Question

我有一个特性，它指定了trait方法的生命周期，并且我有一个结构，其中包含一个也需要生命周期的值。我想让结构实现特征，这意味着生命周期需要匹配。但是，我不确定如何表达这一点。

struct Context<'d> {
    name: &'d str,
}

struct Value<'d> {
    name: &'d str,
}

trait Expression {
    fn evaluate<'d>(&self, context: &Context<'d>) -> Value<'d>;
}

struct Constant<'d> {
    value: Value<'d>,
}

尝试1 - 指定方法的生命周期：

impl<'d> Expression for Constant<'d> {
    fn evaluate<'d>(&self, _unused: &Context<'d>) -> Value<'d> {
        self.value
    }
}

这会导致impl上的生命周期被遮蔽，我会收到如下错误：

$ rustc x.rs
x.rs:18:5: 20:6 help: consider using an explicit lifetime parameter as shown: fn evaluate<'d>(&self, _unused: &Context<'d>) -> Value<'d>
x.rs:18     fn evaluate<'d>(&self, _unused: &Context<'d>) -> Value<'d> {
x.rs:19         self.value
x.rs:20     }
x.rs:19:9: 19:19 error: mismatched types: expected `Value<'d>`, found `Value<'d>` (lifetime mismatch)
x.rs:19         self.value
                ^~~~~~~~~~

尝试2 - 未指定生命周期：

impl<'d> Expression for Constant<'d> {
    fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
        self.value
    }
}

这导致我实际上没有实现该方法：

$ rustc x.rs
x.rs:18:5: 20:6 error: method `evaluate` has an incompatible type for trait: expected concrete lifetime, found bound lifetime parameter 'd [E0053]
x.rs:18     fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
x.rs:19         self.value
x.rs:20     }
x.rs:18:60: 20:6 note: expected concrete lifetime is the lifetime 'd as defined on the block at 18:59
x.rs:18     fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
x.rs:19         self.value
x.rs:20     }

Answer 1

你需要给特质本身一个生命周期参数：

trait Expression<'d> {
    fn evaluate(&self, context: &Context<'d>) -> Value<'d>;
}

impl<'d> Expression<'d> for Constant<'d> {
    fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
        self.value
    }
}

Playpen

您收到错误＃1（除了阴影）之外的原因是您的特征限制context具有返回类型的生命周期，而不是self。要约束self，您需要在特征本身上有一个生命周期参数。

你得到错误＃2的原因很简单：生命周期是类型系统的一部分，不能不匹配。您编写的任何通用代码都应该适用于所有特征的实现者 - 如果每个实现中的生命周期都有不同的约束，那么这将无效。

如何统一结构和特征之间的生命周期？

1 个答案: