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From: daveb@laidbak.UUCP
Newsgroups: comp.lang.c
Subject: enum
Message-ID: <1177@laidbak.UUCP>
Date: Fri, 2-Oct-87 13:31:48 EDT
Article-I.D.: laidbak.1177
Posted: Fri Oct  2 13:31:48 1987
Date-Received: Sun, 4-Oct-87 01:54:41 EDT
Reply-To: daveb@laidbak.UUCP (Dave Burton)
Organization: is pretty bad/My method of
Lines: 67

Trick or Trap?

A program I'm writing uses several ENUMs for variables and function
return types. I have need to enforce a certain ordering of data flow,
so I was comparing ENUM to ENUM as in the program below:

main()
{
	enum { A, B, C } varA, varB;

	printf("A     < B    is %c\n", A <  B ? 'T' : 'F');
	printf("A     > B    is %c\n", A >  B ? 'T' : 'F');
	printf("C    == C    is %c\n", B == B ? 'T' : 'F');

	varA = A; printf("varA = A (%d)\n", varA);
	varB = B; printf("varB = B (%d)\n", varB);

	printf("varA  < varB is %c\n", varA  < varB ? 'T' : 'F');
	printf("varA  > varB is %c\n", varA  > varB ? 'T' : 'F');
	printf("varA == varA is %c\n", varA == varA ? 'T' : 'F');

	exit(0);
}

Both lint and cc complain:

"enum.c", line 12: illegal comparison of enums
"enum.c", line 13: illegal comparison of enums

I can shut them up and make the program work by casting varA and varB
to int for the < and > operations. (i.e. (int)varA < (int)varB ).

According to Harbison & Steele, "C: A Reference Book 2/e", section 5.5:

	"In addition to assigning values of enumeration types, the
	 programmer can test two values for equality.

	 Enumeration types are implemented by associating integer values
	 with the enumeration constants, so that the assignment and
	 comparison of values of enumeration types can be implemented
	 as integer assignment and comparison.
	 ...
	 2. The first enumeration constant receives the value 0 if no
	    explicit value is specified.
	 3. Subsequent enumeration constants without explicit associations
	    receive an integer value one greater that the value associated
	    with the previous enumeration constant.
	 ...
	 As a matter of style, we suggest that programmers treat enumeration
	 types as different from integers and not mix them in integer
	 expressions without using casts."

Questions:
	Is my solution portable? (I think so, but I can't test it.)
	Why does the constant comparison work but not the original variable
		comparison?
	

As an aside:
	If you need an authoritative reference on the C language, the
	book by Harbison & Steele is an *excellent* investment, and is
	'available in finer bookstores everywhere'.
-- 
--------------------"Well, it looked good when I wrote it"---------------------
 Verbal: Dave Burton                        Net: ...!ihnp4!laidbak!daveb
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