无法将实现特征的结构装箱为特征对象

Question

我正在玩 Rust，这里是代码：

pub trait TFilter {
    fn get_text(&self) -> &String;
}
...
pub struct ContentFilter {
    text: String,
    domains: String,
    body: String,
}

impl TFilter for ContentFilter {
    fn get_text(&self) -> &String {
        return &self.text
    }
}

impl ContentFilter {
    pub fn from_text(text: String, domains: String, body: String) -> Self {
        return ContentFilter {
            text,
            domains,
            body
        }
    }
}
...

fn filter_from_text(&mut self, text: String) -> &Box<dyn TFilter> {
...
&Box::new(ContentFilter::from_text(text, domains, body)) // is returned
...
}

我收到错误消息：

expected trait object dyn filter::TFilter, found struct filter::ContentFilter

= note: expected reference &std::boxed::Box<(dyn filter::TFilter + 'static)> found reference &std::boxed::Box<filter::ContentFilter>

这是误导性的：

它确实实现了特征
编译器确实知道 ContentFilter struct

有线索吗？

PS。代码无论如何都不好（因为不清楚谁拥有返回的 Box），但消息具有误导性。

PPS。那么如何实现缓存代理：

pub trait TFilterBuilder {
    fn filter_from_text(&mut self, text: String) -> &Box<dyn TFilter>;
}
...
struct FilterCachingProxy {
    filter_builder: Box<dyn TFilterBuilder>,
    cache: Box<dyn TFilterCache>
}

impl FilterCachingProxy {
    fn new_for_builder(filter_builder: Box<dyn TFilterBuilder>, cache: Box<dyn TFilterCache>) -> Self {
        return FilterCachingProxy {
            filter_builder,
            cache
        }
    }
}

impl TFilterBuilder for FilterCachingProxy {
    fn filter_from_text(&mut self, text: String) -> &Box<dyn TFilter> {
        let boxed_filter = match self.cache.get(&text) {
            Some(found_boxed_filter) => found_boxed_filter,
            _ => {
                // delegate creation to wrapped TFilterBuilder impl (`filter_builder`)
                let boxed_filter = self.filter_builder.filter_from_text(text.clone());
                // ... and put to the cache
                self.cache.put(&text, &boxed_filter); // boxed_filter should be moved to make cache own it?
                &boxed_filter // looks strange: who owns it then?
            }
        };
        return boxed_filter;
    }
}

Answer 1

return 类型应该只是一个 Box，而不是对 Box 的引用：

fn filter_from_text(&mut self, text: String) -> Box<dyn TFilter> {
    ...
    Box::new(ContentFilter::from_text(text, domains, body)) // is returned
}

(Minimal compiling version on the playground)

盒子是在您的函数中新创建的，因此您必须 return 盒子的所有权。不可能只是 return 对临时对象的引用，因为它会在函数结束时超出范围。

此更改的一个副作用是从 Box<ContentFilter> 到 Box<dyn TFilter> 的无限制强制转换现在将真正起作用。强制转换仅适用于 强制转换站点 ，如 return 值。在您的代码中，需要强制转换的类型嵌套在引用中，因此编译器不会执行未确定大小的强制转换。

无法将实现特征的结构装箱为特征对象

Can't box a struct that implements a trait as a trait object

traits

rust

box