Object Oriented Programming with D Language. Private access specifier.

[Fawzi Mohamed]

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