Richard Guenther wrote:
> On Thu, May 21, 2009 at 1:50 PM, Andreas Schwab <sch...@linux-m68k.org> wrote:
>
>> Ian Lance Taylor <i...@google.com> writes:
>>
>>
>>> Consider this C/C++ program:
>>>
>>> extern void **f1();
>>> void f2(const char *p) { *(const void **)f1() = p; }
>>>
>>> If I compile this program with g++ -Wcast-qual, I get this:
>>>
>>> foo2.cc:2: warning: cast from type ‘void**’ to type ‘const void**’ casts
>>> away qualifiers
>>>
>> In a sense this warning is actually correct: this is storing a const
>> char * into a void * object, which is where the qualifier is lost. IMHO
>> having a warning for this questionable operation is a good thing.
>>
>
> I don't think so.
>
> extern char **f1();
> void f(char *p)
> {
> *(const char **)f1() = p;
> }
>
> warns the same. typeof(*(const char **)) should still be const char *.
>
It seems the rules for the warning follow the same rules for whether
such qualifier changes are allowed in implicit conversions (or named
casts other than const_cast).
Let's say char** -> const char** is allowed silently. Then the program
below is silently violating const correctness:
char* pc;
char** ppc = &pc;
const char** cppc = ppc; // Silently allowed?
const char cc = 0;
*ppc = &cc; // Silently allowed, pc now points to cc
*pc = 1; // Silently allowed, but changes cc
The conversion T** -> const T** is unsafe. That's all there is to it.
The warning is correct.
Cast to T const* const* instead.
Sebastian
