CODEWITHSUNDEEP
rusttraits
nathansizemore
nathansizemore

Reputation: 3196

Construct a Vector of any Type with a Trait Constraint

I'd like to have a struct, with a member that is a Vec with a Trait Constraint on the types in the Vector. Right now, this is what I am trying:

pub trait SomeTrait {
    fn some_function(&self);
}

pub struct SomeStruct<T: SomeTrait> {
    member: Vec<T>
}

impl<T: SomeTrait> SomeStruct<T> {
    pub fn new() -> SomeStruct<T> {
        SomeStruct {
            member: Vec::new()
        }
    }
}

fn main() {
    let mut some_struct = SomeStruct::new();
}

And the compiler is giving me:

error: the trait 'sometrait::SomeTrait' is not implemented for the type '_'
let mut some_struct = SomeStruct::new();
                      ^~~~~~~~~~~
note: required by 'somestruct::SomeStruct<T>::new'
let mut some_struct = SomeStruct::new();
                      ^~~~~~~~~~~

I've tried placing <T: SomeTrait> in various places throughout, but just getting even stranger compiler errors, so those attempts must be way off base.

Thanks in advance for any help!

Upvotes: 2

Views: 2608

Answers (1)

DK.
DK.

Reputation: 59005

The problem is that you haven't given the compiler enough information to figure out what T is. That's what the _ is for: it's having to infer the parameter to SomeStruct.

Also, there are no types anywhere in this example that implement SomeTrait. If you fix both of these problems, it works (with some warnings):

pub trait SomeTrait {
    fn some_function(&self);
}

pub struct SomeStruct<T: SomeTrait> {
    member: Vec<T>
}

impl<T: SomeTrait> SomeStruct<T> {
    pub fn new() -> SomeStruct<T> {
        SomeStruct {
            member: Vec::new()
        }
    }
}

impl SomeTrait for i32 {
    fn some_function(&self) {}
}

fn main() {
    let mut some_struct_a: SomeStruct<i32> = SomeStruct::new();
    let mut some_struct_b = SomeStruct::<i32>::new();
}

Upvotes: 2

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