CODEWITHSUNDEEP
rust
Joshua Partogi
Joshua Partogi

Reputation: 16425

Declaring a field of vector in Rust

I want to declare a member with type of Vector of trait in Rust but it is returning this error:

error: explicit lifetime bound required

This is what I have tried so far:

struct Lion {
    legs: int,
}

trait Animal {
   fn eat(&self);
}

impl Animal for Lion {
    fn eat(&self) {
        println!("Eat");
    }
}

struct Zoo {
    animals: Vec<Animal>,
}

I tried to search around the internet on how to fix this error with no luck. Can anyone give me a shed of light? Your help is much appreciated.

Upvotes: 0

Views: 1231

Answers (1)

DK.
DK.

Reputation: 59005

Animal is a trait, which means it could be implemented by just about anything. As such, you have to tell the compiler how long references contained by each thing that implements Animal in the Vec will be valid.

For example, you might have a Dodo that contains short-lived lifetime; you wouldn't want it to go extinct whilst it's in your Vec, leading to memory unsafety!

You need to use either

struct Zoo {
    animals: Vec<Animal + 'static>,
}

or

struct Zoo<'a> {
    animals: Vec<Animal + 'a>,
}

The first states that any reference reachable will be valid for the entire lifetime of the program. The second states that there is some lifetime 'a for which all references reachable will be valid. The problem then, however, is that this code isn't valid anyway because...

error: the trait `core::kinds::Sized` is not implemented for the type `Animal+'static`

You cannot have a Vec of values which are dynamically sized, which includes traits. You can solve this by either using Box<Animal + 'xxx>, or &'xxx Animal + 'xxx. A Box will be a lot easier to manage.

Edit: To be accurate, any type which accepts a Sized? parameter works here. This boils down to any form of reference indirection. Rc and Arc probably work, too.

FYI, that second form basically just says "a borrowed reference which is valid for all of lifetime 'xxx (the &'xxx part) to a value which implements Animal, where any references said value contains are valid for at least the lifetime 'xxx (the + 'xxx part)."

Upvotes: 3

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