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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/Hash.html below:

C++ named requirements: Hash (since C++11)

A Hash is a function object for which the output depends only on the input and has a very low probability of yielding the same output given different input values.

[edit] Requirements

The type T satisfies Hash if

The type T satisfies FunctionObject, CopyConstructible, Destructible, and

Given

h, a value of type T or const T, whose argument type is Key,
k, a value of type convertible to Key or const Key,
u, an lvalue expression of type Key.

The following expressions must be valid and have their specified effects.

Expression Return type Requirements h(k) std::size_t The returned value depends only on the value of k for the duration of the program.

All evaluations of h(k) executed within a given execution of a program yield the same result for the same value of k.

The probability of h(a) == h(b) for a != b should approach 1.0 / std::numeric_limits<std::size_t>::max().

h(u) std::size_t u is not modified. [edit] Standard library hash function object
(class template) [edit] [edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 2291 C++11 same results for same arguments were required in all cases only required within a single execution

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