Rust dyn trait. You cannot create a dyn Trait from a trait definition alone; there must be an implementing base type that you can coerce. The years from 2012 to 2015 were marked by substantial changes to the Rust type system. e. This is crucial pre-requisite for async functions in traits. This article is a slight divergence from my Rust in 2025 series. 注意 dyn 不能单独作为特征对象的定义，例如下面的代码编译器会报错，原因是特征对象可以是任意实现了某个特征的类型，编译器在编译期不知道该类型的大小，不同的类型大小是不同的。 而 & dyn 和 Box<dyn> 在编译期都是已知大小，所以可以用作特征对象的定义。 此外，随着 impl Trait 的到来， impl Trait vs dyn Trait 比 impl Trait vs Trait 更好更对称。 impl Trait 将在下一节进一步解释。 因此，在新版本中，选择使用 trait 对象时，你应该选 dyn Trait 而不是 Trait。 Async fn in dyn trait Welcome! This document explores how to combine dyn and impl Trait in return position. trait MyTrait : Sized { } struct MyStruct; impl MyTrait for MyStruct{} fn my_method(obj: &dyn MyTrait){ } fn main() { my_method(&MyStruct); } This code does not compile Learning Rust A tour of dyn Trait Rust's type-erasing dyn Trait offers a way to treat different implementors of a trait in a homogenous fashion while remaining strictly and statically (i. Unlike generic parameters or impl Trait, the compiler does not know the concrete type that is being passed. This default is stronger than the normal function signature elision rules. Motivation In 此外，随着 impl Trait 的到来， impl Trait vs dyn Trait 比 impl Trait vs Trait 更好更对称。 impl Trait 将在下一节进一步解释。 因此，在新版本中，选择使用 trait 对象时，你应该选 dyn Trait 而不是 Trait。 Advice for learning Rust // Owned or borrowed generics fn foo1 <T: Trait>(t: T) {} fn bar1 <T: Trait + ? Sized>(t: &T) {} // Owned or borrowed `dyn Trait` fn foo2 (t: Box <dyn Trait + '_>) {} fn bar2 (t: & dyn Trait) {} When a function has a generic parameter, the parameter is monomorphized for every concrete type which is used to call the function (after lifetime erasure). , lifetimes, types, trait bounds aka HRTBs)A-impl-traitArea: `impl Trait`. Self不是个固定的type，它是“实现者”类型的占位符，也就是说编译器必须得知道Self到底是什么类型才能生成对应的代码，比如在Rust里dyn trait和trait是不一样的，dyn trait里不知道Self是什么，所以也不能用Self，trait里知道Self是什么，所以可以用Self。 dyn Trait isn't about thread safety, it's about erasing different base types into a common, dynamically dispatched type. The traits defined here enable rust-tuf A-async-awaitArea: Async & AwaitA-auto-traitsArea: auto traits (e. To use the trait this way, it must be dyn compatible 1. So if your function returns a pointer-to-trait-on-heap in this way, you need to write the return type with the dyn keyword, e. Background: why is this hard? Supporting async fn in dyn traits is a tricky balancing act. I wanted to share my latest thinking about how to support dyn Trait for traits with async functions and, in particular how to do so in a way that is compatible with the soul of Rust. Trying to use dyn with an async trait produces the following error: How to downcast a trait back to a concrete type in Rust 使用 dyn 返回 trait Rust 编译器需要知道每个函数的返回类型需要多少空间。这意味着所有函数都必须返回一个具体类型。与其他语言不同，如果你有个像 Animal 那样的的 trait，则不能编写返回 Animal 的函数，因为其不同的实现将需要不同的内存量。 但是，有一个简单的解决方法。相比于直接返回一个 Further reading RFCs: impl Trait (return position) impl Trait (argument position, refinements) dyn Trait finalisation of impl Trait and dyn Trait impl Trait in associated types and type aliases Other: Aaron's blog post 2018 Edition book: impl Trait and dyn Trait The Rust book (and following sections) The Rust reference: dyn Trait and impl Trait. It thrives on static dispatch, where the compiler can inline and optimize code … Dyn traits in Rust in 2 hours Crust of Rust: Dispatch and Fat Pointers I promised to write about things I read, turned out, some of them need video and audio. Instead, it uses a vtable — a runtime lookup table — to call the correct method. Both impl Trait and dyn Trait allow you to work with types that implement a particular trait, providing a form of abstraction or polymorphism. dyn is a prefix of a trait object ’s type. Well, yes, but you can't get at it. Here we look at how this notionally works, and also touch on how this leads to some related limitations around dyn Trait. Box<dyn Animal>. [17]: 18:36 [16] Memory management through the ownership system was gradually consolidated and expanded. g. The dyn keyword is used to highlight that calls to methods on the associated Trait are dynamically dispatched. impl Trait is only existential in function return type position and even there it uses static dispatch, and it can be sized. Rust currently does not support passing unsized parameters, returning unsized values, or having unsized locals. Rust's type-erasing dyn Trait offers a way to treat different implementors of a trait in a homogenous fashion while remaining strictly and statically (i. This means all your functions have to return a concrete type. compile-time) typed. Somewhat relatedly is there a good definition of existential in the Rust context? dyn 是 trait 对象 类型的前缀。 dyn 关键字用于突出显示对关联 Trait 上的方法的调用是 dynamically dispatched。 要以这种方式使用 trait，它必须是对象安全的。 与泛型参数或 impl Trait 不同，编译器不知道要传递的具体类型。 即，类型为 erased。 A trait object is an opaque value of another type that implements a set of traits. Dyn compatibility A dyn-compatible trait can be the base trait of a trait object. In general, traits may only be converted to a trait object if certain criteria are met. That is, every type dyn是trait对象类型的前缀 dyn关键字用于强调相关trait的方法是动态分配的。要以这种方式使用trait，它必须是“对象安全”的。 与泛型参数或植入型特质不同，编译器不知道被传递的具体类型。也就是说，该类型已经被抹去。因此，一个dyn Trait引用包含两个指针。一个指针指向数据（例如，一 Dyn compatibility A dyn-compatible trait can be the base trait of a trait object. And in fact, the indirection is necessary for another reason. Returning Traits with dyn The Rust compiler needs to know how much space a function's return type requires. Rust doesn't allow you to "cross cast" from &dyn A to &dyn Any. However, they differ significantly in how they achieve this: impl Trait uses static dispatch (method calls are resolved at compile time); dyn Trait uses dynamic dispatch (resolved at run time). trait MyTrait : Sized { } struct MyStruct; impl MyTrait for MyStruct{} fn my_method(obj: &dyn MyTrait){ } fn main() { my_method(&MyStruct); } This code does not compile Functions new_foo1 and new_foo2 return the same trait Foo using different patterns. To use the trait this way, it must be ‘object safe’. However, there's an easy workaround Every dyn Trait value is the result of type erasing some other existing value. Feature Name: dyn-trait-syntax Start Date: 2017-08-17 RFC PR: rust-lang/rfcs#2113 Rust Issue: rust-lang/rust#44662 Summary Introduce a new dyn Trait syntax for trait objects using a contextual dyn keyword, and deprecate “bare trait” syntax for trait objects. Use Box<dyn Trait> or &dyn Trait when you want polymorphism Since the concrete types differ, their sizes differ, so you must use a reference or a box (&dyn Trait or Box<dyn Trait>). Rust tries to be as explicit as possible whenever it allocates memory on the heap. Demystifying dyn Trait Objects in Rust: When and Why to Use Them Rust is known for its powerful and strict type system. , `auto trait Send {}`)A-dyn-traitArea: trait objects, vtable layoutA-higher-rankedArea: Higher-ranked things (e. dyn is short for "dynamic" and refers to the fact that trait objects perform dynamic dispatch. A trait is dyn compatible if it has the following qualities: Type erasure for async trait methods The stabilization of async functions in traits in Rust 1. That is what as_any is for; because it's something only implemented on our "concrete" types, the compiler doesn't get confused as to which one it's supposed to invoke. In a future edition, dyn will become a proper keyword and a lint against bare trait syntax will become deny-by-default. By 2013, the garbage collector was rarely used, and was removed in favor of the ownership system. The compiler always supplies that implementation. The set of traits is made up of a dyn compatible base trait plus any number of auto traits. A trait is dyn compatible if it has the following qualities: Where in the dyn version I'm allowed to return different types as long they all implement the Trait, while in the impl version I'm only allowed to return the same type (same applies if I return a reference). Indeed, if thread safety is a concern, you probably want dyn Trait + Send + Sync or the like. Therefore, when interacting with dyn Trait, you will generally be working with some sort of indirection: a Box<dyn Trait>, &dyn Trait, Arc<dyn Trait>, etc. The latest thing I was impressed by Rust tries to be as explicit as possible whenever it allocates memory on the