stdtypeindex

std::type_index is a standard library class that provides a lightweight, comparable wrapper around run-time type information obtained from std::type_info. It is defined in the <typeindex> header and is commonly used to identify and compare types in containers, registries, and dispatch schemes. While the term stdtypeindex is sometimes seen in informal writing, the official name is std::type_index.

A type_index object can be constructed from a std::type_info object, via the constructor std::type_index(const std::type_info&). Because there is an implicit conversion from std::type_info to std::type_index, you can obtain an index directly from typeid, for example: auto ti = std::type_index(typeid(T)); or simply std::type_index ti = typeid(T);.

std::type_index supports equality and ordering through operator==, operator!=, and operator<. This enables its use as a key in ordered containers, such as std::map and std::set. A specialization of std::hash<std::type_index> is provided, making std::type_index usable as a key in unordered containers like std::unordered_map and std::unordered_set.

Internally, std::type_index identifies a type by the underlying std::type_info, which is stable for the program lifetime. It is a convenient way to store and compare type identities without carrying extra type-specific data. Common use cases include runtime type registries, type-based dispatch, and schema-like mappings that depend on type rather than value.

Limitations include that type_index does not provide per-type metadata beyond the identity, and human-readable names from std::type_info::name() are implementation-defined and may be mangled. std::type_index is available in C++11 and later and plays a central role in type-erasure and reflection-like patterns in the standard library.