有没有办法让结构包含可能不再有效的引用？

Question

就像某种Box 持有对值的引用之类的？在读取它之前，我必须检查该值是否仍然存在，例如当 Option 与模式匹配时。

一个模拟示例：

struct Whatever {
    thing: AliveOrNot<i32>,
}
fn main() {
    let mut w = Whatever { thing: Holder::Empty };
    w.thing = AliveOrNot::new(100);
    match w.thing {
        Empty => println!("doesn't exist"),
        Full(value) => println!("{}", value),
    }
}

确切的情况是我正在使用 sdl2 Font 并且我想将该结构的实例存储在另一个结构中。我不想做这样的事情，因为Parent 必须和Font 一样长：

struct Font<'a, 'b> {
    aa: &'a i32,
    bb: &'b i32,
}
struct Parent<'a, 'b, 'c> {
    f: &'c Font<'a, 'b>
}

我想要的是让Parent 工作，无论该字段是否仍然存在。

Answer 1

您可能对std::rc::Weak 或std::sync::Weak 感兴趣：

use std::rc::{Rc, Weak};

struct Whatever {
    thing: Weak<i32>,
}

impl Whatever {
    fn do_it(&self) {
        match self.thing.upgrade() {
            Some(value) => println!("{}", value),
            None => println!("doesn't exist"),
        }
    }
}

fn its_dead_jim() -> Whatever {
    let owner = Rc::new(42);
    let thing = Rc::downgrade(&owner);

    let whatever = Whatever { thing };
    whatever.do_it();

    whatever
}

fn main() {
    let whatever = its_dead_jim();
    whatever.do_it();
}

42
doesn't exist

使用非'static 引用在安全的 Rust 中没有方法可以做到这一点。 Rust 引用的一大特点是不可能引用无效值。

• 您可以泄漏内存，创建一个&'static i32，但如果您需要多次这样做，这是不可持续的。

• 您可以使用unsafe 代码并处理没有生命周期概念的原始指针。然后，您有责任确保不会引入内存不安全。

另见：

• Need holistic explanation about Rust's cell and reference counted types
• Situations where Cell or RefCell is the best choice
• Is there a shared pointer with a single strong owner and multiple weak references?
• Why can't I store a value and a reference to that value in the same struct?
• How to convert a String into a &'static str
• Is there any way to return a reference to a variable created in a function?
• How to solve this rust lifetime bound issue of SDL2?
• Cannot infer an appropriate lifetime for autoref due to conflicting requirements