[#std-hash-07]
= xref:std.adoc[std]::hash<T>
:relfileprefix: ../
:mrdocs:

Hash support so strong types can be keys in unordered containers.

== Synopsis

Declared in `<folly/lang/Strong.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<typename T>
requires (
      folly::is_strong<T> &&
      // Required for `folly::is_hashable_v` to be correct.
      requires(typename T::underlying_type underlying) {
        {
          std::hash<typename T::underlying_type>{}(underlying)
        } ‐> std::convertible_to<std::size_t>;
      })
struct hash<T>;
----

== Description

A strong type is hashable iff the underlying type is.

== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:std/hash-07/operator_call.adoc[`operator()`] 
| Hash a strong value by hashing its underlying value.
|===



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