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From: karl@haddock.UUCP (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: union initialization
Message-ID: <455@haddock.UUCP>
Date: Mon, 27-Apr-87 14:20:08 EDT
Article-I.D.: haddock.455
Posted: Mon Apr 27 14:20:08 1987
Date-Received: Tue, 28-Apr-87 02:55:36 EDT
References: <3290@burdvax.PRC.Unisys.COM> <1722@plus5.UUCP>
Reply-To: karl@haddock.ISC.COM.UUCP (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 44
Summary: summary of possibilities, and a new proposal

In article <1722@plus5.UUCP> hokey@plus5.UUCP (Hokey) writes:
>In my opinion, [ANSI's] mechanism is *worse* than not having *any* way to
>perform compile-time initialization of unions.

First off, let me state that I think that initializing a union is a perfectly
valid thing to want to do.  Let's look at some of the possibilities:

K&R says it can't be done.  This is annoying; it requires one to use run-time
initialization or non-portable kludges (including, perhaps, writing the
definition in assembly language).

ANSI proposes that unions be initializable, but only to their first member.
The sudden introduction of a distinguished member of a (previously unordered)
union bothers me a bit; I feel they're attacking the symptom instead of the
problem.  Also, this proposal doesn't always help: the initialization
    { a[0].asfloat = 17.5, a[1].asstring = "hello" }
still can't be performed at compile time.

A cast to union type would fix this (the example would then read
    union floatstr a[2] = { (union floatstr)17.5, (union floatstr)"hello" };
), but this could still cause problems -- if a union includes a char member as
well as an int member, how does one initialize the char?  (Recall that C has
no rvalues of type char.)

Here's my new proposal:

I think what I'd like to see is a syntax for initializing an aggregate by name
rather than position.  Something like
    union floatstr a[2] = { { asfloat: 17.5 }, { asstring: "hello" } };
(the outer braces are for the array, the inner ones for the union).  This
would also apply to structs, so one could initialize a struct without having
to know the number or order of the members:
    struct tm today = { tm_year: 87, tm_mon: 4, tm_mday: 27 };
(Other elements would, as usual, be initialized to zero (if the struct is
static) or garbage (if auto).)  This could even be extended to arrays:
    int a[30] = { 29: -1 }; /* an array with a -1 in the last position */
I would find this last feature useful when dealing with an array which is
logically subscripted by an enum:
    int val[NCOLORS] = { RED: 0x00f, GREEN: 0x0f0, WHITE: 0xfff };

The primary objection to this scheme, I think, is that it is a significant
addition to the language.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint