16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.4 Requirements on types and expressions [utility.requirements] Cpp17Hash requirements [hash.requirements]

A type H meets the Cpp17Hash requirements if:
  • it is a function object type ([function.objects]),
  • it meets the Cpp17CopyConstructible (Table 32) and Cpp17Destructible (Table 35) requirements, and
  • the expressions shown in Table 37 are valid and have the indicated semantics.
Given Key is an argument type for function objects of type H, in Table 37 h is a value of type (possibly const) H, u is an lvalue of type Key, and k is a value of a type convertible to (possibly const) Key.
Table 37: Cpp17Hash requirements [tab:cpp17.hash]
Return type
The value returned shall depend only on the argument k for the duration of the program.
[Note 1: 
Thus all evaluations of the expression h(k) with the same value for k yield the same result for a given execution of the program.
— end note]
For two different values t1 and t2, the probability that h(t1) and h(t2) compare equal should be very small, approaching 1.0 / numeric_limits<size_t>​::​max().
Shall not modify u.