如何匹配特质实现者

时间:2014-09-30 17:28:58

标签: rust

我有一个由某些结构实现的特征。我想写一个模式匹配,我可以处理每个可能的情况:

trait Base {}

struct Foo {
    x: u32,
}
struct Bar {
    y: u32,
}

impl Base for Foo {}
impl Base for Bar {}

fn test(v: bool) -> Box<Base + 'static> {
    if v {
        Box::new(Foo { x: 5 })
    } else {
        Box::new(Bar { y: 10 })
    }
}

fn main() {
    let f: Box<Base> = test(true);

    match *f {
        Foo { x } => println!("it was Foo: {}!", x),
        Bar { y } => println!("it was Bar: {}!", y),
    }
}

Playground

我收到此编译错误:

error[E0308]: mismatched types
  --> src/main.rs:25:9
   |
25 |         Foo { x } => println!("it was Foo: {}!", x),
   |         ^^^^^^^^^ expected trait Base, found struct `Foo`
   |
   = note: expected type `dyn Base`
              found type `Foo`

error[E0308]: mismatched types
  --> src/main.rs:26:9
   |
26 |         Bar { y } => println!("it was Bar: {}!", y),
   |         ^^^^^^^^^ expected trait Base, found struct `Bar`
   |
   = note: expected type `dyn Base`
              found type `Bar`

3 个答案:

答案 0 :(得分:25)

你做不到。特征不支持向下转换 - Rust不是基于继承/子类型的语言,它为您提供了另一组抽象。此外,你想做的事情是不健全的 - 特征是开放的(每个人都可以为任何事情实现它们),所以即使在你的情况下match *f涵盖所有可能的情况,通常编译器也不能知道。

这里有两种选择。如果您事先知道实现特征的结构集,只需使用枚举,它就是一个完美的工具。它们允许您在一组封闭的变体上进行静态匹配:

enum FooBar {
    Foo(u32),
    Bar(u32),
}

fn test(v: bool) -> FooBar {
    if v {
        FooBar::Foo(5)
    } else {
        FooBar::Bar(10)
    }
}

fn main() {
    let f: FooBar = test(true);

    // Now that we have a `Box<Base>` (`*f` makes it a `Base`),
    // let's handle different cases:
    match f {
        FooBar::Foo(x) => println!("it was Foo: {}!", x),
        FooBar::Bar(y) => println!("it was Bar: {}!", y),
    }
}

Playground

这是迄今为止最简单的方法,应始终优先考虑。

另一种方法是使用Any特征。它是从特征对象到常规类型的类型安全向下转换的工具:

use std::any::Any;

struct Foo {
    x: u32,
}
struct Bar {
    y: u32,
}

fn test(v: bool) -> Box<Any + 'static> {
    if v {
        Box::new(Foo { x: 5 })
    } else {
        Box::new(Bar { y: 10 })
    }
}

fn main() {
    let f: Box<Any> = test(true);

    match f.downcast_ref::<Foo>() {
        Some(&Foo { x }) => println!("it was Foo: {}!", x),
        None => match f.downcast_ref::<Bar>() {
            Some(&Bar { y }) => println!("it was Bar: {}!", y),
            None => unreachable!(),
        },
    }

    // it will be nicer when `if let` lands
    //    if let Some(ref Foo { x }) = f.downcast_ref::<Foo>() {
    //        println!("it was Foo: {}!", x);
    //    } else if let Some(ref Bar { y }) = f.downcast_ref::<Bar>() {
    //        println!("it was Bar: {}!", y);
    //    } else { unreachable!() }
}

Playground

理想情况下应该可以这样写:

trait Base: Any {}

impl Base for Foo {}
impl Base for Bar {}

然后在代码中使用Base,但现在无法完成,因为trait继承不适用于trait对象(例如,从Box<Base>到{{1}是不可能的})。

答案 1 :(得分:3)

您可以使用我的match_cast包:

match_cast!( any {
    val as Option<u8> => {
        format!("Option<u8> = {:?}", val)
    },
    val as String => {
        format!("String = {:?}", val)
    },
    val as &'static str => {
        format!("&'static str = {:?}", val)
    },
});

match_down!( any {
    Bar { x } => { x },
    Foo { x } => { x },
});

答案 2 :(得分:1)

我建议访客模式要与特征匹配。这种模式来自OOP,但在许多情况下仍然很有用。而且,它在避免向下转换的同时效率更高。

这是一个摘要:

struct Foo{ value: u32 }
struct Bar{ value: u32 }

trait Base<T> {
    fn accept(&self, v: &dyn Visitor<Result = T>) -> T ;
}
impl <T>Base<T> for Foo {
    fn accept(&self, v: &dyn Visitor<Result = T>) -> T {
        v.visit_foo(&self) 
    }
}
impl <T>Base<T> for Bar {
    fn accept(&self, v: &dyn Visitor<Result = T>) -> T {
        v.visit_bar(&self) 
    }
}

trait Visitor {
    type Result;
    fn visit_foo(&self, foo: &Foo) -> Self::Result;
    fn visit_bar(&self, bar: &Bar) -> Self::Result;
}

struct StringVisitor {}
impl Visitor for StringVisitor {
    type Result = String;
    fn visit_foo(&self, foo: &Foo) -> String {
        format!("it was Foo: {:}!", foo.value)
    }
    fn visit_bar(&self, bar: &Bar) -> String {
        format!("it was Bar: {:}!", bar.value)
    }
}
fn test<T>(v: bool) -> Box<dyn Base<T>> {
    if v {
        Box::new(Foo{value: 5})
    } else {
        Box::new(Bar{value: 10}) 
    }
}
fn main() {
    let f = test(true);
    println!("{:}", f.accept( &StringVisitor{} ));
}