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What does "can be compared" mean? To me it means that "<=>" [1] is well-defined. It doesn't say anything about numerical values. [1] Spaceship operator. Think

What does "can be compared" mean? To me it means that "<=>" [1] is well-defined. It doesn't say anything about numerical values. [1] Spaceship operator. Think "return value of memcmp" but generalized to any operand types.

YSFEJ4SWJUVU6 9y ago
Any valid pointer values can be compared for equality. Only pointers that point within a single object (like an array, or a struct) can be compared for ordering. Nesting doesn't matter as long as down the line there's a common "parent" of sorts. (That is, if you want to have well-defined behaviour.) (Edit: and for any pointers that you can compare for ordering, you may also calculate their offset from one another. However, like GP said, in practice you can do that for any pointers on common platforms.)
ajross 9y ago
No, real pointer values have numerical values. They are numbers, period. You guys are getting balled up thinking about the language standard as the "ultimate reference of truth" in the argument. When the whole point of the linked article is about pointing out that, no, the behavior of the actual machine being used is a more important "truth" in some contexts.
- adrianN 9y ago
  Well, if it's not in the standard, the compiler might disagree with you and produce output different from what you expect. You're not programming the actual machine, you're communicating intended program behavior to the compiler, who programs the actual machine. If you want to avoid misunderstandings between you and the compiler, you'd better adhere to the language definition.
  - Avernar 9y ago
    If you are writing normal code then yes. But there are always exceptions. One is trying to learn how your current target architecture actually works. This is not production code. It's a "where in memory did the compiler decide to put that" question. You may get the results you expect or you may get someting totally bizzare. But since the purpose is discovering how the compiler works there is no wrong result. Another is if you are writing your own memory manager. Here there are a lot of pointer comparisons to not the same allocated objects. But a memory manager is very OS and architecture specific so you will probably have to rewrite it when porting the code. Having said all that, I wouldn't subtract the two void pointers like the example did. I would have converted them both to integers first and then subtract. Again, since this is for debugging and/or learning purposes then it's OK to bend the language rules a bit.
- pjmlp 9y ago
  That value of truth is what allows C compilers to wipe out your beloved code on -O3, because you just feel entitled to write UB code.

YSFEJ4SWJUVU6 9y ago

Any valid pointer values can be compared for equality. Only pointers that point within a single object (like an array, or a struct) can be compared for ordering. Nesting doesn't matter as long as down the line there's a common "parent" of sorts. (That is, if you want to have well-defined behaviour.) (Edit: and for any pointers that you can compare for ordering, you may also calculate their offset from one another. However, like GP said, in practice you can do that for any pointers on common platforms.)

ajross 9y ago

No, real pointer values have numerical values. They are numbers, period. You guys are getting balled up thinking about the language standard as the "ultimate reference of truth" in the argument. When the whole point of the linked article is about pointing out that, no, the behavior of the actual machine being used is a more important "truth" in some contexts.

adrianN 9y ago
Well, if it's not in the standard, the compiler might disagree with you and produce output different from what you expect. You're not programming the actual machine, you're communicating intended program behavior to the compiler, who programs the actual machine. If you want to avoid misunderstandings between you and the compiler, you'd better adhere to the language definition.
- Avernar 9y ago
  If you are writing normal code then yes. But there are always exceptions. One is trying to learn how your current target architecture actually works. This is not production code. It's a "where in memory did the compiler decide to put that" question. You may get the results you expect or you may get someting totally bizzare. But since the purpose is discovering how the compiler works there is no wrong result. Another is if you are writing your own memory manager. Here there are a lot of pointer comparisons to not the same allocated objects. But a memory manager is very OS and architecture specific so you will probably have to rewrite it when porting the code. Having said all that, I wouldn't subtract the two void pointers like the example did. I would have converted them both to integers first and then subtract. Again, since this is for debugging and/or learning purposes then it's OK to bend the language rules a bit.
pjmlp 9y ago
That value of truth is what allows C compilers to wipe out your beloved code on -O3, because you just feel entitled to write UB code.