带有大小超性状的性状仍然有错误“std::标记::大小不满意”[重复]

Question

我有以下代码：

use std::collections::HashMap;

trait T: Sized {}

struct A;

impl T for A {}

fn main() {
    let h: HashMap<String, T>;
}

但是编译器抱怨道:

error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
  --> src\main.rs:10:12
   |
10 |     let h: HashMap<String, T>;
   |            ^^^^^^^^^^^^^^^^^^ `T` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `T`
   = note: required by `std::collections::HashMap`

error[E0038]: the trait `T` cannot be made into an object
  --> src\main.rs:10:12
   |
10 |     let h: HashMap<String, T>;
   |            ^^^^^^^^^^^^^^^^^^ the trait `T` cannot be made into an object
   |
   = note: the trait cannot require that `Self : Sized`

我不理解错误信息，因为我已经将我的特征< code>T标记为< code > size 。我错过了什么吗？

Answer 1

因为我已经将我的特征< code>T标记为< code > size

不，你没有。你说过任何实现T的类型都必须是Sated。trait本身仍然没有大小。您要么需要一个trait对象（例如Box