Match Box variable types

help
1

I have a variable (Box<Any>) and a struct Null. I want to match that variable to check if it is Null.
I want to do something like this:

struct Null;

fn main() {
    let mybox = Box::new("Box");
    
    match (mybox, Box::new(Null {}))
    {
        (Box(Null), Box(Null)) => println!("It is Null"),
        _ => println!("It is not Null")
    }
}
2

You need to have a look at the Option type. What you want is Option<Box<T>>.
Then you can use match to match against Some or None. See the TRPL and/or the Rust Docs for the Option type.

3 Likes
3

It doesn't work with match like that. You'd have to use downcast_ref, BUT the whole design is very suspicious and un-Rust-like.

Try approaching the problem differently, without the weird Null thing. Perhaps you need an enum? Or Option<Box<SpecificType>>?

4

Is it possible to match and do nothing if it is None, but do something if it is not?

5

That looks like a code smell! Your Null is a zero sized type and Option is the best alternative. Just for pedagogy, see the playground.

6

The Option<Box<TheType>> works with the match, but when I try to the value, I get Some(TheType)?

7

You can extract the contents of a Some with an if let:

let my_box = Some(Box::new("Box"));

if let Some(x) = my_box {
    // x is Box containing "Box"
}

This works on a match as well, but if you just need to handle the case where there's some value it's simpler.

8

let my_box = Some(Box::new("Box"));

if let Some(x) = my_box {
// x is Box containing "Box"
}

I only get TheType when I try to print it

9

Can you post your full code to the playground? What specifically are you printing? What is TheType?

10

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