q for the C hackers

[Tom Fogal]

sorry, again. Always forget to change the 'To:' to the list...

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To: Erik Price <erikprice at mac.com>
Subject: Re: q for the C hackers 
In-Reply-To: Your message of "Mon, 18 Aug 2003 20:13:05 EDT."
             <E7B260C2-D1D9-11D7-A2C1-00039351FE6A at mac.com> 
Date: Tue, 19 Aug 2003 09:10:42 -0400
From: Tom Fogal <tfogal at io.iol.unh.edu>

> When I want to define a constant value in Java, such as a magic number, 
> I usually use "public static final":
> 
>      public static final int NUMBER_OF_UNITS = 8;
> 
> However, what is the convention in C?  There seem to be two fine ways 
> of doing it -- using the preprocessor, or the const keyword:
> 
> #define NUMBER_OF_UNITS 8
> 
> const int NUMBER_OF_UNITS = 8;
> 

the normal 'C' (as opposed to C++) convention is to use #defines.
the difference between the two is that the define is replaced with its value
by the C preprocessor, before any code is compiled. The actual 'text' section
of your executable/binary should be where the number '8' would show up,
although i dont think that behavior is defined by the C standard.

the const int way stores the variable in read only memory, and thus, IMO is a
waste of memory. Also, it would be required to do a memory lookup when 
accessing the value.

> I'm just interested in hearing about whether one is more appropriate 
> than the other in some contexts.  Thanks.

Generally, I would use #defines for anything but function parameters.
Passing things as a constant reference (const type &val) is a good way to
avoid passing a large value without the overhead of actual passing it.
Unfortunately, references do not exist in C alone (they were introduced in C++)

Finally, I've never actually heard of 'const' in C. I know that it was
introduced with C++, although i would not be surprised to see later editions
of the C standard included it.
It is not in K&R ('the white book'), IIRC.

HTH,

- -tom

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