------- Additional Comments From gdr at integrable-solutions dot net
2005-07-15 08:10 -------
Subject: Re: pointer +- integer is never NULL
"falk at debian dot org" <[EMAIL PROTECTED]> writes:
| ------- Additional Comments From falk at debian dot org 2005-07-15 06:41
-------
| Subject: Re: pointer +- integer is never NULL
|
| "gdr at integrable-solutions dot net" <[EMAIL PROTECTED]> writes:
|
| > My indirect observation was that reinterpret_cast is intended for
| > specific needs that cannot adequately be expressed at the purely
| > object type level. The result is intended to be unsurprising to
| > those who know the addressing structure. Consequently it takes a
| > creative compiler to make reinterpret_cast<int*>(0) + 5 undefined.
|
| Sorry, I cannot follow you. I'd find it massively unsurprising if
| reinterpret_cast<int*>(0) produces a null pointer, and if I then get
| undefined behavior for doing something with it that is undefined for a
| null pointer.
But, if I used reinterpret_cast to turn an integer value 0 into a
pointer, there is no reason why the compiler would assume that I do not
know the underlying machine and what I'm doing with the pointer.
| In fact I'd find it very *surprising* if
| reinterpret_cast<int*>(0) behaves different than a normally
| constructed null pointer anywhere.
At least, you get that part of my indirect observation! :-)
| > Furthermore, given the mapping chosen by GCC, it takes even more
| > creative compiler to make (int *)0 + 5 also undefined.
|
| And I don't see how that follows, either.
if follows from your surprise that reinterpret_cast<int*> does
something different from the null pointer constant (int*)0.
| As it seems, arguing with different levels of surprisingness seems to
| be somewhat subjective, so I don't think this leads us anywhere.
I'm not actually arguing on different level of surprisingness. I'm
just looking at reinterpret_cast and its implication.
| > There still are reasonable codes for system programming out there
| > that needs the to go through the play with null pointer -- we, GCC,
| > even used to distribute such things in the past.
|
| This is a more relevant point. I don't think this optimization would
| break offsetof-like macros, since they'd use null pointer *constants*,
^^^^^^^^^^^
For the offsetof *macro*, yes
But that is not the case for codes that uses
reinterpret_cat<int*>(expr), where expr is an integer expression with
value 0. Scanning a region of memory starting from zero, is not
exactly the kind of thing never done in practice.
| which we could easily avoid to tag as non-null.
so you would have to pretend that a null pointer constant is not null?
That is even more bizarre arithmetic.
-- Gaby
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22485
