[英]Why is the downcast_ref method not found for the dyn Any type?
我試圖創建自己的模擬框架,但遇到了這個問題。 當我嘗試向下轉換Any
類型時,找不到downcast_ref
方法:
use std::any::Any;
use std::collections::HashMap;
struct X;
struct Y;
fn main() {
let mut map: HashMap<&'static str, Box<Any + Sync>> = HashMap::new();
map.insert("x", Box::new(X));
map.insert("y", Box::new(Y));
get_x(map);
}
fn get_x(map: HashMap<&'static str, Box<Any + Sync>>) {
let ref any = map["x"];
let res = Any::downcast_ref::<X>(any); // Works
let res = any.downcast_ref::<X>(); // Fails
}
error[E0599]: no method named `downcast_ref` found for type `&std::boxed::Box<(dyn std::any::Any + std::marker::Sync + 'static)>` in the current scope
--> src/main.rs:18:19
|
18 | let res = any.downcast_ref::<X>();
| ^^^^^^^^^^^^
如果我使用關聯的函數語法來調用它,它將找到該函數並且可以正常工作。
為何編譯器無法從變量any
找到dyn Any
類型的downcast_ref()
方法?
這是因為未對dyn Any + 'static + Sync
實施Any::downcast_ref()
,僅適用於:
dyn Any + 'static
dyn Any + 'static + Send
dyn Any + 'static + Send + Sync
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