typeindex

typeindex is the C++ standard library class std::type_index, a wrapper around std::type_info that makes runtime type information usable as keys in associative containers. It is defined in the header <typeindex> alongside std::type_info and the typeid operator. std::type_index provides a stable, comparable representation of a type, enabling type-based lookups and dispatch.

A type_index value is typically created from the typeid expression, for example by std::type_index t(typeid(MyClass)); it can also be constructed from a std::type_info object. The resulting object can be compared, ordered, and used as a key in both ordered and unordered containers.

Key features include support for equality and ordering comparisons (operator== and operator<), which allows std::type_index to serve as keys in std::map and std::set. There is also a specialization of std::hash for std::type_index, enabling its use as a key in unordered containers such as std::unordered_map and std::unordered_set. This combination provides a convenient way to associate metadata or handlers with concrete types at runtime.

Common use cases involve type-based registries, per-type dispatch tables, and maintaining metadata about types in a runtime system. For example, a registry might map std::type_index(typeid(SomeDerived)) to a factory function or a handler object, allowing code to select behavior based on the dynamic type of objects.

Note that std::type_index does not implement a full reflection system; it merely provides a stable, comparable representation of a type for runtime lookups and dispatch. It relies on the type information provided by typeid and the C++ type system.