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