Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: C++ Nested Classes
Message-ID: <759@taumet.com>
Date: 6 Jun 91 16:31:55 GMT
References: <5263@servax0.essex.ac.uk>
Organization: Taumetric Corporation, San Diego
Lines: 69

whisd@sersun1.essex.ac.uk (Whiteside S D B) writes:


>It lets me nest a class within another, but...

>i) If the class is named, then I can access the class outside the
>   enclosing class (or inside it, if I assign a variable to the
>   class declaration). This seems odd. The nested declaration has
>   not "hidden" the class name.

>ii)If I declare an unnamed class, that's acceptable. But then I
>   can't access the class definition to define my member functions!

>It's hard to see a way around this. It there a theoretical reason why
>C++ is set up this way? Is it a remnant of "globalish" C or am I
>just confused?

You have every right to be confused.  This is an area of change in C++.

In the original definition of C++, a type defined inside a class was
exported, so that
	class C {
	    class D {
		...
	    };
	    ...
	};
and
	class D {
	    ...
	};
	class C {
	    ...
	};
were equivalent.

In E&S (and in what will become the ANSI C++ Standard), the two are not
equivalent, and nested type defintions are truly nested.  That is,
under the newest definitions you will get the effect you want: class D
will not be visible except in the scope of class C.

I don't know whether any currently-available C++ compilers fully implement
this language feature.  AT&T's version 2.1 implements a compromise,
whereby class D is exported unless there is already a visible class D;
then it is nested.  Examples:
	============================
	class C { class D {}; };// class D exported

	============================
	class D { };		// global class D
	class C { class D {}; };// class D nested

	============================
	class D;		// shows there is a global class D
	class C { class D {}; };// class D nested
	class D { };		// global class D

	============================
	class C { class D {}; };// class D exported
	class D { };		 // error

This is NOT what the language specification says, but as I said, we
are in a transition period.

I do know that there will soon be a number of commercially-available
compilers which implement fully-nested types.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com