无法搬出借用的内容和Builder模式

Question

我正在学习 Rust。我正在尝试为我的游戏结构创建一个构建器结构。这是代码：

struct Input {
    keys_pressed: HashMap<VirtualKeyCode, bool>,
}

pub struct GameBuilder {
    settings: GameSettings,
    input: Input,
}

impl GameBuilder {
    pub fn new() -> GameBuilder {
        GameBuilder {
            settings: GameSettings {
                window_dimensions: (800, 600),
                title: "".to_string(),
            },
            input: Input {
                keys_pressed: HashMap::new(),
            }
        }
    }

    pub fn with_dimensions(&mut self, width: u32, height: u32) -> &mut GameBuilder {
        self.settings.window_dimensions = (width, height);
        self
    }

    pub fn with_title(&mut self, title: &str) -> &mut GameBuilder {
        self.settings.title = title.to_string();
        self
    }

    pub fn game_keys(&mut self, keys: Vec<VirtualKeyCode>) -> &mut GameBuilder {
        for key in keys {
            self.input.keys_pressed.insert(key, false);
        }
        self
    }

    pub fn build(&self) -> Game {
        let (width, height) = self.settings.window_dimensions;
        Game {
            display: glutin::WindowBuilder::new()
                        .with_dimensions(width, height)
                        .with_title(self.settings.title.to_string())
                        .build_glium()
                        .ok()
                        .expect("Error in WindowBuilder"),
            state: GameState::Running,
            input: self.input,
        }
    }
}

但是这段代码在最后一行 input: self.input 抱怨：

error: cannot move out of borrowed content

我想我明白为什么了。由于在函数中传递的参数是 &self，我不能取得它的所有权，最后一行在做什么。

我认为也许将 &self 更改为 self 会起作用，但随后编译认为我不能改变 self.

据我所知，还有 Copy 特性，也许应该可以解决问题。但 Input 基本上是一个 HashMap，这意味着如果哈希本身太大，副本可能会很昂贵。

如何解决这个问题？

编辑：

我试过这样做：

#[derive(Debug, Copy, Clone)]
struct Input {
    keys_pressed: HashMap<VirtualKeyCode, bool>,
}

但是编译器抱怨：

error: the trait `Copy` may not be implemented for this type; field `keys_pressed` does not implement `Copy`

Answer 1

鉴于您的方法签名的制定方式，您的目标似乎是链接：

let game = GameBuilder::new().with_dimensions(...)
                             .with_title(...)
                             .build();

在 Rust 中，这需要 GameBuilder 按值传递：

pub fn with_dimensions(self, ...) -> GameBuilder {
    // ...
}

并且为了能够在方法内改变 self，您需要使其成为 mut:

pub fn with_dimensions(mut self, ...) -> GameBuilder {
}

如果您将 with_dimensions、with_title、game_keys 和 build 的签名更改为按值取 self（mut self 如果突变是有意的），那么链接应该可以工作。

Answer 2

使用 Option 和 take()

尝试构建器模式

示例：

#[derive(PartialEq, Debug)]
struct Game {
    window: Window,
}

#[derive(PartialEq, Debug)]
struct Window {
    title: String,
    dimensions: (u32, u32),
}

struct GameBuilder {
    window_title: Option<String>,
    window_dimensions: Option<(u32, u32)>,
}

impl GameBuilder {
    fn new() -> Self {
        Self {
            window_title: None,
            window_dimensions: None,
        }
    }

    fn window_title(&mut self, window_title: &str) -> &mut Self {
        self.window_title = Some(window_title.to_owned());
        self
    }

    fn window_dimensions(&mut self, width: u32, height: u32) -> &mut Self {
        self.window_dimensions = Some((width, height));
        self
    }

    fn build(&mut self) -> Result<Game, Box<dyn std::error::Error>> {
        Ok(Game {
            window: Window {
                // `ok_or(&str)?` works, because From<&str> is implemented for Box<dyn Error>
                title: self.window_title.take().ok_or("window_title is unset")?,
                dimensions: self
                    .window_dimensions
                    .take()
                    .ok_or("window_dimensions are unset")?,
            },
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test() {
        let mut builder = GameBuilder::new();
        builder.window_title("Awesome Builder");
        builder.window_dimensions(800, 600);
        let game = builder.build();

        assert_eq!(
            game.expect("build success"),
            Game {
                window: Window {
                    title: "Awesome Builder".into(),
                    dimensions: (800, 600)
                }
            }
        );
    }

    #[test]
    fn test_1() {
        let game2 = GameBuilder::new()
            .window_title("Busy Builder")
            .window_dimensions(1234, 123)
            .build();

        assert_eq!(
            game2.expect("build success"),
            Game {
                window: Window {
                    title: "Busy Builder".into(),
                    dimensions: (1234, 123),
                }
            }
        )
    }
}

无法搬出借用的内容和Builder模式

Cannot move out of borrowed content and Builder pattern

rust

borrow-checker

编辑：