Re: Is it conformant to substract two pointer-to-void ?

On 12 mar, 22:36, Eric Sosman <Eric.Sos...@xxxxxxx> wrote:
Francois Grieu wrote:
Why do you *want* this to work?

My application includes the following:

#define CHK_PTR_TYPE(ptr,basetype) (1?(ptr):((ptr)-(basetype*)(ptr))+
(ptr))

and I am concerned that this wont reliably work when basetype is
void or const void

I guess that by "work" you mean "produce a diagnostic if
the type of *ptr is not compatible with basetype?"

Yes, that's my goal.

As written, the test should elicit a diagnostic if the
types are incompatible (which is what I think you want), or
if ptr is void* or if basetype is void (which I think you'd
regard as false positives).

Yes.

I don't understand what the addition at the end is for

It is here to make the expression correct. (ptr)-(basetype*)(ptr)
is a ptrdiff_t and we need to add a ptr to make that
compatible with a ptr, on the left side of the :


If you'd be happy with a slightly different test (compatibility
of ptr with basetype* instead of *ptr with basetype), you might
try

#define CHK_PTR_TYPE(ptr,basetype) \
(1 ? (ptr) : (basetype*)(ptr))

... the idea being that the second and third operands of ?:
must be of compatible pointer types (6.5.15p3), and the
compiler must diagnose a constraint violation.

Indeed that SHOULD do the job. <OT> I'll check with the
various compilers that I target, but I'm a bit pessimistic,
I think I remember one of them is hapy with mixing int and
pointers on each side of : </OT>

Francois Grieu
.

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