Re: const local variables

[James Youngman]

On Sun, Apr 11, 2010 at 1:08 PM, Bruno Haible <address@hidden> wrote:
> James Youngman wrote:
>> * lib/close-stream.c (close_stream): Make boolean variables const
>> to document the fact that we set but do not change them.
>> --- a/lib/close-stream.c
>> +++ b/lib/close-stream.c
>> @@ -55,9 +55,9 @@
>>  int
>>  close_stream (FILE *stream)
>>  {
>> -  bool some_pending = (__fpending (stream) != 0);
>> -  bool prev_fail = (ferror (stream) != 0);
>> -  bool fclose_fail = (fclose (stream) != 0);
>> +  const bool some_pending = (__fpending (stream) != 0);
>> +  const bool prev_fail = (ferror (stream) != 0);
>> +  const bool fclose_fail = (fclose (stream) != 0);
>
> There are programming languages where this style of using 'const' may
> be customary (like C++ or Java), but I don't think it helps in C code:
>  1) For the person reading the code, it draws the attention away from
>     more importants parts of the code.

On the contrary, it's a signal that there is no need to consider
changes to the values of those variables in the later code.

>  2) For the person modifying the code, it requires additional attention,
>     to remember to remove 'const' here and there. That's frequent in C,
>     because the vast majority of code in C is written in style that
>     modifies local variables (as opposed to functional style).

This statement is true but I don't understand its relevance.  If some
of the variables are modified and some are not, this surely increases
the value of the const qualifier as an item of documentation.


> If you find yourself looking at a function where you don't overlook
> all the uses of a variable, then my advice would be to
>  a) use an IDE which shows you all uses of a local variable on
>     demand (Eclipse can do that, and José Marchesi knows how to do
>     that with Emacs),
>  b) limit the scope of the variables. Create intermediate blocks
>     that declare variables for the minimum possible scope.
>
> Bruno
>