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.

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 (C++11)

hash function object (class template)

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