与Rust编译器的斗争- 6

在上一篇文章中，我们讨论了与trait生命周期相关的编译器错误。特别是，我们看到了Box被隐式转换为Box<dyn Trait + 'static>。

在前面的例子中，我们所使用的所有特征对象都满足'static生命周期，但是如果我们想要将武器特征对象的生命周期限制放宽为非静态呢？

代码如下：

trait Weapon {
    fn fire(&self);
}

struct Player {
    weapon: Box<dyn Weapon>,
}

impl Player {
    fn new(weapon: impl Weapon + 'static) -> Self {
        Self { weapon: Box::new(weapon) }
    }

    fn change_weapon(&mut self, weapon: impl Weapon + 'static) {
        self.weapon = Box::new(weapon);
    }

    fn shoot(&self) {
        self.weapon.fire();
    }
}

struct CustomWeapon<'a> {
    sound: &'a str
}

impl<'a> CustomWeapon<'a> {
    fn new(sound: &'a str) -> Self {
        Self { sound }
    }
}

impl<'a> Weapon for CustomWeapon<'a> {
    fn fire(&self) {
        println!("{}", self.sound)
    }
}

fn main() {
    let impact = String::from("pew pew"); // non-static
    let player = Player::new(CustomWeapon::new(&impact));
    player.shoot(); // pew pew
}

与前面的例子不同的是，现在CustomWeapon有一个对非静态对象的引用。

error[E0597]: `impact` does not live long enough
  --> src/main.rs:42:48
   |
41 |     let impact = String::from("pew pew");
   |         ------ binding `impact` declared here
42 |     let player = Player::new(CustomWeapon::new(&impact));
   |                  ------------------------------^^^^^^^--
   |                  |                             |
   |                  |                             borrowed value does not live long enough
   |                  argument requires that `impact` is borrowed for `'static`
43 |     player.shoot(); // pew pew
44 | }
   | - `impact` dropped here while still borrowed

For more information about this error, try `rustc --explain E0597`.

正如预期的那样，编译器抱怨impact对象不是静态的。修复方法是引入生命周期泛型并替换'static，代码修改如下：

struct Player<'a> {
    weapon: Box<dyn Weapon + 'a>,
}

impl<'a> Player<'a> {
    fn new(weapon: impl Weapon + 'a) -> Self {
        Self { weapon: Box::new(weapon) }
    }

    fn change_weapon(&mut self, weapon: impl Weapon + 'a) {
        self.weapon = Box::new(weapon);
    }

    fn shoot(&self) {
        self.weapon.fire();
    }
}

有了这个改变，玩家结构体不仅可以接受静态的，也可以接受非静态的武器特征对象！