有没有办法要求具有特定参数的构造函数？

Question

我认为我想要做的事很容易，但我没有得到正确的搜索关键词。我想要的是保证所有实现类都具有可以用已知类型的对象调用的构造函数的特征。语法应该是：

trait Message { 
    def this(rdr: msgpack.MsgReader): Message 
} 

但编译器告诉我它期望等号。任何想法如何做到这一点？

Answer 1

使用类型类模式，而不是：

trait Message[T] { 
    def read(reader: msgpack.MsgReader): T 
    // Example of what would be a normal instance method. 
    // thiz is similar to this, except because we're in another object it must be explicitly taken as parameter. 
    // It's in a separate param list for convention and ease of currying 
    def foo(thiz: T)(param1: Int): Boolean 
} 

// "Implementors" don't actually implement it, the behavior exists as its own singleton 
class Foo { ... } 

implicit object FooMessage extends Message[Foo] { 
    // Note that this is not restricted to just constructors. If you want that then you are really out of luck. (And probably doing it wrong.) 
    override def read(reader: msgpack.MsgReader) = ??? 
    override def foo(thiz: Foo)(param1: Int) = thiz.foo(param1) 
} 

// Usage 
// Use an explicit, named implicit param to avoid implicitly 
def readMsg[T: Message](reader: msgpack.MsgReader) = implicitly[Message[T]].read(reader) 

val foo = readMsg[Foo](???)