On 05/29/12 16:20, Michel Fortin wrote:
> On 2012-05-29 13:29:35 +0000, Don Clugston <[email protected]> said:
>
>> On 27/05/12 02:45, Walter Bright wrote:
>>> You could implement it as simply comparing the addresses - you'd be no
>>> worse off than C is, and you would get the correct answer for pointers
>>> both in and out of the array without needing special cases.
>>
>> I think that's a no-go.
>> Implementation-specific behaviour at runtime is bad enough, but at compile
>> time, it's truly horrible. Consider that any change to unrelated code can
>> change the results. Something that makes it really terrible is that the same
>> function can be called in CTFE once before inlining, and once after. Good
>> luck tracking that down.
>> And at runtime, you have a debugger.
>
> Wouldn't it be possible to just catch the case where you compare two pointers
> not assigned from the same memory block and issue an error?
>
> Here's an idea: make each CTFE pointer some kind of struct with a pointer to
> the memory block and an offset. When comparing, if the memory block isn't the
> same, it's an error. If you add or subtract to a pointer, it'll still belong
> to the same memory block but with a different offset, thus it remains
> comparable. When dereferencing, you can make sure the pointer still points
> inside the block, assuming the block knows its length.
>
int a[1024];
int[] da = a[0..1024];
if (whatever)
da = da[3..14];
if (something_else)
da = [42] ~ da;
// etc
if (da_is_a_slice_of_a())
still_inside_a();
How do you implement da_is_a_slice_of_a()?
Disallowing pointer comparisons means normal code needs __ctfe special
casing in order to work at compile time at all, like it was done for the
"bug" mentioned in this thread.
artur
