🚀 go-pugleaf

keagchon@mehta.anu.edu.au (Geoff Keating 9205156) writes:
	...
>Hmmm. Suppose you claim that X[20][20] and Y[20] are not aliases of
>each other. What's wrong with:

>assert(&Y[20] <= &X[0][0] && X[20][20] >= Y[0]);

>(apart from the obvious fact that it's not portable)?

Well, another obvious fact is that you probably meant '||', not '&&':-).

Seriously, the problem with this is that it's not just "not portable",
it's *UNDEFINED BEHAVIOUR* to compare pointers that are not pointing
inside the same array -- a perfectly compliant compiler could make
demons fly out of your nose as a consequence of this code.

I fully agree with the spirit of your proposal -- we don't need more
special-cases, we need a more general solution that can solve several
problems at one stroke.  And that general solution, IMHO, hinges on
a way to attach assertions to classes and methods, exactly as it can
be done in Sather or Eiffel: class invariants, method preconditions,
method postconditions.  A debugging compiler can insert code to test
that (some or all) of the conditions are in fact met, an optimizing
one can accept that the assertions hold and perform all sort of
clever optimizations based on them, etc etc.

This specific problem highlights ANOTHER sore need: a *portable* way
to test for "is this pointer inside this array, or not".  Not only
would this be precious for pre/post/invariant assertions, it would
come in handy in all sorts of low-level programming situations --
a memory allocation subsystem could check to see whether it's
being passed a valid pointer for deallocation, for example (a more
general solution to this latter issue would probably also need a
portable way to test whether a given pointer is, or isn't, suitably
aligned for a given type).

bool isPointerInside(const void* pointerBeingTested,
		     const void* startOfArray,
		     size_t sizeOfArray);
ideally with startOfArray/sizeOfArray allowed to be, in fact, a
_sub_-array, e.g., give "double A[20];",

isPointerInside((const void*)&A[11],(const void*)&A[8], 5*sizeof(double))

would be valid (NOT undefined behaviour) and yield true,

isPointerInside((const void*)&A[14],(const void*)&A[8], 5*sizeof(double))

would be valid (NOT undefined behaviour) and yield false.


and maybe also:

template<class T> isPointerAligned(const void* pointerBeingTested);
(to be called as, e.g., isPointerAligned<double>((const void*)myptr))

would, I feel, be worthy additions, mostly for low-level programming.


Compiler-usable preconditions and postconditions, ones that can be
attached to methods (including pure virtual ones!) _in a class's
DECLARATION_, would be, IMHO, infinitely more precious still.  ("Class
invariants" can be seen as a special case of conditions applying as
both pre and post to all of the class's methods, so they're "merely" a
great notational convenience in this context).


Alex
--
DISCLAIMER: these are TOTALLY personal opinions and viewpoints, NOT connected
in any way with my employer, nor any other organization or individual!
Email: martelli@cadlab.it                            Phone: +39 (51) 597313
CAD.LAB s.p.a., v. Ronzani 7/29, Casalecchio, Italia   Fax: +39 (51) 597120

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