我正在尝试制作类似不可变字典特征的东西,可以在不影响以前版本的情况下添加新项目(引用)并使用它。最小的例子:
#[derive(Clone)]
pub struct SetOfValues<'a> {
value: Vec<&'a i32>,
}
pub trait TheSetAccessor<'b> {
fn with_additional_values(&self, new_set: Vec<&'b i32>) -> Box<dyn TheSetAccessor<'b>>;
fn get_from_set(&self, index: usize) -> &i32;
}
impl<'a, 'b : 'a> TheSetAccessor<'b> for SetOfValues<'a> {
fn with_additional_values(&self, new_set: Vec<&'b i32>) -> Box<dyn TheSetAccessor<'b>> {
Box::new(SetOfValues { value: new_set } )
}
fn get_from_set(&self, index: usize) -> &i32 {
self.value[index]
}
}
fn usage() {
let a = 0;
let set = SetOfValues {
value: vec![&a]
};
// ...
let b = 1;
let extended_set = set.with_additional_values(vec![&a, &b]);
// ...
let got_b = extended_set.get_from_set(1);
}
错误信息如下:
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
--> src/test.rs:13:18
|
13 | Box::new(SetOfValues { value: new_set } )
| ^^^^^^^^^^^
|
note: first, the lifetime cannot outlive the lifetime `'b` as defined here...
--> src/test.rs:11:10
|
11 | impl<'a, 'b : 'a> TheSetAccessor<'b> for SetOfValues<'a> {
| ^^
note: ...so that the expression is assignable
--> src/test.rs:13:39
|
13 | Box::new(SetOfValues { value: new_set } )
| ^^^^^^^
= note: expected `Vec<&i32>`
found `Vec<&'b i32>`
= note: but, the lifetime must be valid for the static lifetime...
note: ...so that the types are compatible
--> src/test.rs:13:9
|
13 | Box::new(SetOfValues { value: new_set } )
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: expected `Box<(dyn TheSetAccessor<'b> + 'static)>`
found `Box<dyn TheSetAccessor<'b>>`
据我了解,新的 SetOfValues 应该具有传递的向量('b)的生命周期,但这部分
首先,生命周期不能超过此处定义的生命周期
...'b
正如我所见,这表明 SetOfValues 的新实例有另一个生命周期(“静态?”),它的生命周期应该比“b”长。我不太明白我该如何限制这一生。我该怎么做才能使这段代码正常工作?
这是因为
dyn Trait
实际上是dyn Trait + 'static
。因此,dyn TheSetAccessor<'b>
实际上是dyn TheSetAccessor<'b> + 'static
,并且不能包含任何非'static
生命周期,因此它需要'b: 'static
。
要放宽这一界限,请为该特征添加一生:
dyn TheSetAccessor<'b> + 'b
。请注意,这可能不是最佳解决方案,具体取决于您的用例。
fn with_additional_values(&self, new_set: Vec<&'b i32>) -> Box<dyn TheSetAccessor<'b> + 'b>;
游乐场。