使用构建器模式时,“借来的值不够长”

问题描述 投票:4回答:1

我有以下代码:

pub struct Canvas<'a> {
    width: isize,
    height: isize,
    color: Color,
    surface: Surface,
    texture: Texture,
    renderer: &'a Renderer,
}

impl<'a> Canvas<'a> {
    pub fn new(width: isize, height: isize, renderer: &'a Renderer) -> Canvas<'a> {
        let color = Color::RGB(0, 30, 0);
        let mut surface = core::create_surface(width, height);
        let texture = Canvas::gen_texture(&mut surface, width, height, color, renderer);
        Canvas {
            width: width,
            height: height,
            color: color,
            surface: surface,
            texture: texture,
            renderer: renderer,
        }
    }

    pub fn color(&mut self, color: Color) -> &mut Canvas<'a> {
        self.color = color;
        self.texture = Canvas::gen_texture(
            &mut self.surface,
            self.width,
            self.height,
            self.color,
            self.renderer,
        );
        self
    }
}

我希望能够做到这一点:

let mut canvas = Canvas::new(100, 100, &renderer).color(Color::RGB(80, 230, 80));

我收到此错误:

错误:借来的值不够长,让mut canvas = Canvas :: new(100,100,&renderer)

为什么退回的Canvas物体的寿命不够长?如果我将结果存储在中间let中,那么它可以工作;为什么?

rust builder lifetime borrow-checker
1个回答
10
投票

这是一个最小的再现:

#[derive(Debug)]
pub struct Canvas;

impl Canvas {
    fn new() -> Self {
        Canvas
    }

    fn color(&self) -> &Canvas {
        self
    }
}

fn main() {
    let mut canvas = Canvas::new().color();
    //             1 ^~~~~~~~~~~~~
    //                           2 ^~~~~
    println!("{:?}", canvas);
}

锈2015年

error[E0597]: borrowed value does not live long enough
  --> src/main.rs:15:22
   |
15 |     let mut canvas = Canvas::new().color();
   |                      ^^^^^^^^^^^^^        - temporary value dropped here while still borrowed
   |                      |
   |                      temporary value does not live long enough
...
19 | }
   | - temporary value needs to live until here
   |
   = note: consider using a `let` binding to increase its lifetime

Rust 2018

error[E0716]: temporary value dropped while borrowed
  --> src/main.rs:15:22
   |
15 |     let mut canvas = Canvas::new().color();
   |                      ^^^^^^^^^^^^^        - temporary value is freed at the end of this statement
   |                      |
   |                      creates a temporary which is freed while still in use
...
18 |     println!("{:?}", canvas);
   |                      ------ borrow later used here
   |
   = note: consider using a `let` binding to create a longer lived value

出现问题是因为您创建了一个临时变量(1),然后将对该变量的引用传递给方法(2),该方法返回引用。在方法链的末尾,您尝试返回引用并将其存储在变量中,但引用指向一个无处可存的临时项! Rust不会让你引用无效的东西。

部分问题是这不是Builder模式,这只是一个使用链式方法调用来修改自身的结构。一些解决方案

  1. 存储“临时”变量,在这种情况下,所有方法都只是之后发生的正常变异方法。
  2. 接受self而不是对self(&self&mut self)的引用,然后最终返回完整的struct。
  3. 在链的末尾有一个build方法,它返回另一个独立的结构,而不是引用。
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