Private Nested Classes?

[Rémy Mouëza]

On Sunday, 31 March 2019 at 20:07:39 UTC, D Man wrote:
> Is it possible to have private inner nested classes?
>
> For example if I have a class that is only ever used inside 
> another class, it would make sense to be able to 'lock' it 
> inside the outer class, so that raw instances of it cannot be 
> created.
>
> But in this example: https://run.dlang.io/is/PfemUV
>
> The `private` specifier seems to have no meaning.
>
> Is there a way to get the desired effect (make a nested class 
> only accessable from within it's outer class)?

In D the unit of encapsulation is the module, not the class.

As the main function resides in the same module as the A.Inner 
class, it is visible and no compiler error should occur.

This rules is meant as a simplification of C++ friend classes and 
functions. In D, instead of defining friends, one put related 
class and function into the same module.

To "lock" the class, we need separate the main function from 
class A. This gives a warning (using dmd v2.085.0):
main.d(7): Deprecation: a.A.Inner is not visible from module main

I notice that Inner is defined as "private static", meaning that 
you don't intend to use any reference to the enclosing class 
through `this.outer`.
By putting Inner outside of class A, and having main outside of 
the module, an error occurs:
main.d(7): Error: module `main` class a.Inner is private
main.d(7): Deprecation: a.Inner is not visible from module main