Object Oriented Programming with D Language. Private access specifier.

[Jacob Carlborg]

DF wrote:
> Fawzi Mohamed Wrote:
> 
>> On 2008-08-21 09:59:35 +0200, DF <deefriend at ymail.com> said:
>>
>>> Robert Fraser Wrote:
>>>
>>>> DF wrote:
>>>>> Why can private fields be accessed from other methods or classes in the 
>>>>> same module?
>>>>>
>>>>> If I wanted to access them from the same module I would make them 
>>>>> package public.
>>>> It's a feature -- a replacement for "friend" in C++. The general idea of
>>>> a module is that it is an autonomous code unit controlled by a single
>>>> developer/team and if you're accessing a private function in the module,
>>>> you have a good reason to. It's all the same file, so if you're changing
>>>> something that accesses a private member, you can change the private
>>>> implementation as well.
>>>>
>>>> "package" isn't implemented (sadly -- I find it very useful in Java so
>>>> that a package has only a single public API).
>>> Ok, thanks for your reply. But I think you've missed one thing. Let's 
>>> now speak of OO systems, about one basic principle of such systems 
>>> which is data abstraction. According to it an object should not expose 
>>> any of its implementation details. This means that you should 
>>> completely hide the way in which an object implements a message handler 
>>> from the rest of the program.That's one reason why all of your instance 
>>> variables (a class's nonconstant fields) should be private.
>>> So what do you think on that D implementation of "private" access 
>>> specifier breaks data abstraction?
>> There can be good reasons to break encapsulation (see C++ friend method).
>> A language should make it easy to respect successful practices, support 
>> them, but not needlessly limit the programmer.
>> A programmer should be a grown up person, as long as it is clear what 
>> is ok and what not, and doing the right thing is easy, all should be 
>> well.
>> In Python for example all variables are actually private just by convention...
>>
>> I find D approach very reasonable, it forces all the things that know 
>> the private interface to be in one place, namely one file.
>> Suppose that you need to write a template specialization that needs 
>> access to private details... D approach is well suited.
>>
>> Fawzi
>>
> 
> Nice reply. "A programmer should be a grown up person..." who told you that? :) Just a joke.
> 
> "There can be good reasons to break encapsulation (see C++ friend method)." - it is sad that you think so.You mixed up a good design solution and a solution. (Here I want to say that PROBABLY you've designed your OO system in a wrong way if you need to break an encapsulation).
> 
> And I don't believe that one can't write "a template specialization that needs access to private details" in Java. (Where "private"  - restricts the access to the class itself. Only methods that are part of the same class can access private members.)
> 
> -- Thanks for your reply. Maybe I am wrong.

In Java you can have several classes in the same file and they have 
access to each others private members, it's called inner classes. It's 
useful for listeners for example.