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From: mike@umn-cs.CS.UMN.EDU (Mike Haertel)
Newsgroups: comp.lang.c++
Subject: Re: g++ core dump from improper use of extern "C"
Message-ID: <18157@umn-cs.CS.UMN.EDU>
Date: 12 Jan 90 18:31:50 GMT
References: <1581@aber-cs.UUCP>
Reply-To: mike@umn-cs.cs.umn.edu (Mike Haertel)
Organization: Free Software Foundation
Lines: 33

In article <1581@aber-cs.UUCP> pcg@cs.aber.ac.uk (Piercarlo Grandi) writes:
>Not really, neither in C nor in C++. Try compiling:
>
>	struct a {struct b *b;} a;
>	struct b {struct a *a;} b;
>
>It shall work, both in C and C++. There has never been a
>requirement that the name of a struct be known before its use,
>if its use is to declare a pointer to the struct.

Just to be pedantic, this is not strictly true.  It is OK to
declare a pointer to an incomplete type, so this works *here*.
But consider the following counterexample:

/*1*/	struct b { int x; };
/*2*/
/*3*/	void foo() {
/*4*/	    struct b;
/*5*/	    struct a { struct b *b; } *a;
/*6*/	    struct b { struct a *a; } *b;
/*7*/
/*8*/	    b = a->b;
/*9*/	}

In this case, line 4 is necessary to make "struct b" an incomplete type.
Otherwise, the "struct b *b" in line 5 would refer to the "struct b"
declared on line 1, and line 8 would be a pointer type mismatch.

The moral of the story is to declare all your types at file scope.
-- 
Mike Haertel <mike@ai.mit.edu>
"Everything there is to know about playing the piano can be taught
 in half an hour, I'm convinced of it." -- Glenn Gould