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

A Hash

Requirements

The type T satisfies Hash if

• The type T satisfies FunctionObject, CopyConstructible, Destructible

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