[英]Rust generics: Expected <T> found <Foo>
I'm trying to use generics but I don't master that topic well enough and I get this error: 我正在尝试使用泛型,但是我对该主题的掌握不够好,因此出现此错误:
error: mismatched types:
expected `book::mdbook::MDBook<R>`,
found `book::mdbook::MDBook<renderer::html_handlebars::HtmlHandlebars>`
(expected type parameter,
found struct `renderer::html_handlebars::HtmlHandlebars`) [E0308]
This is the relevant code 这是相关代码
pub struct MDBook<R> where R: Renderer {
title: String,
author: String,
config: BookConfig,
pub content: Vec<BookItem>,
renderer: R,
}
impl<R> MDBook<R> where R: Renderer {
pub fn new(path: &PathBuf) -> Self {
MDBook {
title: String::from(""),
author: String::from(""),
content: vec![],
config: BookConfig::new()
.set_src(path.join("src"))
.set_dest(path.join("book")),
renderer: HtmlHandlebars::new(), // <---- ERROR HERE
}
}
}
The Renderer
trait is empty at the moment and the implementation for HtmlHandlebars
is 目前, Renderer
特性是空的,并且HtmlHandlebars
的实现是
pub struct HtmlHandlebars;
impl Renderer for HtmlHandlebars {
}
impl HtmlHandlebars {
pub fn new() -> Self {
HtmlHandlebars
}
}
What am I doing wrong? 我究竟做错了什么?
impl<R> MDBook<R> where R: Renderer {
pub fn new(path: &PathBuf) -> Self {
These lines claim that for all types R
that implement Renderer
, there is a method new(path)
that returns MDBook<R>
. 这些行声称, 对于实现Renderer
所有类型R
,都有一个方法new(path)
返回MDBook<R>
。 However, your implementation of the method always returns MDBook<HtmlHandlebars>
regardless of what R
is. 但是,无论R
是什么,方法的实现总是返回MDBook<HtmlHandlebars>
。
You could add a trait bound to R
(or a method to Renderer
) that allows constructing a value of type R
in new
. 您可以添加绑定到R
的特征(或Renderer
的方法),以允许在new
构造类型R
的值。 Alternatively, the method could accept the renderer as parameter, ie fn new(path: &Path, renderer: R) -> Self
. 或者,该方法可以接受渲染器作为参数,即fn new(path: &Path, renderer: R) -> Self
。 Either way, you need a way to get your hands on a renderer (ie, a value of type R
) inside new
. 无论哪种方式,您都需要一种在new
内部使用渲染器(即类型R
的值)的方法。
If on the other hand you want to support something like this: 另一方面,如果您想支持以下内容:
let book = MDBook::new(path);
if some_condition {
book.set_renderer(SomeOtherThing::new());
}
then generics are the wrong tool for the job, since they make the choice of renderer part of the static type of book
. 那么泛型是完成这项工作的错误工具,因为泛型使选择渲染器成为book
静态类型的一部分。 You can remove the R
type parameter completely, keep your trait and simply store a trait object (likely Box<Renderer>
) in MDBook
. 您可以完全删除R
类型参数,保留特征并仅将特征对象 (可能是Box<Renderer>
)存储在MDBook
。
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