[hash.requirements] clarify that Cpp17Hash does not imply stateless #7733
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The normative wording before that note can also be interpreted to say that the value of |
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LWG2291 touched this last, but the problematic part was already there. |
jensmaurer
reviewed
Mar 19, 2025
| @@ -2076,7 +2076,7 @@ | |||
| the program. | |||
| \begin{note} | |||
| Thus all evaluations of the expression \tcode{h(k)} with the | |||
| same value for \tcode{k} yield the same result for a given execution of the program. | |||
| same values for \tcode{h} and \tcode{k} yield the same result for a given execution of the program. | |||
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What if the evaluation h(k) changes h? Then, for the next evaluation, the precondition "same value of h" is not satisfied, but that's not in the spirit of the rule, I think. We should clarify that evaluation of h(k) should not change h.
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A colleague was mislead into thinking that the Cpp17Hash requirements imply hash functions must be stateless, so that every
hwould produce the same value forh(k). I think we can dispel that just by clarifying the note, so that it doesn't seem to imply that only the value ofkmatters.