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einQimiaozi
einQimiaozi

Reputation: 1

How do i access enum values in rust when I don't know the enum type?

I want to compare enum values when the enum types are the same. What should I do?

for example

#[derive(Debug, PartialEq, PartialOrd)]
enum Fruits {
    Orange{weight: u8},
    Banana{weight: u8},
    Pear{weight: u8},
}

fn compare(fruit_a: Fruits,fruit_b: Fruits) -> bool {
    // Fruits::Orange < Fruits::Banana < Fruits::Pear
    // if same type, compare weight
    if fruit_a < fruit_b {
        return false;
    }else if fruit_a == fruit_b {
        todo!()
    }else {
        return true
    }
}

I can't use a method like

let v = if let Fruits::Banana{weight} = fruit {
        weight
    }else {
        0u8
    };

Because I don't know what type follows Frutis::

Upvotes: 0

Views: 213

Answers (1)

Masklinn
Masklinn

Reputation: 42632

I can't use a method like

let v = if let Fruits::Banana{weight} = fruit {
    weight
} else {
    0u8
};

Because I don't know what type follows Frutis::

That... is the point of if let? If it's Fruits::Banana it'll take the first branch, otherwise it'll take the second branch.

For a wider pick, with more flexibility, you can use match:

let v = match fruit {
    Fruits::Orange{weight} => weight,
    Fruits::Banana{weight} => weight,
    Fruits::Pear{weight} => weight,
}

Though do note that what follows Fruits:: is not a type, it's a variant tag (sometimes called "constructor", but more for singleton or tuple variants).

Anyway your entire line of questioning seems useless: what you're asking for is already the behaviour of Eq and Ord so you can just call [partial_]cmp or == on your structures directly, and it'll do exactly what you're looking for, compare the variants first, then compare the variant' values if the values are the same variants:

#[derive(Debug, PartialEq, PartialOrd, Eq, Ord)]
enum Fruits {
    Orange { weight: u8 },
    Banana { weight: u8 },
    Pear { weight: u8 },
}

fn main() {
    println!(
        "{:?}",
        Fruits::Orange { weight: 5 }.cmp(&Fruits::Pear { weight: 2 })
    );
    // Less, because Orange < Pear
    println!(
        "{:?}",
        Fruits::Orange { weight: 7 }.cmp(&Fruits::Orange { weight: 5 })
    );
    // Greater, because 7 > 5
}

Upvotes: 1

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