# C++ concepts: Hash

From cppreference.com
< cpp‎ | concept

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 (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

std::hash

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