Question

是否可以声明代表特征的关联类型？如果没有，我该怎么办呢？试着做：

struct ColorData {
    let name: String
    let value: UIColor
}

class MyViewController: UIViewController {

    let data = [
        ColorData( name: "Blue", value: UIColor.blueColor() ),
        ColorData( name: "Red", value: UIColor.redColor() ),
        ColorData( name: "Green", value: UIColor.greenColor() )
    ]

    @IBOutlet weak var label: UILabel!

    var timer = NSTimer()

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear( animated )

        self.timer = NSTimer.scheduledTimerWithTimeInterval( 1.0, target: self, selector: "update", userInfo: nil, repeats: true )
    }

    func update() {
        let randomIndex = Int(arc4random() % UInt32(self.data.count))
        let colorData = data[ randomIndex ]
        self.label.text = colorData.name
        self.label.textColor = colorData.value
    }

    override func viewDidDisappear(animated: Bool) {
        super.viewDidDisappear( animated )

        // Don't forget to invalidate, otherwise this instance of `MyViewController` will leak
        self.timer.invalidate()
    }

}

我遇到的最大问题是使用trait Foo { /// A trait representing all types that can be returned from baz() type ReturnType; fn baz(&self) -> Self::ReturnType; }特征，因为我想要一个函数来返回实现Sized的类型项的向量：

ReturnType

选项1的问题是trait Foo { type ReturnType; // option 1 fn bar(&self) -> Vec<Self::ReturnType>; // option 2 fn bar<T: Self::ReturnType>(&self) -> Vec<T>; }不会被调整大小，因为它是一个特征，而选项2的问题是编译器不会将关联的类型识别为特征：ReturnType和failed to resolve. Use of undeclared type or module 'Self'（这使我认为相关类型不能指定特征）

编辑：我正在尝试做的一个例子

use of undeclared trait name 'Self::ReturnType'

用户的实施可能是

/// Represents all types that store byte-data instead of the actual
/// element
trait BufferedVec {
    /// the trait representing types that can be converted from the byte-data
    type FromBuffer;
    /// return the data at the given index, converted into a given type
    fn get<T: Self::FromBuffer>(&self, index: usize) -> T;
}

Answer 1

关联类型无法指定特征。您无法在Rust中的任何位置指定特征。您可以要求通用参数（或关联类型）需要实现特征。

trait Foo {
    type ReturnType: Clone;
}

这样Foo的任何实现者都需要确保他们选择的ReturnType也实现了Clone。

impl Foo for Bar {
    type ReturnType: i32;
}

是否可以声明一个代表特征的关联类型？

1 个答案: