强制性状由一个对象实现

Question

如何强制scala中的特征只能被对象（不是类，特征等）扩展？

我有以下星座：

trait Outer { self: SomeOtherClass => 
    def member: Inner.type = Inner 

    /* some other methods... */ 

    trait Inner { iself => 
    def parent: self.type = self 
    /* other methods... */ 
    } 
} 

我的问题是：我怎么能强制执行内部特质必须通过的对象来实现（如对象内延伸内{...}）？

我希望每个实现Outer的类都有一个内部对象，它携带一些状态+从外部获取内部的可能性，反之亦然。

最好的问候，

约亨

编辑：

例子：

// compiles 
class C extends Outer { 
    object Inner extends Inner { ... } 
} 

// doesn't compile 
class C extends Outer { 
    class Inner extends Inner { ... } 
} 

Answer 1

使用Singleton自我类型：

trait Outer { 
    def member: Inner 
    /* other methods... */ 
    trait Inner { self: Singleton => 
    def parent: Outer.this.type = Outer.this 
    /* other methods... */ 
    } 
} 

class OImpl extends Outer { 
    // Note that OImpl's subclasses can't override member this way 
    // You could also do private[this] object member0 extends Inner 
    // and override lazy val member = member0 to restore overridability 
    // The laziness of object and lazy val mean that member0 won't be created 
    // if a subclass doesn't use it. 
    override object member extends Inner { 
    // ... 
    } 
    // ... 
}