heap. Returning Traits with dyn The Rust compiler needs to know how much space every function's return type requires. [16] Other features were removed in order to simplify the language, including typestates, the pure New issue New issue Open Open Coherence bypass using associated type to impl Trait for dyn Trait#152783 Labels This document describes the core repository abstraction layer in rust-tuf, which defines how TUF clients and repository builders interact with storage backends. This guide explains how trait objects work at the memory layout level, focusing on the dyn keyword, fat pointers, and virtual function tables (vtables). Is there a preferred way “Dyn-compatibility” refers to the ability for a trait to be converted to a trait object. It gives you performance and safety — but sometimes you just want to write … Rust tries to be as explicit as possible whenever it allocates memory on the heap. Rustのプログラミングにおいて、トレイトは強力で柔軟な抽象化の手段を提供します。特に、トレイトオブジェクトを用いることで、動的ディスパッチが可能となり、多様なシナリオでの柔軟な設計を実現できます。本記事では、Rustのdynキーワードを使 Learning Rust dyn Trait implementations In order for dyn Trait to be useful for abstracting over the base types which implement Trait, dyn Trait itself needs to implement Trait. A trait object is an opaque value of another type that implements a set of traits. Unlike other languages, if you have a trait like Animal, you can't write a function that returns Animal, because its different implementations will need different amounts of memory. Jun 1, 2018 · This new keyword parallels the impl Trait syntax and strives to make the type of a trait object more obviously distinct from the "bare" trait syntax. I don't see any functional difference between them besides new_foo1 being more verbose. However, I'm confused about why we can't use a trait, bound with Sized constrain like in the following example. That is, the type has been Rust tries to be as explicit as possible whenever it allocates memory on the heap. Static vs Dynamic Dispatch Static Returning Traits with dyn The Rust compiler needs to know how much space every function's return type requires. Rust uses a vtable to find the correct method address at runtime. dyn是trait对象类型的前缀 dyn关键字用于强调相关trait的方法是动态分配的。要以这种方式使用trait，它必须是“对象安全”的。 与泛型参数或植入型特质不同，编译器不知道被传递的具体类型。也就是说，该类型已经被抹去。因此，一个dyn Trait引用包含两个指针。一个指针指向数据（例如，一 Learning Rust dyn Trait lifetimes and Box<dyn Trait> Every trait object (dyn Trait) has an elide-able lifetime with it's own defaults when completely elided. However, there's an easy workaround Apr 5, 2025 · dyn Trait objects in Rust let you work with values of different types that share behavior, without knowing their exact type at compile time. 🌟🌟🌟. I have read about dyn compatibility, "object safety", "trait objects" in the Rust reference, Rust book, etc. Oct 23, 2025 · Understanding Rust trait Objects: dyn Keyword, Fat Pointers, and Vtables Introduction Rust provides two mechanisms for polymorphism: static dispatch via generics and dynamic dispatch via trait objects. Because the different implementations of a trait probably uses different amounts of memory, functions need to either return a concrete type or the same type when using impl Trait, or return a trait object with dyn. As a motivating example, consider the trait AsyncIterator: dyn Trait is always existential, uses dynamic dispatch, and is unsized. When you use trait objects (like &dyn Trait), Rust doesn’t know the concrete type at compile time. 🧩 Understanding Box<dyn Trait> in Rust: Dynamic Dispatch Done Right Rust loves knowing everything at compile time. dyn Trait In addition to using traits for static dispatch via generics, Rust also supports using them for type-erased, dynamic dispatch via trait objects: `dyn` is a prefix of a trait object’s type. `dyn` is a prefix of a trait object’s type. Trait objects implement the base trait, its auto traits, and any supertraits of the base trait. Although you need a Box<dyn Trait> (instead of just dyn Trait) because Rust only wants same size objects on non-template / non-generics functions (1 layout for 1 function, vs N layouts for N functions). 75 did not include support for using traits containing async functions as dyn Trait. kppjf, 7coesx, cmfp, 2xckh, ls2l, ehbb, j3oo, b8far, igfpx, x7azel,