本文介绍了为什么Option< String> .as_ref()不取消引用Option<& str> ;?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我期望这两个代码示例具有相同的结果:
I expect the same result for both of these code samples:
let maybe_string = Some(String::from("foo"));
let string = if let Some(ref value) = maybe_string { value } else { "none" };
let maybe_string = Some(String::from("foo"));
let string = maybe_string.as_ref().unwrap_or("none");
第二个示例给我一个错误:
The second sample gives me an error:
error[E0308]: mismatched types
--> src/main.rs:3:50
|
3 | let string = maybe_string.as_ref().unwrap_or("none");
| ^^^^^^ expected struct `std::string::String`, found str
|
= note: expected type `&std::string::String`
found type `&'static str`
推荐答案
因为 Option::as_ref
已定义:
Because that's how Option::as_ref
is defined:
impl<T> Option<T> {
fn as_ref(&self) -> Option<&T>
}
由于您有Option<String>
,所以生成的类型必须为Option<&String>
.
Since you have an Option<String>
, then the resulting type must be Option<&String>
.
相反,您可以添加 String::as_str
:
Instead, you can add in String::as_str
:
maybe_string.as_ref().map(String::as_str).unwrap_or("none");
或更短:
maybe_string.as_ref().map_or("none", String::as_str);
最终,您还可以使用 Option::deref
.
Eventually, you can also use Option::deref
.
另请参阅:
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