On 27/05/12 02:45, Walter Bright wrote:
On 5/26/2012 3:59 AM, Don wrote:
Yes, that's what happens now. But that doesn't help the programmer.
If it is inside, no problem, the expression is true. But if it is not
inside,
the expression is not false -- it's a compile-time error.
Ok, I understand now what you meant.
So you can't use it as a test for if it is inside the same object.
I was confused about how memmove can work in C without relying on
undefined
behaviour. But I just read
http://www.cplusplus.com/reference/clibrary/cstring/memcpy/
which defines it in terms of an intermediate buffer.
So maybe, the current CTFE implementation is _exactly_ consistent with
the C
spec. If that's true, though, I find it pretty incredible that there
is no way
to find out if a pointers points a particular array, even if you have
pointers
to both the start and end of that array.
(OK, I guess you can iterate from start to end, checking for equality,
but ..
bleah .. it's a terrible abstraction inversion).
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.
It's not hard to make it work in all cases of:
one-sided-comparison && one-sided-comparison
one-sided-comparison || one-sided-comparison
one-sided-comparision:
ptr_expression RELOP ptr_expression
! one-sided-comparision
RELOP:
>
<
>=
<=
where ptr_expression is any expression of type pointer.
And by all cases, I really mean all (the code for the four pointer
expressions need not having anything in common).
It's only when you allow other expressions to be present, that things
get hairy.
