-Wcast-qual and casting away

[Richard Guenther]

On Thu, May 21, 2009 at 1:50 PM, Andreas Schwab <schwab@linux-m68k.org> wrote:
> Ian Lance Taylor <iant@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 *.

For

extern const char **f1();
void f(char *p)
{
  *(char **)f1() = p;
}

it warns with

t.C: In function ‘void f(char*)’:
t.C:4: warning: cast from type ‘const char**’ to type ‘char**’ casts
away constness

which makes sense.

Richard.