Pointer semantics in CTFE

[Michel Fortin]

On 2012-05-29 13:29:35 +0000, Don Clugston <dac at nospam.com> 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.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/