"borrowed value does not live long enough" 使用构建器模式时

Question

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));

我收到这个错误：

error: borrowed value does not live long enough let mut canvas = Canvas::new(100, 100, &renderer)

为什么返回的 Canvas 对象寿命不够长？如果我将结果存储在中间 let 中，那么它就可以工作；为什么？

Answer 1

这是一个最小的复制：

#[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);
}

生锈 2015

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

生锈 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

问题的出现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 return 是引用。在方法链的末尾，您试图 return 引用并将其存储在变量中，但该引用指向一个 无处可去 的临时项！ Rust 不允许您引用无效的内容。

部分问题是这个不是构建器模式，这只是一个使用链接方法调用修改自身的结构.一些解决方案：

存储"temporary"变量，在这种情况下，所有方法都是之后发生的正常变异方法。
接受 self 而不是对自身的引用（&self、&mut self），然后最终 return 完整结构。
在链的末尾有一个 build 方法，return 是另一个独立的结构，而不是引用。

"borrowed value does not live long enough" 使用构建器模式时

"borrowed value does not live long enough" when using the builder pattern

builder

lifetime

rust

borrow-checker