stdhash

Stdhash is a term used to describe the standard hashing interface provided by a programming language’s standard library, enabling the calculation of hash values for objects so they can be stored in hash-based containers such as maps and sets. The idea behind stdhash is to separate the notion of hashing from the objects themselves, allowing reusable and interchangeable hash functions.

A typical stdhash interface centers on a function object or trait that can be invoked to produce a hash value. In many languages, this takes the form of a template or generic type named Hash or Hasher, with an operator or method that accepts a value of type T and returns an integer type, commonly size_t. Specializations or implementations define how different types are hashed. User-defined types can usually provide a custom specialization to enable hashing, aligning with the language’s equality semantics.

In C++, std::hash<T> serves as the canonical example. The standard library provides specializations for common primitives and standard types, and user code can supply a specialization for a custom type (for example, a struct) to enable inclusion in unordered containers. The key requirements are that hashing is deterministic for a given process, equal objects produce equal hashes, and hashes are distributed to minimize collisions.

Hash-based containers rely on stdhash to index elements efficiently, with containers such as unordered_map and unordered_set depending on the hash function and equality comparator to manage buckets and lookups. It is important to note that stdhash is not a cryptographic hash; it is designed for performance and distribution in data structures, not for security.

See also: std::hash, hash function, unordered_map, unordered_set, Hasher.