasks suggested that add additional requirements that we mentioned at
the start of this section to see how working with this version of the code
feels.
Even though Rust is capable of implementing object-oriented design patterns,
there are other patterns like encoding state into the type system that are
available in Rust. These patterns have different tradeoffs than the
object-oriented patterns do. While you may be very familiar with
object-oriented patterns, rethinking the problem in order to take advantage of
Rust's features can give benefits like preventing some bugs at compile-time.
Object-oriented patterns won't always be the best solution in Rust, since Rust
has features like ownership that object-oriented languages don't have.
## Summary
No matter whether you think Rust is an object-oriented language or not after
reading this chapter, you've now seen that trait objects are a way to get some
object-oriented features in Rust. Dynamic dispatch can give your code some
flexibility in exchange for a bit of runtime performance. This flexibility can
be used to implement object-oriented patterns that can help with the
maintainability of your code. Rust also has different features, like ownership,
than object-oriented languages. An object-oriented pattern won't always be the
best way to take advantage of Rust's strengths.
Next, let's look at another feature of Rust that enables lots of flexibility:
patterns. We've looked at them briefly throughout the book, but haven't seen
everything they're capable of yet. Let's go!