Vector of Trait with generic method

help
#1

Hi everyone,
I am learning Rust and trying to implement a small project with it.
I am facing some issues with trait that contains generic method.
Basically, I want to have a vector that contains objects of multiple types which implement a trait.
The point is that the trait has a generic method inside.


use std::collections::HashMap;
trait AwesomeTrait {
    fn awesomeness<B> (&self, x: B) {
        println!("Awesome Trait!");
    }
    
    fn boxed_clone(&self) -> Box<dyn AwesomeTrait>;
}

#[derive(Clone, Debug)]
struct AwesomeStruct;

impl AwesomeTrait for AwesomeStruct {
    fn boxed_clone(&self) -> Box<dyn AwesomeTrait> {
        Box::new(self.clone())
    }
}

#[derive(Clone, Debug)]
struct AwesomeStruct2;

impl AwesomeTrait for AwesomeStruct2 {
    fn boxed_clone(&self) -> Box<dyn AwesomeTrait> {
        Box::new(self.clone())
    }
}

#[derive(Clone)]
struct ListOfAwesomeStruct{
    list: HashMap<String, Box<dyn AwesomeTrait>>
}

impl Clone for Box<dyn AwesomeTrait> {
    fn clone(&self) -> Self {
        self.boxed_clone()
    }
}

impl ListOfAwesomeStruct {
    fn new () -> Self {
        Self {
            list: HashMap::new(),
        }
    }
    
    fn push<S: AwesomeTrait + 'static>(&mut self, key: String,awesome: S) -> &mut Self {
        self.list.insert(key, Box::new(awesome));
        
        self
    }
}

fn main() {
    let mut ooo = ListOfAwesomeStruct::new();
    ooo.push("key1".to_owned(), AwesomeStruct);
    ooo.push("key2".to_owned(), AwesomeStruct2);
}

This is the error:

error[E0038]: the trait `AwesomeTrait` cannot be made into an object
 --> src/main.rs:8:30
  |
8 |     fn boxed_clone(&self) -> Box<dyn AwesomeTrait>;
  |                              ^^^^^^^^^^^^^^^^^^^^^ `AwesomeTrait` cannot be made into an object
  |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> src/main.rs:4:8
  |
3 | trait AwesomeTrait {
  |       ------------ this trait cannot be made into an object...
4 |     fn awesomeness<B> (&self, x: B) {
  |        ^^^^^^^^^^^ ...because method `awesomeness` has generic type parameters
  = help: consider moving `awesomeness` to another trait

Is there any solution to solve this issue? Thank you in advance.

Object safety and generic methods
#2

If you do not need to call awesomeness() on the dyn AwesomeTrait, you can restrict it to where Self: Sized to make the trait object safe:

fn awesomeness<B>(&self, x: B)
where
    Self: Sized,
{
    // ...
}
#3

Another alternative is to type-erase the generic parameter on your method, too.

trait CanAwe {}
// ^^^^^^^^^^^^

use std::collections::HashMap;
trait AwesomeTrait {
    fn awesomeness(&self, x: Box<dyn CanAwe>) {
//                           ^^^^^^^^^^^^^^^
        println!("Awesome Trait!");
    }
    
    fn boxed_clone(&self) -> Box<dyn AwesomeTrait>;
}

Playground.

1 Like
#4

Object safety is pretty confusing! It doesn't help the error looks you to a pretty dense specification of what it is, but not why or how to fix it. The suggested fix is ok, but not if you actually do need to call it!

1 Like
#5

Thanks for your suggestion. Indeed I need to call the method awesomeness() on dyn AwaresomeTrait