'static：std :: marker :: Sized`不滿足 - 我需要Box嗎？

Question

我即將給出一個特徵作爲參數來存儲它通過名爲new的構造方法。

用於結構類型的性狀給定爲參數這裏：

渲染 .RS

use super::shapes::Shape; 

pub struct Renderer; 

impl Renderer{ 
    pub fn set_shape<T : Shape>(&self, shape: T) -> T::Builder{ 
     T::Builder::new(shape) 
    } 
} 

然後由相關聯的類型指定的Builder的構造器將被稱爲

shapebuilder.rs

use super::shapes::Shape; 
use super::shapes::Rectangle; 

pub trait ShapeBuilder{ 
    fn new<T:Shape>(shape: T) -> Self; 
} 

pub struct RectangleBuilder{ 
    shape: Shape<Builder=RectangleBuilder> 
} 

impl ShapeBuilder for RectangleBuilder{ 
    fn new<T:Shape>(shape: T) -> Self{ 
     RectangleBuilder{ 
      shape: shape as Rectangle 
     } 
    } 
} 

在這一點上我已經想指出編譯器輸出

compiler_output

error[E0277]: the trait bound `shapes::Shape<Builder=shapebuilder::RectangleBuilder> + 'static: std::marker::Sized` is not satisfied 
    --> shapes.rs:14:6 
    | 
14 | impl Shape for Rectangle{ 
    |  ^^^^^ 
    | 
    = note: `shapes::Shape<Builder=shapebuilder::RectangleBuilder> + 'static` does not have a constant size known at compile-time 
    = note: required because it appears within the type `shapebuilder::RectangleBuilder` 
    = note: required by `shapes::Shape` 

error: aborting due to previous error 

我發現這裏的SO類似的問題這跟一些關於拳擊。我試圖將每個參數類型都填滿以解決問題。像這樣盒裝它shape: Box<T>。沒有成功。我是否需要裝箱？我瞭解編譯器無法解析特徵大小的問題，因爲特定/具體結構類型可能根據其字段/屬性具有不同的大小。但我仍然無法找到解決方案。希望它是微不足道的。

---

NOT involed模塊（我認爲）上市完整性

shapes.rs

use super::shapebuilder::ShapeBuilder; 
use super::shapebuilder::RectangleBuilder; 

pub trait Shape{ 
    type Builder: ShapeBuilder; 
} 

#[derive(Clone, Copy)] 
pub struct Rectangle{ 
    pub height: usize, 
    pub width: usize, 
} 

impl Shape for Rectangle{ 
    type Builder = RectangleBuilder; 
} 

lib.rs

pub mod renderer; 
mod shapes; 
mod shapebuilder; 

Answer 1

那麼，編譯器並沒有真正指出錯誤的根源。該問題是在這裏：

pub struct RectangleBuilder { 
    shape: Shape<Builder=RectangleBuilder> 
    //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this is an unsized type! 
} 

Shape是性狀，並且用它作爲一個類型產生一個未施膠類型。我們可以將其修復以解決此錯誤：

pub struct RectangleBuilder { 
    shape: Box<Shape<Builder=RectangleBuilder>> 
} 

但是，那麼，我們該怎麼做呢？

impl ShapeBuilder for RectangleBuilder { 
    fn new<T: Shape>(shape: T) -> Self { 
     RectangleBuilder { 
      shape: shape as Rectangle 
      //  ^^^^^^^^^^^^^^^^^^ can't cast a generic type! 
     } 
    } 
} 

如果RectangleBuilder確實將準備接受任何Shape其Builder是RectangleBuilder，那麼我們刪除的演員和添加適當的約束在必要。

pub mod renderer { 
    use super::shapes::Shape; 
    use super::shapebuilder::ShapeBuilder; 

    pub struct Renderer; 

    impl Renderer { 
     pub fn set_shape<T: Shape + 'static>(&self, shape: T) -> T::Builder { 
      T::Builder::new(shape) 
     } 
    } 
} 

mod shapes { 
    use super::shapebuilder::ShapeBuilder; 
    use super::shapebuilder::RectangleBuilder; 

    pub trait Shape { 
     type Builder: ShapeBuilder; 
    } 

    #[derive(Clone, Copy)] 
    pub struct Rectangle { 
     pub height: usize, 
     pub width: usize, 
    } 

    impl Shape for Rectangle { 
     type Builder = RectangleBuilder; 
    } 
} 

mod shapebuilder { 
    use super::shapes::Shape; 

    pub trait ShapeBuilder: Sized { 
     fn new<T: Shape<Builder=Self> + 'static>(shape: T) -> Self; 
    } 

    pub struct RectangleBuilder { 
     shape: Box<Shape<Builder=RectangleBuilder> + 'static>, 
    } 

    impl ShapeBuilder for RectangleBuilder { 
     fn new<T: Shape<Builder=Self> + 'static>(shape: T) -> Self { 
      RectangleBuilder { 
       shape: Box::new(shape) 
      } 
     } 
    } 
} 

的'static束縛它限制可以存儲在特定Shape實例的引用。'static意味着實現不能包含引用，除非它們的生存期爲'static。

但是，如果你需要在RectangleBuilder使用Rectangle的字段，然後RectangleBuilder應該只接受Rectangle S，而不是任何形狀。我們可以再次使用關聯的類型來表達這一點。

pub mod renderer { 
    use super::shapes::Shape; 
    use super::shapebuilder::ShapeBuilder; 

    pub struct Renderer; 

    impl Renderer { 
     pub fn set_shape<T: Shape>(&self, shape: T) -> T::Builder { 
      T::Builder::new(shape) 
     } 
    } 
} 

mod shapes { 
    use super::shapebuilder::ShapeBuilder; 
    use super::shapebuilder::RectangleBuilder; 

    pub trait Shape { 
     type Builder: ShapeBuilder<Shape=Self>; 
    } 

    #[derive(Clone, Copy)] 
    pub struct Rectangle { 
     pub height: usize, 
     pub width: usize, 
    } 

    impl Shape for Rectangle { 
     type Builder = RectangleBuilder; 
    } 
} 

mod shapebuilder { 
    use super::shapes::Shape; 
    use super::shapes::Rectangle; 

    pub trait ShapeBuilder: Sized { 
     type Shape: Shape + ?Sized; 

     fn new(shape: Self::Shape) -> Self; 
    } 

    pub struct RectangleBuilder { 
     shape: Rectangle, 
    } 

    impl ShapeBuilder for RectangleBuilder { 
     type Shape = Rectangle; 

     fn new(shape: Self::Shape) -> Self { 
      RectangleBuilder { 
       shape: shape 
      } 
     } 
    } 
} 

在ShapeBuilder，我們增加了一個Shape相關聯的類型，指定每個ShapeBuilder將要操作的是什麼類型的Shape。 ShapeBuilder::new現在使用此關聯類型來代替類型參數來指定其操作數的類型。請注意，+ ?Sized綁定是必要的，否則會有一個隱含的+ Sized綁定，Rust抱怨Shape並不意味着Sized。解決此問題的另一種方法是將: Sized添加到Shape的定義中。

pub trait Shape: Sized { 
    type Builder: ShapeBuilder<Shape=Self>; 
}