问题

我正在阅读《Rust for Rustaceans》，它讨论了使用零大小类型来建模库的API。它提供了一个应用程序的框架，但没有展示实际的实现，当我尝试实现它时，失败了。

我需要帮助实现默认值，因为它目前抱怨在创建对象时没有为阶段提供值。实际上，我以为由于它是PhantomData，我不需要提供任何值。

我还需要帮助实现launch。我希望能够返回self以重用现有对象，但它抱怨它是不同的类型。将 `self as Rocket<Launched>` 强制转换也不起作用。

如何使这个框架正常工作的正确方法是什么？

```rust
struct Grounded;
struct Launched;

struct Rocket<Stage = Grounded> {
    stage: std::marker::PhantomData<Stage>,
}

impl Default for Rocket<Grounded> {
    fn default() -> Self { }
}

impl Rocket<Grounded> {
    pub fn launch(self) -> Rocket<Launched> { }
}

impl Rocket<Launched> {
    pub fn accelerate(&mut self) { println!("加速"); }
    pub fn decelerate(&mut self) { println!("减速"); }
}

impl<Stage> Rocket<Stage> {
    pub fn color(&self) { println!("蓝色"); }
    pub fn weight(&self) { println!("10公斤"); }
}

fn main() {
    let rocket: Rocket = Default::default();
}

struct Grounded;
struct Launched;

struct Rocket<Stage = Grounded> {
    stage: std::marker::PhantomData<Stage>,
}

impl Default for Rocket<Grounded> {
    fn default() -> Self { }
}

impl Rocket<Grounded> {
    pub fn launch(self) -> Rocket<Launched> { }
}

impl Rocket<Launched> {
    pub fn accelerate(&mut self) { println!("Accelerating"); }
    pub fn decelerate(&mut self) { println!("Decelerating"); }
}

impl<Stage> Rocket<Stage> {
    pub fn color(&self) { println!("blue"); }
    pub fn weight(&self) { println!("10kg"); }
}

fn main() {
    let rocket: Rocket = Default::default();
}

答案1

得分: 2

每个字段都必须被初始化，即使是 ZST 字段，比如 `PhantomData`。
你可以这样做：
```rust
use std::marker::PhantomData;
impl Default for Rocket<Grounded> {
    fn default() -> Self { 
        Self {
            stage: PhantomData,
        }
    }
}

你可以通过简单地派生 Default 来减少一些样板代码：

#[derive(Default)]
struct Grounded;
#[derive(Default)]
struct Rocket<Stage = Grounded> {
    stage: PhantomData<Stage>,
}

由于 Rocket<Grounded> 和 Rocket<Launched> 是两种不同的类型，你不能使用一个来初始化另一个。你必须创建一个新的火箭：

impl Rocket<Grounded> {
    pub fn launch(self) -> Rocket<Launched> {
        Rocket {
            stage: PhantomData,
        }
    }
}

尽管不太清楚为什么你需要 PhantomData，因为这些阶段已经是 ZST，而且在没有它的情况下也能很好地工作。

<details>
<summary>英文:</summary>

Every field has to be initialized that&#39;s also true for ZST fields such as `PhantomData`
You can do so like this:
```rust
use std::marker::PhantomData;
impl Default for Rocket&lt;Grounded&gt; {
    fn default() -&gt; Self { 
        Self {
            stage: PhantomData,
        }
    }
}

You can remove some of that boilerplate by just deriving Default though:

#[derive(Default)]
struct Grounded;
#[derive(Default)]
struct Rocket<Stage = Grounded> {
    stage: PhantomData<Stage>,
}

Since Rocket<Grounded> and Rocket<Launched> are two different types you can't use one to initialize the other. You have to create a new rocket:

impl Rocket<Grounded> {
    pub fn launch(self) -> Rocket<Launched> {
        Rocket {
            stage: PhantomData,
        }
    }
}

Though it's not exactly clear why you need PhantomData at all since the stages are already ZSTs and it all works just fine without it