为什么Manifest在构造函数中不可用？ [英] Why is the Manifest not available in the constructor?

问题描述

请考虑这个代码：

  class Foo [T：Manifest]（val id：String = manifest [T] .erasure .getName）
  

我基本上想在Foo中存储标识符，这通常只是类名。

不需要特殊标识符的子类可以方便地使用默认值。

t even compile，错误消息是：

 错误：没有清单可用于T. 
  
 
 
编辑：
 $ 
 b 
 $ b 
如果清单在主构造函数之前不可用，为什么这样工作？
  class Foo [T：Manifest]（val name：String）{
 def this（）= this（manifest [T] .erasure.getName）
} 
  
解决方案
当语法糖从该上下文中移除时，它被重写为：
  class Foo [T] 
（val id：String = implicitly [Manifest [T]]。erasure.getName）
 implicit ev $ 1：Manifest [T]）= ... 
  
当确定 id 的默认值时可用。我会这样写：
  class Foo [T：Manifest]（id0：String =）{
 val id = if（id0！=）id0 else manifest [T] .erasure.getName 
} 
  
 
 
 
 
 
 
在你的第二种方法（这是一个很好的解决方案，顺便！），期望重写类似：
 
 
  class Foo [T]（val name：String）（implicit x $ 1：Manifest [T]）{
 def this ：Manifest [T]）= this（manifest [T] .erasure.getName）
} 
  
是，在调用清单[T] .erasure  
 
之前，清单 Consider this code:
class Foo[T : Manifest](val id: String = manifest[T].erasure.getName)
I basically want to store an identifier in Foo, which is often just the class name.

Subclass which do not need a special identifier could then easily use the default value.

But this doesn't even compile, the error message is:
error: No Manifest available for T.
Is there another approach which will work?

EDIT:

Why does this work if the manifest isn't available until the primary constructor?
class Foo[T: Manifest](val name: String) { 
  def this() = this(manifest[T].erasure.getName)
}

解决方案
When the syntactic sugar is removed from that context bound, it gets rewritten as:
class Foo[T]
  (val id: String = implicitly[Manifest[T]].erasure.getName)
  (implicit ev$1: Manifest[T]) = ...
So the Manifest evidence simply isn't available when determining the default value of id.  I'd instead write something like this:
class Foo[T : Manifest](id0: String = "") {
  val id = if (id0 != "") id0 else manifest[T].erasure.getName
}




In your second approach (which is a great solution, by the way!), expect a rewrite similar to:
class Foo[T](val name: String)(implicit x$1: Manifest[T]) { 
  def this()(implicit ev$2: Manifest[T]) = this(manifest[T].erasure.getName)
}
So yes, the manifest is available before the call to manifest[T].erasure

                        
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