------- 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