rust 中是否可能存在具有大小特征的多态性

Question

I am using a library that exposes a trait which requires it to be Sized .

I am using an object that implements this trait, and am trying to create an array that include all those instances. With a non-sized trait I would use something like Vec<Box<dyn MyNonSizedTrait>> , but with the Sized trait it's impossible, the compiler complains with the trait MySizedTrait cannot be made into an object .

Is there any way to work around that?

Here is an example of what I'm trying to do:

trait A: Sized {
    fn do_something(&self);  
}
    
struct B;

impl A for B {
    fn do_something(&self){
        println!("This is B")
    }
}
    
struct C;
    
impl A for C {
    fn do_something(&self) {
        println!("This is C")
    }
}
    
    
fn my_do_something(d: &[Box<dyn A>]) {
    for i in d {
        i.do_something(); 
    }
}
    
fn main() {
    let mut d: Vec<Box<dyn A>> = Vec::new();
    d.push(Box::new(B));  
    d.push(Box::new(C)); 
    my_do_something(&d); 
}

Answer 1

dyn Trait is always unsized by definition, because it allows the object behind it to have any size from 0 to isize::MAX .

To have a fixed-size object and polymorphism, use an enum . You can then add impl on the enum itself that dispatches calls to the variants. There are some hacky crates that automate that.