# Trait Sized

1.6.0 · Source

    pub trait Sized: MetaSized { }

Expand description

Types with a constant size known at compile time.

All type parameters have an implicit bound of `Sized`. The special syntax `?Sized` can be used to remove this bound if it’s not appropriate.

    struct Foo<T>(T);
    struct Bar<T: ?Sized>(T);

    // struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]
    struct BarUse(Bar<[i32]>); // OK

The one exception is the implicit `Self` type of a trait. A trait does not have an implicit `Sized` bound as this is incompatible with trait objects where, by definition, the trait needs to work with all possible implementors, and thus could be any size.

Although Rust will let you bind `Sized` to a trait, you won’t be able to use it to form a trait object later:

    trait Foo { }
    trait Bar: Sized { }

    struct Impl;
    impl Foo for Impl { }
    impl Bar for Impl { }

    let x: &dyn Foo = &Impl;    // OK
    // let y: &dyn Bar = &Impl; // error: the trait `Bar` cannot be made into an object

## Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

## Implementors§