On 05/25/12 08:50, Don Clugston wrote: > The current implementation of CTFE strictly enforces C pointer semantics. One > of the restrictions is that you cannot perform ordering comparisons between > unrelated pointers. > This is important for repeatability: if it was permitted, the results would > be arbitrary and might vary unpredictably with subtle changes in the code, or > change between compiler releases. > > But, there's an interesting case from bug 7898: the 'inside' operation. > If p and q are pointers to the same array, then (r >= p && r <= q) is true if > r points inside that array, and false if it does not. > This seems to be a perfectly reasonable operation: it is completely > repeatable and safe, regardless of what r points to. But there doesn't seem > to be any way to rewrite it to avoid the disallowed comparisons. > > I could write code to allow this special case in CTFE. There's a bit of work > to make sure that all the valid cases are detected, because there are quite a > lot of ways to rewrite it, but it's not too terrible. > > But I dunno, I don't like this sort of thing much. Feels a bit clunky. > OTOH it seems like necessary functionality, and I can't see any other way of > doing it. > > Opinions?
It _is_ necessary functionality. Comparing truly unrelated pointers results in undefined behavior, this is not different from normal non-ctfe code - i don't see a reason to even bother with implementing any restrictions. It's undefined, but not unsafe - so the programmer gets to deal with it. artur
