For questions related to Trait Objects (dynamically-dispatched objects) in Rust
Trait objects are the dynamically dispatched objects in Rust. They are unsized, so they are used behind some kind of reference (Box, for example).
Questions tagged [trait-objects]
183 questions
90
votes
4 answers
What makes something a "trait object"?
Recent Rust changes have made "trait objects" more prominent to me, but I only have a nebulous grasp of what actually makes something into a trait object. One change in particular is the upcoming change to allow trait objects to forward trait…
Shepmaster
- 388,571
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- 1,366
68
votes
1 answer
Sending trait objects between threads in Rust
I'd like to send a trait object between threads, but can't figure out if it's possible. It seems like it might not be, as they apparently do not fulfill the Send trait.
The following code demonstrates what I'm trying to do:
use std::{
…
guff
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- 6
47
votes
3 answers
The trait cannot be made into an object
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- ;
}
pub struct MyStruct {
my_trait:…
Alexander
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18
votes
3 answers
Why can't `&(?Sized + Trait)` be cast to `&dyn Trait`?
In the code below it is not possible to obtain a reference to a trait object from a reference to a dynamically-sized type implementing the same trait. Why is this the case? What exactly is the difference between &dyn Trait and &(?Sized + Trait) if I…
w1th0utnam3
- 963
- 7
- 19
13
votes
1 answer
Difference between &mut and ref mut for trait objects
First of all, I'm not asking what's the difference between &mut and ref mut per se.
I'm asking because I thought:
let ref mut a = MyStruct
is the same as
let a = &mut MyStruct
Consider returning a trait object from a function. You can return a…
Rodolfo
- 335
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13
votes
2 answers
What is the difference between Box and &Trait / Box?
When writing code with traits you can put the trait in a trait bound:
use std::fmt::Debug;
fn myfunction1(v: Box) {
println!("{:?}", v);
}
fn myfunction2(v: &T) {
println!("{:?}", v);
}
fn main() {
…
Erik Vesteraas
- 4,675
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12
votes
2 answers
Why are trait methods with generic type parameters object-unsafe?
To quote the Book (emphasis mine),
The same is true of generic type parameters that are filled in with concrete type parameters when the trait is used: the concrete types become part of the type that implements the trait. When the type is forgotten…
nalzok
- 14,965
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- 139
12
votes
1 answer
How to move a value out of an object-safe trait object?
A Mech carries a driver, which is a Named entity. At run-time, an omitted Mech constructor consults external source for the specific type of driver to use.
trait Named {
fn name(self) -> String;
}
struct Person {
first_name: String,
…
mwgkgk
- 173
- 8
12
votes
2 answers
Can't clone Vec> because Trait cannot be made into an object
I'm trying to clone a vector of boxed traits.
Naturally simply deriving Clone on all the structs that implement my trait isn't enough, because the compiler doesn't know at compile time that all the structs implementing the trait have Clone.
Okay, so…
Electric Coffee
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12
votes
1 answer
Function returning a closure not working inside my filter
I cannot get this to compile without using a closure. I'm trying to get the function apply to return the correct kind of closure in the first place.
#![feature(conservative_impl_trait)]
#![allow(dead_code)]
fn accumulate<'a>(tuples: &[(&'a str,…
iopq
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11
votes
1 answer
How to Return a Result with generic error
I want to write a function that reads the contents of a file, and raises an error if it fails. I want to call this function from a python script, so I'm including some mentions of Python below in case it might be relevant.
As I have tried showing in…
Simen Russnes
- 2,002
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11
votes
2 answers
function pointer vs Fn trait object
fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 { // definition
f(arg) + f(arg)
}
do_twice(|x| x + 1, 5) // call
This function accepts both, closures and function pointers. It takes a function pointer as parameter type.
When should I prefer…
Akshay Mariyanna
- 113
- 1
- 5
10
votes
1 answer
Are vtables generated for all types that implement a trait?
If I have a trait Foo, and some implementors Bar, Baz.
impl Foo for Bar {
}
impl Foo for Baz {
}
But say I only use one of them ever as a trait object,
let bar = Bar {..};
let foo: &dyn Foo = &bar;
Then will my binary still have vtable for…
zombiesauce
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10
votes
2 answers
Understanding Traits and Object Safety
I am struggling with the basics of object safety. If I have this code
struct S {
x: i32,
}
trait Trait: Sized {
fn f(&self) -> i32
where
Self: Sized;
}
fn object_safety_dynamic(x: Trait) {}
I receive
error[E0038]: the trait…
left4bread
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9
votes
1 answer
How do I pass Rc>> to a function accepting Rc>>?
I have originally asked this question here, but it was marked as duplicate, although it duplicates only a part of it in my opinion, so I have created a more specific one:
Consider the following code:
use std::rc::Rc;
trait MyTrait {
fn…
Dmitry Uvarov
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