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Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!princeton!allegra!alice!bs
From: bs@alice.UUCP
Newsgroups: comp.lang.c++
Subject: Re: Bug in C++ 1.2 involving the :: operator
Message-ID: <6785@alice.uUCp>
Date: Tue, 7-Apr-87 20:09:06 EST
Article-I.D.: alice.6785
Posted: Tue Apr  7 20:09:06 1987
Date-Received: Sat, 11-Apr-87 08:00:45 EST
References: <867@vaxb.calgary.UUCP>
Distribution: na
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 77
Keywords: also a question of style
Summary: not a :: bug



lomow@calgary.UUCP (lomow @ U. of Calgary, Calgary, Ab.) writes:

 > I have found a bug in release 1.2 of C++. Could you please look
 > at the following description of the problem and either confirm
 > or clear up the problem. I'm sorry if this problem has been
 > previously reported, but it is new to me.
 > 
 > The following program
 > 
 > 	#include <stdio.h>
 > 	#define NULL 0
 > 	
 > 	class q_el {
 > 	  q_el *Next;
 > 	public:
 > 	  q_el *next() { return Next;}
 > 	  q_el(q_el *n) { Next=n; }
 > 	};
 > 	
 > 	class d_q_el : public q_el {
 > 	  int Count;
 > 	public:
 > 	  int count() { return Count;}
 > 	  d_q_el(q_el *n, int c) : (n) { Count = c; }
 > 	};
 > 	
 > 	main()
 > 	{ q_el *q;
 > 	  q = new d_q_el(NULL, 2);	// will be at the end of the list
 > 	  q = new d_q_el(q, 1);		// will be at the head of the list
 > 	
 > 	  if ( ((d_q_el*)q) ->count() == 1) printf("ok\n");
 > 	  if ( ((d_q_el*)q->next()) ->count() == 2) printf("ok\n");

The following line contains an error:

 > 	  if (q->d_q_el::count() == 1) printf("ok\n");

"q" is a pointer to a "q_el" and a "q_el" does not have a base class
called "d_q_el". The member qualification syntax "class_name::member_name"
is a mechanism for disambiguation and not a mechanism for breaking the type
system.
 > 	// *** the following line causes an internal error ****
 > 	  if (q->next()->d_q_el::count() == 2) printf("ok\n");

same bug, sorry about the poor error message. My latest cfront says:
"", line 25: error: d_q_el  is not a base class of  q_el
"", line 32: error: d_q_el  is not a base class of  q_el

 > 	}

 > causes the compiler to issue the following messages:

 > 	"c.err.c", line 31: internal <<cfront 05/20/86>> error:
		bus error (or something nasty like that)

 > I prefer the following style
 > 	  if (q->next()->d_q_el::count() == 2) printf("ok\n");
 > to
 > 	  if ( ((d_q_el*)q->next()) ->count() == 2) printf("ok\n");
 > because the cast in the second if statement is hard to read.
 > Would anyone care to comment? are there other alternatives?

The cast notation is ugly because you are doing something ugly.
Please don't misunderstand me: casts can be necessary, but I see
no point in going out of my way to make them pleasing to the eye.

There is a ``proper way'' of ``raising'' the type of a base class to
that of the appropriate derived class: a virtual function call.

Thank you for reporting the bug/problem.

I have a point about style too: Don't use ``#define NULL 0''; it is redundant
and classical C variations such as ``#define NULL ((char*)0)'' are potentially
harmful.