Understanding the features and security of objects

Question

I am struggling with the basics of object security. 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 get

fn object_safety_dynamic(x: Trait) {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `traits::Trait` cannot be made into an object
= note: the trait cannot require that `Self : Sized`

When I add / change :Sizedas a trait inheritance or binding, fI get slightly different error messages.

Can anyone explain:

• Why does this particular example not work? The Trait Objects chapter says, “What makes a method object-safe? Each method must require that Self: Sized.” Isn't it full?

• What is the difference between Trait: Sizedand where Self: Sized? (Well, yes, one inherits the trait, the other is associated with the parameter, but from the point of view of the Rust object?

• What preferred change did I have to make object_safety_dynamic?

rustc 1.19.0-nightly (01951a61a 2017-05-20), .

- Follow Up:

.

trait TraitB {
    fn f(&self) -> i32 where Self: Sized;

    fn g<T>(&self, t:T) -> i32 where Self: Sized;
}

Answer 1

? " " ", -? " Self: Sized ". ?

: -?

A Trait Object - - :

• ,
• .

, , , "", .

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:

• , , "" - ,
• .

: Sized Self: Sized? (, , , , Rust?)

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• fn method(&self) where Self: Sized , , Sized.

: ; , Self: Sized , .

object_safety_dynamic?

. , , .

Answer 2

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, :

trait Trait {
    fn f(&self) -> i32;
}

fn object_safety_dynamic(x: &Trait) {}

- - , . a Box<T> &T. , , .