從功能返回特征

Question

我看一下生銹lib： https ： //github.com/cyderize/rust-websocket/源文件https://github.com/cyderize/rust-websocket/blob/master/src/client/response .rs我看到方法begin（）：

pub fn begin(self) -> Client<DataFrame, Sender<W>, Receiver<R>> {
    let (reader, writer) = self.into_inner();
    let sender = Sender::new(writer);
    let receiver = Receiver::new(reader);
    Client::new(sender, receiver)
}

所以......我決定創建一些簡單的包裝函數：

fn get_transport(url: &str)  ->  Client<DataFrame, Sender<Write>, Receiver<Read>> {
    let url = Url::parse(url).unwrap();
    let request = Client::connect(url).unwrap();
    let response = request.send().unwrap();
    let mut ws = response.begin();

    ws
}

但不幸的是我有錯誤的結果：

error: the trait `core::marker::Sized` is not implemented for the type `websocket::ws::sender::Sender<std::io::Write>` [E0277]
src/lib.rs:36 fn get_transport(url: &str)  ->  Client<DataFrame, Sender<Write>, Receiver<Read>> {

有人可以形容我為什么不能以這種方式回報價值。 與response.rs中的begin（）方法有什么區別？ 在我的情況下，我應該如何回報價值？

UPD1：在@ker推薦之后我有下一個代碼：

fn get_transport<W: Write, R: Read, S: Sender<W>, RE: Receiver<R>>(url: &str) -> Client<DataFrame, S, RE> {

    let url = Url::parse(url).unwrap();
    let request = Client::connect(url).unwrap();
    let response = request.send().unwrap();
    let mut ws = response.begin();

    ws
}

但是在下一次編譯中收到下一個錯誤：

src/lib.rs:45:5: 45:7 error: mismatched types:
 expected `websocket::client::Client<websocket::dataframe::DataFrame, S, RE>`,
    found `websocket::client::Client<websocket::dataframe::DataFrame, websocket::client::sender::Sender<websocket::stream::WebSocketStream>, websocket::client::receiver::Receiver<websocket::stream::WebSocketStream>>`
(expected type parameter,
    found struct `websocket::client::sender::Sender`) [E0308]
src/lib.rs:45     ws
                  ^~
src/lib.rs:45:5: 45:7 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error

似乎我使用錯誤的發件人和錯誤的接收器：\\經過更正后使用部分我已經得到了結果代碼：

fn get_transport(url: &str) -> Client<DataFrame, Sender<WebSocketStream>, Receiver<WebSocketStream>> {
    let url = Url::parse(url).unwrap();
    let request = Client::connect(url).unwrap();
    let response = request.send().unwrap();
    let mut ws = response.begin();

    ws
}

不幸的是我無法找出為什么我不能使用泛型作為結果值。 像這樣：

fn get_transport<R: Read, W: Write>(url: &str) -> Client<DataFrame, Sender<W>, Receiver<R>>

在這種情況下我收到錯誤消息：

expected `websocket::client::Client<websocket::dataframe::DataFrame, websocket::client::sender::Sender<W>, websocket::client::receiver::Receiver<R>>`,
    found `websocket::client::Client<websocket::dataframe::DataFrame, websocket::client::sender::Sender<websocket::stream::WebSocketStream>, websocket::client::receiver::Receiver<websocket::stream::WebSocketStream>>`
(expected type parameter,
    found enum `websocket::stream::WebSocketStream`) [E0308]
src/lib.rs:45     ws
                  ^~

Answer 1

由於任何類型都可能影響特征，因此您無法通過了解對象實現的特征來了解對象的大小。 你可以做的是，使你的函數分別通用於實現Write和Read的類型。 這樣您就可以返回對象，而不知道對象的實際類型是什么。

fn get_transport<W: Write, R: Read, S: Sender<W>, RE: Receiver<R>>(url: &str) -> Client<DataFrame, S, RE> {
    let url = Url::parse(url).unwrap();
    let request = Client::connect(url).unwrap();
    let response = request.send().unwrap();
    let mut ws = response.begin();

    ws
}

添加泛型的原因之一是因為泛型標簽指向實際類型，而不知道除了它們實現的特征之外的任何類型。

Answer 2

您希望使用具體類型替換返回類型中的Write和Send 。

嘗試更改行let mut ws = response.begin(); let mut ws: () = response.begin() ，您將看到一條錯誤消息，告訴您從begin函數返回的具體類型。