特征对象如何将具有泛型方法的特征作为参数？

Question

所以 trait 对象不能有带有泛型的方法 - 看起来不错。但是在这种语言中，使用抽象机制的唯一方法是通过泛型和特征对象。这意味着对于每个特征，我必须事先决定它是否可以用作对象，并在任何地方使用 dyn 而不是 impl。并且它内部的所有特征都必须以相同的方式来支持这一点。这种感觉非常难看。您能提出什么建议或告诉我为什么要这样设计吗？

fn main() {}

// some abstracted thing
trait Required {
    fn f(&mut self, simple: i32);
}

// this trait doesn't know that it's going to be used by DynTrait
// it just takes Required as an argument
// nothing special
trait UsedByDyn {
    // this generic method doesn't allow this trait to be dyn itself
    // no dyn here: we don't know about DynTrait in this scope
    fn f(&mut self, another: impl Required);
}

// this trait needs to use UsedByDyn as a function argument
trait DynTrait {
    // since UsedByDyn uses generic methods it can't be dyn itself
    // the trait `UsedByDyn` cannot be made into an object
    //fn f(&mut self, used: Box<dyn UsedByDyn>);

    // we can't use UsedByDyn without dyn either otherwise Holder can't use us as dyn
    // the trait `DynTrait` cannot be made into an object
    // fn f(&mut self, used: impl UsedByDyn);

    // how to use UsedByDyn here?
}

struct Holder {
    CanBeDyn: Box<dyn DynTrait>,
}

Answer 1

这意味着对于每个特征，我必须事先决定它是否可以用作对象，并在任何地方使用 dyn 而不是 impl。

你可以这样做，但幸运的是这不是唯一的选择。

您也可以像往常一样编写您的特征，在适当的情况下使用泛型。如果/当您需要 trait 对象时，请定义一个您在本地使用的新的对象安全 trait，并在该位置公开您实际需要的 API 子集。

例如，假设您拥有或使用了非对象安全的特征：

trait Serialize {
    /// Serialize self to the given IO sink
    fn serialize(&self, sink: &mut impl io::Write);
}

该特征不能用作特征对象，因为它（可能是为了确保最大效率）具有通用方法。但这并不需要阻止您的代码使用特征对象来访问特征的功能。假设您需要将 Serialize 值装箱，以便将它们保存在一个向量中，您将把它们保存到一个文件中en masse：

// won't compile
struct Pool {
    objs: Vec<Box<dyn Serialize>>,
}

impl Pool {
    fn add(&mut self, obj: impl Serialize + 'static) {
        self.objs.push(Box::new(obj) as Box<dyn Serialize>);
    }

    fn save(&self, file: &Path) -> io::Result<()> {
        let mut file = io::BufWriter::new(std::fs::File::create(file)?);
        for obj in self.objs.iter() {
            obj.serialize(&mut file);
        }
        Ok(())
    }
}

上面的代码不能编译，因为Serialize 不是对象安全的。但是 - 您可以轻松定义一个新的对象安全 trait 来满足 Pool 的需求：

// object-safe trait, Pool's implementation detail
trait SerializeFile {
    fn serialize(&self, sink: &mut io::BufWriter<std::fs::File>);
}

// Implement `SerializeFile` for any T that implements Serialize
impl<T> SerializeFile for T
where
    T: Serialize,
{
    fn serialize(&self, sink: &mut io::BufWriter<std::fs::File>) {
        // here we can access `Serialize` because `T` is a concrete type
        Serialize::serialize(self, sink);
    }
}

现在Pool 几乎可以正常工作，使用dyn SerializeFile (playground)：

struct Pool {
    objs: Vec<Box<dyn SerializeFile>>,
}

impl Pool {
    fn add(&mut self, obj: impl Serialize + 'static) {
        self.objs.push(Box::new(obj) as Box<dyn SerializeFile>);
    }

    // save() defined the same as before
    ...
}

定义一个单独的对象安全特征似乎是不必要的工作 - 如果原始特征足够简单，您当然可以从一开始就使其成为对象安全。但是有些特征要么太通用，要么太面向性能，从一开始就无法做到对象安全，在这种情况下，最好记住保持通用性是可以的。当您确实需要对象安全版本时，它通常用于具体任务，其中根据原始特征实现的自定义对象安全特征将完成这项工作。

