Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!ukc!dcl-cs!aber-cs!pcg
From: pcg@aber-cs.UUCP (Piercarlo Grandi)
Newsgroups: comp.lang.c++
Subject: Re: g++ core dump from improper use of extern "C"
Summary: Ansi C is like C++, now, as to forward declarations of types...
Message-ID: <1587@aber-cs.UUCP>
Date: 16 Jan 90 18:59:16 GMT
Reply-To: pcg@cs.aber.ac.uk (Piercarlo Grandi)
Organization: Dept of CS, UCW Aberystwyth
	(Disclaimer: my statements are purely personal)
Lines: 61

I wrote:

        typedef short t;
    
        static struct a {struct b *b; long l[1];} a;
    
        main()
        {
	    {
    	      auto t t1;
    	      typedef double t;
    	      auto t t2;
    
    	      printf(" size t1 %u, size t2 %u\n",sizeof t1, sizeof t2);
	    }
    
	    {
    	      struct a;
    	      struct c {struct a *a; long l[2];} c;
    	      struct a {struct c *c; long l[3];} a;
    
    	      printf(" size a %u, size c %u, size c.a %u\n",
    		  sizeof a, sizeof c, sizeof *(c.a));
	    }
    
	    return 0;
        }
    
    This should print, on your typical 32 bit byte oriented machine, something
    like 8 and 2 on the first line and 16, 12, 8 on the second line.
    
    	NOTE: it will fail to compile on some ancient C compilers that
    	erroneously handle a typedef like a #define, e.g.
	Sun's. [ .... ] 

By the way, try it taking out the line 'struct a;'.

It has pointed out to me (a few pieces of Hate Mail :->) that
in Ansi C (which I love a lot :->) now 'struct b' *is* a forward
declaration, just like in C++. Special casing is still strong;
this means that 

On the other hand, of the (few: G++, Sun, Gould, Mips)
compilers that I have tried, *none* could handle correctly the
above code save for G++; they all chickened out at the
redefinition of 't', save for one that allows for type 'short
double' apparently...

G++, meritoriously, handled the above monstrosity with flair,
even issuing the most appropriate warnings, and printing 16, 2,
16 on the second line.

What's amusing is that the presence of the 'struct a;' line
does not seem to affect it. I think its absence should: as far
as I know, the only (regrettable) exception to the one-pass-rule
in C++ is class members, where in a member you can refer to
members declared later.
-- 
Piercarlo "Peter" Grandi           | ARPA: pcg%cs.aber.ac.uk@nsfnet-relay.ac.uk
Dept of CS, UCW Aberystwyth        | UUCP: ...!mcvax!ukc!aber-cs!pcg
Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk