Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!teknowledge-vaxc!mkhaw
From: mkhaw@teknowledge-vaxc.ARPA (Mike Khaw)
Newsgroups: comp.lang.c++
Subject: Re: accessing base class protected members
Message-ID: <26705@teknowledge-vaxc.ARPA>
Date: 15 Feb 89 21:44:35 GMT
References: <1000008@hpclscu.HP.COM>
Organization: Teknowledge, Inc., Palo Alto CA
Lines: 73

<1000008@hpclscu.HP.COM>, by shankar@hpclscu.HP.COM (Shankar Unni):
>> class higher : public bottom
>                  ^^^^^^
>> {
>> protected:
>> 	float	hifloat;
>> 
>> public:
>> 	float	gethi() { return hifloat; }
>> 
>> friend void	mumble(higher&);
>> };
> 
> The [compiler error] message *is* correct.
> 
> Since the "public" qualifier is used in the inheritance, the members of
> "higher" can only access the public members of "bottom". This restriction
> also carries over to any friends of the derived class.
> 
> What Mike needs to do to make this do what he wants is to declare "higher"
> as:
> 
>   class higher : bottom {
>    ...
>   }

That's not the obvious interpretation.  I quote from Stroustrup (Chapt. 7,
7.2.3: Visibility):

		class manager : public employee {
			// ...
		};

	This means that a public member of class _employee_ is also a
	public member of class manager. ...

	...

	... one can declare a _private_ base class by simply leaving out
	the word _public_ in the class declaration:

		class manager : employee {
			// ...
		};

	This means that a public member of the class _employee_ is
	a PRIVATE MEMBER [emphasis added] of class _manager_...
	Friends of a derived class have the same access to base class
	members as do member functions.

In Stroustrup's paper "An Overview of C++", in the section titled
"Visibility Control", it says about _protected_ that:

	a member should be private as far as functions outside the
	class hierarchy are concerned but accessible to member functions
	of a derived class in the same way that it is accessible to
	members of its own class.  Such a member is said to be _protected_.

And in "What is Object-Oriented Programming?", in "Encapsulation"
section, he says:

	Friend functions have access to private and protected
	members just like member functions.

So I concluded that a friend of a derived class should be able to
access a protected member of a base class as though it were a member
of the derived class that it is a friend of.

Mike Khaw
-- 
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