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From: news@cit-vax.Caltech.Edu (Usenet netnews)
Newsgroups: comp.lang.c++
Subject: Re: Constant expressions
Message-ID: <1288@cit-vax.Caltech.Edu>
Date: Fri, 5-Dec-86 18:46:59 EST
Article-I.D.: cit-vax.1288
Posted: Fri Dec  5 18:46:59 1986
Date-Received: Fri, 5-Dec-86 22:48:53 EST
References: <1252@cit-vax.Caltech.Edu> <6426@alice.uUCp>
Reply-To: jon@cit-vax.UUCP (Jon Leech)
Organization: California Institute of Technology
Lines: 60

Organization : California Institute of Technology
Keywords: 
From: jon@oddhack.Caltech.Edu (Jon Leech)
Path: oddhack!jon

In article <6426@alice.uUCp> shopiro@alice.UUCP writes:
>In article <1252@cit-vax.Caltech.Edu>, jon@oddhack.Caltech.Edu (Jon Leech) writes:
>> 
>> 	I recently found that C++ (1.1) does not allow integral casts in
>> constant expressions in some cases; for example,
>> 
>> 	typedef enum { one = (unsigned)1 } ENUM;
>> 
>> Unfortunately, this makes it impossible to include some of the graphics header
>> files on Suns which use this form (it also happens to disagree with 
>> the ANSI C draft, sadly).  Has anyone got a fix for this problem?
>
>This code (or any code that uses casts in constant expressions) is not
>legal C according to Ritchie* or the ANSI draft.  Apparently some C
>compilers will accept it;  I believe there are others that won't.
>C++ has already abandoned complete upward compatability with legal
>C -- it's unreasonable to expect it to be compatible with illegal C.
>
>I grant that it is clear what such casts would mean if they were
>legal, and that there are examples where they might allow you to
>write code that couldn't be written as directly otherwise.  I am
>confident that if this extension were approved by the ANSI committee
>it would also appear in C++.
>
>However, in all the SUN header examples I have looked at, the cast is
>pure noise and can be eliminated with no effect whatsoever.  That is
>what I suggest you do.  (When you're done, I'm sure there will be many
>people who would like a copy).
>
>* Dennnis Ritchie, The C Programming Language - Reference Manual
>	Sep. 1980.

	In my April 30, 1985 copy of the draft, Section C.4 ``Constant
Expressions'' says 

	`` For constant expressions in initializers, in addition to integral 
	constant expressions as discussed previously, one can also use
	floating constants and arbitrary casts...''

	This is not allowed in K&R unless I'm missing something 
(Appendix A, Section 15, pg. 211).

	Unless this has been changed in later drafts, we are both
partially right; you're right that the particular code I quote
above is illegal, and I'm right that the usage is in fact acceptable -
in initializers. What a stupid restriction. Time to comment
to ANSI.

	It turns out to be far easier to write C hooks to the Sun
graphics routines. Annoying, but less pain than changing
header files.

    -- Jon Leech (jon@csvax.caltech.edu || ...seismo!cit-vax!jon)
    Caltech Computer Science Graphics Group
    __@/