如何对 Box 进行模式匹配以获取结构的属性? [英] How to pattern match a Box to get a struct's attribute?

问题描述

我正在尝试访问枚举内装箱结构的属性，但我不知道如何与 std::boxed::Box

I'm trying to access an attribute of a boxed struct inside an enum but I can't figure out how to pattern-match with std::boxed::Box

enum ArithExp {
    Sum {
        lhs: Box<ArithExp>,
        rhs: Box<ArithExp>,
    },
    Mul {
        lhs: Box<ArithExp>,
        rhs: Box<ArithExp>,
    },
    Num {
        value: f64,
    },
}

fn num(value: f64) -> std::boxed::Box<ArithExp> {
    Box::new(ArithExp::Num { value })
}

let mut number = num(1.0);
match number {
    ArithExp::Num { value } => println!("VALUE = {}", value),
}

我收到以下错误:

error[E0308]: mismatched types
  --> src/main.rs:22:9
   |
22 |         ArithExp::Num { value } => println!("VALUE = {}", value),
   |         ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::boxed::Box`, found enum `main::ArithExp`
   |
   = note: expected type `std::boxed::Box<main::ArithExp>`
              found type `main::ArithExp`

访问属性的正确方法是什么?

What is the correct way of accessing the attribute?

推荐答案

您需要取消引用装箱的值，以便您可以访问框内的内容:

You need to dereference the boxed value so that you can access what's inside the box:

match *number {
    ArithExp::Num { value } => println!("VALUE = {}", value),
    _ => (),
}

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