未完成的模块（我的观点）列出了完整性

Question

我准备通过一个名为new的构造方法来存储它作为参数。

结构类型的特征在此处作为参数给出：

渲染器 .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)
    }
}

然后将调用相关类型指定的Builder的构造函数

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
        }
    }
}

此时我已经想要指出编译器输出

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

我在SO上发现了类似的问题，这些问题讲述了拳击事件。我尝试将每个参数类型设置为Box来解决问题。装箱就像这样shape: Box<T>。没有成功。我需要装箱吗？我理解编译器无法解析特征大小的问题，因为特定/具体结构类型可以根据其字段/属性具有不同的大小。我仍然无法找到解决方案。希望它是微不足道的。

未完成的模块（我的观点）列出了完整性

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

好吧，编译器并没有真正指出错误的来源。问题出在这里：

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

Shape是一个特征，并将其用作类型会产生一个未大小的类型。我们可以将其设置为修复此错误：

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

但是，那么我们如何处理演员呢？

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确实准备好接受其Shape为Builder的任何RectangleBuilder，那么让我们删除演员并在必要时添加适当的约束。

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)
            }
        }
    }
}

'static绑定限制了可以存储在特定Shape实例中的引用。 'static表示实现不能包含引用，除非它们具有'static生存期。

但是，如果您需要在Rectangle中使用RectangleBuilder的字段，那么RectangleBuilder应该只接受Rectangle，而不是任何形状。我们可以再次使用相关类型来表达这一点。

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中，我们添加了一个Shape关联类型，用于指定每个Shape将运行的ShapeBuilder类型。 ShapeBuilder::new现在使用此关联类型而不是类型参数来指定其操作数的类型。请注意，需要+ ?Sized绑定，因为否则存在隐式+ Sized绑定，并且Rust抱怨Shape并不暗示Sized。解决此问题的另一种方法是将: Sized添加到Shape的定义中。

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

'static：std :: marker :: Sized`不满意 - 我需要Box吗？

未完成的模块（我的观点）列出了完整性

1 个答案: