Rc::new_cyclic -借用可能未初始化的变量

Question

我有这个Emulator课程。

pub struct Emulator {
    cart: Rc<RefCell<Cartridge>>,
    bus: Rc<RefCell<Bus>>, /* requirs access to cartridge, dma, ppu, and joypad */
    cpu: Rc<RefCell<M6502>>, /* requires access to bus */
    ppu: Rc<RefCell<PPU>>, /* requires access to cartridge */
    dma: Rc<RefCell<DMA>>, /* requires access to cpu, ppu, and bus */
    joypad: Rc<RefCell<Joypad>>
}

您可以看到，它的组件相互依赖。我决定使用Rc和Weak来维护这种循环关系。

我很难弄清楚如何创建这个结构的新实例。

impl Emulator {
    pub fn new(fname: &str) -> Self {
        let cart_ref = Rc::new(RefCell::new(Cartridge::new(fname)));
        let weak_cart = Rc::downgrade(&cart_ref);

        let ppu_ref = Rc::new(RefCell::new(PPU::new(weak_cart.clone())));
        let weak_ppu = Rc::downgrade(&ppu_ref);

        let joypad_ref = Rc::new(RefCell::new(Joypad::new()));
        let weak_joypad = Rc::downgrade(&joypad_ref);

        let mut bus_ref: Rc<RefCell<Bus>>;
        let mut cpu_ref: Rc<RefCell<M6502>>;

        let dma_ref = Rc::new_cyclic(|weak_dma| {
            bus_ref = Rc::new(RefCell::new(Bus::new(
                weak_cart.clone(),
                weak_ppu.clone(),
                weak_joypad.clone(),
                weak_dma.clone()
            )));
            let weak_bus = Rc::downgrade(&bus_ref);

            cpu_ref = Rc::new(RefCell::new(M6502::new(weak_bus.clone())));
            let weak_cpu = Rc::downgrade(&cpu_ref);

            RefCell::new(DMA::new(weak_cpu.clone(), weak_ppu.clone(), weak_bus.clone()))
        });

        Emulator {
            cart: cart_ref,
            bus: bus_ref,
            cpu: cpu_ref,
            ppu: ppu_ref,
            dma: dma_ref,
            joypad: joypad_ref
        }
    }
    ...
}

我犯了这个错误

error[E0381]: borrow of possibly-uninitialized variable: `bus_ref`
  --> src\emulator.rs:38:38
   |
38 |         let dma_ref = Rc::new_cyclic(|weak_dma| {
   |                                      ^^^^^^^^^^ use of possibly-uninitialized `bus_ref`
39 |             bus_ref = Rc::new(RefCell::new(Bus::new(
   |             ------- borrow occurs due to use in closure

error[E0381]: borrow of possibly-uninitialized variable: `cpu_ref`
  --> src\emulator.rs:38:38
   |
38 |         let dma_ref = Rc::new_cyclic(|weak_dma| {
   |                                      ^^^^^^^^^^ use of possibly-uninitialized `cpu_ref`
...
47 |             cpu_ref = Rc::new(RefCell::new(M6502::new(weak_bus.clone())));
   |             ------- borrow occurs due to use in closure

我不知道怎么修好它。我想不出其他的组合。我想知道我是否应该重组我的代码。

Answer 1

您不能在闭包中借用未初始化的变量，因为当它被删除时，编译器必须知道它是否已初始化，但它无法跟踪闭包内的变量(不是完全不能，但无论如何，这是Rust不支持的)。见第41124期。

我处理这个问题的方法是在这里和那里洒一些Options和.unwrap()s：

let mut bus_ref: Option<Rc<RefCell<Bus>>> = None;
let mut cpu_ref: Option<Rc<RefCell<M6502>>> = None;

let dma_ref = Rc::new_cyclic(|weak_dma| {
    let bus_ref_ = Rc::new(RefCell::new(Bus::new(
        weak_cart.clone(),
        weak_ppu.clone(),
        weak_joypad.clone(),
        weak_dma.clone(),
    )));
    let weak_bus = Rc::downgrade(&bus_ref_);

    let cpu_ref_ = Rc::new(RefCell::new(M6502::new(weak_bus.clone())));
    let weak_cpu = Rc::downgrade(&cpu_ref_);

    bus_ref = Some(bus_ref_);
    cpu_ref = Some(cpu_ref_);

    RefCell::new(DMA::new(
        weak_cpu.clone(),
        weak_ppu.clone(),
        weak_bus.clone(),
    ))
});

Emulator {
    cart: cart_ref,
    bus: bus_ref.unwrap(),
    cpu: cpu_ref.unwrap(),
    ppu: ppu_ref,
    dma: dma_ref,
    joypad: joypad_ref,
}

不完美，但有效。