how to make private class member private

[Mike Parker]

On Tuesday, 13 March 2018 at 06:58:08 UTC, psychoticRabbit wrote:

>
> What you're saying, is in D, class encapsulation is really 
> 'module' encapsulation.
>
> I get it. Fine. It's an intersting design decision.

"Enapsulation" in D means the same as it does in every other 
language -- hidden from the outside.

The purpose of encapsulation is to prevent client code from 
breaking when an internal API changes. That's exactly the level 
of encapsulation that D's private provides. Making private 
restrict access to the class would be like other languages, sure, 
but then it becomes over restrictive.

>
> But, in doing that, D has shifted the boundary of class 
> encapsulation, to a boundary that is outside the class.

Nope. It's still hidden from the outside world. No one can read 
or write private class members *from the external API*.

>
> To me, that sounds like D has broken class encapsulation. I 
> don't know how else one could describe it.
>
> I continue to think, that class encapsulation is sacred, a well 
> defined, well understood, concept that has been around for a 
> very long time.

Only if you view encapsulation as something other than "hidden 
from the outside world".


> private could have still meant private, and surely someone 
> could have come up with a different access modifier to mean 
> 'private at module level'.
>
> Was that too hard the language designers?
>
> Was it not hard, but just to complex to implement?
>
> I don't get it.

Any new keywords, or reuse of existing keywords, does make the 
language more complex. Everything that is added must have a 
reason. Private is module level because friend is so common in 
C++, i.e. people find it useful and it would be great to support 
something similar in D. Making modules the lowest level of 
encapsulation does that without the need for an extra keyword for 
friends while still maintaining a strict border between external 
and internal APIs. Moreover, it restricts friends to the same 
module, easing the maintenance burden and decreasing the chance 
of error. It was a great decision.