stdhashstdtypeindex

The std::hash<std::type_index> is a specialization of the std::hash class template defined in the C++ Standard Library. It provides a hashing function for objects of type std::type_index. std::type_index is a class that represents a run-time type obtained from a RTTI (Run-Time Type Information) typeid expression. Hashing a std::type_index allows it to be used as a key in hash-based associative containers such as std::unordered_map or std::unordered_set.

The std::hash<std::type_index> functor generates a hash value based on the underlying type information. This hash value is a unique integral representation of the type. The standard guarantees that if two std::type_index objects represent the same type, their hash values will be identical. This property is crucial for the correct functioning of unordered containers, ensuring that elements with the same type are located in the same bucket.

When you need to store or look up information based on types rather than specific values, using std::type_index as a key in an unordered container becomes very useful. For example, you might want to maintain a registry of factory functions keyed by the type of object they create. The std::hash<std::type_index> specialization is what makes this possible by providing the necessary hashing mechanism.