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From: lomow@calgary.UUCP
Newsgroups: comp.lang.c++
Subject: Bug in C++ 1.2 involving the :: operator
Message-ID: <867@vaxb.calgary.UUCP>
Date: Sat, 4-Apr-87 08:47:34 EST
Article-I.D.: vaxb.867
Posted: Sat Apr  4 08:47:34 1987
Date-Received: Sun, 5-Apr-87 22:51:29 EST
Distribution: na
Organization: U. of Calgary, Calgary, Ab.
Lines: 110
Keywords: also a question of style



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");
	
	  if (q->d_q_el::count() == 1) printf("ok\n");
	
	// *** the following line causes an internal error ****
	  if (q->next()->d_q_el::count() == 2) printf("ok\n");
	}
	//////////

causes the compiler to issue the following messages:

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

The problem seems to be the use of the scope resolution operator
(i.e., '::') with a result returned from a function (or maybe it
has to do with the way inline functions are expanded).

The error can be eliminated by:

	1) Commenting out line 30 (the line before the
		closing curly bracket of main()).

or

	2) Making Next a public data member of q_el and changing
		the calls to next() in main() to references to the 
		variable Next - as shown in the following program.

	//////////
	#include <stdio.h>
	#define NULL 0
	
	class q_el {
	public:
	  q_el *Next;						// MOVED
	  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);
	  q = new d_q_el(q, 1);
	
	  if (((d_q_el*)q)->count() == 1) printf("ok\n");
	  if (((d_q_el*)q->Next)->count() == 2) printf("ok\n");	// CHANGED
	
	  if (q->d_q_el::count() == 1) printf("ok\n");
	
	  if (q->Next->d_q_el::count() == 2) printf("ok\n");	// CHANGED
	}
	//////////

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?

Thank You,

Greg Lomow.

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