C++ named requirements: Hash

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C++ named requirements

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.


The type T satisfies Hash if


  • 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 (since C++14)

All evaluations of h(k) executed within a given execution of a program (since C++14) 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