I'm trying to create a function that returns an instance of the Shader
trait. Here is my drastically simplified code:
trait Shader {}
struct MyShader;
impl Shader for MyShader {}
struct GraphicsContext;
impl GraphicsContext {
fn create_shader(&self) -> Shader {
let shader = MyShader;
shader
}
}
fn main() {}
However I receive the following error:
error[E0277]: the trait bound `Shader + 'static: std::marker::Sized` is not satisfied
--> src/main.rs:10:32
|
10 | fn create_shader(&self) -> Shader {
| ^^^^^^ `Shader + 'static` does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `Shader + 'static`
= note: the return type of a function must have a statically known size
Newer versions of the compiler have this error:
error[E0277]: the size for values of type `(dyn Shader + 'static)` cannot be known at compilation time
--> src/main.rs:9:32
|
9 | fn create_shader(&self) -> Shader {
| ^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `(dyn Shader + 'static)`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: the return type of a function must have a statically known size
This makes sense as the compiler doesn't know the size of the trait, but nowhere can I find the recommended way of fixing this.
Passing back a reference with &
wouldn't work as far as I know because the reference would outlive the lifetime of its creator.
Perhaps I need to use Box<T>
?
fn create_shader(&self) -> impl Shader {
let shader = MyShader;
shader
}
It does have limitations, such as not being able to be used in a trait method and it cannot be used when the concrete return type is conditional. In those cases, you need to use the trait object answer below.
You need to return a trait object of some kind, such as &T
or Box<T>
, and you're right that &T
is impossible in this instance:
fn create_shader(&self) -> Box<Shader> {
let shader = MyShader;
Box::new(shader)
}
See also: