Hash

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.

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.

Standard Library

hash

(since C++11)

hash function object

(class template)

Defect reports

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

LWG 2291 (C++11)

Link	https://cplusplus.github.io/LWG/issues/2291.html
Applied to	C++11
Behavior as published	same results for same arguments were required in all cases
Correct behavior	only required within a single execution