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From: brett@wjvax.UUCP (Brett Galloway)
Newsgroups: net.lang.c
Subject: Re: boolean datatype
Message-ID: <705@wjvax.wjvax.UUCP>
Date: Wed, 4-Jun-86 12:21:28 EDT
Article-I.D.: wjvax.705
Posted: Wed Jun  4 12:21:28 1986
Date-Received: Fri, 6-Jun-86 04:58:20 EDT
References: <5498@alice.uUCp> <1462@mmintl.UUCP> <852@bentley.UUCP> <1126@whuxl.UUCP> <393@peregrine.UUCP>
Reply-To: brett@wjvax.UUCP (Brett Galloway)
Organization: Watkins-Johnson Co., San Jose, Calif.
Lines: 47

In article <393@peregrine.UUCP> mike@peregrine.UUCP (Mike Wexler) writes:
>Another feature I would like to have is a range data type.  I don't
>know of a good syntax for putting this in C but it would be nice to
>use in programs where are know the possible values of a number but
>would like the compiler to figure out an efficient sized slot to put
>it in.
>	...
>	range hour:1..12; /* create a variable named hour that can vary from */
>	typedef range DAY:1..366;
>	DAY day1,day2;
>
>I will let other people argue about how to index an array by a range. 

I would prefer an operator that merely generated an integral type based on
a number of bits requested, not based on a desired range.  For example,
on a machine with 8-bit char, 16-bit short, 32-bit int, and 64-bit long,
bittype(8) == char; bittype(9) == short; bittype(31) == int; and so on.
Bittype() would be evaluated at the same time sizeof() is evaluted, which I
think is in the preprocessor.  The preprocessor would just substitute a type
word (char, int, etc.) for the bittype(n) operator.  Bittype() would NOT,
however, enforce range restrictions; it would just efficiently generate a type
that contains at least the given number of bits, if possible.

Another operator that would be useful in conjunction with the above and useful
elsewhere would be a constant-generation construct that provides knowledge of
the number of bits in each of the integral arithmetic types (char, short, int,
and long).  For example, bitlen(int) == 32 on a 32-bit machine;
bitlen(char) == 8 most of the time.  This would simplify generating efficient
code to simulate bitfield arrays.  For example, to generate a packed
100-element array of unsigned 1-bit fields, one could define an array

	unsigned int wordarray[(100+(bitlen(int)-1))/bitlen(int)];

and define macros to do the bitmask accesses.  In fact, the user could write
a general macro package to handle all this.  My understanding of the current
situation is that, short of a well-known #define in an #include file, the
only way to determine the bit-length of an integral type is at run-time, by
right/left-shifting a bit through an item of that type.  Run-time determination
is much less efficient, however.  It requires the overhead of the run-time
determination, plus it precludes compile-time constant evaluation.

Finally, while we're on the subject of wish-lists, I think a typeof()
operator would be VERY useful.

-------------
Brett Galloway
{pesnta,twg,ios,qubix,turtlevax,tymix,vecpyr,certes,isi}!wjvax!brett