为什么serde_json :: from_reader拥有读者的所有权? [英] Why does serde_json::from_reader take ownership of the reader?

问题描述

我的代码:

fn request_add<T>(request: &mut Request, collection_name: &'static str) -> Fallible<Fallible<String>>
where
    T: serde::Serialize + serde::de::DeserializeOwned,
{
    let add_dao = dao::MongoDao::new(collection_name);
    let obj = serde_json::from_reader::<Body, T>(request.body)?; //cannot move out of borrowed content
    Ok(add_dao.add(&obj))
}

我有一个cannot move out of borrowed content错误，因为request是引用，但是为什么serde_json::from_reader不使用mut引用?为什么需要所有权?我该如何解决?

I have a cannot move out of borrowed content error, because request is a reference, but why does serde_json::from_reader not use a mut reference? Why does it need ownership? And how can I fix it?

推荐答案

因为它是

通用读取器/写入器函数按值(C-RW-VALUE)取R: Read和W: Write

标准库包含以下两个提示:

Generic reader/writer functions take R: Read and W: Write by value (C-RW-VALUE)

The standard library contains these two impls:

impl<'a, R: Read + ?Sized> Read for &'a mut R { /* ... */ }

impl<'a, W: Write + ?Sized> Write for &'a mut W { /* ... */ }

这意味着任何接受R: Read或W: Write泛型的函数 如有必要，可以使用mut引用调用按值设置的参数.

That means any function that accepts R: Read or W: Write generic parameters by value can be called with a mut reference if necessary.

您可以调用 Read::by_ref 或只是自己参考:

You either call Read::by_ref or just take a reference yourself:

serde_json::from_reader(&mut request.body)

serde_json::from_reader(request.body.by_ref())

另请参阅:

• 从实现Read的类型中读取任意数量的字节
• 如何在BufReader中使用文件并且仍然能够对其进行写入?
• 为什么Iterator :: take_while拥有迭代器的所有权?

• Read an arbitrary number of bytes from type implementing Read
• How to use a file with a BufReader and still be able to write to it?
• Why does Iterator::take_while take ownership of the iterator?

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