在trait中定义一个方法，返回Self的默认实现。

Question

我想拥有以下功能

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));
}

(游乐场)

这个不能编译。

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`

能否改变一下方法使其工作？是否可以让特质对象有一个方法来返回一些实现 Self? 如果不是，为什么不是？

Answer 1

落实 default 特质对象类型上，而不是特质上。

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

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

参见:

• 为什么我要在trait上实现方法，而不是作为trait的一部分？

Answer 2

我构建了一个类似的API的变通方法。

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),
        }
    }
}

游乐场

有了这些，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));
}