Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!oliveb!ames!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.c
Subject: Re: hardcoded constants
Message-ID: <33604@think.UUCP>
Date: 16 Dec 88 17:23:07 GMT
References: <1988Dec8.173158.11839@utzoo.uucp> <846@starfish.Convergent.COM> <9134@smoke.BRL.MIL> <1988Dec13.172306.16195@utzoo.uucp> <5146@bsu-cs.UUCP> <1988Dec15.190331.2986@utzoo.uucp>
Sender: news@think.UUCP
Reply-To: barmar@kulla.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 43

In article <1988Dec15.190331.2986@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>Trouble is, often it's almost impossible to devise a meaningful name.
>I'm not talking about hard-coding things like choice of control characters,
>but about things like (in a function to concatenate two strings with a
>'/' in between):
>
>	foo = malloc(strlen(a)+strlen(b)+2);	/* 2 for '/' '\0' */
>
>Now, what's a good name for that "2", and how does naming it improve
>readability?

I have a few ideas:

1) When I've done this in other languages, I've used something like
strlen("/") instead of the 2.  Unfortunately, in C I'd still have to
say "+1", which I'd then want to comment with /* leave room for the
trailing null */, since I don't think there's an expression that will
return the total space taken up by a string.  I like this because I
would expect a reasonable compiler to constant-fold the expression,
and it says exactly what the extra space is there for.

2) Define constants: #define PATH_DELIMITER "/"
		     #define ROOM_FOR_PATH_DELIMITER (strlen(PATH_DELIMITER))
		     #define ROOM_FOR_TRAILING_NULL 1

then use ROOM_FOR_PATH_DELIMITER+ROOM_FOR_TRAILING_NULL.
ROOM_FOR_TRAILING_NULL would probably be useful in other places if the
program does lots of concatenation like this.  Those names are pretty
meaningful.  If you're worried that strlen("/") won't be
constant-folded, put the 1 in the #define, with the expression in a
comment.

3) Word the comment differently: /* allocate room for the two strings,
a separator and trailing null */.  This way, it doesn't sound as if
you're defining the 2, just explaining what the statement as a whole
is doing.  The 2 is implicit, and doesn't really stand for anything.


Barry Margolin
Thinking Machines Corp.

barmar@think.com
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