Answer 2

我使用了@user4815162342 的答案，但制作了我自己的版本，不需要用具体类型替换非对象友好的特征。

struct Holder {
    dyn_traits: Vec<Box<dyn DynTrait>>,
}

// this trait doesn't know that it's going to be used by DynTrait
// it just takes ObjectFriendly as an argument
// nothing special
trait ObjectUnfriendly {
    // this generic method doesn't allow this trait to be dyn itself
    // no dyn here: we don't know about DynTrait in this scope
    fn f(&mut self, another: &impl ObjectFriendly);
    fn f2(&mut self, another: &mut impl ObjectFriendly);
    fn f3(&mut self, another: impl ObjectFriendly);
}

trait ObjectFriendly {
    fn f(&mut self, simple: i32);
    fn f2(&self, simple: i32);
}

// this trait needs to use the trait above as a function argument
trait DynTrait {
    // since that trait uses generic methods it can't be dyn itself
    // the trait cannot be made into an object
    //fn f(&mut self, used: Box<dyn ObjectUnfriendly>);

    // we can't use that trait without dyn either otherwise Holder can't use us as dyn
    // the trait `DynTrait` cannot be made into an object
    // fn f(&mut self, used: impl ObjectUnfriendly);

    // how to use ObjectUnfriendly here?
    // we use our own extension trait that is object-friendly
    fn f(&mut self, used: dyn NowObjectFriendly);
}

// our own object-friendly version
trait NowObjectFriendly {
    // if arguments are ObjectFriendly - we are lucky
    fn f(&mut self, another: &dyn ObjectFriendly);
    fn f2(&mut self, another: &mut dyn ObjectFriendly);
    fn f3(&mut self, another: Box<dyn ObjectFriendly>);

    // if not - we can just accept the specific struct we need
    // fn f3(&mut self, another: SomeImpl);

    // or do the same thing by making an extension trait
    // fn f3(&mut self, another: Box<dyn ObjectFriendly2Ex>);
}

// delegate implementation
impl<T: ObjectUnfriendly> NowObjectFriendly for T {
    fn f(&mut self, another: &dyn ObjectFriendly) {
        self.f(&ObjectFriendly2AsImpl(another));
    }

    fn f2(&mut self, another: &mut dyn ObjectFriendly) {
        self.f2(&mut ObjectFriendly2AsImpl(another));
    }

    fn f3(&mut self, another: Box<dyn ObjectFriendly>) {
        self.f3(ObjectFriendly2AsImpl(another));
    }

    // if not object friendly - we can just accept the specific struct we need
    // fn f3(&mut self, another: SomeImpl) {
    //     SomeImpl::f3(self, another);
    // }

    // or do the same thing for that trait by making another extension trait
    // fn f3(&mut self, another: Box<dyn ObjectFriendly2Ex>) {
    //     self.f3(another);
    // }
}

// for this delegation to work
// we need to make it convertible to impl

// can't implement foreign traits on foreign types
struct ObjectFriendly2AsImpl<T>(T);

impl ObjectFriendly for ObjectFriendly2AsImpl<&dyn ObjectFriendly> {
    fn f(&mut self, simple: i32) {
        unreachable!()
    }

    fn f2(&self, simple: i32) {
        (*self.0).f2(simple)
    }
}

impl ObjectFriendly for ObjectFriendly2AsImpl<&mut dyn ObjectFriendly> {
    fn f(&mut self, simple: i32) {
        (*self.0).f(simple)
    }

    fn f2(&self, simple: i32) {
        (*self.0).f2(simple)
    }
}

impl ObjectFriendly for ObjectFriendly2AsImpl<Box<dyn ObjectFriendly>> {
    fn f(&mut self, simple: i32) {
        (*self.0).f(simple)
    }

    fn f2(&self, simple: i32) {
        (*self.0).f2(simple)
    }
}

如果有此或更轻量级实现的宏，请发表评论。