How to pattern match a Box to get a struct's attribute?

user_9870544 0
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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),
}

I get the following error:

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`

What is the correct way of accessing the attribute?

rust
1 Answers

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),
    _ => (),
}

playground

user_53970090
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