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Path: utzoo!watmath!kpmartin
From: kpmartin@watmath.UUCP (Kevin Martin)
Newsgroups: net.lang.c
Subject: Re: unions inside structures
Message-ID: <11085@watmath.UUCP>
Date: Tue, 22-Jan-85 15:22:37 EST
Article-I.D.: watmath.11085
Posted: Tue Jan 22 15:22:37 1985
Date-Received: Wed, 23-Jan-85 05:09:39 EST
References: <133@ISM780B.UUCP> <10975@watmath.UUCP> <2606@umcp-cs.UUCP>
Reply-To: kpmartin@watmath.UUCP (Kevin Martin)
Organization: U of Waterloo, Ontario
Lines: 41
Summary: 

In article <2606@umcp-cs.UUCP> chris@umcp-cs.UUCP (Chris Torek) writes:
>Concerning the problem of ``invented names'':
>Suppose you need a structure to hold either a character/font pair, or
>an adjustment.  Then you might write
>
>	enum node_type { ntype_char, ntype_adjust };
>
>	struct char_node {
>	    char cn_char;
>	    char cn_font;
>	};
>
>	struct node {
>	    enum node_type n_type;	/* is it a char or an adjust? */
>	    union {
>		struct char_node un_char;/* value if char */
>		short un_adj;		/* value if adjust */
>	    } n_un;
>	};
>
>Then a reasonable compromise for accessing the fields of a struct node
>is to create the definitions
>
>	#define n_char	n_un.un_char
>	#define n_adj	n_un.un_adj
>
>which makes the ``n_un.un'' part of each name vanish.  Admittedly
>this has problems, but it can improve readability---and it requires
>no changes to the language.

The problem with this is that the fake element names 'n_char' and 'n_adj'
have a scope which differs from that of any other struct elements. That is,
the element names 'n_type', 'n_un', 'un_adj', etc. are only recognized when
preceded by 'primary.' or 'pointer->', where the primary or pointer is of the
appropriate type. On the other hand, 'n_char' and 'n_adj' cannot be used
in any *other* context (unless you happen to have a variable called 'n_un'
which happens to be a struct or union...).
 
The other problem is that a symbolic debugger would have no idea what
p->n_adj means.
                     Kevin Martin, UofW Software Development Group