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From: richard@pantor.UUCP (Richard Sargent)
Newsgroups: comp.lang.c++
Subject: Access to protected members
Message-ID: <59.UUL1.3#5109@pantor.UUCP>
Date: 20 Mar 90 14:59:00 GMT
Organization: Pansophic Systems Inc, Graphics Product Company
Lines: 39


(I apologize if this has been seen before, but I have had no
indication that it successfully made its way out into the world.)

We have encountered a problem with C++ 2.0, and diligent reading
of the manual confirms that we are wrong. :-(

I am left with a couple of questions as a result, but first the
problem:

We have a tree of classes, such as various graphics objects. We
want to have some base class members be accessible only to objects
from this tree of classes. No problem you say, just use protected.
There's the nub: any given object can access its protected base
members, but not those of any other object derived from the same
tree. (Reference: AT&T Reference Manual, Select Code 307-146, 
section 11.5 Protected Member Access, page 73)

For example, we have a derived class which we want to be able to
traverse an array of graphics objects (of which class it is, too).
However, this object (and its member functions) cannot access the
protected members of the objects in this array.

Q1: Why was the language specified with this restriction? I am not
a language designer, so I don't understand the decisions that might
be obvious to some one is. I certainly don't presume to think that
Stroustrup is incorrect, but I do want to understand the ratioanle
here.

Q2: Given that this restriction is part of the language, how can I
work around it? The obvious solution is to make the member in question
"public", but then everything has access to it. However, I see
no alternatives.

Any and all help and explanations welcomed and appreciated.
Thanks in advance.

Richard Sargent                   Internet: richard@pantor.UUCP
Systems Analyst                   UUCP:     ...!mnetor!becker!pantor!richard