Question

我希望具有以下功能

trait Policy {
    fn eval(&self, k: u32) -> bool;

    fn default() -> Box<dyn Policy>
    where
        Self: Sized,
    {
        Box::new(MaxPolicy { max: 2 })
    }
}

struct MaxPolicy {
    max: u32,
}

impl Policy for MaxPolicy {
    fn eval(&self, k: u32) -> bool {
        println!("MaxPolicy");
        k < self.max
    }
}

#[test]
fn max_policy() {
    let p = MaxPolicy { max: 2 };
    assert!(!p.eval(3));
}

#[test]
fn default_policy() {
    let p = Policy::default();
    assert!(!p.eval(3));
}

（Playground）

无法编译：

error[E0283]: type annotations needed
  --> src/lib.rs:31:13
   |
4  |     fn default() -> Box<dyn Policy>
   |        -------
5  |     where
6  |         Self: Sized,
   |               ----- required by this bound in `Policy::default`
...
31 |     let p = Policy::default();
   |             ^^^^^^^^^^^^^^^ cannot infer type
   |
   = note: cannot resolve `_: Policy`

是否有可能改变使之起作用的方法？特质对象是否有可能具有返回Self的某些实现的方法？如果没有，为什么不呢？

Answer 1

对特征对象类型而不是对特征执行default：

trait Policy {
    fn eval(&self, k: u32) -> bool;
}

impl dyn Policy {
    fn default() -> Box<Self> {
        Box::new(MaxPolicy { max: 2 })
    }
}

另请参阅：

Why would I implement methods on a trait instead of as part of the trait?

Answer 2

我构建了一个具有类似不错的API的解决方法

trait Policy {
    fn eval(&self, k: u32) -> bool;
}

struct MaxPolicy {
    max: u32,
}

impl Policy for MaxPolicy {
    fn eval(&self, k: u32) -> bool {
        println!("MaxPolicy");
        k < self.max
    }
}

struct MinPolicy {
    min: u32,
}

impl Policy for MinPolicy {
    fn eval(&self, k: u32) -> bool {
        println!("MinPolicy");
        k > self.min
    }
}

enum PolicyEnum {
    Custom(Box<dyn Policy>),
    Default,
}

impl PolicyEnum {
    const DEFAULT: MaxPolicy = MaxPolicy { max: 4 };
}

impl Policy for PolicyEnum {
    fn eval(&self, k: u32) -> bool {
        match self {
            PolicyEnum::Custom(p) => p.eval(k),
            PolicyEnum::Default => Self::DEFAULT.eval(k),
        }
    }
}

Playground

这样就可以做到：

#[test]
fn default() {
    let p = PolicyEnum::Default;

    assert!(p.eval(3));
}

#[test]
fn custom() {
    let p = PolicyEnum::Custom(Box::new(MinPolicy{min: 4}));

    assert!(p.eval(5));
}

在特征中定义一个方法，以返回Self的默认实现

2 个答案: