And even whether it is only UB 'from that point on' or just the whole program is UB, is something only differentiated recently.   When the compiler transforms and reasons about the program, UB can often manifest anywhere and in any way.   -----Ursprüngliche Nachricht----- Von:Thiago Macieira via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Do 31.07.2025 19:23 Betreff:Re: [std-proposals] Standardising 0xdeadbeef for pointers An:std-proposals_at_[hidden]; gb2985_at_[hidden]; CC:Thiago Macieira <thiago_at_[hidden]>; On Thursday, 31 July 2025 10:00:36 Pacific Daylight Time zxuiji wrote: > How's it undefined? Take my MAX_INVALID_ADDRESS for example, let's say NULL > is defined as and nullptr is likewise defined to use 0xdeadbeef. 0xdeadbeef > +- MAX_INVALID_ADDRESS would be the range for the inline to check against. > without or without a 0 based NULL/nullptr the compiler can optimise out the > addition/subtraction applied to NULL & nullptr to check the range when > compiling it for a library. Granted I prefer being able to check the upper > bits but that's something I would leave to a glibc/ucrt function to provide > a extension function for. The standard defines this as UB:  char *ptr = nullptr;  ptr + 1; It doesn't matter that you did not dereference. It doesn't matter that the result was not stored. It's UB and anything past that point is UB. Adding a *Standard* function that can only detect something after UB has already happened is pointless. Just like you can't detect a signed integer overflow *after* the overflow has happened. You need to detect the problem before UB has happened. I would however like: - guaranteed ability to round-trip small numbers through pointer variables - guaranteed that such small numbers never be returned by memory allocation It is similar to what you're asking, but avoids the UB by not doing arithmetic on the null pointer or an invalid pointer. Instead, such pointers are formed by casting an integer to pointer and only used by casting back from pointer to integer. The first request is de facto universal right now and the second is pretty much so but there can be outlier architectures with near-null valid pointers (usually in kernel mode). -- Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org  Principal Engineer - Intel Platform & System Engineering -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals