分享程序员开发的那些事...
更新时间：2023-02-24 07:55:53
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
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)
}
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