impl convert ::从for(mutable)reference
问题描述:
我试图实现From
为我想获取作为可变引用的类型,所以我impl它为&mut TheType
,但那么我该如何正确呼叫from
?尝试我执行失败,因为它试图做一个类型&mut TheType
from
反射(TheType从TheType)或不能(或不知道如何)。impl convert ::从for(mutable)reference
代码将更好地希望解释一下:
enum Component {
Position(Point),
//other stuff
}
struct Point {
x: i32,
y: i32,
}
impl<'a> std::convert::From<&'a mut Component> for &'a mut Point {
fn from(comp: &'a mut Component) -> &mut Point {
// If let or match for Components that can contain Points
if let &mut Component::Position(ref mut point) = comp {
point
} else { panic!("Cannot make a Point out of this component!"); }
}
}
// Some function somewhere where I know for a fact that the component passed can contain a Point. And I need to modify the contained Point. I could do if let or match here, but that would easily bloat my code since there's a few other Components I want to implement similar Froms and several functions like this one.
fn foo(..., component: &mut Component) {
// Error: Tries to do a reflexive From, expecting a Point, not a Component
// Meaning it is trying to make a regular point, and then grab a mutable ref out of it, right?
let component = &mut Point::from(component)
// I try to do this, but seems like this is not a thing.
let component = (&mut Point)::from(component) // Error: unexpected ':'
...
}
是什么,我想在这里做可能吗?上面的impl From
编译得很好,只是它的调用而逃脱了我。要做到这一点
答
的一种方法是指定的component
这样类型:
let component: &mut Point = From::from(component);
由于Simon Whitehead指出的,更地道的方式来做到这一点是使用相应的功能into()
:
let component: &mut Point = component.into();
答
正确语法如下:
let component = <&mut Point>::from(component);
它本质上是没有领先::
的“turbofish”语法。
此外,因为STDLIB包括向对于T IMPL的'许多变型,其中U:从'也可以做到这一点实现'From'后:'让成分:&MUT点= component.into();' –