Re: [Bug-gnulib] coding practices

[Bruno Haible]

Paul Eggert writes:
> a method that I now prefer in software that I help maintain.
> Instead of this:
> 
>       return table[(unsigned char) E];
> 
> I write something like this:
> 
>       unsigned char c = E;
>       return table[c];
> 
> Avoiding casts entirely catches a few more errors.  E.g., if E is
> actually a pointer you'll get a compile-time error if you avoid the
> cast, but typically an unchecked run-time error if you use the cast.

The problem with this idiom is that a commonly used, commonly
correctness-preserving modification

        type var = E;
        return func(var);

  <-->

        return func(E);

now introduces a bug. Or, in the case of characters, one often changes
the type from 'unsigned char' to 'int' (so that c can then also
accomodate the value EOF).

In both cases, your cast-less style introduces a bug. In other words,
the code as you write it is correct, but the maintainers that come
after you are not likely to understand the subtlety of

        unsigned char c = E;

and therefore it presents a pitfall for them.

Bruno