From 75227c98bec5ad0c2c38c4d5f3b1ba7b43bda5e3 Mon Sep 17 00:00:00 2001 From: Zheng Ping Date: Fri, 26 May 2017 18:02:17 +0800 Subject: [PATCH] Update ch17-02-trait-objects.md --- src/ch17-02-trait-objects.md | 25 ++++++------------------- 1 file changed, 6 insertions(+), 19 deletions(-) diff --git a/src/ch17-02-trait-objects.md b/src/ch17-02-trait-objects.md index 85b7a2c..f787727 100644 --- a/src/ch17-02-trait-objects.md +++ b/src/ch17-02-trait-objects.md @@ -263,22 +263,11 @@ in the details"? Thanks! /Carol --> * 该trait要求`Self`不是`Sized`; * 该trait的所有方法都是对象安全的; -`Self`是一个类型的别名关键字，它表示当前正被实现的trait类型或者是方法所属的类型that is an alias for the type that we're implementing -traits or methods on. `Sized` is a marker trait like the `Send` and `Sync` -traits that we talked about in Chapter 16. `Sized` is automatically implemented -on types that have a known size at compile time, such as `i32` and references. -Types that do not have a known size include slices (`[T]`) and trait objects. - -`Sized` is an implicit trait bound on all generic type parameters by default. -Most useful operations in Rust require a type to be `Sized`, so making `Sized` -a default requirement on trait bounds means we don't have to write `T: Sized` -with most every use of generics. If we want to be able to use a trait on -slices, however, we need to opt out of the `Sized` trait bound, and we can do -that by specifying `T: ?Sized` as a trait bound. - -Traits have a default bound of `Self: ?Sized`, which means that they can be -implemented on types that may or may not be `Sized`. If we create a trait `Foo` -that opts out of the `Self: ?Sized` bound, that would look like the following: +`Self`是一个类型的别名关键字，它表示当前正被实现的trait类型或者是方法所属的类型. `Sized`是一个像在第16章中介绍的`Send`和`Sync`一样的标记trait, 在编译时它会自动被放进大小确定的类型里，比如`i32`和引用. 大小不确定的类型包括切片(`[T]`)和trait对象. + +`Sized`是一个默认会被绑定到所有常规类型参数的内隐trait. Rust中要求一个类型是`Sized`的最具可用性的用法是让`Sized`成为一个默认的trait绑定，这样我们就可以在大多数的常规的使用中不去写`T: Sized`了. 如果我们想在切片(slice)中使用一个trait, 我们需要取消对`Sized`的trait绑定, 我们只需制定`T: ?Sized`作为trait绑定. + +默认绑定到`Self: ?Sized`的trait可以被实现到是`Sized`或非`Sized`的类型上. 如果我们创建一个不绑定`Self: ?Sized`的trait`Foo`，它看上去应该像这样: ```rust trait Foo: Sized { @@ -286,9 +275,7 @@ trait Foo: Sized { } ``` -The trait `Sized` is now a *super trait* of trait `Foo`, which means trait -`Foo` requires types that implement `Foo` (that is, `Self`) to be `Sized`. -We're going to talk about super traits in more detail in Chapter 19. +Trait`Sized`现在就是trait`Foo`的一个*超级trait*, 也就是说trait`Foo`需要实现了`Foo`的类型(即`Self`)是`Sized`. 我们将在第19章中更详细的介绍超trait(supertrait). The reason a trait like `Foo` that requires `Self` to be `Sized` is not allowed to be a trait object is that it would be impossible to implement the trait