[#hash_traits]
== Hash traits

:idprefix: hash_traits_

=== Synopsis

[listing,subs="+macros,+quotes"]
-----
// #include <boost/unordered/hash_traits.hpp>

namespace boost {
namespace unordered {

template<typename Hash>
struct xref:#hash_traits_hash_is_avalanching[hash_is_avalanching];

} // namespace unordered
} // namespace boost
-----

---

=== hash_is_avalanching
```c++
template<typename Hash>
struct hash_is_avalanching;
```

A hash function is said to have the _avalanching property_ if small changes in the input translate to
large changes in the returned hash code &#8212;ideally, flipping one bit in the representation of
the input value results in each bit of the hash code flipping with probability 50%. Approaching
this property is critical for the proper behavior of open-addressing hash containers.

`hash_is_avalanching<Hash>::value` is:

 * `false` if `Hash::is_avalanching` is not present,
 * `Hash::is_avalanching::value` if this is present and convertible at compile time to a `bool`,
 * `true` if `Hash::is_avalanching` is `void` (this usage is deprecated),
 * ill-formed otherwise.
 
Users can then declare a hash function `Hash` as avalanching either by embedding an appropriate `is_avalanching` typedef
into the definition of `Hash`, or directly by specializing `hash_is_avalanching<Hash>` to a class with
an embedded compile-time constant `value` set to `true`.

Open-addressing and concurrent containers
use the provided hash function `Hash` as-is if `hash_is_avalanching<Hash>::value` is `true`; otherwise, they
implement a bit-mixing post-processing stage to increase the quality of hashing at the expense of
extra computational cost.

---