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
4
votes
1 answer
Why do some std::fmt::Debug* methods use dynamic dispatch?
These methods use dynamic dispatch (receive a trait object &Debug as parameter):
DebugMap::entry
DebugSet::entry
DebugStruct::field
DebugTuple::field
DebugList::entry
These methods use static dispatch and are written in terms of the related entry…
malbarbo
- 10,717
- 1
- 42
- 57
3
votes
2 answers
Rust serde deserialize dynamic trait
I have a recursive data structure in my pet-project:
(this is a simplified example)
pub trait Condition {
fn validate(&self, s: &str) -> bool;
}
pub struct Equal {
ref_val: String,
}
impl Condition for Equal {
fn validate(&self, s:…
Da_Niel
- 41
- 4
3
votes
2 answers
Supertrait as return traitobject doesn't have a known size at compile time
I am working with the DHT11 library where the argument gpio, based on the esp32 implementation, for new must implement InputPin and OutputPin.
So I created a function that returns a traitobject of a supertrait of those two traits. But when I want to…
elhe
- 122
- 10
3
votes
1 answer
Rust: can't collect an `Iterator- >` into `Vec
>`
I feel like this code should work since Boxs are supposed to be able to be implicitly converted into Boxs in most situations:
struct Dog {}
trait Animal {}
impl Animal for Dog {}
fn main() {
let _: Vec> = [Dog…
A. Kriegman
- 510
- 1
- 4
- 18
3
votes
1 answer
Why is rustc's suggestion here wrong?
I'm trying to implement the following trait, where the <'baz> is required for a later impl<'baz> Baz<'baz> for T where T: OtherTrait†.
trait Baz<'baz>: Clone {
fn baz(&'baz mut self);
fn bazzed(&self) -> Self {
let mut x =…
tuydore
- 33
- 4
3
votes
1 answer
Why using boxed objects over trait objects?
In the book Rust for Rustaceans, the author writes:
Broadly speaking, though, you’ll want to use static dispatch in your libraries and dynamic dispatch in your binaries. In a library, you want to allow your users to decide what kind of dispatch is…
Marcus
- 5,104
- 2
- 28
- 24
3
votes
1 answer
Trait can't be made Object Safe when Hash + PartialEq are Supertraits
I'm pretty new to Rust, so there is the possibility that this will be an easy Question.
I'm trying to create a small registry for Handlers that should return any struct that implements the TransferObject Trait:
pub trait TransferObject: Hash +…
stuchlyf
- 187
- 1
- 2
- 11
3
votes
3 answers
Trait objects from references
The below is (roughly) the trait object example taken from the rust book chapter 17.2. For my use case, I want to continue to use button and select_box after creating screen (see the commented out println!() after declaring screen), however I am not…
Max888
- 3,089
- 24
- 55
3
votes
1 answer
How to implement Mul Trait for a custom struct type to work in both ways
// main.rs
#[derive(Clone, Copy)]
struct A(f64, f64);
impl Mul for A
where
f64: From,
T: Copy, // f64: Mul,
{
type Output = A;
fn mul(mut self, rhs: T) -> Self::Output {
self.0 = self.0 * f64::from(rhs);
…
sherlock sampath
- 61
- 5
3
votes
2 answers
Drop order for when boxed struct contains reference
I'm trying to make a node structure but I have no idea why this won't compile (Rust playground):
trait SomeTrait {}
struct SomeObject<'a> {
something: &'a dyn SomeTrait,
}
impl<'a> SomeTrait for SomeObject<'a> {}
struct OtherObject {}
impl…
Ludvig
- 637
- 6
- 18
3
votes
2 answers
Calling static method on type alias
While using boxed closures I ran into the following issue:
type Test = Rc i64>;
fn test_bad() -> Test {
Test::new(|| 42)
}
fn test_good() -> Test {
Rc::new(|| 42)
}
In the first case, I'm using the type alias to refer to the…
max
- 1,048
- 10
- 20
3
votes
1 answer
Why does Vec.sort() seem to require a static lifetime?
This is a vastly simplified example of the issue I'm running into, but given a trait Thing which implements Ord, and a struct Object which implements Thing, I have the following struct:
pub struct MyStruct<'a> {
my_things: HashMap
NanoWizard
- 2,104
- 1
- 21
- 34
3
votes
1 answer
Rust: Trait RangeBounds cannot be made into an object
I sumbled upon a problem while trying out Rust, that might point to a bigger non understanding of its concepts on my part.
My goal is to programm a variant of Conway's Game of Life. I want the values when cells are created or stay alive not to be…
Rüdiger Ludwig
- 115
- 8
3
votes
1 answer
How do I deal with Sized requirements in a trait object?
I'm having trouble understanding traits and object safety in Rust.
I have a StoreTrait for storing some data and a Resource struct that holds a reference to a StoreTrait.
I want the Resource to have a reference to a store intance, because many of…
joepio
- 4,266
- 2
- 17
- 19
3
votes
0 answers
Why is a value not converted to a trait object with dynamic dispatch in a closure?
Given the following code:
use std::iter::Iterator;
trait Sequence {
type SeqType: Iterator- ;
fn seq(&self) -> Option
;
}
struct Doubler<'a>(Option<&'a [u32]>);
impl<'a> Sequence for Doubler<'a> {
type SeqType…
Rodolfo
- 335
- 1
- 4
- 10