design-patterns - 如何在 Rust 的装饰器模式中使用生命周期说明符？

Question

我是使用rust 的新手，还没有得到生命周期说明符的东西。为了将不同的关注点分离到不同的结构中，我尝试做一些类似于装饰器模式的事情。但是，以下代码无法编译:

trait T {
    fn foo(self) -> u64;
}

struct Inner {}

impl T for Inner {
    fn foo(self) -> u64 {
        42
    }
}

struct Outer<'a> {
    delegate: &'a T,
}

impl<'a> T for Outer<'a> {
    fn foo(self) -> u64 {
        self.delegate.foo()
    }
}

pub fn dec_test() {
    let inner = &Inner {};
    let outer = Outer{delegate:inner};

    println!("Meaning of life: {}", outer.foo());
}

我得到以下错误

error[E0161]: cannot move a value of type dyn T: the size of dyn T cannot be statically determined
  --> src/lib.rs:19:9
   |
19 |         self.delegate.foo()
   |         ^^^^^^^^^^^^^

error[E0507]: cannot move out of borrowed content
  --> src/lib.rs:19:9
   |
19 |         self.delegate.foo()
   |         ^^^^^^^^^^^^^ cannot move out of borrowed content

error: aborting due to 2 previous errors

Answer 1

忽略第一个关于unsized types的错误，后两个错误都是关于同一个问题:“cannot move out of borrowed content”

这个较小的示例会触发相同的错误:

struct Inner {}

impl Inner {
    // consumes self. requires ownership
    fn foo(self) -> u64 {
        42
    }
}

fn main() {
    let my_inner = Inner {};
    let borrow = &my_inner;
    let answer = borrow.foo(); // oops! we are using a borrowed version that we don't own
    println!("meaning of life {}", answer);
}

有关详细信息，请参阅 this answer ，具体关于所有权的部分或参见chapter four of the rust book .

具体针对您的问题，一种解决方法是更改​​ trait T 以便它仅借用其参数:

trait T {
    fn foo(&self) -> u64; // notice the change: `self` -> `&self`
}

// ...

impl T for Inner {
    fn foo(&self) -> u64 { // notice the change: `self` -> `&self`
        42
    }
}

// ...

impl<'a> T for Outer<'a> {
    fn foo(&self) -> u64 { // notice the change: `self` -> `&self`
        self.delegate.foo()
    }
}

// ...