在自身上实现迭代器返回为一个盒子 - 编译错误 关于迭代器未被输入，尽管被嵌入

Question

我正在尝试编写一个库，该库将读取字节（例如从文件中），并解析出对象并返回一个交互器。但是，即使使用虚拟版本，我也在实现迭代器时遇到问题。

这是我的代码：

use std::io::Read;
// One of the objects I want to return
pub struct ObjA {
    data: i8,
}
// There might be other kinds of objects here.
pub enum MyObj {
    ObjA(ObjA),
}
// There will be other input formats, and I want a generic trait for "read bytes and return MyObj's"
pub trait ObjReader<R> {
    // This should create the parser/reader from a std::io::Read source
    fn new(R) -> Self;
    // This will iterate over the objects. (AFAIK this is how to return an iterator)
    fn objects(&self) -> Box<Iterator<Item=MyObj>>;
}
// The data will be in XML, so here's a stupid struct that will (eventually) parse it and return objects. It obv. takes a `Read`
pub struct XMLReader<R: Read> {
    inner_reader: R,
}
// The XMLReader will be an ObjReader
impl<R: Read> ObjReader<R> for XMLReader<R> {
    fn new(reader: R) -> XMLReader<R> {
        XMLReader { inner_reader: reader }
    }
    // Return the iterator for the objects, I'll keep it simple and iterate over itself, rather than create a new struct
    fn objects(&self) -> Box<Iterator<Item=MyObj>> {
        Box::new(self)
    }
}
// Make XMLReader be an iterator
impl<R: Read> Iterator for XMLReader<R> {
    type Item = MyObj;
    // Right now, it's always empty. This is a stub
    fn next(&mut self) -> Option<Self::Item> {
        None
    }
}
// Dummy main to allow it to compile
fn main() {
}

我收到此错误：

$ multirust run beta rustc test.rs
test.rs:24:13: 24:27 error: the trait `core::iter::Iterator` is not implemented for the type `&XMLReader<R>` [E0277]
test.rs:24             Box::new(self)
                       ^~~~~~~~~~~~~~
test.rs:24:13: 24:27 help: run `rustc --explain E0277` to see a detailed explanation
test.rs:24:13: 24:27 note: `&XMLReader<R>` is not an iterator; maybe try calling `.iter()` or a similar method
test.rs:24:13: 24:27 note: required for the cast to the object type `core::iter::Iterator<Item=MyObj> + 'static`
error: aborting due to previous error

错误在于XMLReader如何不迭代，但我已经为它安装了迭代器。我怎样才能做到这一点？

我正在使用来自多锈的 rust 1.7 测试版。（稳定的锈格在我身上断了...

$ multirust run beta rustc --version
rustc 1.7.0-beta.4 (5ed7d4e31 2016-02-26)

Answer 1

答案既简单又烦人：您已经实现了XMLReader<R>而不是&XMLReader<R>的Iterator，或者您必须Box值而不是引用。

前者不会让你走得更远，因为那样你就会尝试围绕引用创建一个Box......有两种可能性：

1. 为读取器和迭代器提供不同的类型
2. 更改objects的签名以按值获取self

然后，您还必须注意约束R以确保它不会引用在您完成迭代之前可能停止存在的对象。'static绑定将更容易开始。