🚀 go-pugleaf

john@polo.demon.co.uk (John Winters) writes:

>In article <KANZE.96Jan17121659@slsvewt.lts.sel.alcatel.de>,
>James Kanze US/ESC 60/3/141 #40763 <kanze@lts.sel.alcatel.de> wrote:
>[snip]
>>
>>Originally, the C standard was going to allow this; in fact, it was
>>going to allow all casts which added const anywhere in the type.  Then
>>someone pointed out that the conversion 'char ** -> char *const *' was
>>unsafe.  As a result, the wording was changed to only allow adding the
>>const at the top level.

>Interesting.  Could you enlighten us (well, me anyway) as to *why* it
>is unsafe.  I don't find it immediately obvious.

Actually 'char ** -> char *const *' is a safe conversion. The related
UNSAFE conversion is 'char ** -> const char **'. This is unsafe
because you could use this conversion to change a constant char.

e.g.

const char a = 'a';
char b;
char *bb = b;
char **bbb = bb;
const char** ccc;
/* The following statement is an illegal cnversion */
ccc = bbb;
/* Assume the preceding conversion were legal - we could now do */
*ccc = &a;
/* bb now points to a */
*bb = 'b';
/* We have just changed the constant character variable a */
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