core::marker::Reflect - Rust
Description: API documentation for the Rust `Reflect` trait in crate `core`.
Keywords: rust, rustlang, rust-lang, Reflect

# Trait core::marker::Reflect [−] [src]

    pub trait Reflect { }

Deprecated since 1.14.0

: Specialization makes parametricity impossible

Unstable (`reflect_marker` #27749)

: requires RFC and more experience

Types that can be reflected over.

By "reflection" we mean use of the `Any` trait, or related machinery such as `TypeId`.

`Reflect` is implemented for all types. Its purpose is to ensure that when you write a generic function that will employ reflection, that must be reflected (no pun intended) in the generic bounds of that function.

    #![feature(reflect_marker)]
    use std::marker::Reflect;
    use std::any::Any;

    fn foo<T: Reflect + 'static>(x: &T) {
        let any: &Any = x;
        if any.is::<u32>() { println!("u32"); }
    }Run

Without the bound `T: Reflect`, `foo` would not typecheck. (As a matter of style, it would be preferable to write `T: Any`, because `T: Any` implies `T: Reflect` and `T: 'static`, but we use `Reflect` here for illustrative purposes.)

The `Reflect` bound serves to alert `foo`'s caller to the fact that `foo` may behave differently depending on whether `T` is `u32` or not. The ability for a caller to reason about what a function may do based solely on what generic bounds are declared is often called the "parametricity property". Despite the use of `Reflect`, Rust lacks true parametricity because a generic function can, at the very least, call `mem::size_of` without employing any trait bounds whatsoever.

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