Xref: utzoo gnu.g++.bug:1319 comp.lang.c++:6111
Path: utzoo!attcan!uunet!mcsun!ukc!dcl-cs!aber-cs!pcg
From: pcg@aber-cs.UUCP (Piercarlo Grandi)
Newsgroups: gnu.g++.bug,comp.lang.c++
Subject: Re: g++ core dump from improper use of extern "C"
Summary: "struct a;" has never been necessary.
Message-ID: <1581@aber-cs.UUCP>
Date: 11 Jan 90 17:56:11 GMT
Reply-To: pcg@cs.aber.ac.uk (Piercarlo Grandi)
Organization: Dept of CS, UCW Aberystwyth
	(Disclaimer: my statements are purely personal)
Lines: 62

In article <494@ucl-cs.UUCP> T.Day@ucl-cs.UUCP writes:
    From: Tim Day <T.Day@uk.ac.ucl.cs>
    
    Piercarlo "Peter" Grandi writes:
    > using 'struct some *' before having seen 'struct some {...'
    > is legal and without problem in both C and C++
    
    Not quite.  You have to say
    	struct frob;
    before you can use pointers or references to frob.

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. The compiler
need not bother; anything that comes after 'struct' must be a
struct name, and a pointer has always the same size.

You are confusing this point with the the fact that in C++,
saying 'struct a;', which in it as well as in C++ is an empty
declaration, and no special construct all, also has the effect
of typedef'ing 'a' as 'struct a'. The compiler needs to know
that a name is a typedef, otherwise parsing declarations cannot
be done.

In this, the C++ practice of saying 'class c;' is NOT a forward
declaration; it i just the old harmless practice of having empty
declarations, coupled with the new side effect of having an
implicit typedef. Saying beforehand 'class c;' in C++ is not
needed if you write 'class c *m;' instead of 'c *m;'.

The effect is the same if you just say 'typedef class a ;'.
With the following preprocessor macro you get the same effect
in C:

	#define mode(FLAVOR,NAME) \
		typedef FLAVOR NAME NAME; FLAVOR NAME

Note that the typedef appears *before* the struct definition, if
any; if you follow this with a ';' the struct definition becomes
a harmless empty delcaration.

You can use this in C as

	mode(struct,complex) { float re,im; };
	complex a,b,c;

or

	mode(struct,clutch);
	mode(struct,car) { ...; clutch *theClutch; ... };

just like in C++, if you have got fed up with using leading
'struct', 'union', 'enum' (enums are special though in C)...
-- 
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