The trait cannot be made into an object

Question

I have the following code:

extern crate futures; // 0.1.24  use futures::Future; use std::io;  struct Context;  pub trait MyTrait {     fn receive(context: Context) -> Future<Item = (), Error = io::Error>; }  pub struct MyStruct {     my_trait: MyTrait, } 

When I try to compile it I get the error message:

error[E0038]: the trait `MyTrait` cannot be made into an object   --> src/lib.rs:13:5    | 13 |     my_trait: MyTrait,    |     ^^^^^^^^^^^^^^^^^ the trait `MyTrait` cannot be made into an object    |    = note: method `receive` has no receiver 

I think I know why it happens, but how do I refer to the trait from the struct? Is it possible? Maybe there are other ways to implement the same behavior?

Answer 1

You can either add a type parameter to your struct, as in Zernike's answer, or use a trait object.

Using the type parameter is better for performance because each value of T will create a specialized copy of the struct, which allows for static dispatch. A trait object uses dynamic dispatch so it lets you swap the concrete type at runtime.

The trait object approach looks like this:

pub struct MyStruct<'a> {     my_trait: &'a dyn MyTrait, } 

Or this:

pub struct MyStruct {     my_trait: Box<dyn MyTrait>, } 

However, in your case, MyStruct cannot be made into an object because receive is a static method. You'd need to change it to take &self or &mut self as its first argument for this to work. There are also other restrictions.

Answer 2

pub struct MyStruct<T> where     T: MyTrait, {     my_trait: T, } 

or

pub struct MyStruct<T: MyTrait> {     my_trait: T, } 

https://doc.rust-lang.org/book/second-edition/ch10-02-traits.html#trait-bounds