protected inner class escapes its defining scope

[HjP]

When I compile the following code I get the error "private class Inner escapes its defining scope..":

class Outer {
  private class Inner {}
  def f = new Inner()
}

Why does this error vanish when I replace "private" by "protected" ?

[Erik Osheim]

So, if 'f' is a public method, what is the type signature of f? Clearly
it has to mention a private class, which is not legal.

By contrast, the following is fine:

trait Widget

class Outer {
private class Inner extends Widget
def ok: Widget = new Inner()
}

Hope this helps!

-- Erik

[Michael Swierczek]

I believe Hjalmar Peters' confusion, which I share, is that
"protected" in Scala means that the declared class, object, method,
val, etc... can only be accessed by itself and its subclasses.
I would expect
class Outer {
protected class Inner {}

def f = new Inner()
}

to still fail to compile because I would expect protected Inner not to
be in scope for 'f'.
What am I missing?

-Mike

[Erik Osheim]

On Thu, Sep 20, 2012 at 10:17:02AM -0400, Michael Swierczek wrote:
> I believe Hjalmar Peters' confusion, which I share, is that
> "protected" in Scala means that the declared class, object, method,
> val, etc... can only be accessed by itself and its subclasses.

I think you're slightly confused. A protected inner class can only be
accessed by its *enclosing* class and subclasses. The Scala compiler
will enforce this constraint (although it won't be reflected in the
Java bytecode AFAIK):

class Outer {
protected class Inner

def inner() = new Inner()
}

class Related extends Outer {
def ok() = new Related().inner()
def fails() = new Outer().inner()
}

class Unrelated {
def alsoFails() = new Outer().inner()
def stillFails() = new Related().inner()

[Michael Swierczek]

Thanks for the clarification. I have Programming in Scala (Odersky,
Spoon, Venners, first edition) and my read of the access modifiers
section was that enclosing classes were not included when using
protected. I was wrong.

-Mike

[Erik Osheim]

On Thu, Sep 20, 2012 at 10:26:18AM -0400, Erik Osheim wrote:
> I think you're slightly confused. A protected inner class can only be
> accessed by its *enclosing* class and subclasses. The Scala compiler
> will enforce this constraint (although it won't be reflected in the
> Java bytecode AFAIK):

Just want to apologize to the list. Paul Phillips pointed out to me
that I must have been doing something wrong, because when Inner is
protected all of these methods compile.

I'm not totally sure what went wrong. I guess the take-away is that
unlike private, protected inner classes don't impose a lot of access
restrictions on the outside world.

Sorry,

-- Erik

[HjP]

So, again my initial question: Why is a protected inner class (unlike a private inner class) allowed to escape its defining scope?